Helicopter Structures Student Resource PDF

Student Resource Subject B1-12b: Helicopter Structures Copyright © 2018 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia CONTROLLED DOCUMENT 2018-08-21 B1-12b Helicopter Structures Page 2 of 7 CONTENTS CONTENTS............................................................................................................................................ 3 DEFINITIONS......................................................................................................................................... 4 STUDY RESOURCES............................................................................................................................... 5 INTRODUCTION.................................................................................................................................... 6 Topic 12.5.1.1 Airframe Structures – General Concepts................................................................ 6 Topic 12.5.2.1 Airframe Structures – Primary Construction.......................................................... 6 Topic 12.5.2.2 Airframe Structures – Secondary Construction...................................................... 7 Topic 12.5.2.3 Airframe Structures – Surface Finishing................................................................. 7 2018-08-21 B1-12b Helicopter Structures Page 3 of 7 DEFINITIONS Define To describe the nature or basic qualities of. To state the precise meaning of (a word or sense of a word). State Specify in words or writing. To set forth in words; declare. Identify To establish the identity of. List Itemise. Describe Represent in words enabling hearer or reader to form an idea of an object or process. To tell the facts, details, or particulars of something verbally or in writing. Explain Make known in detail. Offer reason for cause and effect. 2018-08-21 B1-12b Helicopter Structures Page 4 of 7 STUDY RESOURCES B1-12b Student Handout 2018-08-21 B1-12b Helicopter Structures Page 5 of 7 INTRODUCTION The purpose of this subject is to familiarise you with the basic construction, requirements and maintenance of helicopter airframe structures. On completion of the following topics you will be able to: Topic 12.5.1.1 Airframe Structures – General Concepts State the airworthiness requirements for structural strength. Identify inspection programs associated with ageing aircraft. Describe primary, secondary and tertiary structural classifications. Describe fail safe, safe life and damage tolerance concepts. Describe zonal and station identification systems. Describe the following with regards to airframe structures: Stress Strain Bending Compression Shear Torsion Tension Hoop Stress Fatigue Describe drain and ventilation provisions used in helicopters. Describe provisions used in helicopters for system installations. Describe provisions for lightning protection. Topic 12.5.2.1 Airframe Structures – Primary Construction Identify the following airframe structures and describe their constructions: Stressed skin fuselage. Formers. Stringers. Longerons. Bulkheads. Frames. Doublers. Struts. 2018-08-21 B1-12b Helicopter Structures Page 6 of 7 Ties. Floor Structures. Reinforcement. Methods of skinning and anti-corrosion. Cabin pressure control. Describe the following structure assembly technique: Riveting. Bolting. Bonding. Topic 12.5.2.2 Airframe Structures – Secondary Construction Describe the attachment methods for the following: Pylons. Stabilisers. Landing gear. Describe the general construction and installation of passenger and crew seats. Describe the construction of doors, mechanisms, operation and safety devices. Describe window and windscreen construction and mechanisms. Describe construction of fuel storage tanks. Describe the construction of nacelles and pylons. Describe the construction of firewalls. Describe the construction of engine mounts. Describe methods of alignment and symmetry checks used for inspecting airframes. Topic 12.5.2.3 Airframe Structures – Surface Finishing Describe methods of surface protection, such as chromating, anodising and painting. Describe methods of surface cleaning. 2018-08-21 B1-12b Helicopter Structures Page 7 of 7 Topic 12.5.1.1 – Airframe Structures - General Concepts Table of Contents List of Figures....................................................................................................................................... 3 Topic 12.5.1.1 – Airframe Structures - General Concepts............................................................. 5 Introduction......................................................................................................................................... 5 Stress.................................................................................................................................................... 5 Tension................................................................................................................................................. 6 Compression........................................................................................................................................ 6 Torsion................................................................................................................................................. 7 Bending................................................................................................................................................ 8 Shear.................................................................................................................................................... 9 Hoop Stress........................................................................................................................................ 10 Strain.................................................................................................................................................. 11 Fatigue................................................................................................................................................ 11 Identification Systems.............................................................................................................. 12 Datum................................................................................................................................................. 13 Station Lines....................................................................................................................................... 14 Water Lines........................................................................................................................................ 15 Buttock Lines...................................................................................................................................... 15 Clock Positions................................................................................................................................... 16 Aircraft Airworthiness.............................................................................................................. 17 Introduction....................................................................................................................................... 17 Aircraft Airworthiness........................................................................................................................ 17 FAR-29, CASR 29 and CS-29................................................................................................................ 18 Structural Strength Requirements..................................................................................................... 18 Fail Safe.............................................................................................................................................. 19 Safe Life.............................................................................................................................................. 20 Damage Tolerance............................................................................................................................. 21 Damage Tolerance...................................................................................................................................... 21 Negligible.................................................................................................................................................... 21 Repairable................................................................................................................................................... 21 Unrepairable............................................................................................................................................... 21 Drainage............................................................................................................................................. 22 Ageing Aircraft Inspection Requirements.................................................................................. 24 Task 5 – 14 CFR/JAR 25 Ageing Aircraft............................................................................................. 24 2017-03-09 B1-12.5.1 General Concepts Page 1 of 38 Training Material Only Background........................................................................................................................................ 24 Structural Classifications.......................................................................................................... 27 Primary Structure............................................................................................................................... 28 Secondary Structure........................................................................................................................... 28 Tertiary Structure............................................................................................................................... 28 Installations............................................................................................................................. 29 Quick Disconnect Valves.................................................................................................................... 29 External Power Receptacles............................................................................................................... 30 Avionics Racks.................................................................................................................................... 31 Ground Locking Pins........................................................................................................................... 32 Anchor Nuts....................................................................................................................................... 33 Turnlock Fasteners............................................................................................................................. 34 Electrical Connectors......................................................................................................................... 34 Bonding.............................................................................................................................................. 35 Lightning Protection................................................................................................................................... 35 Bonding Straps............................................................................................................................................ 35 Bonding Straps (cont):................................................................................................................................ 36 Lightning Diverter Strips............................................................................................................................. 37 Lightning Strike Inspection......................................................................................................................... 37 Conditional Inspections...................................................................................................................... 38 2017-03-09 B1-12.5.1 General Concepts Page 2 of 38 Training Material Only List of Figures Figure-1. Formula for Stress............................................................................................................. 5 Figure-2. Tension.............................................................................................................................. 6 Figure-3. Compression...................................................................................................................... 6 Figure-4. Compression on a Rivet..................................................................................................... 7 Figure-5. Torsion............................................................................................................................... 7 Figure-6. Engine Shaft Suffering from Torsional stress.................................................................... 8 Figure-7. Bending............................................................................................................................. 8 Figure-8. Bending of Rotor Blades.................................................................................................... 9 Figure-9. Shear.................................................................................................................................. 9 Figure-10. Shear on a Clevis Bolt...................................................................................................... 10 Figure-11. Hoop Stress..................................................................................................................... 10 Figure-12. Strain............................................................................................................................... 11 Figure-13. Identification Systems..................................................................................................... 12 Figure-14. Datum.............................................................................................................................. 13 Figure-15. Station Lines.................................................................................................................... 14 Figure-16. Water Lines..................................................................................................................... 15 Figure-17. Buttock Lines................................................................................................................... 15 Figure-18. Clock Position.................................................................................................................. 16 Figure-19. Fail Safe........................................................................................................................... 19 Figure-20. Safe Life........................................................................................................................... 20 Figure-21. Damage Tolerance.......................................................................................................... 21 Figure-22. Corrosion......................................................................................................................... 22 Figure-23. Drains.............................................................................................................................. 22 Figure-24. Drain Holes...................................................................................................................... 23 Figure-25. Structural Classifications................................................................................................. 27 Figure-26. Quick Disconnect Valves................................................................................................. 29 Figure-27. External DC Power Receptacles...................................................................................... 30 Figure-28. Avionics Racks................................................................................................................. 31 Figure-29. Ground Locking Pins........................................................................................................ 32 Figure-30. Anchor Nuts..................................................................................................................... 33 Figure-31. Turnlock Fasteners.......................................................................................................... 34 Figure-32. Electrical Connectors....................................................................................................... 34 Figure-33. Bonding Straps................................................................................................................ 36 2017-03-09 B1-12.5.1 General Concepts Page 3 of 38 Training Material Only Figure-34. Braided Bonding Straps................................................................................................... 36 Figure-35. Lightning Protection........................................................................................................ 37 2017-03-09 B1-12.5.1 General Concepts Page 4 of 38 Training Material Only Topic 12.5.1.1 – Airframe Structures - General Concepts Introduction The airframe, or fundamental structure, of a helicopter can be made of either metal or organic composite materials, or some combination of the two. Higher performance requirements will incline the designer to favour composites with higher strength-to-weight ratio, often epoxy (a resin) reinforced with glass, aramid (a strong, flexible nylon fibre), or carbon fibre. Typically, a composite component consists of many layers of fibre-impregnated resins, bonded to form a smooth panel. Tubular and sheet metal substructures are usually made of aluminium, though stainless steel or titanium is sometimes used in areas subject to higher stress or heat. To facilitate bending during the manufacturing process, the structural tubing is often filled with molten sodium silicate. A helicopter's rotary wing blades are usually made of fibre-reinforced resin, which may be adhesively bonded with an external sheet metal layer to protect edges. The helicopter's windscreen and windows are formed of polycarbonate sheeting. Stress Stress is the force or load acting on or through a unit area of material. Stress is the internal force in a body that resists the tendency of an external force to change its shape. When an external force acts on a body, it is opposed by an internal force called stress. The English measure for stress is pounds per square foot, or pounds per square inch. Stress is shown as the ratio: Figure-1. Formula for Stress 2017-03-09 B1-12.5.1 General Concepts Page 5 of 38 Training Material Only Tension Tension describes forces that tend to pull an object apart. It is a stress produced in a body by forces acting along the same line but in opposite directions. Flexible steel cables used in helicopters for ties are designed to withstand tension loads. Steel cable is easily bent and has little opposition to other types of stress; however, when subjected to a purely tensional load it performs exceptionally well. Figure-2. Tension Compression Compression is the resistance to an external force that tries to push an object together is the resultant stress of two forces which act along the same line pushing against each other. Figure-3. Compression 2017-03-09 B1-12.5.1 General Concepts Page 6 of 38 Training Material Only Aircraft rivets are driven with a compressive force. When compression stresses are applied to a rivet, the rivet shank expands until it fills the hole and forms a butt to hold materials together. Figure-4. Compression on a Rivet Torsion Torsion is the stress applied to a material when it is twisted. It is a combination of tension and compression stresses. When an object is subjected to torsional stress, tensional stresses operate diagonally across the object while compression stresses act at right angles to the tension stresses. Figure-5. Torsion 2017-03-09 B1-12.5.1 General Concepts Page 7 of 38 Training Material Only An engine crankshaft is a component whose primary stress is torsion. The pistons pushing down on the connecting rods rotate the crankshaft against the opposition caused by the propeller. The resulting stresses attempt to twist the crankshaft. Figure-6. Engine Shaft Suffering from Torsional stress Bending Bending is the stress in an object caused by load being applied to one end while the other is restrained. Like Torsion, Bending stress is also a combination of tension and compression stresses. Figure-7. Bending Helicopter blades in flight are subjected to bending stress due to lift created; the top surface is subjected to compression and the lower skin is subjected to tension. However when the helicopter is on the ground, bending stress is reversed because of blade sag – top surface tension and lower surface compression. 2017-03-09 B1-12.5.1 General Concepts Page 8 of 38 Training Material Only Figure-8. Bending of Rotor Blades Shear Shear is the force that when exerted on a body, it tries to slice or slide it apart. Figure-9. Shear 2017-03-09 B1-12.5.1 General Concepts Page 9 of 38 Training Material Only Aircraft clevis bolts are subjected to shear stress on an aircraft when the items they are holding together have tension or compression loads applied to them. However, when no force is present, the clevis bolt is free to turn in its hole. Figure-10. Shear on a Clevis Bolt Hoop Stress Hoop stress is stress in a pipe wall acting circumferentially in a plane perpendicular to the longitudinal axis of the pipe produced by the pressure inside the pipe. Cooking sausages are susceptible to hoop stress as the pressure inside the skin increases due to the heat. Cracks along the longitudinal axis are the resultant failure of the skin. Another example is where cracks in car radiator hoses are caused by the pressure from the heated water. Figure-11. Hoop Stress 2017-03-09 B1-12.5.1 General Concepts Page 10 of 38 Training Material Only Strain Strain is the deformation or physical change in a material that is caused by stress. Hooke’s law states that if strain does not exceed the elastic limit of a body, it is directly proportional to the applied stress. This fact allows beams and springs to be used as measuring devices. For example, as force is applied to a hand torque wrench, its deformation, or bending, is directly proportional to the strain it is subjected to. Therefore, the amount of torque deflection can be measured and used as an indication of the amount of stress applied to a bolt. Excessive bending stress causes significant physical change (strain) as displayed by the control rod shown below. Figure-12. Strain Fatigue Fatigue is the condition that exists in metal that causes it to lose some strength. It occurs when metal is subjected to a series of stress reversals. For example, metal is bent back and forth repeatedly. Helicopter blades are susceptible to fatigue due to different bending stresses put on them in the air as opposed to on the ground. 2017-03-09 B1-12.5.1 General Concepts Page 11 of 38 Training Material Only Identification Systems The location of a part on a helicopter is typically specified by fuselage station numbers, water lines, and buttock lines. Another position referencing system - clock positioning often used by engineers or flight crew is to report faults on the aircraft or engine. To help identify specific areas of the helicopter for inspection purposes the helicopter is broken up into zones for inspection purposes. Figure-13. Identification Systems 2017-03-09 B1-12.5.1 General Concepts Page 12 of 38 Training Material Only Datum The datum is an imaginary vertical plane from which all horizontal measurements are taken with the helicopter in a level flight attitude. The reference datum sits at a right angle to the helicopters longitudinal axis. For each helicopter make and model, the location of all items including equipment, tanks, baggage compartments, seats, engines are listed in the Helicopter Specifications or Type Certificate Data Sheets as being so many inches from the datum. There is no fixed rule for the location of a datum. The manufacturer chooses a location for the datum where it is most convenient for measurement, identifying the location of equipment, and for weight-and-balance computation. It may be located on the nose of the helicopter, the fire-wall, or even at a point in space ahead of the helicopter. Figure-14. Datum 2017-03-09 B1-12.5.1 General Concepts Page 13 of 38 Training Material Only Station Lines Fuselage station numbers identify locations fore and aft along the fuselage. All station numbers are measured from a reference called station zero. This reference, often called the datum, is typically on the fuselage or ahead of it. Dimensions are measured aft of the datum reference point along the length of the fuselage. All fuselage frames and bulkheads are identified by fuselage station numbers. For example, if the datum is six inches ahead of the fuselage nose and the centre line of the main bulkhead is 137 inches from the datum; its fuselage station number is 137. Figure-15. Station Lines 2017-03-09 B1-12.5.1 General Concepts Page 14 of 38 Training Material Only Water Lines Waterlines are used to identify structure by vertical measurement. Zero water line is the edge of the base reference plane. Like station numbers, water lines are measured from a datum reference. In this case the zero reference is called water line zero. Waterline zero may be located on or below the fuselage, sometimes even below ground level. Water lines are positive above zero and negative below zero. For example, if the floor of the main cabin must be installed at water line -16, the floor is 16 inches below water line zero. Figure-16. Water Lines Buttock Lines Distances to the right or left of the fuselage centre line are measured by buttock lines and are referenced from an aircraft’s longitudinal centreline (Buttock Line zero). For example, if the tip of a horizontal surface is located at buttock line 108.88, it means that it is 108.88 inches from the fuselage centreline. Figure-17. Buttock Lines 2017-03-09 B1-12.5.1 General Concepts Page 15 of 38 Training Material Only Clock Positions Used to report faults on the aircraft or engine. Figure-18. Clock Position 2017-03-09 B1-12.5.1 General Concepts Page 16 of 38 Training Material Only Aircraft Airworthiness Introduction Before a helicopter can take to the skies, they are required to have an authorised Type Certificate and a Certificate of Airworthiness. Procedural requirements for the issue of Type Certificates and Certificates of Airworthiness can be found in Part 21 of the appropriate regulatory authority. Part of the certification process is satisfying guidelines laid down in separate regulations for the category of aircraft type being applied for. These category regulations include but are not limited to: General requirements Flight requirements Structure requirements Design and construction requirements Power plant requirements Equipment requirements Operating Limitations and Information requirements Structural strength requirements of a helicopter, or a helicopter component, are developed in the initial design phase of the helicopter or component, so they can withstand the operational stresses exerted on them. Aircraft Airworthiness There are 5 separate aircraft categories: Part 22 - Sailplanes and Powered Sailplanes Part 23 - Small Aeroplanes Part 25 - Transport Category Aeroplanes Part 26 - Primary/Intermediate Category Aeroplanes Part 27 - Rotorcraft Part 29 - Rotorcraft (Transport Category) NOTE: To maintain standardisation with ICAO, the numbering system for categories remains the same within EASA Certification Specification (CS), Federal Aviation Authority FARs and the Civil Aviation Safety Authority CASRs. 2017-03-09 B1-12.5.1 General Concepts Page 17 of 38 Training Material Only FAR-29, CASR 29 and CS-29 This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for transport category rotorcraft. Transport category rotorcraft must be certificated in accordance with either the Category A or Category B requirements of this part. A multiengine rotorcraft may be type certificated as both Category A and Category B with appropriate and different operating limitations for each category. Rotorcraft with a maximum weight greater than 20 000 pounds and 10 or more passenger seats must be type certificated as:- Category A Rotorcraft. Rotorcraft with a maximum weight greater than 20 000 pounds and nine or less passenger seats may be type certificated as:- Category B Rotorcraft provided the Category A requirements of Subparts C, D, E, and F of this part are met. Rotorcraft with a maximum weight of 20 000 pounds or less but with 10 or more passenger seats may be type certificated as:- Category B Rotorcraft provided the Category A requirements of 29.67 (a)(2), 29.87, 29.1517, and subparts C, D, E, and F of this part are met. Rotorcraft with a maximum weight of 20 000 pounds or less and nine or less passenger seats may be type certificated as:- Category B Rotorcraft. Each person who applies under Part 21 for a certificate or change described in paragraphs (a) through (f) of this section must show compliance with the applicable requirements of this part. Structural Strength Requirements CS-29 Section 1 - Subpart C - Structures in each aircraft category regulation contains the structural strength requirements that an aircraft or component must adhere to in order to obtain a Type Certificate and a Certificate of Airworthiness. These requirements include but are not limited to: Flight loads Flight manoeuvring loads Control surface and systems loads Ground loads Structural strength requirements are specified in terms of: Limit load:- maximum load expected in service Ultimate load:- limit load multiplied by prescribed factors of safety Ultimate Factor of Safety (U.F.S.):- Ultimate Load/Limit Load The structure must be able to support the limit load without detrimental permanent deformation. The structure must be able to support the ultimate load without failure for at least 3 seconds. Ultimate Factor of Safety (U.F.S.) is a design value. Normally, U.F.S = 1.5 for Aircraft. 2017-03-09 B1-12.5.1 General Concepts Page 18 of 38 Training Material Only Fail Safe Helicopter structural engineers design helicopters to withstand structural loads for the expected lifetime of the airframe. To ensure that the helicopter has a full safe life, engineers include a Fail Safe Structural Analysis in the initial design. That means ensuring the helicopter structure as a whole has sufficient strength to prevent complete structural failure when a component of that structure has failed. This example of a fail-safe structure is shown. The structure is assumed to be a Fail Safe Multiple Load Path Structure and the steps required to satisfy this requirement will be outlined. The structure will be designed to be Fail Safe by virtue of being able to sustain the failure of one load path and still maintain the residual strength and remaining structural guidelines. Figure-19. Fail Safe 2017-03-09 B1-12.5.1 General Concepts Page 19 of 38 Training Material Only Safe Life Safe life is a measure of the useful life of the material and is determined by the number of cycles of loading, of a specified character, that a given specimen of material can sustain before failure occurs In Safe-Life design products are designed to survive a specific design life with a chosen reserve. Employed in critical systems which are either very difficult to repair or may cause severe damage to life and property. These systems are designed to work for years without requirement of any repairs. Typically a Safe Life / Fatigue Life was established by conducting a fatigue test to a helicopter design on either a complete structure or major components and applying a timeframe for life to failure for the fleet. Figure-20. Safe Life 2017-03-09 B1-12.5.1 General Concepts Page 20 of 38 Training Material Only Damage Tolerance In the 1960s it became evident that the original philosophy based on safe life was inadequate for structural components, because it did not account for fatigue cracking arising from damage in the structure from manufacturing processes or from in-service maintenance of the aircraft or for different operating conditions for different aircraft. Damage Tolerance Damage tolerance is applied to the design and modification of aircraft and provides guidance on the application of durability and damage tolerance requirements to aircraft structures. Aircraft designers and manufacturers use Damage Tolerance philosophies to determine NDI schedules for airframe components. Damage Tolerance requires a structure to be strong enough to sustain a defect until it is detected. Negligible Negligible damage is damage which is allowed to exist or stay as is. It is deemed not to have any adverse effects on the helicopter’s airworthiness. Repairable Repairable damage is damage which cannot remain as is. It may have adverse effects on aircraft airworthiness but is not deemed bad enough to warrant replacement parts being installed. Unrepairable Unrepairable damage is damage deemed to be too severe to leave as is, or repair to a serviceable state. Figure-21. Damage Tolerance 2017-03-09 B1-12.5.1 General Concepts Page 21 of 38 Training Material Only Drainage Three conditions must exist simultaneously for corrosion to take place: An anode and a cathode A metallic connector between the anode and cathode An electrolyte Figure-22. Corrosion Controlling the presence of the moisture is usually the most effective means of preventing corrosion. Effective drainage of all structure is vital to prevent fluids from becoming trapped in crevices and causing corrosion. Fluids are directed to these drain holes by a system of longitudinal and cross-drain paths through the stringers and frame shear clips. Figure-23. Drains 2017-03-09 B1-12.5.1 General Concepts Page 22 of 38 Training Material Only Helicopters have small drilled holes in the skin for moisture drainage holes. They also allow fresh air to ventilate through the structure to help dry out any residual moisture. Drain paths and drain holes prevent galvanic corrosion from occurring. Figure-24. Drain Holes 2017-03-09 B1-12.5.1 General Concepts Page 23 of 38 Training Material Only Ageing Aircraft Inspection Requirements The ageing aircraft program requires each transport aircraft to undergo a series of tests and inspections after it has been in service for a specified number of years and/or has accumulated a certain number of flight cycles. These inspections are generally a combination of visual and non- destructive testing methods such as ultrasound, eddy current and x-rays. Corrosion control and structural repairs are made when necessary. Since major disassembly of the aircraft is required to complete the relevant inspections, rigging of the aircraft control system will be necessary. The aged aircraft program must be part of the scheduled maintenance program conducted by an approved Part 145 organisation. Some manufacturers may require major structural disassembly after a certain number of flight hours. For example, some Lear jet models require removal of the wing after approximately 5,000n flight hours for a series of Inspections. The manufacturer of an aircraft may specify this type of disassembly and inspection after a specific period of flight hours, flight cycles, months or years. Task 5 – 14 CFR/JAR 25 Ageing Aircraft Background In 1988, the industry experienced a significant failure of the airworthiness system. This system failure allowed an aeroplane to fly with significant unrepaired multiple site fatigue damage to the point where the aeroplane experienced a rapid fracture and loss of a portion of the fuselage. As a direct result of this accident, the FAA hosted The International Conference on Ageing Aeroplanes on June 1-3, 1988 in Washington D. C. As a result of this conference, an organisation of Operators, Manufacturers and Regulators was formed under the Federal Advisory Committee Act to investigate and propose solutions to the problems evidenced as a result of the accident. This group is now known as the Airworthiness Assurance Working Group (AAWG) (Formally known as the Airworthiness Assurance Task Force). During the 1988 conference, several Airline/Manufacturer recommendations were presented to address the apparent short falls in the airworthiness system including Recommendation 3, which stated: "Continue to pursue the concept of teardown of the oldest airline aircraft to determine structural condition, and conduct fatigue tests of older aeroplanes as per attached proposal" In June 1989, the National Transportation Safety Board (NTSB) made Recommendation 89067 that requested the FAA to pursue necessary tasks to ensure continued safe operations with probable Widespread Fatigue Damage (WFD). WFD was noted by the NTSB to be a contributing cause of the April 1988 Aloha Airlines 737 accident. The NTSB specifically recommended extended fatigue testing for older aeroplanes. In November 1989, the FAA responded by issuing a ‘strawman’ Supplementary Federal Aviation Regulation (SFAR) RE: TWO-LIFE-TIME FATIGUE TEST FOR OLDER AEROPLANES. (The premise behind building a ‘strawman’ – creating a first draft for criticism and testing, and then using the feedback you receive to develop a final outcome that is rock solid.) 2017-03-09 B1-12.5.1 General Concepts Page 24 of 38 Training Material Only In June 1990, the AAWG tasked the formal evaluation of the Aerospace Industries Association of America International Air Transport Association (AIAIATA) Recommendation 3. An alternative approach, to the strawman SFAR was developed by the AAWG and presented to the FAA in March 1991. The FAA accepted this alternative approach in June 1991. The AAWG was informally tasked to institutionalise the position in July. It is urged that any future publications on the subject of widespread fatigue damage should include, or at least reference this standard terminology, in order to avoid possible confusion within the industry. Damage Tolerance is the attribute of the structure that permits it to retain its required residual strength without detrimental structural deformation for a period of use after the structure has sustained specific levels of fatigue, corrosion, accidental or discrete source damage. Widespread Fatigue Damage (WFD) in a structure is characterised by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement (i.e. to maintain its required residual strength after partial structural failure). Multiple Site Damage (MSD) is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in the same structural element (i.e. fatigue cracks that may coalesce with or without other damage leading to a loss of required residual strength). Multiple Element Damage (MED) is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in similar adjacent structural elements. In addition, the AAWG proposes the adoption of the following terminology during discussion of programs to ensure continuing structural integrity: Fatigue Crack Initiation is that point in time when a finite fatigue crack is first expected. Point of WFD is a point reduced from the average expected behaviour, i.e. lower bound, so that operation up to that point provides equivalent protection to that of a two-lifetime fatigue test. Monitoring Period is the period of time when special inspections of the fleet are initiated due to an increased risk of MSD/MED, and ending when the point of WFD is established. Design Service Goal (DSG) is the period of time (in flight cycles/hours) established at design and/or certification during which: 1. The principal structure will be reasonably free from significant cracking 2. Widespread fatigue damage is not expected to occur. Extended Service Goal (ESG) is an adjustment to the design service goal established by service experience, analysis, and/or test during which: 1. The principal structure will be reasonably free from significant cracking 2. Widespread fatigue damage is not expected to occur. Furthermore, certain terminology has been considered by past working groups in relation to the problem of WFD, but was not used in the final ARAC definitions. The following terms have been previously identified as being open to misinterpretation, and should be avoided, or defined carefully if their use is essential. 2017-03-09 B1-12.5.1 General Concepts Page 25 of 38 Training Material Only Threshold has been used in various contexts, such as Fatigue Threshold, which may be defined as the first typical fatigue crack in the fleet for that element Inspection Threshold, which may be defined as the start of supplemental inspections for WFD The AAWG believes that the real meaning of WFD in this context is MSD/MED. Onset has been used as an alternative to Threshold, although the simultaneous use of both terms may cause confusion. Sub-Critical has been used in relation to certain fatigue cracks. However, this may require clarification of what are critical fatigue cracks with reference to occurrence of WFD. There are a number of general conditions and details that must be met in order that a monitoring period concept can be used. These conditions are: No aeroplane may be operated beyond the defined Point of WFD without modification or part replacement The first special inspections, to occur in the monitoring period, should be in line with the estimation of fatigue crack initiation To use a monitoring period for a detail suspected of developing MSD/MED, it must be determined that inspections will reliably detect a crack before the crack becomes critical. If a crack cannot be reliably detected, a monitoring period cannot be used By empirical analysis, evaluation of test evidence and/or evaluation of in-service data, the inspection requirements will be defined for application during the monitoring period The purpose of these inspections is to collect data for reassessment of WFD parameters and to maintain structural integrity (e.g., acceptable level of risk during the monitoring period). Inspections within the monitoring period are mandatory on every aeroplane as well as reporting of inspection results In the case of MSD or MED findings, the Point of WFD will be re-established in accordance to the inspection results. The area of concern will be repaired following a detailed inspection of adjacent areas using NDI technology that will detect small cracks with a high degree of confidence. The remaining aeroplanes may be operated up to the revised Point of WFD, with application of a revised monitoring program. Prior to the Point of WFD, the aeroplane must be repaired, modified, or retired If no MSD/MED cracking is detected by the time the high time aeroplane reaches the predicted Point of WFD, the predicted Point of WFD could be re-evaluated and the special inspection program may be continued after revalidation The monitoring period will terminate at the point in time at which there is sufficient findings to confirm a MSD/MED problem exists and/or the Point of WFD is reached. This will be recommended with the assistance of the STG using an established process 2017-03-09 B1-12.5.1 General Concepts Page 26 of 38 Training Material Only Structural Classifications All helicopter structural components are classified into groups according to their importance for the structural integrity of the helicopter. There are 3 structural classifications: Primary structure Secondary structure Tertiary structure Figure-25. Structural Classifications 2017-03-09 B1-12.5.1 General Concepts Page 27 of 38 Training Material Only Primary Structure The portions of the helicopter structure which if it were to fail in flight, landing or taking off, might cause structural collapse, loss of control, failure of motive power, or serious injury to members of the aircrew. e.g. engine mounts and landing gear. Secondary Structure The portions of structure which would normally be regarded primary but have reserve strength over design requirements that appreciable weakening may be permitted without risk or failure. Portions of the structure are also classified as secondary if their failure would not seriously endanger the safety of the aircraft but may cause significant damage. e.g. fairings. Tertiary Structure The portions of the helicopter would not endanger the safety of the helicopter or cause significant damage if they were to fail. E.g.: loom and piping attachment brackets. 2017-03-09 B1-12.5.1 General Concepts Page 28 of 38 Training Material Only Installations Quick Disconnect Valves Quick-disconnect, or line-disconnect valves, are installed in hydraulic lines to prevent the loss of fluid when units are removed. Such valves may be installed in the pressure and suction lines of the system just in front, and immediately behind the power pump. These valves can also be used in ways other than just for unit replacement. A power pump can be disconnected from the system and a hydraulic test stand connected in its place. Figure-26. Quick Disconnect Valves The top part of the illustration above shows the valve in the disconnected position. The two springs (A and B) hold both poppet’s C and F in the closed position as shown. This prevents loss of fluid through the disconnected line. The bottom part of the illustration above shows the valve in the connected position. When the valve is being connected, the coupling nut draws the two sections together. The poppet valve extension (D or E) forces the opposite piston back against its spring. This action moves the poppet off its seat and permits fluid to flow through that section of the valve. As the nut is drawn up tighter, one piston hits a stop and then the other piston moves back against its spring, opening its poppet valve and, in turn, allows fluid to flow. Thus, fluid is allowed to continue through the valve and on through the system. 2017-03-09 B1-12.5.1 General Concepts Page 29 of 38 Training Material Only External Power Receptacles Because of the heavy drain the starter puts on the battery, a battery cart or external power supply may be plugged in to furnish power for engine starting. Power is brought from a battery cart or rectifier through a standard three-terminal external power plug. Two of the pins in the aircraft receptacle are larger than the third, and are also longer. When the cart is plugged in, a solid contact is made with the two larger plugs. The external power relay in the aircraft remains open, and no current can flow from the external source until the plug is forced all the way into the receptacle, and the smaller pin makes contact. This small pin then supplies power through a reverse-polarity diode to the external power relay. The relay is energised in the closed position and connects the external power source to the aircraft bus. The reverse-polarity diode is used in the circuit to prevent an external power source with incorrect polarity from being connected to the aircraft’s bus. The diode simply blocks current from flowing to the external power relay if the applied power is connected backwards or is offering reverse polarity. Figure-27. External DC Power Receptacles 2017-03-09 B1-12.5.1 General Concepts Page 30 of 38 Training Material Only Avionics Racks Avionics racks are provided on most helicopters for ease of fitment of avionics equipment. They provide for exact positioning and safe housing of the equipment, along with quick removal and installation of the equipment. Several factors to consider when installing avionics equipment include: Sufficient air circulation to prevent overheating. This might require a certain free air space in some cases and the installation of a cooling fan in others Adequate clearance from high temperatures and flammable materials (next to a combustion heater would not be good place to install a radio) Protection from water, fumes, hydraulic fluid, etc. Protection from damage by baggage or seat deflection Sufficient clearance to prevent rubbing or striking upon helicopter structures, control cables, movable parts, etc. Preventing interference and noise. Separate sensitive electronic equipment from inverters, power supplies, strobe lights, motors, etc. If shock mounts will be used, ensure that the equipment does not exceed the weight carrying capability of the shock mounts and install adequate bonding jumpers or straps. Figure-28. Avionics Racks 2017-03-09 B1-12.5.1 General Concepts Page 31 of 38 Training Material Only Ground Locking Pins Ground locking pins are installed into holes specifically placed in the landing gear to prevent retraction during ground and maintenance operations. Figure-29. Ground Locking Pins 2017-03-09 B1-12.5.1 General Concepts Page 32 of 38 Training Material Only Anchor Nuts Anchor nuts simplify the process of installing and removing doors and panels. They are riveted to the aircraft structure so a spanner is not required to hold the nut while the screw is being installed. Figure-30. Anchor Nuts 2017-03-09 B1-12.5.1 General Concepts Page 33 of 38 Training Material Only Turnlock Fasteners These are used on aircraft for quick and easy removal of access panels for inspection and servicing purposes. They include: Camloc fasteners Dzus fasteners Airloc fasteners Figure-31. Turnlock Fasteners Electrical Connectors Electrical connectors provide a great deal of flexibility when attaching electrical wiring to various components. They are installed on wiring that is frequently disconnected. Figure-32. Electrical Connectors 2017-03-09 B1-12.5.1 General Concepts Page 34 of 38 Training Material Only Bonding Lightning Protection Helicopters cannot avoid being struck by lightning. They are metallic objects, often flying in storm conditions. The adverse effects of lightning strike can be minimised. Lightning is unpredictable. It can enter and exit the aircraft at any point. Just like a bullet, the exit damage is always greater than the entry damage. Bonding Straps Bonding between structural components is essential to minimise lightning damage to structure, electronic equipment or human occupants. Bonding or grounding is the process of providing an equal electrical potential between different parts of the airframe. Bonding straps prevent a difference in potential from building to the point that sparks jump from one component to another. In the power plant compartment, shock mounted components are also bonded to the main structure with bonding straps. Any electrical component which uses the helicopter structure as the return path for its current must be bonded to the structure. When selecting a bonding strap, it must be large enough to handle all the return current flow without producing an unacceptable voltage drop. An adequately bonded structure requires bonding straps that hold resistance readings to a negligible amount. For example, CASA AC21-99 Sect 2 Chap 13 states “to accomplish the purpose of bonding or grounding, it is necessary to provide a conductive path where direct electrical contact does not exist. Jumpers are used for this purpose in such applications as between moving parts, between shock-mounted equipment and structure, and between electrically conducting objects and structure. Keep jumpers as short as possible; if practical, under 76mm. Do not use two or more jumpers in series. TESTING BONDS AND GROUNDS Resistance Tests After Connection NOTE The resistance figures provided below are for general electrical and RF bonding. Specific requirements detailed in aircraft or component publications should take precedence. The resistance across a bonding or grounding jumper is required to be 0.1 ohms (100 milliohms) or less for general electrical bonding whether using bonding jumpers or where metallic components are directly attached. Where bonding of RF components is required, the resistance should be a maximum 0.0025 ohms (2.5 milliohms) (Reference – MIL-STD-464). Test is made after the mechanical connection is completed, and consists of a milli-ohmmeter reading of the overall resistance between the cleaned areas of the object and the structure. Aluminium alloy jumpers are recommended for most cases, but copper jumpers should be used to bond together parts made of stainless steel, cadmium plated steel, copper, brass, or bronze, the bonding strap should have enough mechanical strength to withstand constant flexing and should be easily installed to allow removal of the component being bonded.” 2017-03-09 B1-12.5.1 General Concepts Page 35 of 38 Training Material Only Figure-33. Bonding Straps Bonding Straps (cont): Minimise lightning damage where structure is joined or hinged Supply ground path for electrical equipment Reduce radio interference Allow static charges to equalise between different parts of the airframe, reducing fire hazard Figure-34. Braided Bonding Straps 2017-03-09 B1-12.5.1 General Concepts Page 36 of 38 Training Material Only Bonding straps are positioned to: Insolate passenger cabin, fuel tanks, cargo areas and cockpit from risk of sparks or electric shock Divert a charge around electronic equipment Provide a low resistance path for the electrical charge to exit Lightning Diverter Strips Lightning diverter strips are bonding strips mounted on the outside of fibreglass nose radomes. If lightning hits the radome, the diverter strips conduct the charge away and into the aircraft structure. The radome should only be painted with approved types of paint, which will not interfere with the radar frequency signals that must pass through the radome. Lightning Strike Inspection An unscheduled inspection must be carried out after an aircraft has experienced a lightning strike. Lightning damage ranges from minute burn marks to severe damage to the airframe, engine and/or avionics equipment. The inspection checks for damaged structure at entry and exit points, and radio and navigation equipment operation. Figure-35. Lightning Protection 2017-03-09 B1-12.5.1 General Concepts Page 37 of 38 Training Material Only Conditional Inspections A conditional inspection is an unscheduled inspection conducted as a result of a specific over-limit, or abnormal event. Examples of events requiring special inspections include: Hard landings Overstress conditions Flight into severe turbulence Flight into volcanic ash Over-temperature conditions Overweight landings Exceeding placarded speeds for flap or landing gear extension Bird strike Lightning strike Foreign object damage (FOD) 2017-03-09 B1-12.5.1 General Concepts Page 38 of 38 Training Material Only Topic 12.5.2.1 – Airframe Structures – Primary Construction Table of Contents List of Figures....................................................................................................................................... 2 Topic 5.2.1 – Airframe Structures – Primary Construction........................................................... 3 Structural Assembly Techniques.......................................................................................................... 3 Riveting................................................................................................................................................. 4 Rivet Codes................................................................................................................................................... 5 Rivet Head Design......................................................................................................................................... 6 Blind Rivets................................................................................................................................................... 7 Hi-Lok Fasteners........................................................................................................................................... 7 Bolting.................................................................................................................................................. 8 Lock bolts...................................................................................................................................................... 8 Construction............................................................................................................................ 10 Tubular Construction......................................................................................................................... 11 Sheet Metal Construction.................................................................................................................. 12 Stressed Skin Fuselage................................................................................................................................ 12 Bonded Construction......................................................................................................................... 13 Composite Materials and Adhesives.......................................................................................................... 13 Body Structure................................................................................................................................... 15 Bottom Structure........................................................................................................................................ 15 Cabin Section.............................................................................................................................................. 15 Rear Section................................................................................................................................................ 16 Formers.............................................................................................................................................. 16 Bulkheads........................................................................................................................................... 16 Longerons........................................................................................................................................... 17 Stringers............................................................................................................................................. 17 Doubler............................................................................................................................................... 18 Beams................................................................................................................................................. 18 Floor Structure................................................................................................................................... 19 Methods of Skinning: - Metal............................................................................................................ 19 Methods of Skinning: - Composite..................................................................................................... 20 Methods of Skinning: – Sandwich Core............................................................................................. 20 Struts.................................................................................................................................................. 21 2017-03-09 B1-12.5.2.1 Primary Construction Page 1 of 22 Training Material Only List of Figures Figure-1. Riveting.............................................................................................................................. 3 Figure-2. Lockbolt Fastener.............................................................................................................. 4 Figure-3. Jo-Bolt Fastener................................................................................................................. 4 Figure-4. Cherry Fastener................................................................................................................. 5 Figure-5. Hi-Lok Fastener................................................................................................................. 5 Figure-6. Riveting.............................................................................................................................. 6 Figure-7. Blind Rivet Gun.................................................................................................................. 7 Figure-8. Hi-lok Fastener.................................................................................................................. 8 Figure-9. Lock Bolts.......................................................................................................................... 9 Figure-10. Helicopter Construction.................................................................................................. 10 Figure-11. Tubular Construction...................................................................................................... 11 Figure-12. Sheet Metal Construction............................................................................................... 12 Figure-13. Boron Patch..................................................................................................................... 14 Figure-14. Bonded Construction...................................................................................................... 15 Figure-15. Formers........................................................................................................................... 16 Figure-16. Bulkheads........................................................................................................................ 16 Figure-17. Longerons........................................................................................................................ 17 Figure-18. Stringers.......................................................................................................................... 17 Figure-19. Doubler............................................................................................................................ 18 Figure-20. Beams.............................................................................................................................. 18 Figure-21. Floor Structure................................................................................................................ 19 Figure-22. Skinning: - Metal............................................................................................................. 19 Figure-23. Fibreglass Laminate......................................................................................................... 20 Figure-24. Sandwich Core................................................................................................................. 20 Figure-25. Sandwich Core Panel....................................................................................................... 21 Figure-26. Struts............................................................................................................................... 21 2017-03-09 B1-12.5.2.1 Primary Construction Page 2 of 22 Training Material Only Topic 5.2.1 – Airframe Structures – Primary Construction Structural Assembly Techniques As aircraft components are constructed separately, there needs to be processes that are structurally sound to tie them all together. The most common assembly techniques are: Riveting Bolting Bonding Figure-1. Riveting The integrity of a helicopter joint depends upon the fasteners selected and used to secure its parts together. However, not all helicopter joints are made using fasteners. Some joints on newer helicopters are made with composite materials that are held together by adhesives. Although this construction technique is gaining popularity, this method of construction will probably never completely take the place of using fasteners in helicopter assemblies. It is therefore important for a helicopter technician to be thoroughly familiar with the different types of fasteners that are encountered in industry. Although this section provides general guidelines in the selection and installation of various types of hardware and fasteners, it is always advisable to get acquainted with the fastener manufacturer’s technical information before using its product on a helicopter. 2017-03-09 B1-12.5.2.1 Primary Construction Page 3 of 22 Training Material Only Riveting Riveting is the process of installing structural fasteners to tie structure together. The fasteners include but are not limited to: Solid rivets Lockbolt fasteners Hi-Lok fasteners Cherry fasteners Jo-bolt fasteners Figure-2. Lockbolt Fastener Figure-3. Jo-Bolt Fastener 2017-03-09 B1-12.5.2.1 Primary Construction Page 4 of 22 Training Material Only Figure-4. Cherry Fastener Figure-5. Hi-Lok Fastener The solid shank rivet has been used since sheet metal was first utilized in aircraft, and remains the single most commonly used aircraft fastener today. Unlike other types of fasteners, rivets change in dimension to fit the size of a hole during installation. When a rivet is driven, the cross sectional area increases along with its bearing and shearing strengths. Solid shank rivets are available in a variety of materials, head designs, and sizes to accommodate different applications. Rivet Codes Rivets are given part codes that indicate their size, head style, and alloy material. Two systems are in use today: the Air Force - Navy, or AN system; and the Military Standards 20 system, or MS2O. While there are minor differences between the two systems, both use the same method for describing rivets. As an example, consider the rivet designation, AN47OAD4-5 the first component of a rivet part number denotes the numbering system used. As discussed, this can either be AN or MS20. The second part of the code is a three-digit number that describes the style of rivet head. The two most common rivet head styles are the universal head, which is represented by the code 470, and the countersunk head, which is rep resented by the code 426. Following the head designation is a one- or two-digit letter code representing the alloy material used in the rivet. After the alloy code, the shank diameter is indicated in 1/32 inch increments, and the length in increments of 1/16 inch. Therefore, in this example, the rivet has a diameter of 4/32 inch and is 5/16 of an inch long. 2017-03-09 B1-12.5.2.1 Primary Construction Page 5 of 22 Training Material Only The disadvantage of riveting is that each type requires its own specialist tools to install them. E.g. Solid rivets are installed with a rivet hammer gun and require access from both sides of the component. Figure-6. Riveting Rivet Head Design As mentioned, solid shank rivets are available in two standard head styles, universal and countersunk, or flush. The AN470 universal head rivet now replaces all previous protruding head styles such as AN430 round, AN442 flat, AN455 brazier, and AN456 modified brazier. AN426 countersunk rivets were developed to streamline aerofoils and permit a smooth flow over an aircraft’s wings or control surfaces. However, before a countersunk rivet can be installed, the metal must be countersunk or dimpled. Countersinking is a process in which the metal in the top sheet is cut away in the shape of the rivet head. On the other hand, dimpling is a process that mechanically ‘dents’ the sheets being joined to accommodate the rivet head. Sheet thickness and rivet size determine which method is best suited for a particular application. Joints utilising countersunk rivets generally lack the strength of protruding head rivet joints. One reason is that a portion of the material being riveted is cut away to allow for the countersunk head. Another reason is that, when riveted, the gun set may not make direct contact with the rivet head if the rivet hole was not countersunk or dimpled correctly, resulting in the rivet not expanding to fill the entire hole. To ensure head-to-gun set contact, it is recommended that countersunk heads be installed with the manufactured head protruding above the skin’s surface about.005 to.007 inch. This ensures that the gun set makes direct contact with the rivet head. To provide a smooth finish after the rivet is driven, the protruding rivet head is removed using a micro-shaver. This rotary cutter shaves the rivet head flush with the skin, leaving an aerodynamically clean surface. 2017-03-09 B1-12.5.2.1 Primary Construction Page 6 of 22 Training Material Only Blind Rivets A rivet is any type of fastener that obtains its clamping action by having one of its ends mechanically upset. Conventional solid shank rivets require access to both ends to be driven. However, special rivets, often called blind rivets, are installed with access to only one end of the rivet. While consider ably more expensive than solid shank rivets, blind rivets find many applications in today’s aircraft industry. Figure-7. Blind Rivet Gun Hi-Lok Fasteners Hi-Lok Fasteners are used in high strengths application and sometimes replace a solid rivet where access is limited. Hi-Lok bolts are manufactured in several different alloys such as titanium, stainless steel, steel, and aluminium. They possess sufficient strength to with stand bearing and shearing loads, and is available with flat and countersunk heads. A conventional Hi-Lok has a straight shank with standard threads. Although wrenching lock nuts are usually used, the threads are compatible with standard AN bolts and nuts. To install Hi-Lok fasteners, access is required on both sides of the component. The hole is first drilled with an interference fit. The Hi-Lock is then tapped into the hole and a locking collar is installed. An Allen wrench holds the Hi-Lok bolt in place while a combination wrench is used to tighten the locking collar with the attached shear nut. Once the collar is tightened to the appropriate torque value the shear nut breaks away, leaving the locking collar in place. 2017-03-09 B1-12.5.2.1 Primary Construction Page 7 of 22 Training Material Only Figure-8. Hi-lok Fastener Bolting Different bolt types use the same spanners, sockets and socket wrenches for installation saving on cost for tools required. A torque wrench may be required to set the correct tension of the installed bolt. Bolting also utilises safetying devices such as lock nuts, lock washers, split pins and lock wire to ensure the fasteners do not become loose. Lock bolts Lock bolts are manufactured by several companies and conform to Military Standards. These standards describe the size of a lock bolt’s head in relation to its shank diameter, as well as the alloy used. Lock bolts are used to permanently assemble two materials. They are lightweight and as strong as standard bolts. There are three types of lock bolts used in aviation: The pull-type lock bolt The blind-type lock bolt The stump-type lock bolt The pull-type lock bolt has a pulling stem on which a pneumatic installation gun fits. The gun pulls the materials together and then drives a locking collar into the grooves of the lock bolt. Once secure, the gun fractures the pulling pin at its break point. 2017-03-09 B1-12.5.2.1 Primary Construction Page 8 of 22 Training Material Only The blind-type lock bolt is similar to most other types of blind fasteners. To install a blind-type lock bolt, it is placed into a blind hole and an installation gun is placed over the pulling stem. As the gun pulls the stem, a blind head forms and pulls the materials together. Once the materials are pulled tightly together, a locking collar locks the bolt in place and the pulling stem is broken off. Unlike other blind fasteners that typically break off flush with the surface, blind lock bolts protrude above the surface. The stump-type lock bolt is installed in places where there is not enough room to use the standard pulling tool. Instead, the stump-type lock bolt is installed using an installation tool similar to that used to install Hi-Shear rivets. Lock bolts are available for both shear and tension applications: Shear lock bolts:- the head is kept thin and there are only two grooves provided for the locking collar Tension lock bolts:- the head is thicker and four or five grooves are provided to allow for higher tension values The locking collars used on both shear and tension lock bolts are colour coded for easy identification. Figure-9. Lock Bolts 2017-03-09 B1-12.5.2.1 Primary Construction Page 9 of 22 Training Material Only Construction Helicopter structures consist of doors, fuselage, vertical stabilizer, fairings, tail cone, and windows. Doors are for entrance and exit and access to baggage compartments and fuselage structure. Fairings can be opened or removed to get to system components. The framework of the fuselage is of semi-monocoque construction, using the framing and skin plating as structural members. The framing consists of longitudinal beams, longerons, and stringers, which are supported laterally with bulkheads, frames and formers. Figure-10. Helicopter Construction The airframe of the helicopter covers a wide range of materials, mainly due to the advances in technology that have taken place in recent years. For the most part, the materials used in helicopter construction are the same as those used on fixed-wing aircraft. 2017-03-09 B1-12.5.2.1 Primary Construction Page 10 of 22 Training Material Only Tubular Construction Some of the early helicopters used the typical tubular-truss fuselage construction. Although this construction type had a high strength-to-weight ratio, manufacturing such an airframe was quite costly. Each tube was cut, fitted, and welded into place. In addition to these disadvantages, it was difficult to hold dimensions to a close tolerance. The big advantage of this type of construction was the way in which it could be repaired in the field; unless there was severe airframe damage, which would require jigging in order to hold alignment. The criteria for these repairs are all contained in the maintenance manual and the FAA’s AC43.13. Figure-11. Tubular Construction 2017-03-09 B1-12.5.2.1 Primary Construction Page 11 of 22 Training Material Only Sheet Metal Construction At the same time, several of the manufacturers went to aluminium structures. These were of monocoque and semi-monocoque design. This construction type had a high strength-to-weight ratio. In fact, the ratio was higher than the tubular construction of the same size. Sheet metal construction had some advantages in the manufacturing process. Parts could be stamped out and compound curves made rapidly. Helicopters could be constructed in jigs with closer tolerances than those of the tubular fuselage construction. In the field, few additional tools were required for repairs. In general, the structure was slightly more delicate. If large repairs were required, the fuselage required jigs to obtain proper alignment for the various sections. A few helicopters are built using a combination of the sheet metal construction and the tubular construction. Tubular design is used in areas where high strength is required. Sheet metal is used when the strength requirements are not as critical. Figure-12. Sheet Metal Construction Stressed Skin Fuselage The basic structure of the helicopter varies somewhat from that of a fixed-wing aircraft, although they both use the same construction techniques. This is due to the loads and stresses that are placed on the airframes in different locations. It is these differences that should be understood by helicopter maintenance personnel in order to inspect and repair the helicopter airframe properly. The next advance in structural designs came with the development of a construction technique that allowed the helicopter to be formed without a truss frame. This design, generally known as a stressed-skin structure, allowed the helicopter to be built with a more streamlined shape and provided further reductions in weight because the skin itself carried the structural loads. When constructed in this fashion, the helicopter was referred to as having a monocoque design. 2017-03-09 B1-12.5.2.1 Primary Construction Page 12 of 22 Training Material Only The term monocoque is derived from the French meaning ‘single-shell’. Thin aluminium-alloy sheets are used for the exterior of monocoque stressed-skin structures. These sheets have compound curves formed in them by using hydro presses or drop hammers. The formed skins were then riveted onto thin sheet metal formers. The designs provided a lightweight and reasonably durable structure that manufacturers used for many years. In fact, many aircraft constructed in this manner remain in service today. A disadvantage of monocoque designs is that they can fail once subjected to relatively minor dents or creases. To further increase the strength of the structure, manufacturers improved their designs by developing semi-monocoque construction techniques. In these helicopters, the skin is fastened to a sub-structure or skeletal framework, which allows the loads to be distributed between the structural components and the skin of the helicopter. These designs proved to be so successful that they continue to be the primary method of modern helicopter construction. Bonded Construction The third type of construction is the use of fibreglass, honeycomb, and bonded structures. All of these materials and methods are of high strength-to-weight ratios. Honeycomb and bonded structures have reduced construction costs by eliminating some of the riveting and welding. Today most airframes are a combination of various materials and methods of construction obtaining the greatest advantages of each. Composite Materials and Adhesives Some joints on newer helicopters are made with composite materials that are held together by adhesives. The function of the matrix in a composite is to hold the reinforcing fibres in a desired position. It also gives the composite strength and transfers external stresses to the fibres. The ability of the matrix to transfer stress is the key to the strength of a composite structure. A wide range of resin systems are used for the matrix portion of fibre reinforced composites. Resin is an organic polymer used as a matrix to contain the reinforcing fibres in a composite material. Polyester resin, an example of an earlier matrix, used in conjunction with fibreglass has been used in many non-structural applications such as fairings and spinners, The old polyester/fibreglass formulas did not offer sufficient strength to fabricate primary structural members. Newer matrix materials display remarkably improved stress distributing characteristics, heat resistance, chemical resistance, and durability. Resin matrix systems are a type of plastic and include two general categories: thermoplastic and thermosetting. Thermoplastic and thermosetting resins by themselves do not have sufficient strength for use in structural applications. However, plastic matrixes reinforced with other materials form high- strength, lightweight structural composites. Some materials used, like epoxy resins, require to be applied in a temperature and humidity controlled environment using expensive heating and vacuum pressure equipment to cure. The components also need to be extremely clean for the adhesives/resins to stick. 2017-03-09 B1-12.5.2.1 Primary Construction Page 13 of 22 Training Material Only The illustration below is of an F111 upper wing surface. The boron patch was fitted to the wing to strengthen the structure to prevent cracking of the main spar which is attached to the skins through bolting and riveting. Figure-13. Boron Patch In order to minimise the weight in the Tiger helicopter, approximately 80% of the airframe is constructed of composite materials. The frames and beams have been fabricated from Kevlar and carbon laminates. Panels are composed of Nomex honeycomb material with carbon and Kevlar skins. The helicopter blades are of fibre composite construction. Radar reflective structures and surfaces have been minimised. Low infra-red reflection paints have been used and an infra-red (IR) suppressor has been fitted to the engine exhaust. 2017-03-09 B1-12.5.2.1 Primary Construction Page 14 of 22 Training Material Only The self-sealing tanks are equipped with an inert gas system to avoid the danger of an explosive fuel vapour and air mix. The engines are separated by armour plate to prevent the loss of both engines in the event of a single direction hit. The helicopter has nuclear, biological and chemical warfare (NBC) and nuclear electromagnetic pulse protection. Figure-14. Bonded Construction Body Structure The body structure is the main structural member of the fuselage. It not only carries the lift and thrust loads, but also the landing loads. The body structure supports all other members of the fuselage either directly or indirectly. All the forces applied to these other members will be transmitted to the body structure. In actuality; the body structure is a reinforced box with ‘X’ members placed in each side. The transmission assembly, which is connected to the main rotor and absorbs the compression loads of landing, is attached to the bottom of the box. In the middle of this structure is placed the fuel tanks, which should be in the most protected area of the helicopter. Bottom Structure Attached to the body structure on the front of the box is the bottom structure and cabin floor. This section is made of two cantilevered beams extending from the box that are connected to the cross members of the box. These two beams will actually carry the weight of the cabin and transmit it to the box. Cross members are added to these two beams to support the floor and the lower skin panels. The cabin section attaches directly to the floor. Cabin Section The canopy, or cabin section, is made almost exclusively from synthetic materials. This portion is made of subassemblies which are the cabin roof; nose, and vertical members. All of the components are made of polycarbonate reinforced with glass fibres. They are heat moulded and assembled by banding and ultrasonic spot welding. The canopy frame is then bolted to the cabin floor and the body bulkhead. Added to this frame are the upper windows, windshields, and lower window; which are all made of polycarbonate. The transparent polycarbonate is known for its superior strength properties. 2017-03-09 B1-12.5.2.1 Primary Construction Page 15 of 22 Training Material Only Rear Section The rear section of the fuselage connects to the body section. The rear section is made of three frames connected by beams to the body section. This frame, covered with a stainless steel firewall, acts as an attachment point for the engine. The inside of this section acts as a baggage area. The tail boom section is bolted to the rear frame. Formers Formers give the fuselage or wing its shape and the skin is attached to the outside. A series of frames in the shape of the fuselage cross sections are held in position on a rigid fixture. These frames are then joined with lightweight longitudinal elements called stringers. These are in turn covered with a skin of sheet aluminium, attached by riveting or by bonding with special adhesives. Figure-15. Formers Bulkheads Are used as structural partitions to divide the fuselage or wings into bays or compartments, and provide additional strength as well as giving the fuselage shape. Figure-16. Bulkheads 2017-03-09 B1-12.5.2.1 Primary Construction Page 16 of 22 Training Material Only Longerons Longerons give a helicopter capability to carry heavy loads longitudinally. Longerons tie frames together and provide strength to cockpit construction. Figure-17. Longerons Stringers Stringers give the fuselage its longitudinal strengths. They connect the frames (formers) and are joined to the skin. Stringers are manufactured by forming sheet metal into strong cross-sectional shapes or by using extruded aluminium alloy. Figure-18. Stringers 2017-03-09 B1-12.5.2.1 Primary Construction Page 17 of 22 Training Material Only Doubler A doubler is a piece of metal used to strengthen skin structure where a component is attached. Doublers are also used as strengthening repair plates, either on their own or in conjunction with a patch. Any repair shall be done as per the helicopter’s Structural Repair Manual (SRM). Figure-19. Doubler Beams Beams are long structural members in any structure that support/carry loads in bending and shear. Figure-20. Beams 2017-03-09 B1-12.5.2.1 Primary Construction Page 18 of 22 Training Material Only Floor Structure The floor structure of a helicopter consists of a network of longerons and floor beams. The longerons within the floor structure of the passenger cabin often incorporate the seat tracks. Flooring panels are fitted on top of the longerons and beams. They are generally manufactured from composite sandwich constructed materials. The underfloor structure often accommodates an uninterrupted passage for control cable runs and fluid lines. Figure-21. Floor Structure Methods of Skinning: - Metal Helicopters are fitted with formed metal skins for extra strength and less weight. Figure-22. Skinning: - Metal 2017-03-09 B1-12.5.2.1 Primary Construction Page 19 of 22 Training Material Only Methods of Skinning: - Composite As technology progressed, composite fibre skins were introduced to aircraft in positions where the strength of formed metal skins was not required. Composite fibre skins are manufactured by impregnating a fibre (glass, carbon/graphite, Kevlar) with a resin and allowing it to cure. Multiple layers of fibre are impregnated and glued together with a resin matrix, to form a thick fibre skin. Figure-23. Fibreglass Laminate Methods of Skinning: – Sandwich Core Another method used to obtain strength from composite skins is by bonding a thinner fibre or metal skin to a honeycomb core. Figure-24. Sandwich Core 2017-03-09 B1-12.5.2.1 Primary Construction Page 20 of 22 Training Material Only This type of skinning is generally used for removable doors and panels as a high strength low weight alternative to thick metal or fibre composite skins. Figure-25. Sandwich Core Panel Struts Struts are compression resistant member which prevents crushing. Sponson to fuselage struts on a helicopter resists both compression and tension loads. The attached undercarriage strut resists compression loads. Figure-26. Struts 2017-03-09 B1-12.5.2.1 Primary Construction Page 21 of 22 Training Material Only This Page Intentionally Blank 2017-03-09 B1-12.5.2.1 Primary Construction Page 22 of 22 Training Material Only Topic 12.5.2.2 – Airframe Structures – Secondary Construction Table of Contents List of Figures....................................................................................................................................... 3 Topic 5.2.2 – Airframe Structures – Secondary Construction....................................................... 4 Undercarriage...................................................................................................................................... 4 Skids..................................................................................................................................................... 5 Wheeled Landing Gear.................................................................................

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