CASA B1-11b Airframe Structures PDF

MODULE 11A Category B1 Licence CASA B1-11b Aeroplane Structures  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of...

MODULE 11A Category B1 Licence CASA B1-11b Aeroplane Structures  Copyright © 2020 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 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 2 of 232 CASA Part Part 66 - Training Materials Only  Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 3 of 232 CASA Part Part 66 - Training Materials Only  Table of Contents Airframe Structures - General Concepts Part A (11.2.1) 10 Learning Objectives 10 Airworthiness Requirements 11 Introduction to Airworthiness Requirements 11 Transport Category Aircraft 11 Airworthiness Requirements for Structural Strength 12 Structural Design Requirement 15 Structural Classifications 17 Introduction to Structural Classifications 17 Fatigue Design Philosophy 18 Structural Damage 21 Safe-Life 21 Fail-Safe 21 Damage-Tolerance 22 Durability 24 Zonal and Station Identification Systems 26 Aircraft Reference Zones 26 Major Zones 26 Subzones 27 Zones 28 Reference Datum 28 Position Identification and Location 30 Fuselage 30 Wing 32 Engine 33 Clock Position 34 Airframe Structures Loads and Forces 36 Stress, Strain and Fatigue in Aircraft Structures 36 Strain 36 Stress-Strain Curves 36 Tension 38 Compression 38 Shear 40 Torsion 41 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 4 of 232 CASA Part Part 66 - Training Materials Only  Bending 42 Hoop Stress 43 Fatigue 43 Aircraft Drainage 45 Corrosion 45 Drainage 45 Pressurised Fuselage Drain Valve 45 Drain Hole 46 Ventilation 46 System Installation Provisions 48 Provisions 48 Hydraulic Systems 48 Air Conditioning System 48 Electrical/Avionics 48 Fuel Storage 49 Main Landing Gear 49 Waste Disposal 51 Potable Water 52 Auxiliary Power Unit 53 Lightning Strikes 54 Introduction to Lightning Strikes 54 Aircraft Electrical Bonding 57 Bonding 57 Grounding 57 Bonding Leads 57 Airframe Structures - Construction Concepts (11.2.2) 59 Learning Objectives 59 Aircraft Structural Design 60 Structural Design 60 Airframe Structure 62 Structural Members 62 Longerons 63 Stringers 64 Frames 65 Bulkheads 65 Pressure Bulkhead 66 Beams 67 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 5 of 232 CASA Part Part 66 - Training Materials Only  Doublers 68 Struts 68 Ties 69 Floor Structure 70 Reinforcement 72 Methods of Skinning 74 Attachments 77 Engine Attachment 78 Empennage 79 Assembly Techniques 81 Aircraft Assembly Techniques 81 Riveting 81 Bolted Structures 82 Bonding 83 Surface Protection 84 Aircraft Surface Protection 84 Surface Cleaning 87 Aircraft Surface Cleaning 87 Aircraft Cleaning Methods 87 Airframe Symmetry 90 Introduction to Airframe Symmetry 90 Structural Alignment 91 Symmetry Check 92 Aeroplane Fuselage (11.3.1) 96 Learning Objectives 96 Fuselage Construction 97 Aeroplane Fuselage 97 Fuselage Inspection 97 Truss Type Fuselage 98 Stressed-Skin Fuselage 101 Fuselage Structure 105 Bulkheads 106 Keel Beam Structure 107 Nose Landing Gear Wheel Well Box 107 Radome 108 Wing, Stabiliser, Pylon and Undercarriage Attachments 110 Wing Attachment 110 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 6 of 232 CASA Part Part 66 - Training Materials Only  Wing Attachment Points 112 Wing to Body Attachment 113 Stabiliser Attachment 115 Vertical Stabiliser to Fuselage Attachment 115 Horizontal Stabiliser to Fuselage Attachment 116 Pylon and Nacelle to Fuselage Attachment 117 Landing Gear Attachment 118 Pressurised Fuselage 121 Pressurisation Sealing 122 Seat Installation 126 Flight Compartment Seats 126 Cabin Seats 128 Cargo Loading Systems 130 Flight and Passenger Compartment Floors 130 Cargo Compartment 130 Doors 134 Introduction to Aeroplane Doors 134 Entry/Service Doors 136 External Opening Pressure Door 140 Emergency Exit Doors 143 Access Doors 145 Cargo Doors 147 Sealing Control Cables 151 Sealing Electrical Cables 151 Fuselage Structural Protection Under Rapid Decompression 152 Windows 157 Aeroplane Windows and Windscreens 157 Airframe Structures - Wings (11.3.2) 159 Learning Objectives 159 Aeroplane Wings 160 Introduction to Wings 160 Semi-Cantilever Wing 161 Cantilever Wing 162 Aerofoil Section 162 Transmitting Lift into the Structure 163 Principal Components of a Wing 163 Spars 164 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 7 of 232 CASA Part Part 66 - Training Materials Only  Ribs 166 Stringers 168 Wing Skin 169 Wing Design 172 Wing Assemblies 176 Fuel Storage 184 Types of Fuel Storage 184 Rigid Tanks 184 Flexible Tank 185 Integral Tanks 186 Access Doors 188 Wing Attachment Points 188 Landing Gear 190 Aeroplane Landing Gear 190 Pylon 190 Flight Control Surfaces 192 Spoilers (If Fitted) 196 Stabilisers (11.3.3) 198 Learning Objectives 198 Aeroplane Stabilisers 199 Function of Stabilisers 199 Horizontal Stabiliser 199 Vertical Stabiliser 206 Flight Control Surfaces (11.3.4) 208 Learning Objectives 208 Primary and Secondary Control Surfaces 209 Primary Flight Control Surfaces 209 Secondary Flight Control Surfaces 210 Control Surface Balancing 218 Aeroplane Control Surface Balancing 218 Horn Balance 218 Static (Mass) Balance 219 Nacelles and Pylons (11.3.5) 223 Learning Objectives 223 Aeroplane Structures 224 Introduction to Nacelles and Pylons 224 Nacelles 224 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 8 of 232 CASA Part Part 66 - Training Materials Only  Pylons 226 Firewalls 229 Nacelle/Pylon Engine Mounts 231 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 9 of 232 CASA Part Part 66 - Training Materials Only  Airframe Structures - General Concepts Part A (11.2.1) Learning Objectives 11.2.1.1 Describe how airworthiness requirements for aircraft structural strength are derived, including pertinent regulations, standards and other specifications (Level 2). 11.2.1.2 Describe primary, secondary and tertiary structures and identify typical aircraft examples (Level 2). 11.2.1.3 Describe fail-safe, safe-life and damage-tolerance concepts (Level 2). 11.2.1.4 Describe zone identification systems, interpret the meaning of standard aircraft zone identification numbers and the system of station numbering and purpose of datum points (Level 2). 11.2.1.5 Describe the stress, strain, bending, compression, shear, torsion, tension, hoop stress and fatigue and their effects on aircraft structures (Level 2). 11.2.1.6 Describe the reasons for drains and ventilation in aircraft structure and common methods of achieving drainage and ventilation (Level 2). 11.2.1.7 Describe common methods for accommodating system components within aircraft structure (Level 2). 11.2.1.8 Describe techniques for ensuring that aircraft remain safe from lightning strike (Level 2). 11.2.1.9 Describe electrical bonding techniques for airframe structures (Level 2). 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 10 of 232 CASA Part Part 66 - Training Materials Only  Airworthiness Requirements Introduction to Airworthiness Requirements An aircraft is airworthy only when it conforms to the regulations under which it has been certified. Regulatory authorities work together closely to set uniform requirements. Airworthiness standards are organised into sections, called parts, and each part deals with a specific type of activity. As an example: Part 23 covers airworthiness standards for Normal, Utility, Acrobatic and Commuter Aircraft. Part 25 covers airworthiness standards for Transport Category Aircraft. Part 27 covers airworthiness standards for Normal Category Rotorcraft. Part 29 covers airworthiness standards for Transport Category Rotorcraft. Part 33 covers airworthiness standards for Aircraft Engines. Part 35 covers airworthiness standards for Propellers. Part 39 covers Airworthiness Directives. Part 43 covers Maintenance, Preventive Maintenance, Rebuilding and Alteration. Part 65 (Part 66 for EASA and CASA) covers Aircraft Maintenance Engineer Licensing. Part 145 covers Aircraft Maintenance Organisations. Part 147 covers Aviation Maintenance Technicians Schools. Regulatory authorities 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 11 of 232 CASA Part Part 66 - Training Materials Only  Transport Category Aircraft Aircraft registration in the Transport category requires its structure to meet the airworthiness standards specified in the following regulations as applicable for the country in which the aircraft is registered: CASR 1998 Part 25 Subpart 25, as of the 1st Edition – January 2003 for Australia EASA Certification Specifications CS-25 Subpart C, as of Amendment 6 – July 2009 for Europe FAR Part 25 Subpart C, as of the e-CFR Data – August 2009 for the USA, covers four sections relating to the aircraft’s structure: 25.301 Loads 25.303 Factor of Safety 25.305 Strength and Deformation 25.307 Proof of Structure. The content of these four sections is repeated below. Transport category aircraft 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 12 of 232 CASA Part Part 66 - Training Materials Only  Airworthiness Requirements for Structural Strength Loads (a) Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads. The limit load is the maximum anticipated load or combination of loads which a structure may be expected to experience. Ultimate load is the load that a payload must be able to withstand without failure. (b) Unless otherwise provided, the specified air, ground and water loads must be placed in equilibrium with inertia forces, considering each item of mass in the aeroplane. These loads must be distributed to conservatively approximate or closely represent actual conditions. Methods used to determine load intensities and distribution must be validated by flight load measurement unless the methods used for determining those loading conditions are shown to be reliable. (c) If deflections under load would significantly change the distribution of external or internal loads, this redistribution must be taken into account. Load analysis Factor of Safety Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load, which is considered external loads on the structure. When loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 13 of 232 CASA Part Part 66 - Training Materials Only  Strength and Deformation (a) The aircraft structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation. (b) The structure must be able to support ultimate loads without failure for at least 3 s. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-s limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirements, it must be shown that one of the following is true: The effects of deformation are not significant. The deformations involved are fully accounted for in the analysis. The methods and assumptions used are sufficient to cover the effects of these deformations. (c) Where structural flexibility is such that any rate of load application likely to occur in the operating conditions might produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application must be considered. (d) Reserved. (e) The aircraft must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to Vd/Md, (design dive speed), including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope. This must be shown by analysis, flight tests or other tests found necessary by the Administrator. (f) Unless shown to be extremely improbable, the aircraft must be designed to withstand any forced structural vibration resulting from any failure, malfunction or adverse condition in the flight control system. These must be considered limit loads and must be investigated at airspeeds up to Vc/Mc, (design cruise speed). Fuselage structural testing 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 14 of 232 CASA Part Part 66 - Training Materials Only  Proof of Structure (a) Compliance with the strength and deformation of this subpart must be shown for each critical loading condition. Structural analysis may be used only if the structure conforms to that for which experience has shown this method to be reliable. The Administrator may require ultimate load tests in cases when limit load tests may be inadequate. (b), (c) Reserved. (d) When static or dynamic tests are used to show compliance with the requirements of 25.305(b) for flight structures, appropriate material correction factors must be applied to the test results unless the structure, or part thereof, being tested has features such that a number of elements contribute to the total strength of the structure and the failure of one element results in the redistribution of the load through alternate load paths. Proving structural analysis Relevant Youtube link: Boeing 777 wing test 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 15 of 232 CASA Part Part 66 - Training Materials Only  Structural Design Requirement The structural design criteria are determined by the type of aircraft and its intended use. From these criteria, it is possible to define the structural design analysis to consider the types of manoeuvres, speeds, useful loads and gross weights the structure will be subject to. In addition, the criteria must consider such items as inadvertent manoeuvres, effects of turbulent air and severity of ground contact during landing. The strength provided in the airframe structure to meet these conditions must be adequate for the aircraft to perform its intended mission in a safe and profitable manner as operated by qualified personnel under regulated conditions. Certification of airframe structure generally requires the structure to be subject to testing and/or analysis to demonstrate the following capabilities: Static strength Fatigue strength. Static Strength The Design Limit Load (DLL) is the maximum load anticipated on the aircraft during its service life. The aircraft structure must be capable of supporting the limit loads without suffering detrimental permanent deformation or interfering with safe operation. DLL is equal to the design limit load multiplied by a factor of safety. Generally, the safety factor is 1.5. Aircraft wing flex during flight Relevant Youtube link: Airbus A350 ultimate load wing stress test 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 16 of 232 CASA Part Part 66 - Training Materials Only  Structural Classifications Introduction to Structural Classifications Aircraft structures and components are classified according to their level of importance for the part they play in maintaining the structural integrity of the aircraft. This classification determines the significantly different levels of intensity and frequency of inspection and maintenance schedules, component replacement, damage assessment and repair schemes. The classifications are: Primary structure Secondary structure Tertiary structure. Primary Structure Primary structure is structure that is critical to the safety of the aircraft. If a primary structural component fails during take-off, flight or landing, there is potential for structural collapse, loss of control, failure of motive power, fatality or serious injury to aircrew. Examples of primary structures are fuselage, wings, horizontal and vertical stabilisers. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 17 of 232 CASA Part Part 66 - Training Materials Only  Secondary Structure Secondary structure is structure for which failure may cause significant damage that would affect the operation of the aircraft, but not lead to its loss. Examples of secondary structures are wing fixed leading and trailing edges, dorsal fin and nose radome. Structural classification Tertiary Structure Tertiary structure is structure for which failure would not significantly affect the operation of the aircraft. Examples of tertiary structures are brackets, clamps and mounting hardware. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 18 of 232 CASA Part Part 66 - Training Materials Only  Fatigue Design Philosophy Fatigue considerations comprise an important part of aircraft structural design. The common objective is to define aircraft life to ensure flight safety while minimising maintenance and operating costs. Three distinct design approaches protect an aircraft structure from failure due to fatigue damage: Safe-life – Also known as safety by retirement, this approach was introduced in the 1940s after fatigue was recognised as a failure mechanism. It specifies a safe lifespan within which there is no significant risk of structural failure of a component. Fail-safe – The fail-safe design principle was introduced in the 1950s as an improvement to safe-life. A fail-safe structure should be able to sustain the limit load even when one of its elements has failed. Damage-tolerance – Damage-tolerance, or safety by inspection, was developed in the 1970s as an improvement on the fail-safe principle. Damage-tolerance is the present method of achieving structural operating safety. Image by IABG Dresden via Wikipedia Commons Licence Fatigue testing Although the objectives of these three approaches are the same, they vary with regard to their fundamental definitions of service life. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 19 of 232 CASA Part Part 66 - Training Materials Only  Fatigue design history Note: SSIP means Supplemental Structural Inspection Program. The purpose of establishing an SSIP is to reach the safety standard according to FAR 25.571 Amendment 45. MSG refers to Maintenance Steering Group and is the method that aircraft manufacturers, operators and regulators use to develop the manufacturer’s initial maintenance schedule as part of the work towards aircraft certification. The process continues throughout the aircraft type’s life. MSG-3 is the methodology that focuses on aircraft systems and the loss of system function or functions. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 20 of 232 CASA Part Part 66 - Training Materials Only  Structural Damage Safe-Life Safe-life is an approach in which a structure is designed to withstand a certain number of events (flight cycles, landings or flight hours) with a low probability that the strength of the structure will degrade below its designed ultimate strength before the end of its approved life. The structural component must remain crack free during service. This means it must not develop any cracks as a result of fatigue, corrosion or accidental damage during its specified service life. Once the service life of a component has been reached, that component is considered unserviceable and the aircraft’s airworthiness can be maintained only if the component is replaced. Safe-Life Disadvantages There are two major drawbacks to the safe-life approach: Components are taken out of service even though they may have substantial remaining lives, creating unnecessary costs. Despite all precautions, cracks sometimes occur prematurely, creating a safety problem. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 21 of 232 CASA Part Part 66 - Training Materials Only  Fail-Safe Fail-safe is an approach applied to a structure in which it is designed to retain its required residual strength for a period of unrepaired use after a failure or partial failure of a principal structural element. Fail-safe design – multi-spar structure fail-safe disadvantages Despite all precautions, cracks sometimes occur prematurely and may not be detected, creating a safety problem. Fail-safe methodologies do not consider material or manufacturing flaws. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 22 of 232 CASA Part Part 66 - Training Materials Only  Damage-Tolerance This approach can be applied to a structure that is able to sustain a given level of fatigue, corrosion, manufacturing defects or accidental damage and still withstand design loads without structural failure or excessive structural deformation for a predetermined period that allows for a set number of opportunities to detect the damage. Damage-tolerance inspection regime The damage-tolerance approach is based on the principle that while cracks due to fatigue and corrosion will develop in the aircraft structure, the process can be understood and controlled. A key element is the development of a comprehensive program of inspections to detect cracks before they can affect flight safety. That is, damage-tolerant structures are designed to sustain cracks without catastrophic failure until the damage is detected in scheduled inspections and the damaged part is repaired or replaced. In addition, damage-tolerance takes into account initial material or manufacturing flaws by assuming an initial crack, which the fail-safe principle does not do. A damage-tolerant design should allow cracks to be detected before they reach the critical length that will lead to failure. To ensure that this occurs, there should be at least two opportunities to detect the crack prior to it reaching its critical length. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 23 of 232 CASA Part Part 66 - Training Materials Only  The damage-tolerance philosophy uses testing and analysis to determine the critical crack length, residual strength and inspection intervals. Tests include flight testing to determine the loads on the structure and ground testing to determine the fatigue and crack growth characteristics. From the testing and analysis, the critical sites and components susceptible to fatigue can be determined. Fatigue analysis based on flight, ground and pressurisation loads can then be used to determine crack growth performance and residual strength. Non-destructive testing The damage-tolerance philosophy is used for Transport category aircraft. For other categories, this method may be used but is not mandatory; the safe-life or fail-safe methods may be used instead. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 24 of 232 CASA Part Part 66 - Training Materials Only  Durability Durability is the structure's ability to sustain degradation from such sources as fatigue, accidental damage and environment deterioration to the extent that they can be controlled by economically acceptable maintenance and inspection programs. Durability of an aircraft structure comes from having a slow crack growth characteristic and the ability to contain or restrict the progress of damage. Aircraft durability 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 25 of 232 CASA Part Part 66 - Training Materials Only  Zonal and Station Identification Systems Aircraft Reference Zones To facilitate maintenance and component location, an aircraft is divided into major zones. Each zone is further broken down into subzones and zones. Different manufacturers use the term major subzones instead of subzones; however, the identification principle is the same. Each of the zones sequentially produces a three-digit identifier. The identifier is in a standard format as defined by the ATA100 Specification. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 26 of 232 CASA Part Part 66 - Training Materials Only  Major Zones The aircraft is divided into eight major zones. Each zone is numbered from 100 to 800, with the first digit from 1 to 8 and followed by two zeros to create the three-digit identifier. For example, Major Zone 100 is the lower fuselage; Major Zone 200 is the upper fuselage. The empennage is Major Zone 300 and includes the fuselage aft of the rear pressure bulkhead. 100: Lower fuselage 200: Upper fuselage 300: Empennage 400: Power plants and nacelle struts 500: Left wing 600: Right wing 700: Landing gear 800: Doors Major zones 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 27 of 232 CASA Part Part 66 - Training Materials Only  Subzones It is possible to have nine subzones in each major zone. A subzone is identified by the middle digit of the three-digit identifier, using the numbers 1 to 9, and the last digit is a zero. For example, Subzone 820 refers to cargo compartment doors, Subzone 830 is the left-side passenger compartment doors and Subzone 840 is the right-side passenger doors. Subzones Zones The third digit, comprising the numbers 1 to 9, identifies a component or group of components that are in the subzone. For example, Zone 821 identifies the forward cargo door and Zone 822 identifies the aft cargo door. Access doors and panels in a zone are identified by the zone number and a two- or three-letter suffix. This alphanumeric label is different for each access door or panel. For example: 821 is the forward cargo door (right side) 821AR is an access panel on the forward cargo door 821AZ is an access panel on the forward cargo door liner. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 28 of 232 CASA Part Part 66 - Training Materials Only  Reference Datum A reference datum or datum is established by the manufacturer for each aircraft model. The datum is an imaginary vertical plane or line that sits at a right angle to the aircraft’s longitudinal axis. The datum is the point from which all horizontal measurements are taken, with the aircraft in a level flight attitude. The location of all items, including equipment, tanks, baggage compartments, seats, engines and propellers, is listed as being so many inches or millimetres from the datum. Information about the location of the datum is found in the Aircraft Specifications or Type Certificate Data Sheets. Reference datum There is no fixed rule for the location of a datum. It may be located on the nose of the aircraft, the firewall, the leading edge of the wing or even a point in space ahead of the aircraft. The manufacturer chooses a location for the datum where it is most convenient for measurement, equipment location, and weight and balance computation. For example, the datum plane in the B737NG is perpendicular to the fuselage centreline and 130.0 in. forward of the aeroplane nose. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 29 of 232 CASA Part Part 66 - Training Materials Only  Datum line (B737NG) Position Identification and Location Using the datum line as the prime reference point, it is possible to identify an exact point, for example, a specific rivet, anywhere on the aircraft. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 30 of 232 CASA Part Part 66 - Training Materials Only  Fuselage For the fuselage, the three planes that give an exact location are named as follows: The Fuselage Station line (FS) measured in millimetres from the datum line (Airbus), or Body Station (BS), measured in inches from the datum line (Boeing) The Left Buttock Line (LBL) or Right Buttock Line (RBL), measured from the longitudinal centreline of the fuselage The Waterline (WL), measured vertically from an imaginary Waterline 0 placed some distance below the fully extended undercarriage of the aircraft. Body Station Line A fuselage station is a vertical line perpendicular to the body centreline. In the case of the 747 aircraft, the datum line is 90 in. forward of the radome. B STA 90 refers to the body or fuselage station (BS) which is 90 in. back from the datum line. Likewise, it is possible to determine that Body Station 2360 is where the stabiliser leading edge contacts the fuselage. Fuselage station lines Buttock Line A body buttock line (BL) is a vertical line which establishes lateral distance to the left and right of the fuselage’s vertical centreline. Buttock Line 0 is the centreline of the aircraft. Distances left (LBL) or right (RBL) identify positions on the left or right side of the aircraft. Left and right designations are always referenced facing forward in the aeroplane. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 31 of 232 CASA Part Part 66 - Training Materials Only  Waterline A waterline (WL) is a horizontal line which establishes vertical distances from the top to the bottom of the aeroplane. Waterline 0, in the case of a 747, is 91 in. below the lowest point of the fuselage. Buttock lines and waterlines 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 32 of 232 CASA Part Part 66 - Training Materials Only  Wing Wing Buttock Line The use of a Wing Buttock Line (WBL) is one method of providing a location reference along the wing. These stations are measured from the centreline of the aircraft, or Buttock Line 0. They indicate the distance in inches along the wing towards the wing tip. More accurate references are obtained using the term wing station (WS), in which the reference is against the wing’s rear spar. Wing buttock lines 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 33 of 232 CASA Part Part 66 - Training Materials Only  Engine The engine station numbers show the locations of the structural components and features on the engine. Engine station lines 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 34 of 232 CASA Part Part 66 - Training Materials Only  Clock Position Another position referencing system often used by engineers or flight crew is clock positions. These are used to report faults on the aircraft or engines by using a clock face as a reference, e.g. damage at the two o’clock position. The reference point is from within the aircraft looking forward. Clock positions Being able to identify the zone and an exact point anywhere within that zone provides important information, especially when writing up defects found during inspections. Correctly identifying the location of components prevents the possibility of replacing an incorrect component. If the defect found requires a structural repair, it ensures that the correct repair scheme for the structural classification is selected. Finally, correctly identifying the location of a defect ensures that the certification of the work performed directly refers to that work. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 35 of 232 CASA Part Part 66 - Training Materials Only  Airframe Structures Loads and Forces Stress, Strain and Fatigue in Aircraft Structures Aircraft structures must be able to withstand all flight conditions and operate under all payload conditions. In aircraft structures, stress is defined as the force or load (F) applied to an element (beam, bulkhead or skin) divided by its cross-sectional area (A), that is, σ = F/A, where the l indicates the longitudinal direction. The SI unit for stress is the Pascal (symbol Pa), which is equivalent to 1 N (force) per square meter (unit area). The unit for stress is the same as that of pressure, which is also a measure of force per unit area. An aircraft is subjected to five main types of stress loads: Tension Compression Shear Bending Torsion. Strain The stresses within a structure must be kept below a defined permitted level. Where stress is, so is strain. It is impossible for an object to be subjected to stress without experiencing strain. Strain (ε) is the linear deformation of the element divided by its original length or size. When considering length deformation only, the applicable formula is: change in length Strain = original length 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 36 of 232 CASA Part Part 66 - Training Materials Only  Stress-Strain Curves Stress-strain curves are used by engineers to interpret material strength. To obtain this type of curve, it is necessary to carry out a tensile test on a ductile material, such as a piece of low-carbon steel. Stress-strain curves A test piece of known dimensions is placed in the testing machine. To determine its strength, a load (or force) is applied to stretch the test piece. The size of the load is measured, and the stress induced in the piece is calculated by dividing the applied load by the cross-sectional area of the test piece. The calculated value, stress, is represented on the vertical axis of the graph. As the load increases, the material stretches, or elongates. The elongation is measured and converted to strain by dividing the change in length by the original length of the test piece. The calculated value, strain, is represented on the horizontal axis of the graph. As the stress increases, the material stretches. Initially it is elastic. That is, if the load is removed, the test piece returns to its original length. This occurs until the material reaches its proportional limit as shown on the curve. This part is a straight line. The deformation is called elastic deformation. As the stress increases beyond the proportional limit, many materials show a definite yield point, and this is often used as a basis (yield stress) for design calculations. For materials without a definite yield point, such as many brittle materials, an offset method is used to calculate a theoretical yield, which is used for design calculations. After the material reaches the proportional limit, if the stress increases, the material continues to stretch, but the elongation is now permanent. This is called plastic deformation: if the load is removed, the material will not return to its original length. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 37 of 232 CASA Part Part 66 - Training Materials Only  The point of ultimate stress on the curve represents the material’s greatest ability to support a load. In engineering, it is often called the ultimate tensile stress (UTS) and is the figure quoted when we compare the strengths of different materials. Once the UTS has been reached, the cross-sectional area of the test piece reduces (a process known as necking) and the applied load drops accordingly. Fracture occurs at the end of the curve. When aircraft are designed, the size of the parts is determined by the loads those parts have to withstand. Other factors, such as the cyclic nature of the loads, are also considered. As mentioned earlier, depending on the design process being applied, a ‘factor of safety’ can be applied to either the UTS or the yield strength to provide a maximum allowable stress from which the minimum safe size of components can be calculated. Tension Tension is force that tends to pull an object apart. It is a stress produced in a body by forces acting along the same line, but in opposite directions. For example: Flexible steel cables used in aircraft control systems are designed to withstand tension loads. Wing struts of a high-wing aeroplane are under tension during normal flight. The bolts used to fasten a windscreen to the fuselage are under tension stress during normal flight. In the BAC1-111 accident in June 1990, the error was fitting the wrong bolts to the windscreen. The accident happened when the aircraft was climbing through 17 300 ft on departure from Birmingham, England. The left windscreen, which had been replaced prior to the flight, was blown out under the effects of cabin pressure when it overcame the retention of the securing bolts, of which 84 out of a total of 90 were of smaller-than-specified diameter. The commander was sucked halfway out of the windscreen aperture and was restrained by cabin crew while the co-pilot flew the aircraft to a safe landing at Southampton Airport. This illustrates well the category of ‘wrong parts’. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 38 of 232 CASA Part Part 66 - Training Materials Only  Compression Compression is the opposite of tension. It is the resultant stress of two forces which act along the same line, pushing against each other. Wing struts of a high-wing aeroplane are under compression while on the ground, stationary or taxiing since the wings are not producing lift. 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 the materials together. Compression stresses on rivets 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 39 of 232 CASA Part Part 66 - Training Materials Only  Aircraft rivets are driven with a compressive force A landing gear oleo strut is under compression when an aircraft is on the ground. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 40 of 232 CASA Part Part 66 - Training Materials Only  Shear Shear in an aircraft structure is a stress exerted when two pieces of fastened material tend to separate. Shear stress is the outcome of sliding one part over the other in opposite directions. The rivets are designed to withstand shear stresses when aluminium skin panels are riveted to stringers. Shear forces try to rip the rivet in two, and therefore selection of rivets with adequate shear resistance is critical. Generally in aircraft structure, rivets are subjected to shear only, but bolts may be stressed by shear and tension, and in particular material, shear strength is less than tensile or compressive strength. Rivets under shear loads 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 41 of 232 CASA Part Part 66 - Training Materials Only  Torsion Torsion is the stress applied to a material when it is twisted. It is a combination of tension and compression loads. For example, torsional stress on the fuselage is created by the action of the ailerons when the aircraft is manoeuvred. Torque (also a twisting force) works against torsion. Torsional stress on a fuselage Bending Bending is the stress in an object caused by a load being applied to one end while the other is restrained. Like torsion, bending stress is a combination of tension and compression stresses. When an aircraft is on the ground, there is a bending force on the fuselage. This force occurs because of the weight of the aircraft. The bending action creates a tension stress on the lower skin of the fuselage and a compression stress on the top skin. These stresses are transmitted to the fuselage when the aircraft is in flight. Bending force on a fuselage 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 42 of 232 CASA Part Part 66 - Training Materials Only  When the aircraft is in flight, lift forces act upwards against the wings, tending to bend them upwards. The wings are prevented from folding over the fuselage by the resisting strength of the wing structure. The bending action creates a tension stress on the bottom of the wings and a compression stress on the top of the wings. Bending force on wings Hoop Stress Considering a thin cylindrical shell subjected to an internal pressure as shown below, tensile stress acting in a direction tangential to the circumference is called hoop stress or circumferential stress. Pressurised aircraft are subjected to hoop stress when the pressure inside the fuselage increases. The aircraft skin tries to expand and split along the longitudinal axis. Hoses are also susceptible to hoop stress. Hoop stress to fuselage 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 43 of 232 CASA Part Part 66 - Training Materials Only  Fatigue Fatigue is progressive localised structural damage. It occurs in a material subjected to repeated or fluctuating strains at stresses having a maximum value less than the ultimate tensile strength of the material. There are three requirements for a fatigue crack to form and spread in metals: There must be a local plastic stress. There must be a tension stress. There must be a cyclic (repeated or fluctuating) stress. If we can eliminate any one of these three requirements, we can stop the fatigue process. Note that composite materials can fatigue under compression loads. The insidious feature of fatigue failure is that there is no obvious warning. A crack forms without appreciable deformation of the structure, making it difficult to detect the presence of growing cracks. Fractures usually start from small nicks, scratches or fillets which cause a localised concentration of stress. Failure can be influenced by a number of factors, including size, shape and design of the component; condition of the surface; and operating environment. On April 28, 1988, Aloha Airlines Flight 243 suffered a massive structural failure as part of the fuselage tore away from the Boeing 737 while in the air en route to Honolulu. It was later determined that the failure was caused by widespread fatigue damage in the aluminium skin of the fuselage. It was calculated that this particular aircraft had experienced 89 090 flight cycles over its 19-year life span. The investigation into this failure concluded that the widespread fatigue damage that led to the ultimate failure was caused by the aircraft’s exceptionally large number of flight cycles and accelerated by the exposure of the aircraft to corrosive saltwater vapour during its inter-island flights. The combination of these two factors led to the creation of small cracks throughout the fuselage lap joints that eventually linked to form larger cracks and eventually complete failure. Fatigue damage 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 44 of 232 CASA Part Part 66 - Training Materials Only  Aircraft Drainage Corrosion Corrosion is one of the biggest threats to the integrity of aircraft structure. 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 such as water. Water cannot be avoided, but it can be controlled with drain paths, drain holes, sealants and corrosion-inhibiting compounds. Controlling the presence of water is usually the most effective means of preventing corrosion. Small quantities of water collect in the cabin because of condensation. Drains are installed in the fuselage structure to let the water out and further protect the structure from corrosion. The centre fuselage drains are located at the lowest point of the cabin, below the cabin floor. Drainage Effective drainage of all structure is vital to prevent fluids from becoming trapped in crevices. The entire lower fuselage of a pressurised aircraft is drained by a system of valved drain holes. Fluids are directed to these drain holes by a system of longitudinal and cross-drain paths through the stringers and frame shear clips. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 45 of 232 CASA Part Part 66 - Training Materials Only  Pressurised Fuselage Drain Valve As the cabin pressurises, the pressure-sensitive rubber seal moves down to close the drain hole. When the hole is closed, loss of cabin pressure through the drain hole is prevented. When the cabin pressure decreases, the rubber seal moves up to its usual position and opens the drain hole. Any water that has collected drains away through the main gear bay. The drain valves may be electrically heated to prevent freezing in a pressurised aircraft. Pressurised fuselage drain valve Drain Hole In non-pressurised aircraft and in non-pressurised components on pressurised aircraft (flaps, ailerons, rudder, elevators), small drilled holes in the aircraft skin act as moisture drainage holes. They also allow fresh air to ventilate the structure and help dry out any residual moisture. Drain hole 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 46 of 232 CASA Part Part 66 - Training Materials Only  Ventilation Although partial fuselage and wing-to-body fairings are not pressurised structures, differential pressures can build up between the internal sections. Equal pressure in the sections is maintained by the pressure relief valves located in the structure. The valves open, against spring tension, to release any overpressure build-up in the fairing sections. When the valves open, they remain open until the pressure equalises between the sections of the structure. Ventilation 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 47 of 232 CASA Part Part 66 - Training Materials Only  System Installation Provisions Provisions A number of provisions are made on aircraft which help make a maintenance engineer’s job safer and easier. System installations are designed to operate under all aircraft attitudes and flight loads. Systems need to pass through holes in frames and bulkheads. Fluid hoses and tubes are supported where they pass through the structure. Grommets made from nylon or rubber compounds protect the components, moving control systems and structure from damages and wear caused by fretting. Hydraulic Systems Most transport category aircraft have hydraulic systems located in dedicated panels. These panels are commonly located in an unpressurised zone of the aircraft. Fluid leakage drains overboard rather than contaminating the cabin. Having the components grouped together simplifies servicing and maintenance. Some aircraft do have hydraulic systems components located within the pressure cell, such as the F28/F100 and BAE146. In these aircraft, fluid leaks tend to pool under the cabin floor. Hydraulic fluid misting from a small leak under very high pressure (3000 psi) has been known to enter the air conditioning system to the discomfort of the passengers and crew. For this reason, most hydraulic components are eliminated from the pressurised area. Air Conditioning System Air conditioning systems are also located in an unpressurised zone. The cabin air distribution system includes air ducts, filters, heat exchangers, silencers, non-return (check) valves, humidifiers, mass flow control sensors and mass flow meters. Components have lugs and stays which attach to brackets mounted on the aircraft structure. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 48 of 232 CASA Part Part 66 - Training Materials Only  Electrical/Avionics Avionics and electrical equipment centres are commonly located in the pressurised cell of the aircraft. Some of these systems have their own cooling and temperature monitoring systems. Equipment centres are usually located below the cockpit or cabin floor, and some equipment centres can be accessed from the cabin. However, entry via the cabin is not intended for flight crew access in flight. Normal access is through an external door. The battery is stowed in a compartment designed to support its weight in all aircraft attitudes and under high g-loads. The inside of the compartment should be adequately protected with a tar-based paint or with polyurethane enamel to protect against corrosion. Battery cables should be adequately supported by clips to protect them from chafing and flexing. All hardware in the battery compartment should be corrosion-resistant. Fuel Storage In small aircraft, the fuel tanks are located near the centre of gravity so the balance changes very little as the fuel is used. In large aircraft, fuel tanks are installed in every available location, and fuel valves allow the flight crew to keep the aircraft balanced by scheduling the use of the fuel from the various tanks. The weight of the fuel is a large percentage of an aircraft’s total weight, and the balance of the aircraft in flight changes as the fuel is used. These conditions add to the complexity of aircraft fuel system design. Fuel tank system 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 49 of 232 CASA Part Part 66 - Training Materials Only  Main Landing Gear Fittings for the wing-mounted main landing gear (MLG) are installed on the aft face of the rear spar and on an auxiliary or false spar. These fittings include: The gear support rib The trunnion fitting The side stay attachment bracket The actuator attachment brac Main landing gear fittings 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 50 of 232 CASA Part Part 66 - Training Materials Only  Main landing gear components 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 51 of 232 CASA Part Part 66 - Training Materials Only  Waste Disposal The subsystems of the waste disposal system, which discard waste products and fluids from the galleys and the lavatories, are: The toilet system The wastewater drain system. Wastewater drain system The toilet system removes waste from the toilet bowl through a vacuum drain to an underfloor waste holding tank. The system uses potable water from the aircraft pressurised water system to flush the toilet. During ground service, the waste holding tank is emptied, cleaned and filled with a prescribed quantity of sanitary fluid. The wastewater drain system discards wastewater from the lavatory washbasins and the sinks of the galley through the aircraft's heated drain masts. Waste storage tanks are located in the rear of the aircraft, in the pressurised zone. The support structure must support the heavy full tanks at all aircraft attitudes and during high-g manoeuvres. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 52 of 232 CASA Part Part 66 - Training Materials Only  Potable Water The aircraft potable water system is another heavy storage system which supplies drinking water to the galleys and the lavatory wash basins. The potable water storage tanks are located in various locations within an aircraft’s pressurised zone. The support structure must support the weight of the full tanks at all aircraft attitudes and during high-g manoeuvres. Potable water system Heating is required to prevent the water freezing where plumbing passes through unpressurised areas. Galley and wash basin wastewater is drained overboard via heated drain masts. Auxiliary Power Unit Each engine and auxiliary power unit mount and its supporting structure must be designed for a limit load factor in the lateral direction. The side load on the engine and auxiliary power unit mount must be at least equal to the maximum load factor obtained in the yawing conditions, but not less than 1.33. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 53 of 232 CASA Part Part 66 - Training Materials Only  Lightning Strikes Introduction to Lightning Strikes Aircraft cannot always avoid being struck by lightning. To minimise the potential danger of a lightning strike, principally arcing and electrical shock, the electrical current created must be able to find a path of least resistance from the point of impact to a suitable discharge point. Lightning strike on a 737 This path is normally created by the metal skin and frame of the aircraft, provided that the individual structural components that make up the aircraft are adequately connected electrically. Composite materials, which are not electrically conductive, must have provision to conduct electrical current incorporated into their design and manufacture. To provide a suitable path to dissipate a lightning strike safely, the following procedures are employed: High-capacity electrical conductors, called bonding straps, link together all parts of the airframe to provide a low-resistance path, thereby reducing the possibility of arcing. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 54 of 232 CASA Part Part 66 - Training Materials Only  Bonding strap attached to flight control The radome, which must be manufactured of a non-conductive composite material, has conductive lightning diverter strips incorporated to collect the charge built up on the radome surface and safely transfer it to the airframe structure. Radome lightning diverter strips (F28) 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 55 of 232 CASA Part Part 66 - Training Materials Only  Static electricity or a lightning strike generally discharges from the sharp trailing edges of the airframe structure. To prevent arcing damage to these edges, static wicks are placed at these locations to harmlessly dissipate the electrical charge to the atmosphere. Static discharge wicks 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 56 of 232 CASA Part Part 66 - Training Materials Only  Aircraft Electrical Bonding Bonding Bonding is electrically connecting components of an aircraft structure together which are not otherwise adequately connected. It prevents static electricity (including lightning) from building up on one part of the structure to a point where it is high enough to jump to another part. Bonding failure can cause electromagnetic interference (EMI), damage mechanical and electronic components, and increase the risk of electrical shock. Grounding Grounding, in reference to aircraft electrical circuits, is using conductive parts of the structure to provide a return path instead of using an insulated wire to complete an electrical circuit normally. Also referred to as earthing, it provides an alternative return path if a two-wire electrical circuit becomes faulty, reducing the potential for electric shock. Bonding ensures a continuous path between different parts of a conductive structure. On an aircraft, the primary structure is commonly referred to as ground. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 57 of 232 CASA Part Part 66 - Training Materials Only  Bonding Leads To accomplish grounding, it is necessary to provide a conductive path where direct electrical contact does not exist. Jumpers or bonding straps are often used for this purpose in such applications as between moving parts, between shock-mounted equipment and structure, and between electrically conducting objects and structure. Bonding straps (or leads) are prefabricated from braided copper or aluminium terminated with crimps. The maximum permissible resistance of a bonding strap is 0.003 Ω. Radome bonding straps (F28) 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 58 of 232 CASA Part Part 66 - Training Materials Only  Airframe Structures - Construction Concepts (11.2.2) Learning Objectives 11.2.2.1.1 Describe several methods of aircraft construction including the use of stressed skin fuselage, formers, stringers, longerons, bulkheads, frames, doublers, struts, ties beams and floor structures (Level 2). 11.2.2.1.2 Describe several methods of aircraft construction including reinforcment, skinning, wing attachments, empennage and engine attachments (Level 2). 11.2.2.1.3 Describe the methods of anti-corrosive protection used in aircraft construction (Level 2). 11.2.2.2 Describe methods of structural assembly including riveting, bolting and bonding (Level 2). 11.2.2.3 Describe methods of surface protection including chromating, anodising and painting (Level 2). 11.2.2.4 Describe common methods of surface cleaning (Level 2). 11.2.2.5.1 Describe common methods used for airframe alignment and achieving airframe symmetry (Level 2). 11.2.2.5.2 Describe common airframe symmetry checks (Level 2). 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 59 of 232 CASA Part Part 66 - Training Materials Only  Aircraft Structural Design Structural Design Early aeroplanes were made with a truss structure of wood or bamboo, and the lifting and control surfaces were covered with cotton or linen fabric. This structure was lightweight, but difficult to streamline. Truss structure construction Up through World War I, most aeroplanes were built with a truss structure that used struts and wire- braced wings. The occupants sat in open cockpits within a fabric-covered fuselage. Almost all of these aeroplanes had the engine installed up front and auxiliary surfaces mounted aft of the wings to form the tail, or empennage, of the aeroplane. With increased knowledge of flight and the experience gained in building durable structures, designers constructed a superstructure of wooden formers and stringers over the framework to produce a more streamlined shape. Formers provide the contoured cross-sectional shape to a structure, while stringers run the length between the formers to fill in the shape. One of the major breakthroughs in structural design was made in the latter years of World War I, when thin-walled steel tubing was welded together to form the fuselage truss. When fabricated in this fashion, the structure reduced the overall weight of the aircraft while increasing its structural strength. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 60 of 232 CASA Part Part 66 - Training Materials Only  The next advance in structural design came with the development of a construction technique that allowed the aircraft to be formed without a truss frame. This design, generally known as a stressed- skin structure, allowed the aircraft to be built with a more streamlined shape and further reduced weight because the skin itself carried the structural loads. When constructed in this fashion, the aircraft was referred to as having a monocoque design. The term monocoque is derived from the French meaning “single-shell.” Thin aluminium-alloy sheets were next used for the exterior of monocoque stressed-skin structures. These sheets had compound curves formed in them by using hydro-presses or drop hammers to forge complex shapes. 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. Monocoque structure 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 aircraft, 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 aircraft. These designs proved to be so successful that they continue to be the primary method of modern aircraft construction. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 61 of 232 CASA Part Part 66 - Training Materials Only  Semi-monocoque structure Airframe Structure The term airframe refers to the mechanical structure of an aircraft, and as generally used does not include the propulsion system. The airframe of a fixed-wing aircraft is generally considered to consist of five principal units: the fuselage, wings, stabilisers, flight control surfaces and landing gear. The airframe components are constructed from a wide variety of materials and are joined by rivets, bolts, screws, and welding or adhesives. The aircraft components are composed of various parts called structural members (i.e. stringers, longerons, ribs, bulkheads). Aircraft structural members are designed to carry a load or to resist stress. A single member of the structure may be subjected to a combination of stresses. In most cases, the structural members are designed to carry end loads rather than side loads. That is, they are subjected to tension or compression rather than bending. Strength may be the principal requirement in certain structures, while others need entirely different qualities. For example, cowling, fairing and similar parts are usually not required to carry the stresses imposed by flight or the landing loads. However, these parts must have such properties as neat appearance and streamlined shapes. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 62 of 232 CASA Part Part 66 - Training Materials Only  Structural Members The longitudinal structural members are referred to as longerons and stringers. They are used to give the fuselage structure its longitudinal strength. Primary bending loads are taken by the longerons, which usually extend across several points of support to tie frames together. The longerons are supplemented by other longitudinal members, called stringers. The vertical structural members are referred to as bulkheads, frames or formers. The heavier vertical members are located at intervals to allow for concentrated loads. These members are also found at points where fittings are used to attach other units, such as the wings and stabilisers. The skin is attached to the longerons, bulkheads and other structural members, and it carries part of the load. The fuselage skin thickness varies with the load carried and the stresses sustained at particular locations. The airframe is all of the structural components (bulkheads, formers, longerons, stringers, etc.) attached together. Structural members 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 63 of 232 CASA Part Part 66 - Training Materials Only  Longerons Longerons are the major longitudinal load-carrying members. These can run the whole length of the fuselage or across several points of support. They are primarily used to take bending loads. Longerons 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 64 of 232 CASA Part Part 66 - Training Materials Only  Stringers Stringers give the fuselage its longitudinal strength. They connect the frames (formers) and are joined to the skin. Stringers are more numerous and lighter in weight than longerons. They are span-wise structural members designed to stiffen the skin and aid in maintaining the contour of the structure. Stringers also transfer stresses from the skin to the bulkheads and ribs to which they are attached. Stringers are not continuous throughout the structure like longerons and are not subject to as much stress. Stringers Frames Frames (also called formers in some textbooks) are transverse members used to give cross-sectional shape to the fuselage. Frames are made of wood or metal, which gives the fuselage or wing its shape, and the skin is attached to the outside. Frames 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 65 of 232 CASA Part Part 66 - Training Materials Only  Bulkheads 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. Bulkheads 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 66 of 232 CASA Part Part 66 - Training Materials Only  Pressure Bulkhead Pressure bulkheads are designed to withstand pressurisation loads during flight. They are used at the front and rear of pressurised aircraft: A forward pressure bulkhead is typically manufactured from aluminium alloy sheet and reinforced by a structural framework. A rear pressure bulkhead can be manufactured from sheet aluminium alloy sections or composite materials such as Kevlar® and carbon fibre. Pressure bulkhead 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 67 of 232 CASA Part Part 66 - Training Materials Only  Beams Beams are long structural members in the airframe and are designed to support/carry heavy loads. Beams Doublers A piece of metal used to strengthen skin structure where a component is attached is called a doubler. Doublers 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 68 of 232 CASA Part Part 66 - Training Materials Only  Struts A strut is a compression-resistant member made from steel tubing or heavy-walled aluminium alloy tubing which keeps beams or spars separated. It is installed to combat compression loads and keep the main load-carrying beams apart. Struts are also used on light aircraft to hold the wing up off of the fuselage. The wing-to-fuselage strut supports compression loads on the ground and tension loads in the air. Wing strut Another area where struts are used is on the undercarriage to resist compression loads during landing. Struts are also used to support the landing gear. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 69 of 232 CASA Part Part 66 - Training Materials Only  Ties A tie is a tension-resistant member which is used to keep the structure from pulling apart. Examples of ties on an aircraft are drag and anti-drag wires. They are high-strength, solid steel wires that cross the bays of a wing and oppose forces that tend to drag against the wing. Drag wires run diagonally from inboard to outboard to prevent the wing tip going backwards. Anti- drag wires run diagonally from outboard to inboard to prevent the wing tip going forward. Another tension-resistant tie is a tie rod. They do the opposite job of compression struts. Instead of keeping parts of the structure apart, they tie them together to prevent separation under tensional loads. Some aircraft structural components resist both tension and compression forces. Aircraft wing-to- fuselage struts act as a strut and a tie. They are designed to withstand tensional stress while the aircraft is flying and compression stress while the aircraft is on the ground. Tie rod 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 70 of 232 CASA Part Part 66 - Training Materials Only  Floor Structure The floor structure of an aircraft consists of a network of longerons and crossbeams. Flooring panels are fitted on top of the longerons and beams. They are generally manufactured from composite sandwich materials. Floor support structure Aircraft are susceptible to rapid decompression between compartments, causing structural failure. Blow-out panels or decompression doors are placed under the floor to prevent the cabin floor from collapsing during rapid decompression. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 71 of 232 CASA Part Part 66 - Training Materials Only  Floor support structure Floors for toilets and galleys must be watertight to prevent any spills entering the structure and causing corrosion. Freighter aircraft should also have watertight flooring to prevent spills from fluid cargo and waste from livestock contacting the structure. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 72 of 232 CASA Part Part 66 - Training Materials Only  Reinforcement At certain locations on the skin or fuselage, it is necessary to reinforce the structure due to excess stress concentrations. The following pages give some examples. Freighter aircraft floor structure is reinforced to carry the extra weight of cargo. Reinforcement around fuselage openings The fuselage of freighter aircraft is reinforced to accommodate the larger door openings. The internal structures of cargo aircraft doors are also reinforced structures. 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 73 of 232 CASA Part Part 66 - Training Materials Only  Fuselage protection Fuselage skin is reinforced with Kevlar panels on some wing-mounted propeller aircraft to prevent the penetration of propeller ice. Where flight controls attach with hinges, the structure is reinforced. Reinforcement at attachment points 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 74 of 232 CASA Part Part 66 - Training Materials Only  Methods of Skinning Fabric Skinning Older style aircraft have a fabric skin for aerodynamic smoothness attached over the structure. Today, fabric skins are used only on special-purpose aircraft such as aircraft used in agriculture applications, antique aircraft restorations and amateur aircraft. Fabric skinning 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 75 of 232 CASA Part Part 66 - Training Materials Only  Metal Skinning Some all-metal aircraft have retained fabric-covered flight control surfaces. This reduces weight and allows for quick repairs. Metal skinning Composite Skinning Composite fibre skins were introduced to aircraft in positions where the strength of stressed metal skins was not required. They are manufactured by impregnating a fibre (glass, carbon/graphite, Kevlar) with a resin and allowing it to cure. Composite skinning 2022-10-13 B1-11b Turbine Aeroplane Aerodynamics, Structures and Systems Page 76 of 232 CASA Part Part 66 - Training Materials Only  Multiple layers of fibre are impregnated and glued together with a resin matrix to form a thick fibre skin. Composite panel Another method used to obtain streng