Airframe Structures: Key Concepts

Airframe Structures - General Concepts

• Airworthiness requirements dictate the structural strength of an aircraft, based on regulations, standards, and specifications.
• Aircraft structures are categorised into primary, secondary, and tertiary structures.
• Understanding fail-safe, safe-life, and damage-tolerance is crucial for aircraft safety.
• Zone identification systems and station numbering aid in the location of components within the aircraft.
• Stress, strain, bending, compression, shear, torsion, tension, hoop stress, and fatigue all affect aircraft structures.
• Drains and ventilation help manage moisture and pressure, essential for structural longevity.
• System components need specific accommodation methods within the aircraft structure.
• Lightning strike protection techniques ensure safety.
• Electrical bonding methods are applied to airframe structures.

Airworthiness Requirements

• An aircraft needs to conform to the regulations under which its been certified to be airworthy.
• Regulatory authorities set uniform airworthiness criteria, organised into parts that address specific activities.
• Part 23 covers Normal, Utility, Acrobatic, and Commuter Aircraft.
• Part 25 addresses Transport Category Aircraft.
• Part 27 relates to Normal Category Rotorcraft.
• Part 29 involves Transport Category Rotorcraft.
• Part 33 is for Aircraft Engines.
• Part 35 covers Propellers.
• Part 39 relates to 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 pertains to Aircraft Maintenance Organizations.
• Part 147 involves Aviation Maintenance Technicians Schools.
• Transport category aircraft must meet airworthiness standards specified in regulations, based on the country of registration.
• CASR 1998 Part 25 Subpart 25 (Australia, January 2003) covers airworthiness: 25.301 Loads, 25.303 Factor of Safety, 25.305 Strength and Deformation, 25.307 Proof of Structure.
• EASA Certification Specifications CS-25 Subpart C (Europe, July 2009) covers airworthiness.
• FAR Part 25 Subpart C (USA, August 2009) covers airworthiness: 25.301 Loads, 25.303 Factor of Safety, 25.305 Strength and Deformation, 25.307 Proof of Structure.
• Strength is determined by limit loads (maximum expected in service) and ultimate loads (limit loads multiplied by safety factors).
• Limit load is the anticipated maximum load, while ultimate load is what the structure must withstand without failure.
• Air, ground, and water loads must be in balance with inertia forces, accurately representing conditions.
• Load determination methods should be validated by flight load measurement.
• Deflections significantly changing load distribution require redistribution consideration.
• A safety factor of 1.5 is applied to the prescribed limit load, considered external for structural integrity.
• Aircraft structure must support limit loads without permanent deformation, and this deformation can’t affect safety.
• Structures need to withstand ultimate loads for at least 3 seconds.
• Dynamic tests simulating actual load conditions exempt the 3-second limit.
• Static tests to ultimate load must include ultimate deflections and deformation.
• Analytical methods must show deformation effects are insignificant, fully accounted for, or adequately covered.
• Structural flexibility should account for transient stresses due to load application rates.
• Aircraft must withstand vibrations and buffeting in likely operating conditions up to Vd/Md (design dive speed).
• Aircraft must withstand forced structural vibration from failures or adverse flight control conditions at airspeeds up to Vc/Mc (design cruise speed).
• Compliance with strength and deformation standards must be shown for each loading condition.
• Structural analysis is acceptable if previous experience indicates reliability.
• Static or dynamic tests must apply appropriate material correction factors, unless the tested structure has features that redistribute load upon element failure.

Structural Design & Classifications

• Structural design depends on the aircraft type and use, covering maneuvers, speeds, loads, and ground contact severity.
• Safety is provided by airframe structure strength that allows aircraft to safely perform its intended mission while correctly operated by qualified personnel
• Airframe certification requires testing to demonstrate static and fatigue strength.
• DLL (Design Limit Load) is the maximum load expected including a safety factor of 1.5
• Aircraft components are classified based on their importance to structural integrity, affecting inspection and maintenance.
• Primary structures are critical for safety; failure during flight can cause collapse, loss of control, or fatalities (e.g., fuselage, wings, stabilizers).
• Secondary structures can cause significant damage affecting aircraft operation (e.g., wing leading/trailing edges, dorsal fin, nose radome).
• Tertiary structures do not significantly affect operation upon failure (e.g., brackets, clamps, mounting hardware).

Fatigue Design Philosophy

• Fatigue considerations are crucial for aircraft structural design
• A common objective is to balance safety and cost.
• Safe-life, introduced in the 1940s, specifies a safe lifespan, or safety by retirement.
• Fail-safe, introduced in the 1950s, ensures the ability to sustain limit load even after element failure.
• Damage-tolerance, developed in the 1970s, uses inspection and is now used to achieve structural operating safety.
• SSIP (Supplemental Structural Inspection Program) ensures safety in accordance with FAR 25.571 Amendment 45.
• MSG(Maintenance Steering Group) is a methodology used to develop initial maintenance schedules.
• MSG-3 focuses on aircraft systems and loss of system function.
• Safe-life designs withstand a certain number of events before strength degrades; the component must stay crack-free.
• Two drawbacks exist: components may be removed with remaining life, and cracks can occur prematurely creating a safety problem.
• Fail-safe designs retain residual strength after a structural failure.
• Cracks can occur prematurely and may not be detected creating a safety problem.
• Damage-tolerance designs sustain a level of fatigue and damage and still withstand design loads for a set period that allows opportunities to detect the damage.
• The approach of damage-tolerance ensures that cracks can be understood/controlled which ensures an affect safety
• Damage-tolerant structures are designed to sustain cracks until detected and replaced.
• Key is scheduled crack inspections during scheduled maintenance.
• Damage-tolerance takes into consideration initial material or manufacturing flaws which the fail-safe principle does not
• Cracks can be detected before the crack becomes critical and leads to a failure
• Testing determines critical crack length, residual strength, and inspection intervals.
• Fatigue analysis, based on flight, ground, and pressurisation loads, determines crack growth performance and residual strength.
• Damage-tolerance is used for transport aircraft, while safe-life or fail-safe is used for other aircraft.
• Durability refers to a structure's ability to resist degradation such as fatigue and accidental damage

Identification Systems and Datum Reference

• Aircraft are divided into major zones for maintenance and component location for ease of finding parts on a schematic when doing maintenance
• Further broken down into sub zones and then into numbered zones
• Each zone produces a 3 digit identifier as defined by the ATA100 Specification
• Zone numbers range from 100 to 800, designating aircraft sections like fuselage (100-200), empennage (300), and wings (500-600).
• Subzones are identified by the middle digit (1-9), like 820 for cargo doors and 830/840 for passenger doors.
• Zones are identified by the third digit identifying a specific component or panel within the subzone.
• Suffixes can be added to the ID, like 821AR, helping maintenance personal correctly identify each panel.
• A Datum Line or “Reference Datum” is an imaginary point or line the manufacturer establishes to give the locations of all parts of the aircraft.
• No fixed rule exists for where the datum line needs to be placed, but should conveniently aid engineers in locating parts from it.
• Key locations are as follows, Fuselage, Fin/Stabilizer, and Engine.
• The three planes that define point intersect at the reference point: Fuselage, Buttock, Water Line.
• Fuselage station, FS, is the measurement in mm from the Datum Line (Airbus).
• A Body station is in inches from the datum line (Boeing).
• Butt Line measures left(LBL), right(RBL) measurements from the center line of the aircraft to give lateral locations.
• Waterline is an imaginary line giving vertical distances, with the bottom being zero.
• The Wing Buttock Line, is measured in inches going outward from Buttock Line Zero(centerline)
• Precise measurements of the wing use a term called Wing Station, which goes along the rear spar.
• The station numbers show the locations of structural components on engines
• Clock position is another way of referencing location of errors with locations are the 1-12 on a clock face with 12:00 being the front.

Aircraft Loadings

• Aircraft can withstand all flight and payload conditions, stress being the force or load divided by its cross section area.
• SI Unit for stress is measured in Pascals, being equal to one Newton per square meter.
• Aircraft withstand 5 Main Types of Stress Load, Tension, Compression, Shear, Bending, Torsion.
• The stresses within a structure must be kept below a prescribed level, leading to stress which is an experience when being subjected to stress(deformation over original length/size).
• A Stress-Strain Curve can be used by engineers to assess a materials yield strength.
• When aircraft parts designed, size determined by withstanding loads with engineers applying a “safety factor” to UTS(ultimate tensile stress) or yield strength to find a maximum allowable stress from which a minimum safe size can be calculated.
• Tension is the force that attempts to pull an object apart with examples being, steel cables in control systems, wing struts and windscreen bolts.
• Compression is the opposite of tensions and is a resultant stress of two forces which act against each other.
• Wing Struts of a high wing airplane, Stationary when not producing lift, aircraft rivers using compressive force.
• Shear- occurs when two attached materials attempt to slide past one another. In aircraft structures, sheer is present when aluminum skin in riveted to supporting stringers.
• Torsion- is the stress applied when a material is twisted. Example, Torsional stress on the fuselage is created by the action of the ailerons when the aircraft is maneuvered.
• Bending- is the stress in an object caused by a load being applied to one end while one is restrained.
• Hoops Stress- When a thin cylindrical shell is subjective to internal pressure tensile stress acting in a direction that is tangential to the circumference is called hoop stress.
• Fatigue- Progressing Localized Structural Damage from the result of reoccurring or fluctuating strains at stresses at maximum value less than ultimate tensile strength.
• Cracks will start at small nicks scratches of filets which leads to small concentrated area’s-Failure influence by size, shake the condition of Operating environment.

Aircraft Drainage, Ventilation, and Protection

• Corrosion needs Anode/Cathode, Metallic Connector, Electrolyte to form to be avoided using drainage, sealant, corrosion inhibitors
• Condensation will form in small quantities which forces engineers to install drains to protect from corrosion where floor is lowest, such as the centre fuselage drains.
• Drainage is vital with holes to drain and direct flow that might be prevented by pressure by small drilled holes for ventilation.
• Valve drainage can be prevented by heating units
• Ventilation occurs in partial fuselage and wing-to-body fairings.
• If Differential pressures can build with pressure relief valves help with overpressure relief
• System installations designed operate at all attitudes and flight loads where some must have Grommets to isolate structures
• Some Transport Aircraft have hydraulic located in unpressurised Zone for leak prevention while having the group to help simplify servicing
• Conditioning systems located in Zone and components secured in bracket
• Avionics need cooling and Temp controls while also having batteries secured, corrosion prevent cables chaffing. Located below in cockpit or Cabin.
• System installations designed operate at all attitudes and flight loads where some must have Grommets to isolate structures
• Some Transport Aircraft have hydraulic located in unpressurised Zone for leak prevention while having the group to help simplify servicing
• Conditioning systems located in Zone and components secured in bracket
• Avionics need cooling and Temp controls while also having batteries secured, corrosion prevent cables chaffing

Aircraft Lightning Strikes and Protection

• Minimize electrical shock from strike by finding path through impact point.
• Made metal skin aircraft by connecting everything structurally.
• Composite materials need special material to conduct current incorporated through design. Electrical protection is High Capacity Electrical Conductors and Bonding Straps
• Radome must be made of nonconductive with lighting diverting strips incorporated to Collect the charge with static whips at sharp edges to stop Arcing.

Aircraft Electrical Bonding and Grounds

• Bonding is connecting components that are not adequate and are used to prevent build ups that jump, that damage parts or EMI, with grounding the structural parts being used as part of the return.
• Bonding Leads are used when contacted cannot be found with braided copper and minimum resistance of 0.003.