The Landing Gear being an integral part of the airframe system enters an early design phase. As well as being optimised for the physical installation, The Landing gear must to comply with all regulatory and airworthiness requirements. Many found in FAR 25 / CS 25 for Large Aircraft and FAR 23 / CS 23 for “Non Large” Aircraft.
Major design drivers include
a) Performance,
b) Safety,
c) Cost considerations
d) Time line to deliver is constrained
e) Technology and
f) Resources available and required.
g) Kinematics (landing gear geometry)
h) The steering methodology – nose / body gear
Preliminary Design Phase
The actuation mechanisms and “Ground” & “Air” loads must be calculated for the selection of the optimum component sizing and material selection, typically calculated using dynamic simulations
Detailed Design Phase
During the Detailed Design all interfaces and associated systems are considered as well as the design of all the landing gear components. Computer Aided Design (CAD) plays a significant role in this process. (Complex Components will be computer machined during the production phase.)
Using Computer Simulation, Dynamic Analysis is carried out to ensure the detailed design work is completed for
a) Energy absorption,
b) Shimmy suppression and
c) Retraction/Extension.
Using Computer Modelling it is possible at this stage to produce a “Virtual Prototype” – Digital Mock-up of the landing gear.
Stress & Fatigue
Landing Gear is typically considered an Airworthiness Limitation and is designed as a safe life structure, therefore fatigue analysis methods are used to facilitate the prediction of life.
A typical safe for a commercial landing gear would provide for around 60,000 landings
Note -It is not possible for Aircraft Landing Gear to be designed as “Damage Tolerant” because of the utilisation of high strength materials which have the potential of critical flaw sizes which are too small to be detected by present day NDT techniques.
Testing & Certification
The certification testing of the various components and full assembly includes functional tests, structural and fatigue life tests, full cycle load checks are conducted on rigs with load cell platform, wheel spinning facility and lift simulation devices to verify shock absorber performance.
Loads are applied through loading actuators in required directions and strain data is acquired through strain gauging
Note – Final tests of the landing gear are carried out after installation on the aircraft as well as taxi tests, braking and steering tests.
Whilst fine tuning of certain design parameters are done during this phase, changes could be very expensive!
This is followed by flight testing phase where the full capability of the landing gear is evaluated.
Finally environmental tests including vibration, acceleration, temperature, altitude, salt spray, sand and dust etc. are performed.
Reliability & Maintainability
Failure mode and effect and criticality analysis (FMECA) is performed to assess reliability.
To optimise the serviceability and minimise operational costs both mean time between failures (MTBF) and mean time to repair (MTTR) are optimised.
Data on failure modes and failure rate are collected from previous designs to conduct this analysis and reliability is predicted before the design freeze.
Manufacturing & Assembly
Reliability of all products is assured through stringent quality control processes. Employing several techniques and materials including forgings, machined components from ultra-high strength steels, titanium and aluminium alloys. titanium, steel, mild steel, chromoly steel, stainless steel & exotic metals. As well as machined components & precision machine parts. Precision tolerances will be found in many components for example actuator cylinder, piston, shock absorber parts and axle.
Sofema Aviation Services offers EASA compliant regulatory training covering Aircraft Landing Gear Engineering and Maintenance Management Considerations. For details please see www.sassofia.com or email office@sassofia.com
