The guidelines followed by aircraft manufacturers when designing landing gear are not sufficient to deal with crash landings made outside of the runway, according to the Air Accident Investigation Branch (AAIB).
In a report investigating the crash landing of a British Airways Boeing 777-236ER, the AAIB has urged regulatory authorities to review the requirements of landing-gear failures by taking into account the effects of landing on different surfaces.
The initial impact of the B777-236ER, which crashed short of the runway at London’s Heathrow airport in January 2008, resulted in only partial separation of the landing gear beneath each wing causing passenger injury and damage to the aircraft.
‘Typically, aircraft of this size have their landing gear and engines attached via mechanical fuse-pin systems, which release under predetermined impact loads in order to protect adjacent wing fuel tanks from impact damage,’ explained Andrew Walton, senior project engineer at the Cranfield Impact Centre (CIC).
However, Walton said that previous modelling of landing-gear failure had been based on impact on hard runway surfaces.
The B777-236ER’s landing gear was manufactured to comply with this and therefore could not deal effectively with a ‘gear down’ grass landing.
‘This issue is of wider significance because the majority of potentially survivable aircraft impacts are within the airport environment on takeoff or landing and it is almost inevitable that some interaction with soft soil will take place,’ he said.
Prior to releasing the report, Walton and the team at the CIC were commissioned by the AAIB to employ finite element analysis (FEA) on the aircraft in order to reconstruct the dynamics of impact.
Due to the relatively large amount of soil deformation, the group initially considered using a meshless approach called Smoothed Particle Hydrodynamics (SPH) to model the ground surface. However, after modelling they found that a conventional FE mesh could represent the soil deformation more effectively.
This FE mesh was generated by a software package known as LS-DYNA, with soil tests and measurement data from the crash site contributing to the simulation.
Walton said: ‘In the case of this crash model, impact simulation with the soft soil indicated a different and less favourable behaviour in the fuse pins when compared to an identical simulation with a hard runway.’
While the soft-ground simulation did not match the exact fuse-pin behaviour seen during the crash, the CIC believes the simulation results combined with actual data could be significant in modifying future landing-gear design.
The AAIB report is due to be reviewed by the regulatory authorities, who will decide whether to conduct further research and simulations of aircraft impact on soft soil.