What happened
On 15 September 2016, a Spitfire Mk IX, registration G-LFIX, was performing a commercial flight at Sywell Aerodrome in Northampton. During the landing roll, the left landing gear collapsed, causing the aircraft to veer sharply to the left. The pilot attempted to maintain control using full rudder and aileron, but the aircraft departed the runway and came to rest on the grass. The aircraft sustained heavy damage to the propeller, along with minor damage to the engine cowls, landing gear doors, and wingtip. The pilot and one passenger were able to exit the aircraft without injury.
The investigation
The investigation focused on why the landing gear failed despite the pilot receiving indications that the gear was down and locked. Examination of the aircraft revealed that the left landing gear actuator had not reached its full extension. It was discovered that the chevron seals within the left landing gear actuator had failed. Debris from these deteriorating seals had migrated into the space between the actuator ram and the seal support plate, effectively jamming the mechanism.
Technical analysis of the seals showed that the failure was progressive, with material being gradually stripped from the seal lip over several operating cycles. While the maintenance organisation had installed new seals in 2/013, the investigation could not identify any specific chemical or physical defect that made these particular seals more susceptible to failure than others. Furthermore, the investigation confirmed that the landing gear position indication system was functioning correctly, but the specific configuration of the indicator bulbs allowed for a misleading visual signal.
Findings
- The left landing gear failed to achieve a locked position because the actuator chevron seals failed, causing debris to jam the actuator ram.
- The hydraulic system pressure increased enough to trigger the pressure switch, which changed the status indicator from 'down' to 'idle', providing the pilot with a false sense of security regarding the gear's status.
- Due to a modification in the indicator panel, light from the right-side bulb was sufficient to partially illuminate the letters 'D' and 'O' in the 'DOWN' caption, even though the left gear was not locked.
Safety action
To prevent future occurrences of misleading indications, the maintenance organisation modified the landing gear indicator panel. This modification ensures that the complete word 'DOWN' will not illuminate if any part of the landing gear fails to reach the fully locked position.