What happened
On 11 November 2009, a Falcon 2000, registration CS-DFE, was undergoing maintenance at Biggin Hill Airport. To troubleshoot a reported braking defect, a maintenance team requested a series of high-speed taxi trials. The flight crew, accompanied by maintenance personnel, performed eight consecutive accelerate/stop runs on the runway, incrementally increasing the target speeds.
During the eighth run, the crew noticed the aircraft veering and suspected a tyre deflation. As the aircraft slowed, the crew was alerted by air traffic control that a fire had broken out under the left wing. The crew and passengers successfully evacuated the aircraft via the airstair door. The fire, which caused damage to the tyres, fuselage, landing gear, and wing, was extinguished by the airport fire and rescue service.
The investigation
Investigators examined the aircraft's flight data recorder and the physical damage to the landing gear. The inspection revealed that the left main landing gear had suffered extreme heat damage, with the hydraulic system losing fluid. The investigation established that the cumulative energy from the eight braking runs had exceeded the design limits of the brake units.
Specifically, the left brakes absorbed approximately 18 MJ of energy, surpassing the 15 MJ limit stated in the aircraft's flight manual. This intense heat caused the hydraulic seals to fail and released pressurized fluid, which then ignited upon contact with the overheated brake components. The investigation also noted that the aircraft's wheel thermal fuses had activated during earlier runs, causing the tyres to deflate.
Findings
- The primary cause of the fire was the release of hydraulic fluid from the brake units due to extreme heat and seal failure.
- The cumulative effect of multiple high-speed braking runs without sufficient cooling periods led to brake temperatures that caused the hydraulic fluid to ignite.
- The maintenance activity was not formally classified as an Operational Check Flight, meaning specific risk assessments and formal test schedules were not implemented.
- There was a lack of clear procedures regarding the coordination between maintenance and flight operations for high-speed ground testing.
- The aircraft's flight manual contained ambiguous guidance regarding brake energy limits and the necessity of turnaround times for cooling.