What happened
On 17 May 1999, a Fokker F2SB Mark 050, registration G-UKTH, was operating a public transport flight at London Stansted Airport. Prior to the flight, the incoming crew had reported that the left wheel brakes had a tendency to 'snatch'. Despite this, the commander elected to accept the aircraft for the outbound sector after discussions with ground engineers.
During taxiing toward Runway 05, the crew utilized additional engine power to counteract a strong tailwind, using the brakes to regulate speed. While holding at point HA, a cabin crew member alerted the flight deck to a fire involving the left-hand wheels. The commander immediately initiated an evacuation, instructing the first officer to manage ground drills and notifying Air Traffic Control of the brake fire.
Upon inspecting the situation through the cockpit window, the commander observed a deflated outer tyre and a small fire localized around the outer brake unit. The commander determined that activating engine fire extinguishers was unnecessary. The evacuation was completed without 0 injuries to the 18 passengers or 4 crew members. The fire had largely extinguished itself by the time the Airport Fire Service arrived. Following the incident, passengers were permitted back on board under supervision to retrieve personal belongings before being transported to the terminal via bus.
The investigation
The investigation focused on the mechanical failure of the braking system. It was noted that the specific brake unit had a history of involvement in a previous incident on 27 October 1998, where a crosswind landing had caused the tyre to deflate due to high energy absorption.
Technical examination of the brake assembly following the May 1999 event revealed that the unit had seized. This seizure caused the piston seals to disintegrate, which subsequently allowed brake fluid to leak onto the overheated brake components, thereby initiating the fire. While the anti-skid valve was found to be functional, the brake control valve was sent to the manufacturer for detailed strip examination.
Findings
- The fire was triggered by brake fluid leaking onto a hot brake unit following the disintegration of piston seals.
- The brake unit was in a seized condition.
- The aircraft had been operating with a known tendency for the brakes to 'snatch' during previous sectors.
- High power settings used during taxiing to combat a strong tailwind contributed to the thermal conditions of the brakes.