What happened
On 18 May 2005, an Airbus A320-211, registration JY-JAR, was performing a landing on Runway 14 at Leeds Bradford Airport. The aircraft touched down just beyond the end of the marked touchdown zone with the autobrake set to a low setting. Shortly after the main wheels made contact with the runway, the crew began manual braking. However, once the aircraft reached a groundspeed of approximately 70 knots, the braking system ceased to function for a duration of about 17 seconds.
During this period, a dip in the runway profile obscured the view of the runway end from the flight deck. Realising that the aircraft would not stop within the remaining paved surface, the commander opted not to select alternate braking—as doing so would have disabled the nosewheel steering—and instead used the steering to execute a sharp right turn. The aircraft subsequently skidded sideways and came to a stop with its nosewheels off the runway, just before the start of a steep downward slope.
The investigation
The AAIB investigation examined the aircraft's braking and steering systems, as well as the flight recorder data. Investigators looked into the mechanical condition of the tachometer driveshafts and the electrical signals used to monitor groundspeed. The investigation also reviewed the aircraft's maintenance history and existing known anomalies within the brake control and monitoring systems.
Findings
- The investigation could not definitively confirm the exact cause of the braking loss, but the evidence was consistent with excessive noise in the electrical signals from the mainwheel tachometers.
- Two tachometer driveshafts were found to be bent, which created a resonant condition capable of producing signal errors at certain groundspeeds.
- These signal errors could lead the brake control system logic to calculate an incorrect aircraft reference speed, triggering the anti-skid system to release the brakes.
- Fluctuations in these signal errors prevented the aircraft's monitoring systems from detecting the loss of braking or alerting the crew.
- The investigation noted that several known anomalies existed within the brake and monitoring systems that could lead to similar braking failures or wheel locking.