What happened
During the landing phase of a de Havilland Canada DHC-8-200, a failure of the radio altimeter occurred, which subsequently caused the failure of both the TCAS and GPWS. As the aircraft touched down, the power levers were moved into the beta range. At that precise moment, the weight on wheels sensors recorded a momentary in-air condition. Because the radio altimeter had failed, the beta lockout system relied entirely on these sensors, causing the system to activate and trigger engine manual caution warnings for both engines.
This activation caused the engine control units to revert to manual mode, eliminating the availability of reverse thrust. To avoid the restricted propeller speed range, the crew had to manually increase the power levers to above 7 and 80 RPM. This increased power setting, combined with a long taxi down a downhill slope, placed excessive demand on the braking system. After taxiing approximately 3.3 km, the brakes overheated, failed, and ignited, necessitating an emergency evacuation.
The investigation
The investigation examined the interplay between the failed radio altimeter and the beta lockout system. It was determined that the failure of the radio altimeter left the beta lockout system dependent solely on weight on wheels indications. The investigation also reviewed the crew's use of the BCF fire extinguisher during the evacuation. While the use of the extinguisher did not cause injury, it was noted that using BCF on high-temperature metal fires can produce toxic fumes.
Findings
- The radio altimeter failure was the primary driver of the subsequent system failures.
- The simultaneous occurrence of the power levers entering beta range and a momentary in-air sensor reading triggered the beta lockout.
- The loss of reverse thrust necessitated higher power settings, which increased the workload on the wheel brakes.
- The operator's Quick Reference Handbook (QRH) lacked specific guidance for handling a radio altimeter failure or a dual engine manual caution warning.
- The QRH also lacked instructions regarding the risks of using BCF extinguishers on high-temperature brake fires.