What happened
On October 24, 2004, a Dassault Falcon 2000, registration N105LF, was performing a private flight from Geneva to Moscow-Vnukovo. While cruising at flight level 360 near the BAMUR waypoint, the crew experienced a sudden cabin pressure warning accompanied by significant physical pressure changes felt in their ears.
In response to these sensations and the visual alarms, the crew donned oxygen masks and initiated an emergency descent. During this process, the left generator failed. The crew requested permission from Munich Air Traffic Control to descend and divert to Zurich. While the crew attempted to restore the left generator, it only functioned briefly before failing again.
Upon landing at Zurich, the crew attempted to start the auxiliary power unit (APU), but the attempt failed. Simultaneously, the nose wheel steering malfunctioned, leaving the aircraft with limited maneuverability. The aircraft was eventually moved to a parking stand via a tow tug.
The investigation
Investigators examined the aircraft's digital flight data recorder (DFDR) and cockpit voice recorder (CVR). The DFDR did not contain relevant data regarding the electrical system or cabin pressure during the event. The CVR recordings were of limited use due to high volume levels caused by the use of oxygen mask microphones and low volume levels during the final approach.
Technical analysis focused on the electrical and pressure regulation systems. The investigation looked into the cause of the generator failure, the sequence of the pressure fluctuations, and the subsequent failure of the nose wheel steering and APU start attempt.
Findings
- The primary cause of the incident was a faulty generator caused by a temporary interruption in a plug within the exciter circuit.
- This electrical malfunction triggered a false cabin pressure warning and caused the main pressure regulation valve to close. Because the reserve valve was also closed, the pressure differential increased, leading to the physical pressure fluctuations felt by the crew.
- The crew's decision to perform an emergency descent was a prudent safety response to the perceived loss of cabin pressure.
- The investigation suggests the crew likely left the right bus tie rotary switch in the 'FLIGHT NORM' position rather than switching it to 'TIED' as prescribed by the checklist. This prevented the essential bus from being powered by the second generator and led to the depletion of the aircraft battery.
- The failure of the APU to start was likely due to the insufficient charge level of the aircraft battery following the period of discharge.