What happened
On November 23, 1998, a BAe 146-300 operating a scheduled flight from Hannover to Milan experienced a cascading series of electrical failures. Shortly after takeoff, the flight instruments (PFD and ND) on the co-pilot's side failed, and both yaw dampers disengaged. The pilot in command (PIC) noted the loss of several electrical buses (AC 2 and DC 2) as the landing gear was retracted.
During the approach, after the landing gear was extended, the aircraft suffered a total electrical failure. This resulted in the loss of the Electronic Flight Instrument System (EFIS) on the pilot's side, the loss of all radio communications, and the loss of instrument lighting. The flight data recorder and transponder also ceased functioning.
Despite extremely limited visibility and low cloud ceilings, the PIC completed the circuit and landed the aircraft. The landing was highly unstable; all four main tires burst upon touchdown. The pilot found that the spoilers had failed to deploy, the braking system was non-functional, and the engines could not be moved to idle, necessitating a manual engine shutdown. Due to the failure of the nosewheel steering, the aircraft struggled to remain on the runway and eventually came to a halt approximately 400 meters before the end of the runway. All 5 passengers and 4 crew members evacuated via the onboard stairs without injury.
The investigation
The BFU investigation focused on the electrical system and the sequence of events leading to the power loss. Investigators analyzed flight data from the flight recorder and radar/radio recordings provided by DFS. A technical examination of the aircraft was also conducted by the operator and the manufacturer.
Investigators discovered that the cockpit voice recorder (CVR) was non-functional and contained data from a previous flight. The investigation also revealed that the primary cause of the initial instability was a maintenance error performed prior to departure. While investigating the battery systems, it was found that the technician had mistakenly disconnected the healthy battery (BAT #1) instead of the faulty one (BAT #2) due to poor visibility in the battery compartment and non-standard labeling.
Findings
- The aircraft was operated in an unauthorized system configuration because the primary battery had been inadvertently disconnected.
- The initial electrical instability was caused by corroded contacts on BAT #2.
- The total electrical failure during approach was caused by the shutdown of the APU and the simultaneous failure of the hydraulic standby generator.
- The loss of electrical buses (AC 2 and DC 2) after takeoff occurred because the engine-driven generators were not yet connected to the electrical supply, leaving the aircraft reliant solely on the APU.
- The pilot's decision to continue the flight under difficult weather conditions, while attempting to maintain visual contact with the airport, limited the crew's ability to fully diagnose the deteriorating systems.