What happened
On 31 August 1999, a BAe 146-300, registration EI-CLG, was performing a scheduled passenger flight from London Stansted to Dublin. While climbing through FL190, the aircraft's autopilot disconnected, and warning flags appeared on the commander's Attitude Direction Indicator (ADI). Simultaneously, a 'GEN OFF LINE' message appeared for the No 1 electrical generator, accompanied by an 'ELECT' master warning.
Following the electrical failure, the crew experienced a noticeable change in ear pressure. Monitoring the pressurisation panel, they observed a cabin descent rate of approximately 1,800 to 2,000 feet per minute. In response, the crew transitioned the pressurisation controller to manual mode, closed the cabin air discharge valves, and donned oxygen masks as a precaution. After descending to FL80, the crew declared an emergency due to the concurrent electrical and pressurisation issues. They subsequently requested a diversion to Manchester to inspect the generator. The aircraft landed safely in Manchester with no injuries to the 3 crew or 72 passengers.
The investigation
Investigators examined the aircraft's electrical and pressurisation systems to determine the cause of the simultaneous failures. The investigation established that the No 1 electrical generator had tripped offline because of high temperatures within the oil supply to the Constant-Speed Drive (CSD).
While the pressurisation controller was removed for testing, subsequent examinations revealed it was fully serviceable and free of defects. The investigation focused on how the loss of the generator could impact the pressurisation system, noting that the aircraft's electrical architecture relies on a Bus-Tie Contactor (BTC) to transfer load from a failed generator to the remaining active generator.
Findings
- The primary cause of the electrical failure was high temperature in the oil supply to the CSD of the No 1 generator.
- The loss of the No 1 generator caused a transient interruption of AC power on AC BUS 1 during the period required for the No 2 generator to take over the load via the BTC.
- This momentary power interruption affected sensitive components, including the autopilot, the commander's ADI, and the pressurisation controller.
- The manufacturer suggested that a momentary period of power supply outside normal limits likely occurred before the protection circuits tripped, which may have caused the pressure surge.
- The crew's immediate manual intervention was deemed premature, as they did not allow the automatic mode time to stabilize the cabin altitude, though their actions were considered a reasonable response to the unexpected pressure change.