History of the flight
The aircraft departed Nice en route to Luton and was cleared to climb to FL360. As it passed FL340, the crew noticed SPD LIM annunciators on both Electronic Attitude Director Indicators (EADIs). In the absence of a Quick Reference Handbook (QRH) procedure for this indication, they continued the climb, checked for other abnormal signs, and inspected circuit breakers. Three amber lights—SPEED TRIM FAIL, MACH TRIM FAIL, and AUTO-SLAT FAIL—were visible on the left System Annunciator Light Panel, but the MASTER CAUTION light was not illuminated.
The crew checked hydraulic indications (normal) but observed blank engine N1 and fuel flow gauges and a WXR FAIL annunciator on the Electronic Horizontal Situation Indicators (EHSIs). They completed QRH procedures for the three amber lights and noted a speed limitation of 0.74 Mach due to the MACH TRIM FAIL indication. The crew requested a descent to FL300, informing ATC of a technical problem.
Realizing the failures were linked to a general electrical problem, the crew continued descent. The Standby Attitude Indicator began to topple, followed by loss of background color on both EADIs. They selected Equipment Cooling Supply and Exhaust switches from Normal to Alternate, restoring color to the EADIs after a few seconds. The crew declared a PAN call and diverted to Lyons (Satolas).
During diversion, the Senior Cabin Crew Member reported that all Passenger Address and interphone communications in the cabin were inoperative. The commander briefed the cabin crew and instructed them to prepare for a precautionary landing. The flight deck door remained unlocked.
At some point, the flight deck clocks failed, and the co-pilot recalled a blue COWL VALVE OPEN light for the right engine. The autopilot and autothrottle remained engaged to reduce workload. The crew established Visual Meteorological Conditions (VMC) at 4,000 ft and received radar vectors for an ILS approach to Runway 18L at Lyons.
When the aircraft was established on the localiser, they could not arm the autobrakes and discussed manual braking. On selecting landing gear down, a red nose wheel disagreement light appeared. They requested an overflight of the runway at 2,000 ft; ATC confirmed the landing gear appeared extended. The crew then requested vectors for a second ILS approach to allow time to check the Main and Nose Gear Viewers, confirming the gear was locked down.
On the second approach, as the aircraft captured the glideslope, all electrical systems returned to normal and failure indications cleared. The commander disengaged the autopilot and landed manually without incident. After landing, he made a PA announcement to passengers. The Airport Fire Service was stood down, and the aircraft taxied to a stand and shut down.
Investigation
Following the incident, the operator contacted the aircraft manufacturer, who suggested a fault in the R1 or R326 relays. The R1 relay was replaced; subsequent checks showed normal electrical system operation, and the aircraft returned to service. The removed relay was sent to a UK maintenance organisation for investigation. Upon receipt, the relay was found to rattle. Testing revealed that switching operations were audible, but contacts A1 and A2 remained open circuit. Internal inspection showed a contact post had broken off, possibly due to fatigue from the stresses of contacts opening and closing. The relay had been on the aircraft since its delivery in 1989, accumulating 16,680 flight hours.
Battery Bus description
The Battery Bus is supplied from Transformer-Rectifier Unit (TRU) No 3. In the event of a main AC failure, the DC and AC elements of the Standby power system are supplied by the Battery Bus and static inverter respectively. The failure of the R1 relay caused loss of power to the Battery Bus, resulting in the progressive failures reported.
Effect of relay failure on aircraft systems
Loss of the Battery Bus resulted in the immediate loss of all connected systems, though the crew perceived progressive failures. This perception may have been due to time lags, such as between power loss and the toppling of the Standby Attitude Indicator's internal gyroscope, and the loss of color on the EADIs.
The aircraft's electrical system was checked using the AC and DC Metering Panel; no output from the Battery Bus and Static Inverter was noted, while other readings were normal. No abnormal procedure existed for these failures, prompting the crew to divert.
Probable Cause
No explicit probable cause was stated in the source.