What happened
On 13 August 2006, a Boeing 737-36Q, registration G-THOJ, was cruising at FL350 on a passenger flight from Cardiff to Malaga when the crew heard a 'click' from the cockpit panel, followed by a failure of the DC Battery Bus. This electrical failure resulted in the loss of several aircraft systems, including engine N1 and fuel flow indications, as well as the standby attitude indicator (SAI). Additionally, the primary Electronic Altitude Direction Indicators (EADIs) transitioned to a monochromatic mode.
While the crew was able to restore some systems by switching the cooling fan power supply to 'ALTERNATE', the loss of the DC Battery Bus also caused the aircraft'-s interphone to become inoperative. The crew managed communications via a cabin crew member and continued the flight to Malaga. However, during taxiing after the flight, the aircraft suffered a second failure of the same nature, which was traced to a failure of the R1 relay.
The investigation
The AAIB investigation focused on the electrical failure and the subsequent loss of instrument functionality. Investigators examined the aircraft's defect history, noting a similar in-flight failure had occurred on 11 August 2006. During the investigation, it was established that the loss of the DC Battery Bus prevents the equipment cooling low-flow sensors from functioning. This means the 'equipment cooling OFF' light does not illuminate, leaving the crew unaware that the EADIs are at risk of overheating.
Further examination of the R1 relay revealed that while the manufacturer had previously issued a service bulletin to improve reliability, the specific relay involved in this incident was one of the recommended types. The investigation also noted that the loss of the standby attitude indicator's gyroscope was a direct consequence of the power loss to the SAI from the DC Battery Bus.