What happened
On 22 March 2005, a Boeing 737-33V, registration G-EZYN, was climbing through FL340 en route from Nice to Luton when the flight crew observed several abnormal indications. Initially, the crew noted speed trim, Mach trim, and auto-slat failure lights on the left system annunciator panel. While performing checks, the crew discovered that both engine N1 gauges and fuel flow indicators were blank, and the weather radar display had failed.
Due to the lack of specific procedures for these simultaneous indications, the commander declared a PAN call and initiated a diversion to Lyons (Satolas). During the descent, the situation progressed as the Standby Attitude Indicator began to topple and the background color on both Electronic Attitude Director Indicators (EADIs) was lost. The crew also noted the failure of the flight deck clocks and a nose wheel disagreement light upon landing gear extension. To prevent total EADI failure due to overheating, the crew transitioned the equipment cooling supply and exhaust switches to the alternate position, which successfully restored the EADIs' background color.
The investigation
The investigation focused on the aircraft's electrical system. Analysis of the AC and DC metering panels revealed that there was no output from the Battery Bus. The investigation established that the failure originated from a contact post that had broken off within the R1 relay, which is responsible for supplying power to the Battery Bus. This component had been in service since 1989 and had accumulated over 16,000 flight hours.
Findings
- The primary cause of the electrical degradation was the failure of a contact post in the R1 relay associated with the Battery Busbar.
- The loss of the Battery Bus resulted in the loss of several critical systems, including engine fire detection, the Master Caution light, and the inverter control relay.
- The loss of power to the equipment cooling fans created a risk of EADI display shutdown due to high temperatures.
- There were no specific Quick Reference Handbook (QRH) procedures available for this particular relay failure on this aircraft model.
Safety action
- Safety Recommendation 2005-65: It is recommended that the Federal Aviation Administration require the Boeing Company to examine various in-service Boeing 737 configurations with the intent of providing operators with an Operations Manual procedure for managing a loss of power from the Battery Busbar.