A319 electrical failure a mystery

 - February 26, 2008, 6:14 AM

The UK Air Accidents Investigation Branch was unable to determine why Airbus A319 G-EUOB suffered a major electrical failure while flying at night from London to Budapest on Oct. 22, 2005. According to the AAIB final report, the airplane was en route from London Heathrow to Budapest and was approaching FL200 in clear conditions. Both crewmembers heard a “clunk” and the flight deck became dark as the crew lost all flight displays, autopilot, intercom and radio and most of the flight-deck lighting (including standby instrument integral lighting). Autothrust was also disabled, triggering the associated warning horn.

The commander took control, using the external night horizon and the poorly illuminated standby horizon and altimeter for reference. Meanwhile, the copilot worked through the checklist display that had appeared on the lower ECAM display, the only available electronic display. Most of the affected systems were restored after about 90 seconds as the ac ess feed push-button switch was selected to alternate.

The commander made a PAN call to ATC, advising them of the problems, and requested a holding pattern while the crew reviewed the situation. The aircraft held for about 40 minutes while the commander conferred with the company, and it was agreed that the flight should continue to Budapest. At the destination the commander completed the technical log and discussed the problem with a local engineer, who was not an employee of the operator.

The aircraft remained in service until Oct. 28, 2005, when an air safety report completed by the commander reached the company flight operations safety department by surface mail. The aircraft was then removed from service for examination and testing overseen by the AAIB, accompanied by representatives from the French BEA. Investigators conducted extensive testing over the next four months, but they could not identify any faults or reproduce the symptoms the crew reported.

Electro-mechanical Backups
The electrical power generation system of the A320 series underwent a number of design changes and there are two configurations of the system. The original is referred to as “classic” and the more recent is called the enhanced system. G-EUOB is fitted with the classic system.

If both the primary flight instrument displays fail, there are standby instruments, including an electrical standby horizon, standby ASI, standby altimeter and compass. They can be either mechanical, as installed on G-EUOB, or an electronic display known as the integrated standby instrument system (ISIS).

For aircraft without that system the standby horizon is powered from the DC ESS bus, which is powered by AC BUS 1. If either system fails, the standby horizon will lose power and become unusable after about five minutes. At mid-April 2006 1,664 A320-series aircraft did not have the ISIS wiring system. G-EUOB was manufactured with the ISIS wiring loom, but it was fitted with electro-mechanical standby instruments to meet customer requirements. The standby instruments would have remained powered.

Although the AAIB could not trace the source of the fault, it did issue 13 safety recommendations as a result of the incident. Among those recommendations: flying on standby instrumentation should be included in flight simulator training programs.