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Safety: Electrickery and Nil Redundancy

By David Evans | February 1, 2006
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The flight crew lost nearly all of their instrument displays and used both visual and standby horizons at night for attitude reference until the unexplained electrical failure resolved itself. Then, instead of landing, they pressed on to the destination, where maintenance engineers reset all affected systems. But the reason for the glitch remained unresolved and unreported.

It is a tale of nil redundancy.

According to a recent special bulletin put out by the UK’s Air Accidents Investigation Branch (AAIB), the aircraft was a British Airways flight in October 2005 from London’s Heathrow Airport to Budapest. The Airbus A319 was approaching 20,000 feet after takeoff when the pilots heard a "clunk," and the flightdeck suddenly became dark. To put it mildly, they lost a lot. Both the captain’s and first officer’s primary flight displays, navigation displays and upper electronic centralized aircraft monitor (ECAM) went dark–they were left with only the lower ECAM display. The autopilot was lost, and the master warning tone sounded. Autothrust was lost, and the master caution alert sounded. The cockpit-cabin intercom was lost. Most of the flightdeck lighting, to include glareshield, overhead and pedestal lights, was out.

The captain, who was the pilot flying, attempted to transmit a MAYDAY, but the radio was unpowered. He manually flew the aircraft, maintaining attitude by referring, in combination, to the external night horizon and other standby instruments he could see. The AAIB account says, "It is probable that the electrically driven standby horizon was not powered or lighted, [although] according to the flight manual, it does remain usable for five minutes."

The first officer carried out the ECAM-directed troubleshooting actions and restored the primary flight systems, radios and most other systems. Elapsed time of the outage was about two minutes.

With communications restored to air traffic control, the flight crew requested a holding pattern to review the status of their aircraft.

After 40 minutes holding, the crew deemed the electronics situation sufficiently stable to press on to the destination, although during the landing roll-out the pilots noticed an amber caution for the port engine thrust reverser. Maintenance technicians at Budapest reset all affected systems, and the aircraft continued operation in revenue service for six days.

At that juncture, the AAIB received a mandatory occurrence report (MOR) of the incident, and a number of components were removed for detailed investigation, including the display management computers. The airplane was returned to service while those components are being investigated. With the assistance of operator British Airways and manufacturer Airbus, the investigation is proceeding, and the AAIB promises a "further report" when the investigation is completed.

Meanwhile, a few thoughts about the case come to mind. If a single bus is lost, one would think that no more than half the cockpit screens would go blank. That was not the case here. To be sure, the one remaining screen, the ECAM, carried a drill that told the crew to press the button that turned almost everything back on. Even so, losing the primary flight displays for both captain and first officer raises a question of independent redundancy.

The AAIB should further explore the standby attitude indicator. The AAIB says, "the standby horizon, which was unpowered [and unlighted], would only have been available for five minutes." Other than for charging its own battery, what is the point of having the standby system dependent on the aircraft’s electrical system? As one pilot quipped, "might as well save 5 pounds and leave [the standby instrument] at home."

Redundancy, in this case, rested on the fact that the "electrickery," as it were, was backed up by visual meteorological conditions outside and at the destination. AAIB surely will examine the certification of this system. What might have happened during the two-minute power outage if the weather was instrument meteorological conditions or if the failure occurred again during approach and landing? Lastly, with an unexplained electrical failure, the crew held for 40 minutes to sort things out, and then pressed on, in effect conducting an air test with passengers on board.

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