Synopsis

On 9 January 1998, a Boeing 767-300 operated by United Airlines experienced an electrical systems malfunction subsequently attributed to arcing in a faulty electrical loom. The crew elected to divert to London Heathrow where emergency evacuation was carried out on a taxiway upon landing.

Description

The following is an extract from the UK AAIB Report on the Accident:

"Whilst in cruising flight near Paris during an ETOPS flight from Zurich to Washington DC, abnormal warnings appeared on the flight deck instrumentation and circuit breakers began tripping. The commander, in consultation with the operator’s maintenance control centre at London Heathrow Airport, decided to divert and land at Heathrow. The aircraft subsequently landed safely, but during the landing ground roll the right thrust reverser failed to deploy fully and smoke appeared at the forward end of the passenger cabin. As a result, the commander ordered an evacuation when the aircraft was on the taxiway, adjacent to the landing runway. The right off-wing escape slide failed to deploy and alternative slides were used. Several minor injuries occurred during the evacuation."

Cause

The investigation identified the following causal factors:

• "The tripping of multiple circuit breakers had been caused by the occurrence of electrical arcing and associated thermal damage to a wiring loom adjacent to the aft/upper inboard corner of the forward galley chiller unit within the Electronic and Equipment (E&E) bay, with resultant thermal damage to an adjacent loom and smoke generation.
• Prior damage to the wiring loom insulation adjacent the aft/upper corner of the chiller unit had occurred due to contact with such units during associated removal and installation; this chiller unit had been replaced on the day before the accident.
• Aluminium alloy swarf was present within the E&E bay prior to the accident and had probably assisted the onset of arcing between adjacent damaged wires in the loom.
• Incorrect installation of the chiller unit, with its heat exchanger exhaust fitted with a blanking plate, would have caused warm exhaust air to discharge from an alternative upper vent which was capable of blowing any aluminium swarf around the wiring looms.
• The crew were unaware of the potentially serious arcing fire in the E&E bay during the flight due to failure of the bay smoke warning system to activate on the flight deck, because the density of the smoke emitted by the arcing wiring in the bay was not apparently sufficient to be detected by the only smoke sensor, which was located in the card and rack cooling system exhaust duct.
• The jamming of a severely worn latch, associated with the right off-wing slide compartment, prevented that escape slide from operating during the evacuation; such latches exhibited vibration-induced wear on other aircraft."

Recommendations

Eleven recommendations were made as a result of the investigation. Below is a summary of the recommendations:

• The Boeing Commercial Airplane Company (BCAC) and the Federal Aviation Administration (FAA) should issue advice to pilots that whenever a series of apparently unrelated electrical/electronic system failures occur within a short period of time, they should consider a fire or smoke situation with following declaration of emergency and diversion to the nearest airport. Such situations may not be accompanied by warning indications on the flight deck. In general, circuit breakers which have tripped should be reset only, after a suitable cooling period, but only if considered essential to the safe operation of the aircraft.
• The FAA should require immediate inspection for damage, metallic debris and moisture in a specific area of the E&E bay. BCAC should emphasise in the Aircraft Maintenance Manual (AMM) that the appropriate handling equipment is used whenever a galley chiller is replaced within the E&E bay and include a specific warning for potentially critical wiring damage, in addition to a requirement for inspection after such unit installation. BCAC company should introduce as soon as possible additional protection of the wiring looms to prevent such damage.
• The FAA should require the installation of smoke or heat detectors within the E&E bays of Boeing 767 aircraft and other modern jet transport types. Manufacturers and Airworthiness Authorities should investigate the feasibility of installing smoke and/or heat detectors within remote areas of high wiring and equipment density. These institutions should ensure that during maintenance activities which are likely to produce conductive debris, wiring looms and electrical equipment are provided with temporary protection against such contamination. Follow-up inspection should confirm the lack of such debris.
• Aircraft manufacturers should devise simple additional methods for the physical protection of all wiring looms in areas of high maintenance activities, with the ultimate aim of protecting them from insulation damage from conductive debris, maintenance activities and collateral damage from adjacent loom failures.
• Boeing and other major manufacturers should conduct an assessment of the technology which may be available, to provide enhanced computer management/monitoring of electrical/electronic systems. The aim of the assessment would be to improve upon present protection against arcing/fire occurrence and to provide timely warning systems for associated overheat/fire situations.
• The FAA, in conjunction with the manufacturer, should require operators of the Boeing 767 aircraft, equipped with off-wing slide systems, to visually inspect the slide door latches for signs of wear. The BCAC should expedite action to improve the functional reliability of these latches.

Related Articles

• Airworthiness
• Ageing Aircraft - Electrical Wiring
• Damage Tolerance
• Defect Assessment

Further Reading

• Aircraft Accident Report 5/2000 - UK AAIB