Ageing Aircraft - Electrical Wiring

Ageing Aircraft - Electrical Wiring

Description

Deterioration in an aircraft electrical wiring interconnection system (EWIS) is often difficult to identify and repair. The EWIS on many older aircraft still in service was often designed on the ‘fit and forget’ principle, but both age itself and inadvertent collateral damage during unrelated maintenance or routine inspections cause airworthiness problems. Both deterioration and damage are often associated with the difficult-to-detect condition of wiring within the bundles of wires routed together as in ‘looms’. These looms and aircraft wiring generally are often in locations which are difficult to access and, even where they are accessible, only the condition of the outer wires can be properly checked. A lot of effort has therefore been put into developing more effective inspection processes for wiring loom integrity in particular, but also into practical methods of confirming wiring circuit integrity generally. Attention has also been focused on good maintenance practice in respect of wiring looms which have in the past often been vulnerable to undetected damage inadvertently caused during base maintenance.

TWA 800

A widely known example of an accident in which the probable initiating factor was arcing due to damaged insulation on electrical wiring occurred in 1996, when a 23-year-old Boeing 747-100 on an international revenue passenger flight exploded in mid-air shortly after take off from New York. See: B741, en-route, East Moriches NY USA, 1996

Solutions

Improved inspection and diagnostic processes for EWIS in aging aircraft are being progressively mandated but the prospect of longer term progress lies in new technology. Live-wire testing of aircraft EWIS during flight is now possible and can detect intermittent faults that cannot be located during maintenance on the ground. Arc Fault Circuit Interrupter technology is being developed to provide additional safety measures when a fault occurs. Moving forward, nanoscale sensors embedded within emerging ‘smart’ wire systems will detect and correct faults in real time. In the longer run, fibre optics and wireless technologies will reduce the need for bulky wiring looms. While these and other techniques are being developed and tested, fleets must rely on diligent application of the array of currently available diagnostic technologies.

In some instances, where wiring deterioration has been found in locations where the consequences could be instantly disastrous, such as in the FQIS systems inside fuel tanks, means of protecting against the consequences of ignition, such as nitrogen inerting, have been examined, although not yet implemented.

Safety Reporting

Finally, as with Ageing Aircraft - Structural Failure, it also appears that there has often been ineffective safety reporting to the NAA. That authority, based on consideration of individual reports, has potentially approved an aircraft operator or maintenance organisation to take corrective actions to seemingly minor but possibly significant incident or inspection findings which, when considered together, could have helped identify interventions capable of preventing a significant Incident or Accident.

Accidents and Incidents

The following events involved an airworthiness factor associated with Electrical Power:

On 31 January 2011, a Singapore Airlines Airbus A380-800 was in the cruise when there was sudden loud noise and signs of associated electrical smoke and potential burning in a toilet compartment with a corresponding ECAM smoke alert. After a fire extinguisher had been discharged into the apparent source, there were no further signs of fire or smoke. Subsequent investigation found signs of burning below the toilet floor and it was concluded that excessive current caused by a short circuit which had resulted from a degraded cable had been the likely cause, with over current protection limiting the damage caused by overheating.

On 29 July 2011 an oxygen-fed fire started in the flight deck of an Egypt Air Boeing 777-200 about to depart from Cairo with most passengers boarded. The fire rapidly took hold despite attempts at extinguishing it but all passengers were safely evacuated via the still-attached air bridge access to doors 1L and 2L. The flight deck and adjacent structure was severely damaged. The Investigation could not conclusively determine the cause of the fire but suspected that wiring damage attributable to inadequately secured cabling may have provided a source of ignition for an oxygen leak from the crew emergency supply

On 7 January 2013, a battery fire on a Japan Air Lines Boeing 787-8 began almost immediately after passengers and crew had left the aircraft after its arrival at Boston on a scheduled passenger flight from Tokyo Narita. The primary structure of the aircraft was undamaged. Investigation found that an internal short circuit within a cell of the APU lithium-ion battery had led to uncontained thermal runaway in the battery leading to the release of smoke and fire. The origin of the malfunction was attributed to system design deficiency and the failure of the type certification process to detect this.

On 12 October 2018, the crew of a Boeing 737-400 already released to service under MEL conditions with an inoperative No 1 engine generator encountered a loss of services from the No 2 electrical system en-route to East Midlands which created a situation not addressed by QRH procedures. The flight was completed and both the new and existing defects were subsequently rectified relatively easily. The Investigation concluded that the operator involved appeared to be prioritising operational requirements over aircraft serviceability issues and made a range of Safety Recommendations aimed at improving company safety culture and the effectiveness of regulatory oversight.

On 24 August 2016, an ATR 72-600 experienced a static inverter failure which resulted in smoke and fumes which were identifiably electrical. Oxygen masks were donned, a MAYDAY declared and after the appropriate procedures had been followed, the smoke / fumes ceased. The Investigation noted a long history of capacitor failures affecting this unit which continued to be addressed by successive non-mandatory upgrades including another after this event. However, it was also found that there was no guidance on the re-instatement of systems disabled during the initial response to such events, in particular the total loss of AC electrical power.

On 14 October 2017, a Boeing 777-300ER en route to Sydney declared a MAYDAY and diverted to Adelaide after the annunciation of a lower deck hold fire warning and the concurrent detection of a burning smell in the flight deck. The remainder of the flight was completed without further event and after landing a precautionary rapid disembarkation was performed. The Investigation found that the fire risk had been removed by the prescribed crew response to the warning and that the burning which had occurred had been caused by chafing of a wiring loom misrouted at build.

On 25 May 2016, an Embraer ERJ 190 experienced a major electrical system failure soon after reaching its cruise altitude of FL 360. ATC were advised of problems and a descent to enable the APU to be started was made. This action restored most of the lost systems and the crew, not having declared an emergency, elected to complete their planned 400nm flight. The Investigation found that liquid contamination of an underfloor avionics bay had caused the electrical failure which had also involved fire and smoke without crew awareness because the smoke detection and air recirculation systems had been unpowered.

On 13 March 2013, smoke and fumes were immediately evident when the cable of an external GPU was connected to an ERJ170 aircraft on arrival after flight with passengers still on board. A precautionary rapid disembarkation was conducted. The Investigation found that a short circuit had caused extensive heat damage to the internal part of the aircraft GPU receptacle and minor damage to the surrounding structure and that the short circuit had occurred due to metallic FOD lodged within the external connecting box of aircraft GPU receptacle.

On 5 June 2015, a DHC8-200 descending towards Bradley experienced an in-flight fire which originated at a windshield terminal block. Attempts to extinguish the fire were unsuccessful with the electrical power still selected to the circuit. However, the fire eventually stopped and only smoke remained. An emergency evacuation was carried out after landing. The Investigation was unable to establish the way in which the malfunction that caused the fire arose but noted the continuing occurrence of similar events on the aircraft type and five Safety Recommendations were made to Bombardier to address the continuing risk.

On 16 January 2013, a main battery failure alert message accompanied by a burning smell in the flight deck was annunciated as an ANA Boeing 787-8 climbed through FL320 on a domestic flight. A diversion was immediately initiated and an emergency declared. A landing at Takamatsu was made 20 minutes later and an emergency evacuation completed. The Investigation found that the battery had been destroyed when thermal runway followed a suspected internal short circuit in one of the battery cells and concluded that certification had underestimated the potential consequences of such a single cell failure.

On 25 April 2010, a Beech King Air touched down at Nadi with its landing gear in the transit position after flying a night approach during which a significant electrical system failure had occurred. The landing gear retracted and the aircraft left the runway to the side and came to a stop resting on its fuselage. The Investigation attributed the electrical failure, which directly affected the landing gear operating system and required two diodes to have both failed was likely to have meant that one would have failed on an earlier occasion with no apparent consequence.

On 22 September 2008, a Boeing 757-200 being operated by American Airlines on a scheduled passenger flight from Seattle/Tacoma WA to New York JFK lost significant electrical systems functionality en route. A diversion with an emergency declared was made to Chicago O Hare where after making a visual daylight approach, the aircraft was intentionally steered off the landing runway when the aircraft commander perceived that an overrun would occur. None of the 192 occupants were injured and there was only minor damage to the aircraft landing gear.

On 15 March 2009, an Airbus A319-100 being operated by British Airways on a scheduled passenger flight from London Heathrow to Edinburgh experienced an electrical malfunction which blanked the EFIS displays following engine start with some electrical fumes but no smoke. The engines were shut down, a PAN was declared to ATC and the aircraft was towed back onto the gate where passengers disembarked normally via the airbridge.

On 4 November 2010, a Qantas Airbus A380 climbing out of Singapore experienced a sudden and uncontained failure of one of its Rolls Royce Trent 900 engines which caused considerable collateral damage to the airframe and some of the aircraft systems. A PAN was declared and after appropriate crew responses including aircraft controllability checks, the aircraft returned to Singapore. The root cause of the failure was found to have been an undetected component manufacturing fault. The complex situation which resulted from the failure in flight was found to have exceeded the currently anticipated secondary damage from such an event.

On 7 January 2008, a Boeing 747-400 being operated by Qantas on a scheduled passenger flight from London Heathrow to Bangkok was descending through FL100 about 13.5 nm NNW of destination in day VMC when indications of progressive electrical systems failure began to be annunciated. As the aircraft neared the end of the radar downwind leg, only the AC4 bus bar was providing AC power and the aircraft main battery was indicating discharge. A manual approach to a normal landing was subsequently accomplished and the aircraft taxied to the designated gate where passenger disembarkation took place. None of the 365 occupants, who included two heavy crew members who were present in the flight deck throughout the incident, had sustained any injury and the aircraft was undamaged.

Related Articles

Further Reading

  • In 2004, the UK Air Accidents Investigation Branch (AAIB) published its findings on three serious incidents involving electrical wiring damage and took the exceptional opportunity to review all three incidents together in the context of industry developments and make generic rather than the conventional incident-specific safety recommendations. The two areas addressed were wiring damage and circuit breaker design. See AAIB Bulletin: Incidents resulting from damaged electrical wiring
  • The FAA Aircraft Certification Service on-line training course "Lessons Learned From Transport Airplane Accidents"
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