Ageing Aircraft - Electrical Wiring
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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.
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
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.
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:
- A319, London Heathrow UK, 2009 (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.)
- A319, en-route, Nantes France, 2006 (On 15 September 2006, an Easyjet Airbus A319, despatched under MEL provision with one engine generator inoperative and the corresponding electrical power supplied by the Auxiliary Power Unit generator, suffered a further en route electrical failure which included power loss to all COM radio equipment which could not then be re-instated. The flight was completed as flight planned using the remaining flight instruments with the one remaining transponder selected to the standard emergency code. The incident began near Nantes, France.)
- A319, south of London UK, 2005 (On 22 October 2005, a British Airways Airbus A319 climbing en route to destination over south east England at night in VMC experienced a major but temporary electrical failure. Most services were re-instated within a short time and the flight was continued. However, during the subsequent Investigation, which took over two years, a series of significant deficiencies were identified in the design of the A320 series electrical system and the manufacturer-recommended responses to failures in it and in response, Airbus developed solutions to most of them.)
- A321, en-route, Northern Sudan, 2010 (On 24 August 2010, an Airbus A321-200 being operated by British Midland on a scheduled public transport service from Khartoum to Beirut experienced, during cruise at FL360 in night IMC, an electrical malfunction which was accompanied by intermittent loss of the display on both pilots’ EFIS and an uncommanded change to a left wing low attitude. De-selection of the No 1 generator and subsequent return of the rudder trim, which had not previously been intentionally moved, to neutral removed all abnormalities and the planned flight was completed without further event with no damage to the aircraft or injuries to the 49 occupants.)
- A388, en-route Batam Island Indonesia, 2010 (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.)
- A388, en-route, north east of Singapore, 2011 (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.)
- Ageing Aircraft - Structural Failure
- Accident and Serious Incident Reports: AW - a selection of reports concerning events where airworthiness was a causal or contributory factor.
- 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"