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 can 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 takeoff 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 19 November 2020, the police operator of a DJI Matrice M210 UA lost control of it over Poole when it drifted beyond Visual Line Of Sight (VLOS) and communication ceased. It was subsequently damaged when colliding with a house in autoland mode. The Investigation found that a partial power failure had followed battery disconnection with its consequences not adequately communicated to the pilot. It faulted both the applicable UA User Manual content and the absence of sufficient UA status and detected wind information to the pilot. A failure to properly define VLOS was identified but not considered directly causal.
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 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 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.
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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