Aircraft The Burning Issue Of Aging Wire




Aircraft The Burning Issue Of Aging Wire  

Ten years ago when United Airlines Flight 811 took off from Honolulu for Sydney, two thin wires rubbed together and an electrical short circuit powered open the latches on the cargo door in the belly of the Boeing 747.  When the aircraft reached 23,000 feet the huge door burst open and ripped off in the slipstream, tearing away cabin structure.  Nine passengers were blown out through the hole, their bodies were never recovered. So began the saga of aging aircraft wire. 

Since then wiring faults may have killed hundreds more in the crash of TWA Flight 800 in July 1996 and Swissair Flight 111 in September 1998. Every airliner has hundreds of kilometers of wire and more is being added to support sophisticated passenger entertainment systems.  All wire deteriorates to some degree and much of it is not accessible for inspection.  Slowly, ever so slowly the aviation industry is acknowledging that a hazard exists.  Finding remedies will take much longer.


The cause of the United Airlines accident never attracted the attention it deserved partly because blame was initially directed at the cargo handler.  He was accused of not closing the door properly; just another human error.

It took eighteen months to find the door and recover it from 5 km down on the ocean floor. The technology to do this was the same as that recently used to find the Kennedy aircraft; trajectory analysis from radar records, assessment of ocean currents, sonar search, unmanned and manned submersible vehicles.

After studying the door and its internal wiring the National Transportation Safety Board changed its earlier findings. The probable cause became “…a faulty switch or wiring in the door control system which permitted electrical actuation of the door latches toward the unlatched position after initial door closure and before takeoff.  Contributing to the cause of the accident was a deficiency in the design of the cargo door locking mechanisms, which made them susceptible to deformation, allowing the door to become unlatched after being properly latched and locked.” (followup)

Physical clues from the accident were supported by a later incident when a cargo door opened uncommanded on a plane about to board passengers.  Again the insulation on some wires was abraded and burned where the bundle going into the door flexed every time the door opened.

Another reason why wire defects attracted little attention at that time was because the industry was preoccupied with fixing problems of aging aircraft structures.  A year before the United Airlines accident, an old Aloha Airlines Boeing 737 lost a large portion of its fuselage during a short inter-island flight.  Failure of riveted skin joints exposed dangers of fatigue, corrosion, faulty repairs and undue reliance on inspections, instead of fixing known defects.

First Officer Madeline Tompkins was flying the Aloha plane when it burst apart.  She told investigators how her head was jerked back by the blast and the Captain told how he looked back into the cabin and saw blue sky where the roof had been.  Six metres of cabin structure had disappeared; 89 passengers were strapped in but being battered by the slipstream and speared with debris.  Some had seen flight attendant Clarabelle Lansing blasted from the plane.  Miraculously the plane landed safely but most passengers were injured. (Read Story)

Reasons for the fuselage failure were soon determined, none were unexpected, and some had been known for twenty years.  In particular fuselage assembly was faulty on early 737’s.  Airlines had to strengthen the joints or repeatedly inspect them; few did the modification and many became complacent about the inspections.

Disagreement over the risks had gone on for years and just two weeks before the accident Boeing rejected warnings from the Australian regulators.  Afterwards it took a great effort by the US Congress, enlightened airlines and a few regulators to convince the industry that the underlying problems were endemic and not limited to Boeing or to the 737.

A memorable document from the US Federal Aviation Administration rejected an (Australian) assertion that failures on the 737 could equally happen on the Boeing 727.  FAA said there was no evidence.  The very next sentence admitted that a similar defect had just occurred on the 727 so action was needed.  Obviously the first sentence was written before the incident and should have been deleted before the document got published!

Eventually aging structure problems were acknowledged and addressed by the whole industry.  Every structural defect was reassessed and rectified; inspection alone was no longer allowed.  Corrosion prevention and control programs were instituted and maintenance was intensified.

Fixing aging structures has cost the industry billions and billions of dollars.  But the cost per ticket is less than $5, and well worthwhile.

Meanwhile the hazards of aging wire were disregarded until TWA 800 exploded off the coast of Long Island.  Flammable vapor in a near empty fuel tank ignited.  Why is not known, but low voltage wires that go to quantity sensors in the tank might have shorted to high voltage wires elsewhere in the plane.

Again action was forced at a political level.  The White House Commission on Aviation Safety and Security insisted that “in cooperation with airlines and manufacturers, the FAA’s Aging Aircraft program should be expanded to cover non-structural systems”.

Belatedly spot checks of maintenance on old aircraft found that the aging structures program overlooked corrosion of control systems and wiring too was deteriorating with age.  Also alarmingly, wiring bundles were being contaminated by metal shavings; usually debris from fixing the structural problems!

So research began into better detection of faulty wires and technical interchange started between FAA, Defense and NASA on wire deterioration.  Meanwhile evidence emerged that a cockpit fire that caused the crash of Swissair 111 may have been due to defective insulation on aging wires.

The suspect insulation material is caused KAPTON.  It is light and flexible when new but embrittles and cracks over time.   Also when wire short circuit their outer sheaf chars and becomes highly conductive.  Thus bundles of wire can quickly burn along their length.  For these reasons KAPTON has been banned from most military aircraft and is avoided in new civil aircraft.  Nevertheless KAPTON is used on about 65 % of airliners in service.

KAPTON was not implicated in the United 811 accident or TWA 800.  Both of these had older types of wire that too deteriorated.

Aging wire concerns are supposed to be addressed jointly by manufacturers, airlines and regulators.  The first two may not be keen to probe deeply because of horrendous cost implications if something is seriously wrong.  And regulators around the world are being downsized, outsourced and stripped of technical expertise.  The US Government watchdog, the General Accounting Office recently found that FAA’s inspection initiatives were a shambles.

Do not hope for pro-active intervention by our Civil Aviation Safety Authority.  Its policy now is to slavishly mimic the FAA.  Though CASA sneaks in a few rules that are more lax than elsewhere.

Something else has changed.  Whereas professionals in private resolved aging structures, aging wire is getting the full glare of the Internet.  A site sponsored by the widow of a Swissair victim, provides more technical data than any person can digest.  She says she is spending her “blood money” pay-out to ensure informed public debate.

Now everyone can learn the risks of aging wire in aircraft, but whether the risks can be managed and reduced remains to be seen.

By Martin B Aubury
Fellow of the Royal Aeronautical Society
Former Head of Aircraft Structures at (Australian) Civil Aviation Authority
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