Fewer aircraft crashes and an increasing preoccupation withterrorism seem to have muted public concern about air-safety issues. However, the recent mid-air electrical failure on a Qantas 747 and imminent industrial action by Qantas maintenance engineers may reawaken interest.
Electrical faults are far more prevalent and dangerous than generally reported. Eleven years ago they downed Trans World Airways Flight 800 on take-off from New York. Since then, wiring faults have killed hundreds and come within a hair's breadth of killing hundreds more. Slowly the aviation industry acknowledged that a hazard exists, but implementing remedies is taking years too long; not nearly enough has been done yet to minimise known risks.
The saga of ageing wire did not begin with TWA 800. In 1989, as United Airlines Flight 811 left Honolulu for Sydney, two thin wires rubbed together and a 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 part of the fuselage; nine passengers were blown out through the hole.
The cause of the 1989 accident never attracted much attention, partly because, at first, a cargo handler was blamed. He was accused of not closing the door properly; typical human error. It took 18 months to find and recover the door from 5km down on the ocean floor; a stunning feat achieved using radar records, trajectory analysis, appraisal 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 take-off. 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.
Another reason why wire faults did not get attention at that time was because of aviation's preoccupation with fixing ageing structures problems.
A year before the door accident, an old Aloha Airlines Boeing 737 lost much of its fuselage in flight. Flight attendant Clarabelle Lansing was blasted from the plane and most of the 89 passengers were injured, but miraculously the plane survived.
Reasons for the fuselage failure were soon clear, none were unexpected, some had been known for 20 years. Riveted skin joints failed because of fatigue, corrosion, faulty repairs and undue reliance on inspections, instead of fixing known defects.
United States Congress, enlightened airlines and a few regulators persuaded the aviation industry that the underlying problems were endemic and not limited to Aloha, Boeing or to the 737. Eventually, every known defect caused by aging structures had to be reassessed and rectified; inspection alone was no longer allowed. Maintenance was intensified and better corrosion control instituted.
Meanwhile, the hazards of ageing wire were disregarded until TWA Flight 800 crashed off Long Island in July 1996. Fuel in a near-empty tank of the Boeing 747 exploded, according to the board probably because of a short-circuit outside the centre wing tank that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indicating system. The board also blamed the basic design, because 747s (and other Boeing airliners) have heat exchangers under the centre tanks which inevitably warm the fuel, making vapour much more flammable.
Again action was forced at a political level. The Clinton administration set up a Commission on Aviation Safety and Security that insisted in cooperation with airlines and manufacturers, and that theFederal Aviation Administration's ageing aircraft program should be expanded to cover non-structural systems. Spot checks of wiring on old aircraft found poor design, shoddy installation, defective maintenance and deterioration of insulation. Concerns escalated when Swissair Flight 111, an MD-11, crashed in September 1998 after a cockpit fire, apparently triggered by defective insulation on ageing wires. It also emerged that standards for flammability, particularly of thermal insulation blankets, did not account for temperatures generated by an electrical short-circuit and wiring of a recently installed passenger entertainment system was faulty.
Unlike major accidents, many dangerous incidents like the recent one on Qantas flight QF 2 usually escape media scrutiny. For instance, tanks exploded on two Boeing 737s, just like TWA 800, but they happened on the ground and were largely ignored. NASA, too, got involved in 1999 when wiring faults on Space Shuttle Columbia failed two computers during launch. With little fanfare, the fleet was grounded and investigators reportedly found 3500 wiring defects on the fleet.
In June 2003, Concorde's demise was hastened by a wiring fire. Fuel seeped from a tank into a fairing and was ignited by a fuel pump wire chafing and sparking against structure. Fortunately, the fire self-extinguished and only became known a week later during routine investigation of an electrical discrepancy.
All this prompted the Federal Aviation Administration to convene the Ageing Transport Systems Rulemaking Advisory Committee, which recommended changes to wiring system design, certification and maintenance, and initially led to a swath of airworthiness directives to address obvious hazards on specific aircraft types. Then in October 2005, 16 years and nearly 500 deaths after UA 811, the Federal Aviation Administration proposed new rules for enhanced safety of electrical wire interconnection system from both design and maintenance perspectives. These new rules did not become law until December 2007. Full implementation will take another three years and cost over $400million. Even then, the most contentious safety improvement may never happen.
The Federal Aviation Administration proposes that in addition to minimising the risk of a spark, fuel tank voids should be filled with inert gas to prevent explosions. This is already done on military aircraft. Boeing plans to do so on its new 787 but Airbus has designed the A380 super jumbo without a centre tank and sees no need for inerting. Existing aeroplanes are never likely to be fixed. Every airliner has hundreds of kilometres of wire; all of it deteriorates and much of it is not accessible. Faults remain a daily occurrence; so too the risks.
Martin Aubury has worked as an aeronautical engineer for 43 years. This article is loosely based on a lecture he gave last year to a masters course at Australian Defence Force Academy.