Brighter sparks

As they age, aircraft break down, much like any other piece of machinery. But fixing a large airliner is a major task. There are so many complex systems spread over such a large area that repairing an intermittent electrical fault is extremely difficult; especially if, for safety reasons, it is imperative to find the fault before, rather than after, it becomes dangerous.

Researchers at the US Sandia National Laboratory have now developed a system called Pulsed Arrested Spark Discharge, which uses a small and extremely brief electrical spark to help locate breaks in the insulation of wiring that lead to intermittent faults.

The PASD product, licensed to the Washington-based company Astronics Advanced Electronic Systems, is expected to be launched this autumn.

Intermittent electrical faults are usually caused by short-circuits between exposed wiring and any other conductor, which could be another wire or part of the airframe. These can be caused by insulation breaks too small to be seen by the eye and their effects can be annoying -blinking cabin lights, wheezy air conditioning -or catastrophic.

Electrical faults have been blamed for several fatal air crashes, such as the fire that downed Swissair 111, a McDonnell-Douglas MD-11, off the Canadian coast in 1998.

As well as being minute, the faults are often difficult to detect because wiring can move. Short-circuits can be caused by vibration when the aircraft is in the air or on the ground with the engines running, but when the aircraft is on the ground, the wires may have shifted into a non-shorting state.

PASD, paradoxically, works by making the short-circuit manifest before it would usually do so. A suitcase-sized generator fires a nanosecond pulse at extremely high voltage along the aircraft wiring bundles. This means that the driving force behind the pulse is so high that it can jump across gaps in damaged insulation but there is so little energy in the pulse itself that it is harmless. The system detects the so-called 'pre-emptive spark' and calculates how long it takes for the current to travel back to its source, which indicates how far the damaged wiring is from the current entry point.

'Reacting to a problem, these systems can find a fault before it manifests into a catastrophic event,' said Larry Schneider, who led the PASD development team at Sandia. 'Rather than ripping apart the fuselage for access to a faulty wiring harness that may run the length of the plane, airline mechanics will be able to use this new tool to efficiently locate and repair the intermittent fault.'

Astronics has incorporated PASD into a fault-finding system called ArcSafe, which uses a DC current to locate breaks in wires. The hybrid system uses the DC current to provide support for the PASD pulse, allowing it to work up to 30m from the source. 'We really value PASD technology,' said Mike Ballas, Astronics team leader. 'We licensed it, turned it into a practical, portable unit targeted for the aviation industry to find intermittent faults and we believe it's the best way now to do the job.'

Although initially intended for aerospace, the system is likely to find applications elsewhere, according to Sandia. The automotive industry has shown interest and it could be used to detect electrical faults in new homes.

In the military sector, it could also be used in systems where wiring is hard to access, behind the armour of tanks or the heavy steel bulkheads of submarines.