Geoff Thomas/DUBAI
The average single-aisle 150-seat aircraft contains more than 80km of cabling to help meet today's demands for 'touch of a button' cockpit information and user-friendly electronic features.
This increase in cabling creates a new set of engineering challenges because 80km of cable weighs around 600kg. In an Airbus A320, the weight of the cabling is equivalent to around 7.5 non-fare-paying passengers. But not only does the additional weight increase fuel burn, some airport authorities calculate takeoff and landing fees on aircraft weight.
In creating cables to meet weight and size specifications, it is vital that internationally recognised standards of construction and performance are met. The problem is which standards?
Aerospace cables operate in hostile environments with temperatures cycling from -65C° to 260°C, humidity varying from 0 to 100%, and all with a background of constant and varying vibration. The most extreme environment is in wings where - particularly at the leading edge - all the most extreme conditions are found.
Cables used in aircraft wings tend to be composites constructed with PTFE allowing temperatures up to 260°C while those used in the fuselage use Polyamide or Tefzel which is useful up to 200°C. Cabling manufacturers use sophisticated tests to simulate a lifetime's wear and thermal endurance at high temperatures for up to 12 months, equivalent to 25 years' service. The tests also help the makers to calculate failure rates.
Europe and the USA tend to use different cable construction techniques. While the Europeans use tapes (Polyamide or PTFE) to cover the conductors, US makers have generally opted for extruded sheaths.
Techniques
Recent press coverage - particularly concerning the Swissair MD-11 crash off Nova Scotia in September 1998 - has focused on the various types of cable construction and their relative resistance to the phenomenon known as 'arc tracking'. This occurs when a cable is damaged - with the conductor exposed - although the damage must have occurred adjacent to a suitable conductor. Ever since the accident, journalists have speculated that the aircraft's wiring system may have been involved.
It is possible under the right conditions to induce arc tracking in all types of cable. All aerospace cables are designed to meet agreed standards but it's the aircraft manufacturer that chooses which of the dozens of standards to apply. A number of specifications have been agreed for cables over the past 30 years, PAN, ASNE and MIL standards have been in place nearly 30 years while ACT and D Stan are almost 15 years old.
Why so many different standards? Firstly, many of them grew from national defence programmes so there was no need for cross-referencing and some of these, in turn, have been picked-up and used by other bodies around the world.
The second reason is the great diversity in cable usage. A cable used on a fighter aircraft may only see 4,000h service (less than a 10th of a civilian cable) but it will see higher temperatures, more vibrations and face the possibility of damage under attack.
In future there may be a more coherent set of international standards for aircraft cabling with fewer variations between the major manufacturers. But for now, it's inevitable that the plethora of manufacturing techniques and standards will continue to bedevil an industry that can't afford to gain a reputation for jeopardising the travelling public simply through a perceived inability to create a coherent set of internationally agreed standards.
Source: Flight Daily News
