Aircraft wiring is just as important as airframes, engines, and avionics. The consequence of aircraft wiring failure can literally be the death of all onboard.
Unfortunately, unlike an airplane’s single airframe or one to four engines, there is a lot of wiring to be inspected and maintained during an aircraft’s life span. Acase in point: According to Boeing, there are about 42 miles of wire on its 737-600/700/800/-900ER (Extended Range) models. If you find this factoid daunting, take heart! That’s four miles less of wiring that can be found on Boeing 737-300/400/500 models.
The 737 isn’t the only aircraft that has miles of wire. This level of wiring is common on all modern aircraft, simply because so many airplane systems are either electrically controlled, or interact with electrical instruments to display sensor data in the cockpit.
Current aircraft wire bundles are generally protected in line with the exposure they receive to heat, vibration and other potentially damaging influences during flight.
Deteriorated short-circuiting wires, improper wiring and deficiencies
in the door’s design were blamed for opening a 747 cargo door
at 23,000’ on United Airlines Flight 811 on February 24, 1989, as
The most rugged wiring is found in high heat areas such as the engines. “For the harshest environments, we manufacture a metal convoluted conduit design that provides the necessary protection from extreme heat,” said Sam Symonds. He is president and CEO of Co-Operative Industries Aerospace & Defense, a manufacturer and repair facility of aircraft wiring products. “This conduit or bent tube design, coupled with a compatible hermetic connector design, serve to protect against the most severe conditions,” Symonds continued. “For less extreme temperature, but still high vibration areas, a less rigid type design is utilized to react more fluidly with the environment. For forward turbine engine sections where chaffing and fluid intrusion is a concern, a double braided design is incorporated to protect from fluid wicking, EMI and abrasion.”
“Connector platings can greatly increase resistance to corrosion in harsh environments,” added Bob Gannon. He is a design engineer with Harco, a designer/ manufacturer of electrical cable assemblies. For instance, “500 hour or 1000 hour salt spray resistance prolongs the flying hours of connectors,” Gannon said. “Inside the cable assembly, Harco has engaged in improvements such as the sealing of cable entry points into the connector and backshell. Thorough experimentation with epoxies to match thermal expansion of connectors has yielded improvements in prevention of movement of corrosive water and salt spray into the areas where cables are joined to connectors.”
The Dangers of Stress
All wiring, from protected bundles in engines to those within the airframe itself, are exposed to various stress factors during its operational lifetime. “Factors include vibration, isolation cracks, dust, heat, humidity, mechanical stress, bending, liquids, grease and foreign object damage (FOD),” said Roland Arntz, SR Technics’ head of Avionics. Add aging after decades of service, and even the most protected wiring can begin to deteriorate over time.
“So many things can go wrong over time,” Symonds explained. “For example, a breakdown in the engine ducting can expose the harness and wiring to heat levels far beyond what the design was intended for. A loose P-clamp can cause excess vibration and chaffing against nearby components.”
Another concern is the proximity to corrosive fluids, which can leak onto the harness, wicking down the wires and cause them to degrade over time or cause shorting in the connector. “While AP mechanics are highly trained and responsible, there is always the danger of foreign object damage (FOD) being introduced during maintenance,” said Symonds. “Any materials left in an interconnect can result in damage to the contacts or conductor insulation. The fact that there are so many possible scenarios that can result in compromised harness integrity reinforces the importance of sound inspection and maintenance programs.”
Deteriorated internal fuel tank wiring is believed to have caused the demise of TWA Flight 800. It was a 25 year-old Boeing 747-100 that exploded after takeoff from New York City on July 17, 1996, with all 230 onboard killed.
The NTSB came to the conclusion that the probable cause of the TWA 800 accident was an explosion of flammable fuel/air vapors in a
fuel tank, and, although it could not be determined with certainty, the most likely cause of the explosion was a short circuit.
“The National Transportation Safety Board determines that the probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank,” said the official NTSB Aircraft Accident Report. “The source of ignition energy for the explosion could not be determined with certainty, but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the CWT that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.” Deteriorated short-circuiting wires have also been blamed for opening a 747 cargo door at 23,000’ on United Airlines Flight 811 (February 24, 1989), blowing nine passengers through the resulting airframe hole to their deaths. Aging faulty wiring is also thought to have caused the cockpit fire on Swissair 111 (September 2, 1998), resulting in the MD-11 crashing into the Atlantic Ocean crash with all 229 on board lost.
The Inspection Process
Ongoing wiring inspection is part of any aircraft’s regular safety check process. “In various checks (A/C/D-check) wiring is controlled visually for cleanness, cracks, chafing, color change and installation,” Arntz said. “This is done according to Original Equipment Manufacturer Standard Practice Manuals and EWIS (Electrical Wiring Interconnection System) tasks incorporated into the Aircraft Maintenance Program.”
Still, unless something obvious happens—shortly before the explosion on TWA Flight 800, the captain was recorded as saying, “Look at that crazy fuel flow indicator there on number four, see that?”—electrical problems can go unnoticed. This is why such problems may not be found until the C- or D-Check, when “an aircraft is pretty much disassembled down to its bones,” said Frank Correro, StandardAero’s avionics manager in Springfield, Ill. “This is when technicians have their best opportunity to look at all of the aircraft’s wiring, to spot and rectify problems.” The only exceptions are self controlling systems built into an aircraft system that identify faults through BITE (Built-in Test Equipment) tests, and power wires that are specifically monitored with load control units (circuit breakers) to indicate system failure and protect wiring.
Sometimes equipment manufacturers can help when aircraft wiring problems are identified in the shop. “Recently, HARCO was asked to look at a harness that had been in service for 20 years,” Gannon said. “The harness, which measured exhaust gas temperatures mated to probes, required exposed ring terminals to be fastened to the probe stud.” Now such an exposed ring terminal can invite moisture, which can reduce the insulation resistance of a wire harness. To address this, “Harco introduced some features to prevent the harness from absorbing water that improved the insulation resistance properties of the harness, and prevented false warning indicators from being triggered in the cockpit,” he said.
What to Look For
Unfortunately for aircraft maintenance technicians, there is no advanced handheld device that can be waved over aircraft wires, to detect faults quickly and reliably. Instead, it takes careful visual inspections of wiring bundles, along with manipulation of wires for flexibility and signs of cracking, to detect problems before they become serious.
“The problem is that most mechanics are not given extensive training in wiring inspection,” said Paul Sneden. He is an instructor at Global Jet Services. Based in Weatogue, Conn., Global Jet Services offers a range of professional development and continuing educations courses for aircraft technicians, including a week-long course in wiring inspection and maintenance that is used by MROs such as StandardAero. “They need extra hand-on training to identify and deal with the many signs of deteriorating aircraft wiring.”
So what should mechanics be looking for when inspecting aircraft wiring? In general, anything that doesn’t look like factory-standard, Sneden replied. Ideally, wiring bundles should be secure but not under stress, with all clamps in place and properly locked. Exterior insulation should be unbroken and uncracked, and it should continue to be when flexed by hand to spot any hidden damage.
Aging, faulty wiring is also thought to have contributed to
the cockpit fire on Swissair 111 on September 2, 1998. While
suggestive, the Canadian TSB investigation was unable to
confirm if arcing from wiring of the in-flight entertainment
system was the main event that ignited the flammable
covering on insulation blankets that quickly spread across
other flammable substances.
Any form of staining is bad news. It could point to fluid leaking onto the wires, or deterioration of the wire’s insulation. “Similarly, any sign of chafing, charring, burning or arcing is not to be dismissed,” said Sneden. “The bundle needs to be removed and inspected, and if need be replaced.”
That’s not all. Any signs of damage on wiring could be evidence of failures in other parts of the aircraft’s systems and airframe. The causes for wiring damage need to be tracked back to the source, so that these problems can be dealt with as well.
A rule of thumb is the older and/or more used the aircraft, the more likely that the wiring is suffering from age-related deterioration. Since aircraft 20 years or older fall into the ‘aging’ category, mechanics need to be extra-vigilant when working on anything made in 1993 or earlier.
Unfortunately, until the current wave of airline fleet renewals is over, MROs will find themselves coping with an increasing number of aging aircraft on a daily basis. The problem of wire deterioration is thus considered to be so serious, that “EWIS has been incorporated as a preventive measure to monitor wire aging,” said SR Technics’ Arntz. “Therefore it can be stated that on condition maintenance has been changed to a more preventive maintenance concept for wiring.”
So far, “a complete re-wiring of aged wires is not yet a part of the rulemaking agenda,” he added. But this might change as active air fleets get older and if more aging wire issues emerge.
Vigilance is Vital
If there is a moral to this tale, it is that aircraft wiring is a difficult-to-service element that must be monitored, inspected and maintained as rigorously as engines and avionics. The losses of TWA Flight 800 and Swissair Flight 111 point to the devastating consequences that can occur should this not happen.
Co-Operative Industries is an AS9100C and ISO9001:2000 registered company based in Fort Worth, Texas. The company develops and manufactures electrical wiring interconnects, ignition leads, and flexible conduits for airframe, engine, ground support equipment, and space applications. In addition to manufacturing capabilities, Co-Operative Industries also provides Part 145 repair services such as check & test, overhaul and repair, and S/B incorporation for many of the commercial aircraft wiring harnesses in service today (FAA No.: OI0R891N, EASA: EASA.145.5897, CAAC No.: F00100406).
For additional information on Co-Operative Industries Aerospace or the services offered, please contact:
Co-Operative Industries Aerospace
1401 South Cherry Lane
Fort Worth, TX 76108