Horror Stories Surface From Unheeded Calls For Improved Wiring Safety

The terms "accelerate" and "expedite" characterize safety recommendations issued this month by the UK's Air Accidents Investigation Branch (AAIB) to improve the safety of aircraft electrical wiring.

To illustrate the AAIB's sense of urgency, draft advisory circulars (ACs) produced by the U.S. Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) in 2002 have yet to be issued by the U.S. Federal Aviation Administration (FAA). The ATSRAC has devoted its effort to the safety of electrical wiring and interconnection systems in aging transport-category aircraft. Those ATSRAC-developed ACs contained new guidance to the industry on wiring installation and maintenance practices, as well as guidance for training technicians so as to minimize contamination and accidental damage to wiring while working on aircraft. But the effort has not translated into action.

Meanwhile, wiring-related accidents and incidents continue to occur apace. The AAIB publishes a monthly bulletin of its ongoing activities. The June issue is unprecedented in two respects. First, it is devoted entirely to aircraft wiring faults, the range of malfunctions, and the seemingly unrelated effects of these errors, which can confound troubleshooting. The June bulletin presents four cases involving electrical wiring damage on airliners. Second, the AAIB's June bulletin begins with a letter from Ken Smart, chief inspector of air accidents. Smart observed that the four case studies, which we call "horror stories", are emblematic of some of the "wider issues of maintenance related and aging problems with aircraft wiring systems."

In these respects, the AAIB's June bulletin is a call to action. The agency wants informational material provided to the industry to prevent damage to wiring, and it seeks accelerated development and deployment of circuit breakers that will provide better protection. From its seven-page overview of wiring safety, the AAIB has identified a common thread, not only between the four recent cases, but to earlier accidents and incidents, notably the 1996 TWA B747 disaster involving arcing in a fuel tank, the 1998 Swissair** MD-11 crash from a runaway electrically-stoked fire, and the 1998 incident involving severe arcing in a United Airlines B767 from faulty maintenance work in the electronics and equipment (E&E) bay (see ASW, May 17). Two citations in the AAIB's overview of all these cases present the heart of its dual concerns:

Damage to wiring:

"All these incidents show how prone electrical wiring is to damage, occurring over time or being introduced during maintenance or modification action. Periodic zonal inspections are carried out but damage and debris is often hidden within wiring bundles and is difficult to detect without disturbing the looms."

Circuit breaker (CB) design:

"There are many reports of wiring loom damage where sustained arcing within/between looms occurred, or probably occurred, where CBs have failed to operate.

"Electrical circuits are protected ... by thermal/

mechanical types of circuit breaker. This [type] is most suitable for a 'solid' and continuous short-circuit but less reliable for transient arcing faults, which develop high energy over a very short period of time insufficient to trip the circuit breaker. An 'intelligent' circuit breaker, which could directly replace the circuit breakers presently in widespread use, can recognize the rapid current and/or voltage signature associated with arcing faults.

"It is recommended that the FAA expedite a requirement for the replacement of existing thermal/mechanical type circuit breakers [with] arc fault circuit breakers."

Herewith, highlights of the four cases:
Horror story #1:

Date of event -- November 2002

Aircraft type and age -- B737-400, 11 years.

Wiring type -- Aeromatic polyimide (Kapton). As is well understood in the industry, "carbon arc tracking" associated with Kapton can spread the effects of arcing in a bundle.

The combination -- Chafing, electricity and water.

Circumstances -- Electrical burning smell, electrical "crackling" sounds and cockpit aural warning on climb out of London's Heathrow for a flight to Kiev. Maintenance workers had improperly secured a braided steel water hose to a wire bundle over the forward galley. Resultant chafing led to sever arcing. Arcing ate through wiring, causing multiple failures, to include the cockpit/cabin interphone, and 25 tripped CBs. Aircraft returned and emergency landing at Heathrow.

Related issues -- Maintenance manual -- Did not state precisely how the water hose should be disconnected and secured when the galley is removed for refurbishment.

Flight recorders -- The digital flight data recorder (DFDR) and the quick access recorder (QAR) both record more than 300 parameters, but the only one captured in this event was "lavatory smoke," when the circuit was broken by arcing.

Cockpit door -- In response to the 9/11 attacks, the airplane was fitted with a locked door. With loss of the cabin/cockpit interphone, there was no way for the cabin staff and pilots to communicate. With water pouring out of the ceiling, the cabin services director (CSD) resorted to banging on the cockpit door, observing through the peephole that both pilots were wearing their oxygen masks. AAIB: "Faced with smoke from an undetermined source coupled with failure of the cabin interphone system, the [captain] determined that his only option was to unlock the flight deck door and speak directly to the CSD."

Checklists -- The year before this incident, the operator dropped its own "Smoke" checklist and adopted the manufacturer's quick reference handbook (QRH) checklists. The word "Smoke" did not appear on the front cover. Rather, the relevant checklists came under "Unannunciated Checklists" or "Fire Protection." Neither title led intuitively to the smoke event checklist. The findings reflect the concerns expressed recently by pilots at Delta Air Lines (see ASW, March 15).

The pilots later expressed their concern to the AAIB abut how difficult it would be to find the correct checklist in a cockpit filled with a significant amount of smoke. The AAIB noted, "As a result of this incident, the operator has revised the manufacturer's original QRH and the relevant checklists now come under the title of 'Smoke' on the index page."
Horror story #2:

Date -- May 2003.

Aircraft type and age -- B737-400, 12 years old.

Wiring type -- Kapton.

The combination -- Fretting, arcing, burning.

Circumstances -- Loss of pressurization in cruise on a flight from Marseille, France, to London's Gatwick Airport with immediate diversion landing at Lyon, France. Loss of pressurization caused by overheated and burned wiring loom running across the top of the aft cargo hold and below the cabin floor.

Possible damage to bundle during maintenance, starting the fretting process.

Related issues -- Circuit breakers -- The thermally activated conventional breakers did not trip.

Redundancy -- Wires running in the same bundle controlled all four means of controlling cabin pressurization. All four means were thus lost. In addition, the bundle contained circuits to the aft drain mast heater, the aft door warning system and the potable water tank quantity system. Moreover, high power 28V DC and 115V AC and low-power signal wires were mixed in the same bundle.

The AAIB said the incident "highlighted the problem of routing of the wiring for redundant systems, in this case the primary (AUTO) and secondary (STBY) systems for controls of the aircraft's pressurization, in the same loom."

"This defeats the object of having such alternative systems should a single point failure of the wiring loom occur," the AAIB said, recommending re-routing to better separate and protect the intended redundancy.
Horror story #3:

Date -- July 2003.

Aircraft type and age -- B737-300, 15 years old.

Wiring types -- Kapton and polytetrafluoroethylene (PTFE, a.k.a. Teflon).

The combination -- Chafing, arcing, burning.

Circumstances -- In preparation for pushback at Newcastle Airport for a flight to London's Gatwick Airport, the crew noticed popped circuit breakers and an electrical smell. More breakers popped during crew trouble-shooting. When smoke was observed, the captain ordered the airplane shut down and the passengers debarked. The problem was traced to 115V AC power feeder cables, the Teflon insulation of which was likely damaged during maintenance.

The AAIB said the incident highlights airworthiness issues "which reflect broader concerns on all aircraft types regarding wiring condition, particularly as aircraft age and modifications are introduced."

Related issues -- Material vulnerability -- Arcing of galley power feeder cables may have started the arc tracking of Kapton wiring.

Cascading, spreading damage -- Among the systems affected by the damage: autothrottle, stall warning, engine idle control, engine ignition, thrust reverser control for right engine, right wing slat and flap indicating systems, altitude information from No. 2 air data system, inertial reference systems, EFIS (electronic flight instrument system) and weather radar.
Horror story #4:

Date -- June 2003

Aircraft type and age -- Concorde, 28 years old

Wiring type -- PTFE.

The combination -- Loose wire, fretting, arcing, fire

Circumstances. Faulty fuel quantity indication system (FQIS) for tank No. 7 during Heathrow to New York Flight. Investigation revealed damaged to FQIS wiring from a short duration fire inside the wing-to-body fairing below fuel tank No. 3. Fuel had seeped into this space. The ignition source was a chafed 115V AC wire for the main tank No. 3 fuel pump. The pump wiring had likely been damaged in maintenance two years before, when the space was entered to repair structural cracks. Subsequent examination showed this wire was loose within its bundle and had chafed on a fairing panel, arcing and searing a hole in it.

Related issues -- Circuit breakers -- The thermally activated conventional breaker for the fuel pump did not trip.

Fault isolation -- Multiple fuel gauge and center of gravity (CG) computer failure flags occurred in flight, again illustrating the confusing panoply of apparent malfunctions that can complicate pilot troubleshooting.

Out of these cases, some basic points emerge. One, wiring systems may not be designed with adequate separation, segregation and shielding to prevent single point failures of supposedly independent and redundant systems. Recall the veteran sergeant's command to neophyte soldiers, "Spread out or one grenade will get you all." The same ethic applies when it comes to arcing and the designed routing of cable and wiring.

Second, for maintenance activity in and around wiring, the watchwords are "extreme caution."

Third, the degree of caution required is a function of training and awareness. The AAIB is calling for more of both.

Fourth, there is even more wiring in newer aircraft. As measured by linear feet, the need for wiring safety is increasing (see ASW, May 17, 'Evolution of Complexity'). Remember the phrase "loose lips sink ships." For airplanes, loose wires can lead to loose electrons and dangerous arcing.

Fifth, as perennially upgraded add-in systems such as IFE (in flight entertainment) become the norm over the life of an airplane, the options for routing new wiring become increasingly constrained. This is particularly so where wiring is easily tacked onto existing wire-bundles, strapped to handily adjacent tubing or must pass through the limited width portals provided for wiring's passage through bulkheads. Such confluences of wiring is where the trouble normally starts and also where out of sight is out of mind.

Sixth, a welter of apparent component failure warnings in the cockpit may mask wiring failures, confounding pilot trouble-shooting. On the other hand, if pilots are inundated with seemingly unrelated failure warnings, that very pattern may strongly indicate the presence of a vicious arcing gremlin. (ASW note: The letter from the AAIB's chief inspector of air accidents, the overview and the four case studies may be found at http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafet y_029072.hcsp)

** Poster's note: Entertainment System Wiring