More Separation Needed In Design of Aircraft Wiring Systems

'The Lord didn't design the human body then throw the veins in afterwards'
Electrical wires connecting flight-critical electrical systems need to be more separated for protection against the effects of arcing. When arcing occurs in a packed bundle containing both power wires and signal data wires, the damage can lead to a cascading sequence of failures that can vastly compound the hazard created by the original arcing event.

"There is no substitute for distance" when it comes to wiring connecting flight-critical systems, maintained Jim Shaw in his recent presentation on wire separation from the pilot's perspective to the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). Shaw is head of the In-Flight Fire Team for the Air Line Pilots Association (ALPA).

"Do we need a hard and fast number?" Shaw asked, with respect to a minimum wire separation standard. "Probably not," he said, going on to explain, "Whatever is used should have the dielectric equivalent of two inches of air." Moreover, Shaw suggested, any wire separation standard should address the potential for severe electrical arcing to create damage. In this respect, even a two-inch dielectric equivalent distance may be optimistic. The temperature at the center of an electrical arc is about 5,000? C (9,032? F), which is more than sufficient to melt most aeronautical metals.

That kind of intense heat will make short work of any stainless steel high-pressure oxygen lines anywhere in the vicinity. And as soon as such a high-pressure oxygen line is pierced, the oxygen rushing out of the pinhole creates a blowtorch-like effect than can spread fire. A similar concern relates to high-pressure hydraulic lines. The jet of mist emerging from a pinhole opening in a steel tube carrying hydraulic fluid can have an effect similar to that of escaping oxygen - an impressive increase in the fierceness of a fire.

Documentation during design
In calling for the establishment of minimum separation standards for aircraft wiring, Shaw may be well ahead of efforts in the Federal Aviation Administration (FAA). Its latest policy statement talks about the need simply to have a better understanding of where all wires in an aircraft originate and where they terminate. The FAA's February 2002 "Notice of Final Policy" said there have been too many instances where the wiring data on which officials must approve aircraft designs, modifications thereto, or post-production installations, such as in-flight entertainment systems (IFE), "did not adequately and clearly define the configuration for the model to be certificated." The lack of documentation "left an inordinate portion of the installation to the discretion of the installer," the policy notice said.

In other words, the FAA wants more than silhouette drawings. The new policy says installation drawings should depict the exact path a wire takes from starting point to termination, around spars, through bulkheads, its proximity to other systems, and so forth. Under the new policy, the documentation must show if a wire is routed close to an oxygen line, or in the same bundle as high-current-carrying cables.

The new policy also touches on the safety aspect that is Shaw's primary concern as a pilot. When applicants submit their design packages for FAA approval, a system safety assessment should be included. "The analysis should consider the possible effects wire system failures would have on systems for safe flight and landing due to damage in collocated wiring bundles and the possibility of smoke and/or fire events," the policy intoned.

However, the call to do better may have been undercut by the new policy's own closing remarks. The actions called for are "not intended to establish a binding norm," the policy said. "It does not constitute a new regulation." The FAA seems to be trying to improve substandard practices short of issuing new regulations. In a way, this course may reflect a sense of urgency. Regulations can take years to put into effect. Policy can be put in place by pronouncement.

Furthermore, regulatory teeth may be coming from the work of a government-industry committee under the guise of the Enhanced Airworthiness Program for Airplane Systems (EAPAS). This effort is a direct outgrowth of ATSRAC activity on wiring over the past three years. recommendations for rulemaking are expected this coming August.

"These [new] regulations and associated advisory circulars (ACs) are a giant leap forward," Shaw said. Present regulations, he added, are "woefully inadequate," and fall under the heading of "Thou shalt build safe aircraft."

Indeed, Shaw does not appear to be overstating his case. The FAA recently requested bids for a three-year, $18 million research project, titled "Evaluation of Aircraft Wiring Separation and Segregation Requirements and Practices." The April 30, 2002, announcement said with revealing candor, "Today, there are no available criteria for what is a safe separation from a wiring fault perspective ... Should the wiring for two redundant channels be 4 inches apart or 5 feet apart?".

It is not clear how the findings of this research project will be integrated into the EAPAS effort. Separation standards proposed this summer for incorporation into regulations could be overcome or negated by the results of the three-year research effort.

In the meantime, Shaw's concern about the vulnerability to flight-critical circuits remains. Among them, he includes:

Flight and engine controls
Cockpit instruments for the captain and first officer
Navigation systems
Pressurization
Fire detection and suppression circuitry
Others may come to mind, but the wiring associated with these items provide a rudimentary start. These are the ones that should feature a performance standard equivalent to a separation standard of two inches, Shaw believes. That standard also should apply to heavy current-carrying cables, to include all bus power supplies, to include 115-volt AC power and 10-15 amp 24-volt DC power supply circuits. Redundant circuit separation also would be a feature.

continued Air Safety Week:

Damage at a distance
Shaw's call for greater separation stems from the regularity with which arcing events damage other wires. Moreover, many wires can be found running through a single node. Arcing damage is not always confined to a small area, and the hazard is heightened when wires are run close together, presenting the possibility that all flight-essential equipment could be lost. Shaw is not alone in his concern. The National Transportation Safety Board (NTSB) believes separation standards are not uniform, at the very least, and that there is a real potential for arcing to damage wiring at a distance. In the NTSB's investigation into the TWA Flight 800 accident, damaged wires in the fuel quantity indication system (FQIS) were suspected as the source of arcing that caused an explosion in the plane's center wing fuel tank. In their examination of other airplanes of similar vintage, the safety board found cases of collateral damage more than 1.5 inches away, which tends to support Shaw's 2-inch standard:

"Safety Board investigators reviewed the general wire separation standard and practices of several airplane manufacturers and found that ... standards are not uniform. For example, Douglas Aircraft Company specified that wiring for certain systems (including FQIS and other fuel system wiring, fire warning system wiring, primary generator feeder cables, and electro-explosive devices) must be separated by at least 3 inches from other electrical wiring. In contrast, Boeing specifications do not require protection of some systems specified by Douglas (such as the FQIS) and, for those systems that are designated as protected, the required separation distance is only 1/4 inch in pressurized areas and 1/2 in unpressurized areas." (Page 286 of NTSB Final Report)

A retired electrical system specialist, who served 23 years in the Royal Air Force, recalled his experience: "I have come to realize how little respect or attention was given to the design, configuration and installation of electrical wiring on aircraft. I constantly found myself designing procedures to fix badly designed wiring installations."

"The RAF has spent an enormous amount of money making aircraft wiring safer," he added. That effort has included replacing aromatic polyimide (Kapton) with XL-ETFE (cross- linked Tefzel), separate AC and DC power distribution panels, removal of all wire ties and replacing them with lacing cord, and separating all wiring from the aircraft structure by 10 mm (0.4 inch). For the fleet of Harrier combat jets, this veteran of wiring woes recalled, "Every wiring fault on the Harrier fleet is now recorded on a data sheet and monitored on every unit [airplane] by a dedicated wiring specialist.

"The one lesson to be learnt [sic] from all this is that wiring is as crucial to flight safety as the airframe, mission computer or propulsion system," he said. "If you design it right, you save enormous man-hours and rework at 'the sharp end' of operations." With trenchant eloquence, this expert said, "The Lord didn't design the human body then throw the veins in afterwards. He evolved them at the same time."

continued Air Safety Week:

Options for operators
It may well take years before greater separation standards take hold in the industry. In the meantime, with hundreds of wires packed in close proximity, what can operators do to minimize the hazard? The options range from major to modest. At the major end, Swissair's "Modification Plus" program for its MD-11 fleet features complete rerouting of critical electrical bundles and power feeder cables to provide greater separation (see ASW, July 30, 2001). At the more modest end of the options, operators may wish to consider an anti-corrosion lubricant for electronics. One such, manufactured by Lektro-Tech., Inc., dubbed Super Corr-B. Is used by the U.S. Air Force to protect connectors and circuit boards on the fly-by-wire F-16 fighter jet. It protects from moisture while eliminating surface static electricity. It also can be sprayed on wiring to protect against chafing and wear.

"This may not solve all problems associated with wiring systems, but it may be a low-tech solution that will work in appropriate areas until some of the longer-term solutions become more commonplace," said Lektro-Tech vice president Lewis Krosner. "We currently protect Embraer and Gulfstream equipment," he added, by way of pointing to the product's wider application for wiring protection on commercial airliners.

"We have a viable product that can save large amounts of O&S [operations and support] time and dollars," Krosner asserted. >> Shaw, e-mailjdshaw@compuserve.com; Krosner, e-mail lkrosner@lektrotech.com <<

continued Air Safety Week:

For Every Wire - A Source and Destination
Notice of Final Policy (extracts)

"The FAA expects the applicant to provide engineering drawings instead of merely statements such as 'install in accordance with industry standard practices' ... The FAA considers such statements inadequate because the standard practices cannot define the location or routing of the wiring.

"Type design data packages that were submitted did not include wiring diagrams showing the source and destination of all wire associated with the installation. Also, wire installation drawings showing airplane wire routing, grounding, shielding, stand-offs, clamping, conduits, etc., either were missing or lacked sufficient detail.

"The system safety Assessment should include an assessment of the effects of failures of the airplane wire and its associated wire bundle for equipment installed on the airplane. The analysis should consider the possible effects wire system failures would have on systems required for safe flight and landing due to damage in collocated wiring bundles and the possibility of smoke and/or fire events."

Source: FAA, Policy Statement Number ANM-01-04, System Wiring Policy for Certification of Part 25 Airplanes, February 2002

continued Air Safety Week:

'No Available Criteria'
Evaluation of Aircraft Wiring Separation and Segregation Requirements & Practices, FAA request for proposal (extracts)

Objective

"The contractor shall investigate whether current wiring practices adequately address wire separation and/or segregation ... [and] shall develop minimum requirements."

Definitions

"Separation is a measure of distance and addresses physical hazards resulting from potential electrical failure modes [spanning] two or more bundles, or [that] occur between an electrical bundle and a surrounding non-electrical system.

"Segregation involves the actual segregation of wires into separate bundles ... to eliminate potential functional and physical hazards between systems.

Discussion

"Today, there are no available criteria for what is a safe separation from a wiring fault perspective [emphasis added]. For example, in order to prevent a common mode failure of a critical system, should the wiring for two redundant channels be 4 inches or 5 feet apart? Alternatively, can a common mode failure due to a wiring fault of two redundant channels be effectively precluded using barriers within the wire bundles, such as conduits or protective sleeves? [ASW note: One pilot observed that in a fly-by- wire aircraft, critical systems redundancy may be in jeopardy due to vulnerability stemming from inadequate wire spacing. "It is not alarmist to theorize about the possibility of an electrical sequel to the United Flight 232 crash at Sioux City, Iowa, in 1989, where the DC-10 lost all three of its triply redundant hydraulic flight control systems following an uncontained failure of the center engine. There was no manual back-up flight control system," this pilot recalled.]

"Separation and/or segregation requirements due to wire bundle internal faults are subjective with little guidance available.

"What are safe clearances between power wires and other essential wiring? The Canadian Transportation Safety Board raised the question during the Swissair 111 accident investigation."

Source: FAA, Solicitation No. DTFA03-02-R-00018. For more information on this research project, contact Phyliss Schlegel, tel. 609/485-4380, e-mail phyllis.schlegel@tc.faa.gov

conclusion Air Safety Week:

'Ensure Adequate Separation'
The National Transportation Safety Board issues its call to action:

"The Safety Board concludes that existing standards for wire separation may not provide adequate protection against damage from short circuits ... the FAA [Federal Aviation Administration] should review the design specifications for aircraft wiring systems of all U.S.-certified aircraft and (1) identify which systems are critical to safety, and (2) require revisions, as necessary, to ensure that adequate separation is provided for the wiring related to those critical systems."

Source: NTSB, TWA 800 Final Report (No. NTSB/AAR-00/03), p. 286, Aug. 23, 2000

Arcing Events That Damage Other Wires Occur - With Regularity
With fatal results:

July 17, 1996. TWA Flight 800. Center wing tank explosion most likely caused by inadequate separation of current-carrying wires from signal wires associated with fuel quantity indication system (FQIS)
Sept. 2, 1998. Swissair Flight 111. Electrical arcing followed by spreading fire in concealed/inaccessible areas of the aircraft between ceiling panels and outer fuselage. Entire wiring system under scrutiny

With non-fatal but frightening results. A sampling:

Jan. 14, 1985. Monarch Airlines B757. Kapton wire power feeders chafed and arced. Wire bundle vaporized. Adjacent wire bundles damaged. Boeing Alert Service Bulletin SB757-24A0025
March 17, 1991. Delta Air Lines L-1011. Flames erupted from base of the cabin sidewall. Wires burned. Five circuit breakers tripped. Diverted to Goose Bay, Canada. Ref. DOT/FAA/CT-TN94/95
Oct. 8, 1993. MD-11. Lost ground spoilers. During inspection, numerous chafed and burned wires were found in the forward avionics bay. Ref. FAA SDR (Service Difficulty Report) No. 1993111000022 (Item 5)
May 19, 1995. DC-10. Circuit breakers popped. Number 1 and No. 2 generators lost. Unscheduled landing. Burnt-out wire bundle found in ceiling area of forward cargo bay. Ref. FAA SDR 1995072100418 (Item 9)
April 28, 1996. MD11. Flight arrived with five avionics circuit breakers tripped. Wire bundle chafing found. Ref. FAA SDR 1996060600 (Item 25)
Dec. 14, 1996. B737. During inspection, 15 of 32 wires found melted in wire bundle in left wing. Ref. Boeing service letter 737-SL-24-27. Ref. FAA SDR 1996122600319 (Item 82)
Jan. 9, 1998. United Airlines B767. Severe arcing damage to wire bundle in avionics bay leading to precautionary landing at London's Heathrow. Multiple display errors and tripped circuit breakers occurred. Dozens of wires were damaged, some more than 11/2 inches away.
Nov. 9, 1998. Delta Air Lines L-1011. Wire arcing and fire led to 33 multiple systems failures involving the autopilot, cabin pressurization, auto-spoilers, and thrust reversers. Ref. NTSB Recommendation Letter of Dec. 15, 1999, Recommendations A-99-104 through 106. See http://www.ntsb.gov/recs/letters/1999/A99_104_106.pdf
Nov. 11, 1998. MD-11. At the gate, a fire erupted in the forward cargo hold. Wiring damage led to a Mylar insulation blanket fire. Ref. NTSB Recommendation letter of Dec. 15, 1999, Recommendations A-99-104 through 106. See http://www.ntsb.gov/recs/letters/1999/A99_104_106.pdf

These articles from Air Safety Week are best read from the bottom up.

The site for Air Safety Week is at:

http://www.aviationtoday.com/cgi-bin/aviation/avnews.pl

Some interesting revelations about where they're at with wiring:
click this link

called up from here

Dagger, Thanks for the interesting articles. No matter how many times I read about how the industry/agencies knew about the dangers of cracked aircraft wiring I remain shocked and disgusted at how little has been done to correct the situation. I keep coming back to the same thought. Don't people who are in charge of all this have children of their own that fly on these airplanes? Are they willing to 'live' the rest of their lives in pain if their lack of action and callous disregard for human life directly affects them? I can tell them first hand they don't want to 'live' at the place that I've been (and I'm pretty certain other families as well), since this terrible tragedy killed my daughter.

I remain appalled that over 3 yrs later, those responsible for the installation of the IFEN have still not had to answer for their actions as well. The deeper you delve into that story, it only gets worse when you look at some of the 'players'. I will always believe that if they had not been installed on the swissair MD-11s, I would have my daughter today. Of course it certainly didn't help that the FAA still permits airplanes to fly around with combustible materials. I just wish someone would get that story out there- it absolutely needs to be told. I'd better stop here because I am just enraged by the fact that my daughter is dead today. I would like to see some justice for a change regarding an air crash directed at those responsible. I don't however expect to see that in my 'life'time. God do I miss my daughter. She meant everything to me.

POGO's "TRIPWired"
Also looking for a copy of The Centre for Public Integrity's "Errant Arcs" (the link here at http://www.public-i.org/dtaweb/downloads/ErrantArcs.pdf is broken). That's from this page

The only way to fight the situation is to gather together more incriminating evidence. The China Northern MD-82 crash is starting to look like a replay of SR-111. See this link

Thanks Dagger for the information.