Aircraft Grounded for Wiring Inspections
Are FAA wiring installation standards adequate?
by David Evans
The massive grounding of American Airlines MD-80 aircraft is over a one-inch variation in the intervals at which wiring lacing cord must be tied. However, there may be other wiring issues of greater importance.
The airline has grounded this week hundreds of its MD-80 aircraft to re-inspect and if necessary correct the wiring in the airplanes main-gear wheel wells. The work will require several hours to complete on each aircraft. All aircraft are expected to return to service within a few days. Over the past several weeks the FAA has been conducting an audit of all airlines’ maintenance records for compliance with airworthiness directives (ADs). During the audit of American Airlines (AA), a joint team of AA and FAA inspectors raised questions regarding an already accomplished AD concerning how a certain bundle of wires is secured.
The AD required corrective action to prevent wires from chafing or arcing, which could cause the airplane to lose hydraulic power or cause a fire. As the AD said, repairs were required of the wire bundles to “reduce the potential of an ignition source adjacent to the fuel tank, which, in combination with flammable fuel vapors, could result in a fuel tank explosion.”
AA instituted an Engineering Change Order (ECO) to accomplish the AD. Wrapped around the bundle of auxiliary hydraulic pump wiring there is a protective sleeve (see photo at bottom). There are two issues concerning application of the ECO to accomplish the AD:
The aircraft are being re-inspected to ensure that the wiring is installed and secured exactly according to AA directives. The AD affects 1,100 aircraft worldwide, of which 300 are operated by American Airlines. Media reports of thousands of AA flights cancelled stem from the fact that a single MD-80 will make five or more flights per day.
It is not clear if the original joint AA/FAA inspection was of the ECO paperwork or included an actual examination of the wiring in the wheel wells. It is suspected that the original inspection was restricted to paperwork, and when questions were raised about the ECO, the actual airplane inspections were directed by the airline.
Nor is it clear if any violation and civil penalties will result, as the airline’s action appears to be one of self-disclosure.
It seems that a lot of inconvenience has been caused by inspections to make sure not only that the lacing cord is wrapped every inch but also that the bundles are clamped at 5-inch intervals instead of 6-inches apart. After all, the AD was issued in 2006 with an 18-month compliance period. Given the generous allowance of time, why does a day or two now make a significant difference?
That said, it should be mentioned that the AD was issued as part of the review of fuel tank safety the manufacturers conducted under Special Federal Aviation Regulation 88 (SFAR 88). The review was intended to eliminate hazards of the sort believed to have caused the loss in 1996 of TWA flight 800; the B747 suffered electrical arcing in a wire bundle that led to an explosion of the center wing tank. However, imperfect application of this AD for MD-80s is further evidence that all potential ignition sources cannot be reliably removed and that some form of inerting explosive fuel-air vapors must be applied. While the FAA has expressed a commitment to inerting, it is not clear if this technology will be required of existing aircraft or will only be applied to newly manufactured aircraft like the B787.
The photograph also raises other issues. First, the clamping of the electrical bundle onto the pipe immediately below it. The angle to the clamp does not appear to assure adequate separation between the wire bundle and the adjacent line.
Second, the routing of electrical cables in the wheel well without benefit of shielding. Recall that the Air France Concorde jet was lost in July 2000 from a tire bursting, the shrapnel from which severed electrical lines in the wheel well. The lines, whipping about in the turbulent air, touched, arced, and ignited fuel-air vapors. Following the fiery crash, British Airways, also a Concorde operator, installed protective shielding around all wheel well wiring in its Concorde aircraft. Burst tires also occur in subsonic aircraft, and wiring in the wheel well needs to be protected from this eventuality.
Third, the routing of electrical lines in close proximity to fuel and/or hydraulic lines, as shown in the picture, is problematic. The FAA has no standard for the separation of electrical, fuel, hydraulic, oxygen, or other lines. In forthcoming wiring inspections, all electrical lines within 2-inches of critical flight controls are to be inspected, according to an FAA notice of proposed rulemaking (NPRM). The U.S. Navy does have a standard. Known as Naval Air 01-1A-505-1, the document specifies installation and repair practices for aircraft wiring. This 2004 directive says specifically: “When wiring must be routed parallel to flammable lines for short distances, maintain as much fixed separation as possible, six inches or more (Figure 7). When a six inch clearance cannot be maintained, the bundle and flammable line shall be clamped … so that there will be no relative motion between them … Space clamps so that if a wire is broken at a clamp it will not contact the line. …”
Fourth, there are limits to visual inspection. Visually checking can identity certain conditions, such as heat damaged or burn wiring, and vibration damage such as chafing. But visual inspections cannot be relied upon to find other conditions, such as cracked insulation, arcing, insulation delamination, and degraded repairs or splices. There are non-destructive tests to detect such conditions, but they are not required by the FAA.
Basically, the photograph reveals not only the misapplication of good wiring practices in terms of clamping and whatnot – it also provides evidence of the absence of realistic FAA standards for the design and installation of wiring in the first place. The MD-80 is a 20-year old design, but there is nothing on the books from preventing a brand new airplane from being designed with the same wiring installation. g
Wiring a Problem for
All Aging Aircraft,
Not Just MD-80s