Swissair Flight 111 Accident Puts Spotlight on Wiring Practices
Nov 16, 1998

Decision to Connect Entertainment System to Essential Electrics Questioned
The interactive inflight entertainment system on the Swissair Flight 111 accident aircraft was installed in a manner contrary to the manufacturer's standard procedures.

Every carrier can profit from configuration control practices Swissair officials are now privately declaring should be avoided in future.

At this juncture in the investigation, the manner in which that inflight entertainment system was installed is under close scrutiny. Investigators from the Transportation Safety Board of Canada (TSB) presently are in Zurich, inspecting the manner in which these systems were installed in the carrier's other MD-11s. For comparative purposes, TSB officials also are examining a Delta Air Lines [DAL] MD-11 now undergoing heavy maintenance which does not have the system installed.

The system's heat-seared wiring presently is the prime suspect. The prevailing scenario is a short circuit in the cockpit wiring of the inflight entertainment system, which led quickly to an electrical fire and the massive and rapid breakdown of the airplane's defenses-in-depth against a total electric failure (see related story). The set-up for this catastrophe may have been the way the inflight entertainment system was installed by contract technicians about six years after the airplane's original 1991 delivery to Swissair from the factory. The installation may have involved wiring errors that have led to fires in other cases. According to the January 1997 issue of SAir Group News, a company newspaper (translated from the German), "The time pressure was tremendous" to install the system.

Reconsidering the wiring philosophy
Of more direct concern, the system was connected to an essential component of the electrical system, AC Bus 2, which provides power to vital cockpit systems. According to documents obtained by Air Safety Week, Swissair officials believe now, even if the entertainment system is absolved of any fault in the tragedy, it should not be connected to essential power sources. Connecting the system to the non-essential cabin bus, for example, would make more sense, as the first item on the Swissair checklist for combating smoke calls for the crew to pull the breaker on the cabin bus.

Swissair is now in the process of disconnecting the system, first by pulling all the relevant circuit breakers and, second, by disconnecting and capping wires.

Officials at Boeing's Douglas Products Division said it is standard practice for factory-installed inflight entertainment systems to be connected to the cabin bus or the ground service bus. "Both are turned off when the cabin bus is killed," said a Douglas Products official.

Out of the loop
This official said the company was "never consulted" about the proposed installation in the Swissair jet. "We weren't involved in any way with this installation," the official said, adding, however, that such non-involvement is standard practice when the installation has received an FAA supplemental type certificate (STC).

The FAA position is different. "The FAA approval process (for an STC) verifies not only the design but also that there is no safety impact on other aircraft systems. That is when consultation would come into play with the (aircraft) manufacturer," declared an official in the FAA's Seattle office. The entire STC approval process and the decision to connect the system to an essential bus seems certain to be thoroughly examined by accident investigators.

Faulty rheostats likely a red herring
Meanwhile, the FAA issued an airworthiness directive (AD 98-24-02) Nov. 13th requiring all MD-11 operators to replace a faulty dimmer control. Under high lighting, the capacitor has ruptured, causing smoke in the cockpit. The Swissair accident jet was affected by this AD, and in response to a June 1995 Douglas service bulletin on this item, had already replaced the dimmer control with the improved model. The actions called for in the AD are not believed to be related to the chain of events that brought down Flight 111.

Swissair, tel. 516/844-4561; Boeing Douglas Products, tel. 562/593-8253; FAA NW Region, tel. 425/227-1203

A Portrait of the Participants
A snapshot of the players involved in the installation of interactive inflight entertainment systems on Swissair aircraft:

Company A: Interactive Flight Technologies of Phoenix, Ariz., built the system.
Company B: Santa Barbara Aerospace, Calif., a Designated Alteration Station (DAS), received the Supplemental Type Certificate (STC), signed on behalf of the FAA by the company's DAS Coordinator, which authorized...
Company C: Hollingshead International of Garden Grove, Calif., to install the system into...
Company D: 21 long haul Swissair aircraft.