Positions Staked Out on Eve of TWA 800 Hearing

Air Safety Week, August 21, 2000

The principal vested interests have declared their positions regarding the vulnerability of aircraft fuel tanks to explosions. The latest development was the release last week of a three-year study by the Air Transport Association (ATA), assuring that existing fuel tank systems are safe, and that further tweaks to design and maintenance practices will make them even safer. The ATA is the industry lobby group representing major U.S. airlines and some international carriers.

The ATA declaration came on the eve of this week's hearings by the National Transportation Safety Board (NTSB) capping its four-year investigation into the fatal 1996 center wing tank explosion that destroyed TWA Flight 800, a 25-year old Boeing 747-100. Early in the investigation the NTSB recommended that flammable vapors in fuel tanks need to be greatly reduced or eliminated outright. The Federal Aviation Administration (FAA) has slowly come to essentially agree with the Safety Board. This sea change marks a major departure in the traditional design philosophy, which has been oriented to the elimination of all conceivable ignition sources while accepting the presence of flammable vapors in the tanks.

As of this writing, the various positions may be summarized thusly:

* All is well - The Air Transport Association: "Fuel tank systems of the world fleet are soundly designed and do not tend to degrade as airplanes age." Source: Aircraft Fuel System Safety Program Report, Aug. 4, 2000, p. 81.

* Flammable vapors pose a hazard - The Federal Aviation Administration: Ground-based inerting to reduce the potential for fuel tank explosions "is the closest for possible deployment." Source: Beth Erickson, FAA director of aircraft certification (see ASW, July 17).

* The design philosophy is sound; some improvements needed - The Boeing Co. [BA]: "The fundamental design philosophy for fuel system safety is to diligently preclude the presence of ignition sources. This philosophy is sound...The extensive and comprehensive investigation of TWA 800 has identified areas to further enhance fuel system safety." Source: Boeing Co. April 28, 2000 submission to NTSB, p. 4.

* Threats are lurking - The Air Line Pilots Association:

"ALPA believes latent failures are a central issue to the resolution of fuel tank safety. It is highly likely that one or more latent failures were involved in the sequence of failures leading to the ignition of the (TWA 800) center wing tank. We share FAA's concerns regarding a latent condition placing the airplane one failure from a catastrophic event and, specifically, we believe that substantive methods must be applied to identifying latent failures within fuel tanks immediately." (p. 17)

In addition:

"Existing circuit protection devices are not adequate to prevent electrical failures from becoming ignition sources at any location in an airplane.

During the safety inspections conducted by the FAA since the TWA 800 accident, one area of notable concern has to do with fuel pump wiring routed through conduit within the tank. The conduit is used to prevent physical damage to the wires, which could result in pump failure or electrical malfunctions. However, the FAA found that both the 747 and the 737 exhibited wiring insulation degradations which led to actual arcing between the wires and the conduit. While some damage discovered was minor, nine cases in the 737 resulted in arcing to the fuel pump conduit, and one case...resulted in actual burn-through of the conduit into the fuel cell.

In 1970, the FAA issued the first advisory material addressing (this problem): 'Where electrical wires are routed through conduits in a fuel tank, high surface temperature can be created by short circuits. A critical electrical wiring condition might be one in which the insulation is cracked, broken or of low dielectric strength, allowing intermittent or constant arcing to occur without consuming enough power to cause the circuit protection device to open.' Thus, the argument for an arc-fault circuit interrupter (AFCI) was made 30 years ago." Source: ALPA April 30, 2000 submission to NTSB.