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Computer-Related Incidents with Commercial Aircraft

High-Intensity Electromagnetic Radiated Fields; TWA 800, and Swissair 111 (1996 and 1998)

17 July 1996, and 2 September 1998

Synopsis On 17 July, 1996, TWA Flight 800, a Boeing B747, suffered a catastrophic in-flight break-up at about 13,500 ft altitude, off Long Island, N.Y., on climbing out of New York's John F. Kennedy Airport for a transatlantic flight. The results of the NTSB investigation can be found in the docket on the NTSB WWW site at http://www.ntsb.gov/events/TWA800/default.htm.

The cause of the accident was determined by the NTSB's Bob Swaim to have been a rapid combustion of flammable vapors in the Center Fuel Tank (in the fuselage, underneath, in the position where the wings are joined to the fuselage), which caused the fuselage to rupture close to this point and the aircraft to break in two. The concentration of flammable vapor was mostly caused by the low level of fuel in the tank and sitting in the hot sun on the ground at JFK. But flammable vapor and a ready supply of oxygen do not alone suffice for combustion: one needs a source of ignition. The NTSB determined that the most likely source of ignition was an electrical fault in a wiring bundle allowing sufficient current to pass to and through the in-tank fuel-level sensor wiring (which is uninsulated, being designed to handle tiny electrical currents) to create a spark and ignite the flammable vapors. Because the voltage required to initiate a spark over such distances varies with air pressure, it follows that, for a given voltage, a spark may be generated at a particular altitude, but not below, which could for why the event happened when it did, and not sooner.

The possibility of an electrical fault led to an investigation of the electrical cabling on aging aircraft, notably B747, B727 and older B737 models, and the NTSB found many disturbing signs of insulation damage and arcing traces on older aircraft. This substantiated claims to which my attention had been drawn before the accident by John King, who sent me inter alia the arc-fault studies of Patricia Cahill at the FAA Technical Center in New Jersey, that aircraft wiring posed a significant source of risk of in-flight fire in aging aircraft.

There were many cover-up and conspiracy theories associated with this accident. One of the most favored was that the aircraft was hit by a missile (an alternative source of ignition of the combustible vapors, even, if the missile were to have been live, of the combusion itself). Another was offered by a Harvard humanities professor, Elaine Scarry, in the pages of the New York Review of Books. Her article (available now only to subscribers or for a fee) suggested that high-intensity radiated electromagnetic fields, in particular such fields produced by U.S. military activity in the vicinity of the accident, could have affected the aircraft systems, and in effect ignited the vapors. She proposed that the NTSB should investigate this possibility more thoroughly, and a discussion ensued in the letters pages. The NTSB engaged NASA to study the electromagnetic radiation, not only from fields outside the aircraft but also from fields generated inside the "Faraday Cage" of the aircraft shell.

On 2 September, 1998, Swissair Flight 111, an MD-11 enroute from New York's JFK to Geneva, experienced an in-flight fire and crashed into the ocean on an attempted approach to an emergency landing at Halifax, Nova Scotia. The Final Report on the investigation is available from the Synopsis Page of the Accident on the Transportation Safety Board of Canada WWW site. An excellent in-depth article on the report was written by David Evans and published in Air Safety Week 17(14), April 7, 2003

Professor Scarry wrote another article for the New York Review of Books, Swissair 111, TWA 800, and Electromagnetic Interference, on September 21, 2000, suggesting that there were significant commonalities between TWA 800 and Swissair 111 that the accidents might be linked. Take-off point, time of day, route of flight, altitude and point of possible onset of problems, and so on.

I noticed that she had misintepreted some of the NASA findings, which had determined that the amount of energy from high-intensity radiated fields (HIRF) impinging on TWA 800 was some three to four orders of magnitude below that which would have to be delivered to the fuel vapors in the CFT to ignite them. (There is a third NYRB article by Scarry, TWA 800 and Electromagnetic Interference: Work Already Completed and Work that Still Needs to be Done, October 5, 2000, which was published on the WWW only. Professor Scarry also extended her inquiry to the crash of Egyptair 990, which most cognoscenti, but not necessarily the Egyptian government or Egyptair, attribute to a murder/suicide. The U.S. NTSB performed the investigation. See in particular the Summary section of the Final Report.)

I was not necessarily very impressed by the quality of the physical reasoning in Professor Scarry's articles, and neither was my correspondent Didier de Fontaine, a materials scientist ex una mearum almarum matrium, the University of California, Berkeley, who wrote an article, Air Crash Cover-Up? which appeared in the journal Commentary in May 2001, and may be found by searching the magazine archives. A lengthier version of the article, Concerning the Fall of TWA 800, Swissair 111 and EgyptAir 990; The Unfriendly Skies Scenario. is available.

There were two main points to my concern with Scarry's argumentation:

The implausibility arguments, inter alia, were sufficient for most of my professional interlocutors to dismiss Scarry's speculation. But Professor Scarry had asked for a proof: she had asked that the hypothesis be investigated to the point that it could be definitively ruled out, if it were not to be true. Fair enough. Implausibility is one thing, but proof of impossibility is another and no one had yet expended the effort on it. I got together with my colleague Willi Schepper in the Faculty of Physics at Bielefeld. Willi obtained his Ph.D. in electrical engineering and is an expert in HIRF. His investigations were responsible for the Bielefeld University tram line being routed 200 meters away from the university buildings, as they showed that the HIRF produced by the trams would unavoidably disrupt experimental apparatus in the Faculty of Physics, which the building had been upgraded to accomodate at considerable expense. With Willi's help, I produced a proof that ruled out Scarry's hypotheses, published in the paper EMI, TWA 800 and Swissair 111 (PDF).

Willi Schepper and I summarised the results of our work in an article EMI, TWA 800 and Swissair 111 in the Risks Digest, Volume 21 Number 08, 11 October, 2000.

One upshot of this work was an invitation from Mike Sharon, a senior investigator for the Transportation Safety Board of Canada working on the Swissair 111 case, to review the TSB's work on the HIRF to which Swissair 111 had been subjected. We performed that task for the TSB with pleasure, and a certain amount of pride that we were able to help.

The Swissair 111 accident is computer-related to the following extent. The aircraft had been installed with an after-market in-flight digital entertainment system (IFES), which involved extensive additional cabling. The cabling used in aircraft wiring bundles must be installed with great care, to avoid inappropriate bending and stretching, in particular in and around the harnesses, which may damage the insulation and potentially lead to break-down and arcing behavior as the aircraft ages. An IFES requires large amounts of new cabling to be installed. The likely origin of the Swissair 111 fire was a cockpit bulkhead that had substantial IFES cabling running through it. (Note: that is not to say that the IFES cabling was at fault. The origin of the fire will never be known with certainty.)

In general, the extensive electronic systems on more modern aircraft are requiring substantial amounts of electrical power. It is not clear to me that we yet understand, or can yet adequately manage, the physical risks associated with the enormous, powerful and complex electrical systems being installed on modern "electric aircraft".

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