Challenging Assumptions and Making Aviation Safer

By Tom Haueter

Fifteen years ago today, on a warm summer evening, a Boeing 747 took off from New York’s Kennedy airport. Within minutes, the airplane exploded and plunged into the Atlantic Ocean. All 230 people onboard perished.

What happened? TWA Flight 800 led to the NTSB’s biggest, longest, and most complicated investigation. After four years and many theories played out in the press about missile attacks and government cover-ups, the NTSB determined that the probable cause of the TWA flight 800 accident was the center wing fuel tank’s explosion due to tank’s flammable fuel/air mixture igniting. The ignition source remains a mystery. Our investigators determined that the most likely source was a short circuit outside of tank that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.

After this deadly crash, the NTSB issued a host of recommendations. Included among them was recommending the development of nitrogen fuel-inerting systems. Initially, the response from the aviation community was negative. Industry believed that the solution lay in reducing ignition sources, not inerting. All the models and the research, we were told, showed that inerting was too complicated, heavy, and expensive.

Over the years, fuel tank safety remained on the NTSB’s Most Wanted list. The FAA issued 70 advisory circulars and more than 100 airworthiness directives aimed at mitigating risks, but most of them addressed reducing sources of ignition. It did not release a proposed rule on fuel inerting until 2005.

What led up to that rule — the final rule was issued in 2008 — is a fascinating story. At the NTSB’s December 1997 public hearing on the accident, Ivor Thomas, Boeing’s chief engineer of fuel systems, testified. He said it was a “given” that fuel tank contents are flammable. It was up to engineers to reduce ignition sources.

The next year, Ivor Thomas retired from Boeing and went to work for the FAA on the science of fuel systems. It was time, he decided, to explore the chemistry and flammability of fuel — in other words, inerting. He questioned industry assumptions that inerting systems were too expensive, heavy, and unreliable. Years went by, but Thomas and an FAA team persevered.

Finally, along with FAA’s Dick Hill and the help of test engineers at FAA’s Technical Center in New Jersey, there was a breakthrough. The team figured out a way to reliably and economically use an onboard inerting system.

That, in turn, led to FAA’s final rule requiring fuel inerting for center wing fuel tanks, which are now making air travel safer thanks to persistent engineers who challenged assumptions.


Tom Haueter is Director of the Office of Aviation Safety.

2 thoughts on “Challenging Assumptions and Making Aviation Safer”

  1. We are often told that the “golden age” of air travel is now well behind us. To the contrary, I firmly believe that today we live in the golden age of air travel as history is sure to support.

    We can click on a mouse and obtain a plane ticket from almost anywhere to almost anywhere. Furthermore, we can travel with near absolute assurance that depsite the age of the aircraft or the age of the pilots that we will arrive safely.

    Commercial air travel is made safe by the hard working and talented staff of the NTSB. Every time I travel in the air, I always feel blessed that America’s air travel is held to such a high standard.

    My family thanks all of you at the NTSB.

  2. “… Challenging Assumptions …”

    For investigators, one other lesson was learned immediately after July 17, 1996: Investigator must acknowledge the role of post-accident rumors (eg, the “Boeing Scenario” after the mysterious inflight upset of 4Apr79).

    TWA-pilots had been the victims of such rumors during the first week a April 1979, after a mysterious inflight upset of a B727. In 1996 we were not willing to let RUMOR misdirect the investigation into TWA800: Investigator-err can be “forced” by the overwhelming persuasion of a reasonable rumor.

    Investigators must stick with the direct evidence: Trail of Debris, observation of any surviving crew, examination of wreckage. We’re still waiting for the correction to PC in AAR 81-8.

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