No Fire in the Hole

More than a decade of change later


Author’s Note: AMT magazine has chronicled change in our aviation industry. This is the latest in a series of articles that was first published in February 2000 with “No Fire in the Hole” and again in July 2001 with “No Fire in the Hole — Revisited, New regulations affecting aircraft fuel tanks.” There was an electronic update in October 2003 titled “No Fire in the Hole, New developments on aircraft fuel tanks.” The current article “No Fire in the Hole — More than a Decade of Change Later” shows an evolution of regulatory response and technological change both deliberate and slow. It tells the story of a maturing technology over 13 years through innovation to meet a specific aviation concern for safety and security for potentially hazardous environments like fuel tanks, cargo compartments, and confined areas of an aircraft. Technology has matured so that the percentage of oxygen in the air above the fuel is measured and nitrogen is controlled and directed only as needed to keep the ullage inerted or for fire suppression in inaccessible areas. This increases the safety margin so that no internal ignition source can cause ignition and external threats are decreased from shoulder fire missiles or high power sniper rifles.

The Trans World Airlines (TWA) 800 accident was the start of an extensive review by the FAA and the industry that has focused on three areas of concern: preventing ignition from any source within the tank, fuel flammability, and fuel tank inerting. During the NTSB hearing, those of us in the room in Baltimore, that heard the technical testimony about the in-flight explosion of the center fuel tank and observed the families of those lost, were deeply moved and we hoped that a loss like this would never happen again.

Fuel tank and cargo compartment protection
The military has had OBIGGS (on board inerting gas generating systems) on its transport aircraft and some fighters for some time. These generators are referred to as PACKS. The overriding need for safety and security trumps the concerns about fuel usage to support high bleed air usage to provide air that is separated into usable nitrogen that floods all the fuel tanks.

In order to get an OBIGGS that has in tank monitoring for control and distribution of the inerting gas on a commercial transport aircraft these are just some of the hurdles: STCs (supplemental type certificates), PMAs (parts manufacturing authority), environmental testing (RTCA/DO-160 Series {MIL Standard 810F}), environmental test methods for airborne equipment, RTCA/DO-178, guidance for the orderly development of software for airborne digital computer-based equipment and systems, and ground and flight testing plans of all system components.

For commercial aircraft the early goal was to have a system weighing about 800 pounds that would provide enough nitrogen to support the inerting of the center fuel tank of the Boeing 747.

With a lot of industry input the FAA developed a lighter weight system of less than 400 pounds for the center fuel tank application, but again it flooded the tank with nitrogen which because of its constant generation of nitrogen, reduced its effective life on the aircraft.

The change over time is that we now have the ability to measure accurately the percentage of oxygen in the tank ullage and control the output of the nitrogen generating packs and distribution of nitrogen to the fuel tanks only as needed to inert all tanks and wing vent boxes. Also, we can now use nitrogen to supplement existing fire suppression systems like Halon or replace part of an existing system, like the second Halon bottle, in cargo compartments by routing nitrogen to the cargo compartment as needed according to the feedback from probes in the ceiling of the cargo compartment. One possible change might use inerting with nitrogen or CO2 with a water mist from a 5- to 6-gallon source. The water wets the flammable material and takes away some heat while the inerting puts out the fire and keeps it out. This may be the Halon replacement because it is easy to clean up and does not cause excessive corrosion or toxicity.

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