Safety Board Calls for Action On Fuel Tank Inerting

In the 18 months since Marion Blakey, head of the Federal Aviation Administration (FAA), announced that fuel tank inerting would be required for about 3,800 Boeing and Airbus jets, nothing has happened. The initiative remains tied up at the Department of Transportation (DOT).

According to DOT’s website on significant rulemaking, the initiative is considered “economically significant” and “unanticapated impacts requiring further analysis” are necessary.

On the eve of the ninth anniversary of the fuel tank explosion that destroyed TWA Flight 800, a B747-100, on July 17, 1996, officials at the National Transportation Safety Board (NTSB) are clearly frustrated that fuel tank inerting has dropped into a procedural black hole.

Dan Campbell, managing director of the NTSB, said that as far as the flammability of the tanks, “We’re no different today than we were in ’96. This is frustrating.” Fuel tank safety remains on the NTSB’s “Most Wanted” list of improvements.

In February of 2004, Blakey announced that a notice of proposed rulemaking (NPRM) would be published “later this year” calling for inerting of center wing tanks with air conditioning packs located under them. These “heated” center wing tanks, like that on the TWA, are considered particularly at risk because they get hot as the heat generated from the air conditioning system rises. The heating of the tank causes the fuel-air vapors inside to become explosive. The FAA and Boeing had developed new lightweight inerting systems that would provide a flow of nitrogen-enriched air to the tank to displace the oxygen, thereby inerting the flammable vapors. The system has been tested on both Boeing and Airbus jets. Blakey hailed this technology as a breakthrough in terms of weight and cost, which has made inerting possible.

Campbell noted that “these systems have been flight tested, but the rulemaking still needs to come out, more than a year after first promised.”

Without the NPRM, he said, the arguments about inerting, such as the costs and performance of the system, are not evident. And without the NPRM, there is no progress on the regulatory front to require inerting.

The situation recalls previous FAA Administrator Jane Garvey’s stance on child restraint seats. She announced in January 2001 that the FAA would call for child restraint seats, ending the practice of “lap children,” but the NPRM was never issued, and the situation remains unchanged to this day (see ASW, Jan. 8, 2001).

The same fate could befall fuel tank inerting systems. An FAA official said the proposed rule “has been drafted and is getting final clearance.” Virtually the same assurances were given regarding child restraint seats.

This is not to say that there has been no progress. Flammable vapors require an ignition source, and reducing the potential ignition sources in fuel tanks has been a major FAA effort. It has been carried out under the auspices of SFAR 88, a special federal aviation regulation under which manufacturers have undertaken design reviews of their fuel tanks, and made changes to provide greater safety regarding fuel pumps, wiring and such. Some 32 separate actions have been identified, and airworthiness directives have been issued or proposed. Of interest, there are no B717 or B777 aircraft modifications listed; apparently Boeing did sufficient electrical proofing in the design and build of the fuel tanks on these aircraft, in comparison to the long-winded evolution of general SFAR 88 fixes that is still ongoing.

In addition, wiring adjacent to fuel tanks will be addressed in a forthcoming directive, expected this fall, as a result of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) review (although some of the SFAR 88-related activity seems to address wires running outside of fuel tanks). Note that TWA 800 was believed to have been destroyed by the passage of high current into a low-power signal wire associated with the center tank’s fuel quantity indication system (FQIS).

Indeed, this passage of current is the reason for the transient suppression units (TSUs) installed in many B747 classics after TWA 800. However, problems with these devices and associated issues of shorting across terminal blocks due to build-ups of silver sulfide deposits accelerated the retirement of some airframes.

About 6 percent of U.S. carriers further restrict heat going to the center tanks by using ground-based air conditioning systems when parked at the gate. This enables the on- board air conditioning packs to be turned off while on the ground.

However, there has been little in the way of mandated changes to operational procedures, according to Campbell.

He and other NTSB officials want to see inerting practices put into place to minimize the presence of ignition sources, as the SFAR 88 effort may not have eliminated all of them. Moreover, publishing the NPRM would provide the NTSB with an opportunity to review and comment on the adequacy of the propounded inerting system. For example, the safety board recommended that all tanks be inerted, but the FAA is evidently only going to require inerting for heated center wing tanks. The FAA is also apparently allowing for a 12 percent oxygen concentration instead of the 9 percent traditionally used to define an inert tank, in the belief that this relaxation still provides an acceptable margin of protection against a fuel tank explosion.

For future aircraft, NTSB officials noted with pleasure that Boeing’s new 787 aircraft will feature inerting. They are less sanguine about the Airbus A380, which will not.

Actions Under SFAR 88 To Reduce Ignition Sources in Fuel Tanks

Note: The absence of date means the item has yet to come out, but the FAA was not able to confirm that by press time.