NTSB Disappointed That A380 Fuel Tanks Will Not Be Inerted

Air Safety Week, July 11, 2005

The Airbus A380 will not feature fuel tank inerting, but rather a comprehensive effort to minimize ignition sources. National Transportation Safety Board (NTSB) officials say the system safety assessments upon which the fuel system for the A380 is predicated, with the focus on ignition sources, is an incomplete approach.

"The special review under SFAR 88 shows that the system safety assessment doesn't work," an NTSB official said. He hastened to add that it is not believed the A380 "will be more vulnerable than any plane flying today," but he hoped that "new production would be harmonized" in terms of European Aviation Safety Agency (EASA) and Federal Aviation Administration (FAA) requirements. Here is where the failure to produce a notice of proposed rulemaking (NPRM) on this side of the Atlantic has now undercut the U.S. position. Last year, the FAA said a flammability reduction system (e.g., inerting) is being considered for the A380 and Boeing's new B787. At that time, FAA officials said they wanted to avoid surprises, and that manufacturers should plan on including the systems. The B787 will feature inerting, although regulatory requirements are evidently years away, if that.

As EASA notes, "Regulatory action to address both current and future aircraft fuel system certification standards has been proposed by FAA ... but this is not yet in place."

Moreover, "EASA is not aware of any Supplemental Type Certificates [STCs] already issued for nitrogen inerting systems." In other words, no requirement, no STCs, no mandate for inerting.

"Airbus will demonstrate that the ignition source prevention features are sufficient, in and of themselves, to meet the aircraft safety objectives considering a permanent flammable fuel tank environment," EASA declares. "Airbus will further demonstrate through its flammability assessment that it has further upgraded its safety level by implementing design features, which minimize heat transfers to the fuel tanks, avoiding unnecessary increases in the fuel tank flammability."

There is no center wing tank on the A380, although one could be fitted on subsequent models of the aircraft (e.g., the 800-passenger version). Moreover, the air conditioning packs on the A380 are located on the forward side of the front wing spars, with insulation. Thus located, they are not directly under any fuel tanks, which is the case on the B747 and the B737; these models have experienced three fuel tank explosions in recent years - TWA 800 involving a B747, a Philippine Air Lines (PAL) B737 at Manila, and a Thai International Airways (THA) B737 at Bangkok. All three sustained explosions of the center wing tanks.

"The A380 air conditioning packs are in the 'shoulder' of the wing," the NTSB official conceded, but on a hot day on the ground, as was the case in the Philippine Air Lines and Thai Airways explosions, the airplanes didn't have the benefit of airflow to dissipate the heat of the packs, and this also would be the case with the A380 on a hot day.

The safety board registered its objections to EASA, which rebuffed the complaints. The Europeans seem bent on issuing special conditions for the A380 that focus on three sources of ignition, and the need to minimize them: electrical arcs, friction sparks resulting from mechanical contact of rotating equipment in the fuel tank, and hot surface ignition. "The applicant should perform a Safety Assessment of the fuel system showing that the presence of an ignition source within the fuel system is Extremely Improbable [that is, 1 x 10- 9, or one in a billion flight hours] and does not result from a single failure," the EASA said.

Airbus must account for a variety of fuel pump failure modes that can cause ignition (e.g., debris lodged inside pumps, poor bonding of components to structure, etc.). It's generally conceded that a pump immersed in fuel cannot cause ignition due to any failures. Indeed, that is why so many airworthiness directives (ADs) have been issued limiting the running of in-tank pumps to a certain minimum fuel level. This ensures that fuel levels in heated tanks (those with air conditioning packs located below) with flammable ullage would never drop to a level where fuel pumps would not be immersed - no matter what the aircraft's pitch, roll or yaw attitude. Yet, knowing this, Airbus apparently is not being required to locate fuel pumps outside the fuel tanks, where they would never be "not immersed" nor exposed to in-tank heated ullage. This logical avoidance step would also allow high voltage/high current pump wiring to remain outside of the tank.

Regarding specific FAA and NTSB objections to its special conditions, EASA was pretty clear in its view:

FAA: "Flammable conditions within the [Thai and Philippines jets] would have been reached in flight after these airplanes began to climb, even if the tank had not been heated."

EASA: "EASA does not consider any of the A380 fuel tanks as exhibiting high flammability characteristics."