Along with UPS and FedEx, Boeing is opposing an FAA-proposed safety retrofit in the center fuel tank of 757 cargo jets, calling the risk of an explosion like the one that downed TWA Flight 800 in 1996 "less than extremely improbable."

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The Federal Aviation Administration (FAA), continuing a stream of safety regulations flowing from the 1996 explosion aboard TWA Flight 800, is proposing fuel-tank wiring retrofits on 352 cargo jets operated in the U.S.

In an unusual standoff, Boeing insists the risk is negligible and is balking at the fix.

It says it won’t design the wiring modification the agency wants installed on in-service 757 cargo planes, instead offering only a more expensive solution.

In turn, industry trade group Airlines for America (A4A) cites the lack of a Boeing wiring design and the cost of the alternative fix as reasons to stall the proposed FAA rule.

The cargo-jet dispute reflects a serious disagreement over the scale of risk involved.

The FAA says the planes have an “unsafe condition” that “could result in fuel-tank explosions and consequent loss of the airplane.”

It projects the wiring modification in the center fuel tank would cost between $100,000 and $200,000 per airplane. Cargo operators would have five years to comply.

Boeing insists that’s unnecessary, calling the risk of such an explosion “less than extremely improbable.”

If the regulator makes the rule mandatory, Boeing says it will offer cargo-jet operators only a more expensive fix — a nitrogen-gas flammability-suppression system in the tanks.

Boeing and the cargo operators also worry the FAA could later require similar modifications on other cargo planes, including 747-400 and 767 freighters.

Last week, after the window for initial input had closed, the FAA gave the industry an additional two-month period to offer further argument.

The proposed FAA retrofit is the latest safety measure stemming from the fuel-tank explosion that killed 230 people aboard Paris-bound TWA Flight 800 over the Atlantic Ocean off Long Island, N.Y.

The new rule for cargo aircraft would mirror similar fuel-tank safety rules for passenger jets introduced after that disaster. But implementation of the rules even for those jets has been slow and is still incomplete.

The FAA in the late 1990s first mandated shielding and separation of wiring going into passenger-jet fuel tanks. Then in 2008, it further required installation of a flammability-suppression system in the tanks — such as Boeing’s nitrogen generating system (NGS).

All new Boeing airplanes today roll out with the NGS as standard. Airplane operators were required to retrofit such a system on half their fleet by the end of 2014, and on the remaining half by the end of 2017.

But the airlines may not meet even those stretched-out deadlines.

In March, trade group A4A asked the FAA to extend the deadlines because the agency and Boeing have not yet certified kits needed to retrofit the system on two models: 757s and 767s powered by Rolls-Royce engines. (The FAA has not yet responded to the A4A request.)

Boeing spokesman Miles Kotay said retrofit kits for other models, including those with General Electric engines, are already certified and the missing kits will be certified “no later than the end of October.”

U.S. carriers are today operating more than 2,000 passenger jets still awaiting the fuel-tank retrofit, said A4A spokesman Steve Lott.

Until now, cargo planes were exempt from both the initial wiring modifications and the 2008 requirement for an NGS.

As the FAA now moves to bring in just the wiring-modification rules for the much smaller freighter fleet, Boeing has been joined by Airbus, A4A, UPS and FedEx in opposing the proposal.

In contrast, the union that represents some 500 safety engineers and aircraft-certification personnel inside the FAA has asked the agency to strengthen the proposed rule further.

The safety committee of the National Air Traffic Controllers Association (NATCA) called for the rule to be applied not only to the 757 center-fuel tank but also to the fuel tanks in the jet’s wings. And it recommended the compliance period be shortened from five years to three.

The European Aviation Safety Agency, meanwhile, said that since it never extended the FAA’s 2008 requirement to 757 passenger jets in Europe, it may have to issue its own airworthiness directive that would extend the rule “to the entire Boeing 757 fleet in Europe,” not just the cargo jets.

The 2000 National Transportation Safety Board (NTSB) report into the TWA Flight 800 disaster concluded that “the most likely ignition source” was a combination of two electrical failures.

One was a failure inside the center fuel tank that had gone undetected for some time — perhaps caused by corrosion of wire terminals or debris accumulating on the probes that measure how much fuel remains in the tank. The other was a subsequent short circuit in the wiring outside the tank.

The proposed FAA cargo-jet rule therefore mandates that the fuel probe wiring entering the tank should be separated and shielded from other high-power wires on the airplane.

In a detailed response to the FAA, Boeing estimated the probability of this combination of failures in any 757 at 1-in-a-billion per flight hour.

The risks are even lower for 757 cargo planes, it said.

Also by the time, five years out, when the modifications would have to be implemented, the newest 757s would be almost 12 years old, and Boeing estimated the remaining flying hours in the life of the U.S. all-cargo fleet at less than 5 million hours.

Combined, those estimates mean even a single disastrous explosion in the 757 cargo fleet is “unlikely to occur in the operational life of a fleet,” Boeing’s response stated.

“A risk this low does not require further mitigation,” Boeing said.

And though Boeing noted it “does not use a cost-benefit comparison for safety-related issues,” given that in this case it considers the risk negligible, it proceeds to question the FAA cost estimates and says a wiring modification would cost well in excess of $200,000 per airplane.

Boeing says it doesn’t plan to design such a wiring fix, and instead will offer its NGS as a means of compliance.

This system pumps inert nitrogen gas into the free space inside a fuel tank as the fuel level falls, thus reducing flammability.

The FAA acknowledges that installing an NGS would make a wiring fix unnecessary.

But it would be more costly: UPS, which operates 75 all-cargo 757s, noted that in 2010 Boeing charged $323,000 per airplane for the nitrogen system.

That means the total cost to retrofit UPS’ 757 fleet with NGS would exceed $24 million. For perspective, UPS had a $3.8 billion profit in 2011.

The airline trade group is asking the FAA to withdraw the proposed safety rule until it does “thorough cost-benefit and risk analyses.”

In contrast, the safety committee of the traffic controllers’ group said it wants to know why “this important safety issue has been allowed to go unaddressed for the nearly 16 years since the TWA 800 accident.”

Dominic Gates: 206-464-2963 or