Campaign Continues To Prevent Fuel Tank Explosions

Hunt for ignition sources does not eliminate dangerous vapors

The danger of commercial airline fuel tanks exploding persists, as evidenced by the effort to stamp out ignition sources through a mounting pile of U.S. airworthiness directives enjoining operators to take precautions.

A forthcoming research paper shows that the toll from exploding fuel tanks is higher than generally thought and is likely to climb unless the explosive vapors are eliminated.

The hazard was eliminated in the fuel system of a pioneering military jet design a half-century ago by inerting the explosive vapors. However, these vapors continue to threaten the safety of commercial airline operations, where their presence has been an accepted fact of life and fuel system safety has been geared toward stamping out ignition sources.

At least 10 regulatory actions have been taken in the past 10 months to prevent heat sources inside fuel tanks from causing vapors to explode. If the vapors were inerted, for example by diluting their oxygen content with an infusion of inert gas, the danger of catastrophe would virtually be eliminated. Nullifying the oxidizer would break the simple physics of the explosion triangle (fuel, oxidizer, and ignition source), even if latent ignition sources lurked. It is for this reason that the National Transportation Safety Board (NTSB) has called for inerting, why it is among the board’s “Most Wanted” list of safety improvements, and why the Federal Aviation Administration (FAA) has a prototype system based on gas separation under development (see ASW, Dec. 23, 2002). The FAA system won’t be test flown until this summer and does not provide sufficient nitrogen enriched air to inert all tanks (center tank plus wing tanks), or to inert them during descent, the most demanding phase for an inerting system. Nor does the FAA system in development provide sufficient capacity to inert engine pylons, dry bays and compartments. These, too, are vulnerable to fuel-air explosions triggered by ignition sources outside of the fuel tank. Indeed, there is an ongoing effort to stamp out these sources paralleling the pursuit and elimination of ignition sources inside the tank.

The FAA inerting system program is an outgrowth of the view expressed in two rulemaking advisory committees convened since the fatal 1996 explosion of the center wing tank on TWA Flight 800. Both committees concluded that suitable inerting technology does not exist (see ASW, August 13, 2001 and March 18, 2002). The assertion appears to ignore technology that was employed a half-century ago on the Boeing [BA] B-47 bomber. This airplane, with its swept wings, fuel tanks installed inside the fuselage and engines mounted in pods under the wings, is the forebear of all commercial airliner designs, starting with the B707. This first-generation jet featured many of the design innovations of the B-47, although not the bomber’s fuel tank safety system.

A 1951 maintenance manual for the B-47 indicates that ullage, or vapor, in the bomber’s fuel tanks was inerted by carbon dioxide (CO₂). Half-inch chips of frozen CO₂ (known as “dry ice”) were loaded into insulated canisters located in the wheel wells. The system operated electrically via controls at the copilot’s refueling control panel. Electric coils in the canisters heated the dry ice, which sublimated into gaseous form. The gas was piped to the fuel tanks, inerting the ullage. Even the piping for air-to-air refueling plumbing was inerted.

It is clear from the airplane manual that the U.S. Air Force was acutely conscious of the danger posed by ignition sources in fuel tanks. For example, the wording of a statement in the B-47 manual is strikingly similar to that reiterated in airworthiness directives issued by the FAA over the past months:

CAUTION: Do not let pumps run for any appreciable period after fuel falls below pump intake level.

In other words, don’t let the pumps run dry, lest frictional heating cause an explosion of fuel vapors.

“Boeing knew enough about the explosion hazard to do something about it in 1947,” said Jim Johnson, referring to the year the B-47 was designed. In 1951, he was an aircraft maintenance instructor on the B-47. As he recalled in a recent telephone interview, “I taught everything from the fuel filling inlet to the engine exhaust.” As such, he was intimately knowledgeable with the airplane’s built-in purging system. Given the simplicity and effectiveness of the system in the B-47, Johnson said, “It is inexcusable that TWA Flight 800 blew up.”

The B-47 inerting system was straightforward, Johnson recalled:

• There was no increase in flight crew workload. Purging took place any time the fuel boost pumps were operated (to include cross feeding during flight).
• Sublimation of crushed dry ice took place in gas-tight containers, which were heated electrically by selection of a fuel tank.
• The CO₂ gas was plumbed to the selected tank to inert the space vacated of liquid fuel. Gas expulsion created a pressure of not more than 2 psi and dynamic flow was completed through the fuel tank vent system.
• Servicing with crushed dry ice was not critically time sensitive. The ice could be loaded within a window of up to three hours before takeoff.

The canisters for the dry ice were located at eyeball level. “Replenishment was a simple task for maintainers,” Johnson recalled. Some 2,000 B-47s were built.

“I am a minority of one who suggests that a proven purge [inerting] system with millions of operational hours logged may have prevented past fuel system failures. I’m concerned that more losses may result while the industry searches for a ‘perfect’ high-tech solution,” Johnson said.

In this regard, at least three generations of inerting technology have yet to find their way into commercial application, in part because there was and is no requirement in the Federal Aviation Regulations to minimize or eliminate the presence of flammable vapors in the fuel tanks of transport category aircraft:

1^st generation: 1950, B-47 inerting system based on CO₂.

2^d generation: 1970, FAA successfully demonstrates a liquid nitrogen (LN) based inerting system in a DC-9, thereby advancing the technology from blocks of crushed dry ice to a liquid that would turn into a gas when metered into the inerting plumbing.

3^d generation: Boeing patents a membrane-based technology to produce nitrogen-enriched air (NEA) to inert fuel tanks (see ASW, Dec. 23, 2002).

The system now in development by the FAA is based on the same membrane technology featured in Boeing’s 1983 patent, although it has been stripped of a storage bottle and other features (such as dry bay protection) to save weight, with a concomitant reduction in capability and protection.

Inerting may be even more imperative given the “substantial explosion risk” of present aviation industry design practices. According to a forthcoming paper to be presented at the American Institute of Chemical Engineers’ annual loss prevention symposium this week in New Orleans, recent fuel-system related aviation accidents show that in the absence of more aggressive action, one in every 14 jetliners with heat-generating air conditioning equipment located below the center wing tank (CWT) contains flammable vapors. According to Erdem Ural, of Massachusetts-based Loss Prevention Science and Technologies, Inc., and author of the paper, the public may perceive this risk exposure with alarm. He points out that in the chemical industry vessels containing flammable liquids reaching temperatures within 30� F of their flashpoint are inerted. Moreover, vapors can explode when exposed to ignition sources well below the 0.2 millijoule (mJ) minimum ignition energy assumed by the aviation industry in fuel system design.

In fact, Ural asserts that the industry has not included the full risk history, thereby underestimating the risk of future fuel tank explosions.

Ural believes fuel system safety can be improved significantly for a few dollars per ticket.

>>Johnson, tel 805/525-8544; Ural, e-mail [email protected] <<