It is now more than 20 years since the third-deadliest accident in U.S. aviation history, the explosion of TWA Flight 800 (TWA 800), but the legacy of that tragedy continues in the engineering of fuel tank technology for commercial aircraft.

After years of deliberation, the FAA ruled in 2008 that airlines must act to reduce the flammability levels of fuel tank vapors on the ground and in the air. Fuel tank inerting was proposed. These systems decrease the probability of combustion of flammable materials stored in a confined space by maintaining a nonreactive gas such as nitrogen in the fuel tank of an aircraft.

The new rule marked a step-change from previous efforts at preventing fuel tanks from exploding, which had focused on reducing sources of ignition within the tank such as those in the fuel-quantity-indicating system, in the electrical signal running to the fuel gauges or a lightning strike. The new rule required the retrofitting—at a cost of hundreds of millions of dollars—more than 3,200 Boeing and Airbus aircraft with fuel tank inerting systems. It also decreed that all future fixed-wing aircraft designs for more than 30 passengers would have to feature these systems.

One of the newer aircraft to feature fuel tank inerting technology is Boeing’s 787, which relies on systems manufactured by British engineering group Cobham. Stephen Matthews, vice president of business development and sales at Cobham Mission Systems, says the company originally developed fuel tank inerting systems based on nitrogen gas in the early 1980s, as part of a program funded by the U.S. government for the Boeing KC-135 Stratotanker, a military aerial refueling aircraft. Inerting systems were then widely provided by Cobham for the Apache AH-64 attack helicopter.

Within the U.S. military, the systems, known as Obiggs (onboard inert gas generation system) were subsequently supplied by the company for the V-22 Osprey tiltrotor aircraft and Boeing C-17 military transport, among others. “The hollow fiber membrane system solution used for the C-17 is considered the genesis of our commercial aircraft fuel tank inerting systems,” says Matthews.

Following the crash of TWA 800, U.S. regulators were conscious that these military inerting systems could protect aircraft even in the event of the fuel tank being hit by gunfire. Their review proposed mitigating every single foreseeable ignition source plus adding nitrogen inerting systems—such as the technology Cobham was already supplying to the C-17 and other military aircraft.

The fundamental principle of Cobham’s fuel tank inerting system is that the ullage—the space in which there is fuel vapor, or the volume in the fuel tank above the fuel—is kept at a certain ratio of flammability by pushing out oxygen and replacing it with nitrogen, making it impossible to ignite the vapor. The heart of the system is an air separation module, a unit that separates air through a permeable membrane. “Oxygen flows out of the system, and nitrogen is allowed to flow through,” Matthews explains.

The ullage consists of nitrogen-enriched air, at a ratio of up to about 97% or greater nitrogen, compared to just over 78% in the atmosphere. It is maintained at a bulk-average above 88%. In the case of commercial aircraft, an air source is required, either via the engine bleed or, in the case of the 787, a bleedless system. “The Dreamliner features a United Technologies unit [that] compresses air and feeds it to the Cobham air separation module,” explains Matthews. “There, a system of filters, heat exchangers, valves and sensors work together to make sure the appropriate level of nitrogen is supplied to the fuel tank.”

“If you think about an Apache helicopter, the crew sits right on top of the fuel tank,” Mike Donahue, Cobham senior business development manager, adds. They get shot at all the time by machine gun and small arms fire. But an Apache will literally fly back to base dripping fuel and land safely. There is no fire, no explosion.”

Mindful Maintenance

When the aircraft is on the ground, safety is also of paramount importance while cleaning and inspecting fuel tanks. At Lufthansa Technik, fuel tanks are usually inspected by flashlight and inspection mirror. Sometimes nondestructive testing is used, with ultrasonic equipment and various high-frequency eddy-current inspection probes. For areas that cannot be reached directly for visual inspection, borescope and X-ray equipment may be applied, says Grigor Peshkalov, bay manager at Lufthansa Technik in Sofia, Bulgaria.

At Lufthansa Technik, fuel tanks are cleaned with lint-free rags to remove residual fuel before maintenance begins. They are also ventilated to ensure fuel vapor is removed, providing a safe work environment for fuel tank technicians and inspectors. The process is controlled with special equipment that measures fuel vapors and oxygen concentration inside the tank. Some inspections require sealant removal inside the tank, which is carried out with plastic scrapers and special pneumatic sealant removal tools. When sealant is detached from the tank structure, it is disposed of with vacuum cleaners, and the area is cleaned with special chemicals such as methyl ethyl ketone solvent, isopropyl and others.

Keeping fuel tanks safe from ignition on the ground is critical. “Before fuel tanks are opened they have to be drained and aircraft electrical power has to be switched off. It is mandatory also to electrically ground the aircraft,” says Peshkalov. Turning on electrical power is strictly forbidden before the tank access panels have been replaced. All maintenance activities are carried out with spark-free tools specially certified for fuel tank usage. This includes not only special test equipment, but even simple wrenches and all sources of light. Inspectors’ clothes are manufactured from materials that prevent static electricity buildup when they are moving inside the tanks.

Future technologies will allow fuel tanks to be inspected in detail without opening them, says Lufthansa Technik. “This will decrease the risk of ignition, prevent fuel dumping, minimize pollution and even cut down on usage of hangars,” says Peshkalov.

At Air France Industries KLM Engineering and Maintenance, one new method for detecting leakages is helium equipment, which enables a dry check of the tank. The MRO is also developing new sealants that are stronger and less sensitive to degradation or dry cracking.

At Cobham Mission Systems, the commercial aircraft market, rather than the military, is driving new developments in fuel tank inerting components. R&D at the company has focused on development and engineering of the air separation module to make it more robust. Long-term, this is helping reduce maintenance costs.

Most major U.S. airlines use a new type of engineered air separation module first used three years ago, Cobham says. “It is far more robust than anything that has been introduced into the industry before. Five out of the six major airlines globally have signed up to the new technology, and it is saving those operators in excess of $100 million across the life of the fleet,” says Matthews.

Aircraft manufacturers will continue to focus on the removal of all potential sources of ignition within the fuel tank, as well as the retrofitting—and introduction to new aircraft—of nitrogen fuel inerting systems.

Fundamentally, flammability reduction is the rule, with various ways to achieve it.

While Cobham’s focus has been on making the air separation module more reliable, it is “looking at fuel inerting systems that are 40% lighter and 30% smaller in terms of their packaging,” says Matthews.