Flying today is far safer than it was 10 years ago. In fact, this is the safest period in aviation history due in part to the actions the FAA has taken since the TWA 800 accident that have completely changed the way fuel tanks are designed, operated, and maintained.
Since 1996, the FAA has issued more than 100 Airworthiness Directives (ADs) and a Special Federal Aviation Regulation (known as SFAR 88) to eliminate ignition sources. FAA engineers and scientists also made a major breakthrough for commercial aviation safety by developing the first practical ‘fuel tank inerting’ system for reducing the flammability of center wing fuel tanks on commercial airplanes.
Prior to TWA 800
Prior to the TWA 800 accident, the prevailing philosophy among the world’s aviation experts was that minimizing ignition sources was the best way to avoid a fuel tank explosion. However, the ignition source for the TWA 800 accident remains unknown. The accident fundamentally altered the assumptions held by the FAA, airlines, manufacturers, and the National Transportation Safety Board (NTSB). The FAA is pursuing the right safety solution: eliminate ignition sources and reduce the flammability of the tank.
Eliminate Ignition Sources
The aviation community has focused on precluding ignition sources to prevent fuel tank explosions. Since 1996, more than 100 directives have addressed issues such as pump manufacturing discrepancies, wire chafing, protection of the Fuel Quantity Indication System (FQIS), and overheating solenoids.
On May 7, 2001 the FAA issued a far-reaching safety rule called ‘SFAR 88’ to minimize ignition sources in fuel tanks. SFAR 88 changed the way airplanes are designed, operated and maintained. By December 6, 2002, manufacturers completed reviews of each airplane model and identified potential ignition sources. This effort resulted in the identification of more than 200 previously unknown ignition sources.
While the work accomplished by the industry to comply with SFAR 88 has certainly improved safety, the FAA believes that the added safety net of reducing the flammability of the tank is also necessary. For example, the FAA has issued two ADs for ignition sources discovered after the SFAR 88 reviews. These failure modes were not identified during the SFAR 88 reviews.
Reduce Fuel Tank Flammability
Beginning in 1998, the FAA tasked the Aviation Rulemaking Advisory Committee (ARAC), comprised of world-class aviation safety experts, to evaluate options for reducing the flammable vapors in fuel tanks on existing and future commercial airplanes. The first of two ARAC working groups determined that on-board inerting was too costly. The groups recommended further study of ground based inerting and directed ventilation, a method of reducing the heating of high flammability center wing tanks that then reduces the flammability exposure. In June 2001, a second ARAC working group concluded that their ground based inerting model presented a new set of implementation issues at airports and that an on-board system was preferred. However, they believed that an on-board inerting system remained too complex, heavy, unreliable, and costly.
Despite the ARAC findings, the FAA strongly believed that a practical on-board inerting system could be developed and the agency moved forward an aggressive FAA research program.
Fuel Tank Inerting
For 30 years, the world’s experts have said that inerting was too complicated, too heavy, and too expensive for commercial airplanes. FAA scientists and engineers challenged those assumptions and ultimately developed the first prototype inerting system for commercial airplanes.
An inerting system replaces the oxygen in the fuel tank with an inert gas such as nitrogen, preventing the potential ignition of fuel vapor.
Inerting systems have been used on military aircraft since World War II, primarily to minimize combat explosions and damage. Military airplane inerting systems cannot be installed on commercial airplanes. The military does not have to meet the robust safety standards set for commercial airplanes. Military systems are too heavy and too big for commercial airplanes and may operate only a few hours per day or week compared to the average 14 hours per day flown by a commercial airplane.
Many different techniques have been used in the inerting systems on military aircraft. On World War II-era airplanes, engine exhaust was typically used to produce the inert gas. More recently, nitrogen has been used to render the fuel tank inert. Various techniques exist for separating nitrogen from air for use in inerting, the simplest and most reliable being the membrane technology that is used in the FAA prototype inerting system for commercial airplanes.
FAA’s Breakthrough Prototype
In May 2002, the FAA unveiled a prototype on-board inerting system that replaces oxygen in the fuel tank with inert gas, which prevents the potential ignition of flammable vapors. ARAC had assumed that bleed air from the engines could not be used, requiring a compressor. The FAA developed a simple, reliable prototype that uses bleed air without a compressor or other moving parts. In addition, FAA research demonstrated that a higher level of oxygen — 12 percent can be used versus the 10 percent used by ARAC. The FAA system installed on a 747SP weighed about 200 pounds compared to the complex inerting systems used by the military that weigh thousands of pounds. It also takes up very little “real estate” on an airplane.
Boeing has since developed its own system, which will be installed on new airplanes rolling off the production lines today. All airplanes designed with a center wing tank are susceptible to flammability risk, including both Airbus and Boeing models.
FAA Proposes Rule to Reduce Fuel Tank Flammability
On November 23, 2005, the FAA proposed a rule that would require more than 3,200 existing and certain new large passenger jets to reduce flammability levels of fuel tank vapors. The Notice of Proposed Rulemaking (NPRM) would require aircraft operators to reduce the flammability levels of fuel tank vapors to remove the likelihood of a potential explosion from an ignition source. Fuel tank inerting is the best solution to meeting the news standards outlined in the agency’s proposal.
The FAA’s proposal applies to new large airplane designs. Boeing 737, Boeing 747, and Airbus A320 models would be retrofitted first. The preliminary estimate for the total cost for the U.S. fleet is approximately $808 million over 49 years, including $313 million for retrofitting the existing fleet. The following is the projected U.S. aircraft fleet that would be retrofitted:
|Airbus Models||Number of Aircraft |
|Airbus A320 ||906 |
|Airbus A330 ||44 |
|Boeing Models||Number of Aircraft|
The comment period closed on May 8, 2006. The FAA’s proposed rule and public comments may be viewed online at http://dms.dot.gov, docket FAA 2005-22997.
- Watch a prototype fuel tank inerting installation. (8:00 minute video)
- Listen to John Hickey, Director of Aircraft Certification, discuss improvements since the TWA 800 accident (:35 second audio)