Introduction

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in July 1996. After the accident, the NTSB determined that the "probable cause of the TWA flight 800 accident was an explosion of the center wing fuel tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank (NTSB Report). The NTSB further concluded that contributing factors to the accident were that the design and certification of the aircraft required only the preclusion of all potential ignition sources in order to prevent a fuel tank explosion.

Following the accident, the Federal Aviation Administration (FAA) has issued numerous Airworthiness Directives and has enacted a comprehensive regulation to correct potential ignition sources (SFAR 88) in fuel tanks as well as conducting research into methods that could eliminate or significantly reduce the exposure of transport airplanes to flammable vapors. The latter has been in response to a new FAA policy that strives to eliminate or reduce the presence or consequences of flammable fuel tank vapors. This has included fuel tank inerting, which is commonly used by the military. Fuel tank inerting is the process of replacing potentially flammable gas space above the fuel tank (ullage) with a non-flammable atmosphere. However, the systems weight, resource requirements, and relatively low dispatch reliability have indicated that military fuel tank inerting systems would not be practical for application to transport airplanes.

A fuel tank inerting working group was formed by the Aviation Rulemaking Advisory Committee (ARAC) in response to a task assigned by the FAA to evaluate a proposed rule that would require a reduction in flammability of some or all commercial transport fuel tanks. A previous ARAC working group (1998 ARAC Report) has stated that a potentially cost-effective method of fuel tank flammability reduction was ground-based inerting (GBI). The new working group was charged with examining fuel tank inerting methods to reduce or eliminate the flammability of all or some fuel tanks in the commercial transport fleet while developing regulatory text as well as determining the cost and benefit of the proposed rule change. Under much contention this working group published a final report ( 2001 ARAC Report ) which recommended that no rule making actions be taken at this time and stated that additional reserach and development was needed. Since the inception of the 2001 ARAC WG the FAA has performed extensive research into the lower oxygen concentration (LOC) required to render a fuel tank ullage inert as well as the equipment and methods needed to develop a fuel tank inerting system.

The Fuel Tank Protection Task has two research areas working closely together in an attempt to find practical solutions to this problem. The Fuel Flammability Research examines and defines the effects of various parameters on the flammable vapors existing within a fuel tank ullage, while the Fuel Tank Inerting Research is aimed at the validation of inerting requirements and the design of an economical and practical method of rendering inert the CWT of a commercial transport airplane.