By Fred Workley
On May 15, 2002 the National Transportation Safety Board (NTSB) revised its Most Wanted List strengthening its call for fuel tank safety. The revised language calls for long-term solutions to explosive mixtures in fuel tanks on Transport Category Aircraft.
Fuel tank flammability
It is impossible to predict all possible ignition sources in a fuel tank. Therefore tank design must not concentrate solely on the elimination of ignition sources, but must also incorporate design elements to eliminate the presence of flammable fuel/air mixtures.
FAA Safety recommendations A-96-174 and A-96-175 issued Dec. 31, 1996 respectively address long-term and short-term solutions to minimize fuel tank flammability.
The NTSB is not fully content with the FAA's goal to minimze flammability. Rather, they would like to see fuel tank flammability eliminated. Some operational measures could be taken immediately to reduce current levels of flammable vapors including limiting the on ground operating time of air-conditioning packs and instead using a ground-based cool air supply and cooling or ventilating the pack bay. The use of these types of ground sources for conditioned air can provide feasible short-term benefits, but the NTSB sees fuel tank inerting as a promising solution to fuel tank flammability.
Fuel tank inerting
On board inert gas generation systems (OBIGGS) have been developed for some military aircraft and are under study for use in commercial aircraft. OBIGGS eliminate the potential for explosions.
OBIGGS have the potential to eliminate fuel tank explosions, however the necessary technology to measure the oxygen level in the fuel tank ullage during flight did not exist until recently and is still being perfected.
This real-time monitoring of oxygen in multiple locations in the fuel tank is essential to determine the inertness or "quality" of the ullage gases as well as diagnosing the health of fuel ventilation systems during flight. Real-time measurements of oxygen could be used for control of OBIGGS, which could lead to the development of a truly inert ullage.
Earlier sensor systems only allow for the measurement of oxygen in the nitrogen enriched air (NEA) generated by OBIGGS prior to its entry into the fuel tanks. Fuel tank ullage is very dynamic; frequent/rapid changes in the altitude can result in large changes in the oxygen levels in the fuel tank, regardless of the quality of NEA supplied by OBIGGS. Inerting of the ullage can only be accomplished by supplying NEA in response to the detection of explosive fuel: oxygen mixtures in the fuel tank by an in-situ real-time sensor.
Fuel tank environments are characterized by large variation in temperature and pressure, the presence of hydrocarbon vapors, and the probability of liquid fuel splashing on sensors. Sensors must be designed to withstand and perform in this harsh environment.
Considering all the constraints associated with fuel tank monitoring, there is an immediate need for an oxygen sensor systems with the following characteristics:
- Real-time monitoring of oxygen in a fuel tank.
- Easy tank penetration.
- No electrical current or potential for electrical/electrostatic sparks.
- No physical, chemical, or functional effects when exposed to fuel.
- Operable in a temperature range of -50 to 80 degrees Celcius.
- Infrequent calibration requirements or self-calibrating.
- Lightweight and small.
- Capable of being multiplexed to a single control unit.
- Dynamic range of at least 0 percent (total inertness) to 25 percent (in excess of air) oxygen concentration.
- Resolution of 0.1 percent and an accuracy of 0.5 percent oxygen.
- Power requirements and data generation must be compatible with automatic control systems or accessed by the cockpit crew.
- Must meet realistic cost objectives for acquisition, installation, and ownership.
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