Explosive Mixture

Explosive Mixtures

By Fred Workley

Fred WorkleyOn 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.

Current commercial oxygen sensors include Clark type electrodes, semiconducting metal oxide-based sensors, and optical oxygen sensors. Clark electrodes generally do not lend themselves to fuel applications because they require frequent calibration; they have long-term stability problems and the membrane swells when exposed to hydrocarbons.

Semiconducting metal oxide electrodes using metal oxides such as ZrO2, TiO2, which are currently used as exhaust gas oxygen sensors in feedback control systems to control the air-to-fuel ratio in combustion processes. Although very stable and sensitive, these electrodes need to be heated between 300 and 800 C to become active and therefore they could not be used directly in a fuel tank.

Optical oxygen sensors are generally divided into two types including near infra-red (NIR) sensors and sensors using photoluminescence transducers. NIR sensors are based on the absorbance of NIR light by oxygen molecules using tunable diode lasers. The drawback of this sensor is the requirement of a long optical path length to obtain sufficient sensitivity and also the overlap of absorption bands of hydrocarbons and water vapor with the oxygen absorption band.

In photoluminescence (i.e. fluorescence) optical sensors, a fluorescent indicator molecule is immobilized in a polymer or sol-gel matrix and coated on an optical fiber or other optical surface, the indicator molecule is excited by low wavelength radiation and fluoresces at a higher wavelength. The excited molecules also release energy in a non-radioactive process when they collide with molecular oxygen that diffuses into the matrix from the local environment. This quenching is detected as a decrease in either the fluorescence intensity or the fluorescence decay time. The amount of intensity or decay time quenching is correlated with oxygen partial pressure. Some of the advantages of optical sensors over electrodes include lack of electrical spark possibility, immunity to EMI interference, no consumption of oxygen, not subject to errors from fouling, fluid dynamics, and other factors that influence diffusion rate, small size, large dynamic range, ease of multiplexing, ease of calibration, and low power consumption.

The most promising solution is a commercial oxygen sensor based on the quenching of a fluorescent transducer. A fluorescent Ruthenium-organic complex is trapped in a sol-gel matrix and applied as a thin film on the tip of an optical fiber. The optical fiber is interfaced to a blue LED for excitation and a miniature spectrometer for detection of the fluorescence intensity spectra. This advancement has happened with the development of extremely stable fluorophores and matrices for probes, which has enabled the use of spectroscopic intensity detection. The system has met with great success in a variety of medical, industrial, environmental applications as well as in aircraft fuel tanks. Adding a protective "overcoat" of silicone provides some protection, especially from wetting by solvents.

There is an ongoing program to improve existing fiberoptic oxygen sensor technology for in-situ monitoring of oxygen in a fuel tank environment. The usefulness of the new sol-gel formulations and the overcoats will require the complete characterization of the effects of these modifications on sensor performance. Generally, the protective coatings will impede the oxygen diffusion and therefore sensor response time. PLASM coating techniques will minimize the overcoat thickness, achieve good mechanical stability, uniformity and adhesion, and optimize the chemistry.

Characterization of different overcoating materials has facilitated design of multi-layer systems, where for example the first layer is chosen for adhesion and mechanical properties and the next layer for chemical inertness. Experience indicates that the accuracy and stability of multiple temperature, multiple concentration calibrations is a direct indication of the mechanical stability and chemical compatibility of sensors. The technology now exists for a completely self-calibrating, temperature-compensating system to detect the oxygen content in the fuel tank environment.

Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Benton City, WA, and Indianapolis, IN. He is on the technical committees of PAMA and NATA and participates in several Aviation Rulemaking Advisory Committees. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.

Additional ReSources

FAA Safety Recommendations
A-96-174 and A-96-175

NTSB Most Wanted List