No Fire in the Hole

More than a decade of change later

The control is accomplished through the logic of the software in the system computers. The programmable read only memories (PROMs) software ensures that safety margins are maintained in the fuel tanks and backs up existing smoke detector systems in cargo compartments. By limiting nitrogen production as needed, instead of flooding the tanks, we reduce maintenance and extend the life of the nitrogen generating packs and the amount of bleed air flow required thus saving fuel. System logic for control and distribution sends nitrogen to the area of the airplane, including inaccessible structure, only where it is needed.

It took five years of research to develop a coating for the in-tank probe that would stay in the hazardous fuel environment (Jet A and JP-8) for 5,000 hours or three years to coincide with the heavy “C” check schedule of many aircraft. In order to meet this goal the probe must perform without degradation for 60 calendar months in tests of both submission to fuel vapors and immersed in fuel.

It has taken many years of extensive testing to establish a flammability minimum. Oxygen content in the air above the tank of 13 percent offers some protection in limited circumstances. 12 percent is safe with no margin of error. At 11.5 percent some tests still have ignition. In some military applications 11.5 percent oxygen content gave a 25 percent safety margin. To be full inerted to prevent ignition from an internal tank source of ignition or an external source such as a missile or rifle round, the military suggests 9 percent which is the recommendation of the Mine Safety Authority.

Further testing has shown that the most dangerous times in the flight profile are during climb and descent. Along with generating nitrogen for center tanks that are subjected to heat sources from the air conditioning system, additional packs might generate nitrogen only for climbs and decent. System logic would control these packs by accessing the conditions at long range cruise at altitude while monitoring cooling of the fuel because of low ambient temperatures and lower pressure. Over time there has been a lot of testing of how rapidly heat is dissipated to the outer aluminum wing structure based on the fuel capacity of a tank. One of the early ARAC working group opinions regarding aluminum aircraft was that the center tank was flammable 35 percent of the time. While the wing tanks are flammable 7 percent of the time.

One example of change over the last decade is the production of composite aircraft with composite wing structure. Recent testing has shown that composite wing structure provides much less heat dissipation and that the composite structure holds heat longer. Inerting of a composite wing may have to be done for a greater portion of the flight profile. These findings may affect the ETOPS (Extended-range Twin-engine Operational Performance Standards) time range for composite wing aircraft and thus inerting may be an operational factor to be considered.

I talked about Special Federal Aviation Regulation (SFAR-88) in AMT back October of 2003. Industry implementation of this regulation, in the years since, has revealed numerous short comings in design as related to lightning strikes, wiring failures, and flame arresters on fuel probes. There is an ever present possibility of lightning strikes igniting fuel vapors.

Composites don’t prevent lightning strikes.

In either aluminum or composite wing structures, the fiber optic probes in the tank have to be intrinsically safe. The probe bodies are of corrosion-resistant stainless steel completely grounded to the structure. The fiber optics does not conduct any electricity, only light. No physical sample is required that must be disposed of. The oxygen sensing, temperature, and pressure are taken at a single point on the probe with several redundant probes in each tank. Instantaneous sensing takes place when light is sent out and returned through the fiber optics to the computers that continuously perform trend analysis.

Flammability characteristics
The flammability characteristics of the various fuels approved for use in transport airplanes result in the presence of flammable vapors in the ullage, vapor space of fuel tanks at various times during the operation of the aircraft. Vapors from Jet A fuel (the typical commercial turbojet engine fuel) at temperatures below approximately 100 F are too lean to be flammable at sea level and at temperatures below minus 45 F. After thorough cold soaking at 35,000 feet altitude it takes a high energy source to ignite fuel vapors. However, when jet fuel is 120 F and above fuel vapors are highly flammable, or when a higher percentage of oxygen is present it takes a much lower energy source to cause ignition.

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