SPECIAL REPORT: MILITARY AVIATION FUEL FILTRATION TECHNOLOGY
Fuel Contamination, Aircraft Safety and Filtration Marushka Dubova, Defence Correspondent
IATA: Jointly Operated Systems “Fuel should be clear, bright and visually free from solid matter and undissolved water at ambient temperature. For guidance on contamination limits for into-plane fuelling, refer to IATA Guidance Material for Aviation Turbine Fuels Specifications, 5th Edition, January 2004 (Part III).”
So civil and military fuel engineers have to balance the gains of using FSII to inhibit ice crystal formation with the risk of “apple jelly” or “contaminated water bottom” sludge slowing down fuel flow to engines in flight.
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HE SPECIFICATIONS for civil and military aviation fuel are stringent and monitored by frequent checks by suppliers and users on transfer, fuelling and during use. Military aviation fuels tend to be of a higher specification than civilian fuels because of their use in more extreme circumstances for combat and sustained activities outside routine hub to hub flights. According to Shell, JP-4, the military equivalent of Jet B (with the addition of corrosion inhibitor and anti-icing additives) is rarely used now. It has been replaced by JP-8, which is the military equivalent of Jet A-1 with the addition of corrosion inhibitor and anti-icing additives; it meets the requirements of the U.S. Military Specification MIL-T-83188D. (The UK also have a specification for this grade namely DEF STAN 91-87 AVTUR/FSII (formerly DERD 2453). NATO Code F-34. JP-5 is a high flash point kerosene for use in aircraft carriers (the UK Military specification for this grade is DEF STAN 91-86 AVCAT/FSII formerly DERD 2452). As was demonstrated by the incident with BA 777, anti-icing additives are potentially of high value to aviation fuel. But the anti-icing additives have their own associated risks as the military research conducted for the Department of Defense on the incidence of contamination by “an apple jelly like substance” reveals. These sludge or “apple jelly” like formations in the fuel in the airplane fuel tank or in storage can increase maintenance costs and impede fuel flow. In a report for the Defense Energy Support Center (DESC) J. Andrew Waynick and Steven R. Westbrook reported on their extensive research work on the issue on the nature of this “apple jelly” type substance or “contaminated water bottoms”8 that was found in fuel storage tanks and onboard fuel tanks in military aircraft.
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The Nature of the Sludge “This work has demonstrated that apple jelly is a complex mixture. It begins with water and DiEGME (diethylene glycol monomethyl ether). This mixture reacts with its environment, extracting and dissolving compounds from the materials with which it comes in contact.” DIEGME is a Fuel System Ice Inhibitor (FSII) additive used in military aircraft fuel. It is added to JP-8 and JP-5 jet fuels at concentrations of 0.1% to 0.15% 9. However, DIEGME separates from the jet fuel and collects in concentrations high enough to cause FTTP (fuel tank topcoat peeling). Such coating failures have lead to additional cost, requiring unscheduled maintenance, decreased safety, and decreased mission readiness and overall capability.10 So civil and military fuel engineers have to balance the gains of using FSII to inhibit ice crystal formation with the risk of “apple jelly” or “contaminated water bottom” sludge slowing down fuel flow to engines in flight. Worse for filtration specialists, the defense fuel researchers found the presence of sodium in the apple jelly. This sodium an additional contaminant may have originated from the fuel filtration systems used to reduce the presence of contaminants, the ultimate Catch-22. “We know that the majority of the sodium comes from the water-adsorbing filters. They are the only source sufficiently concentrated in sodium to provide the sodium levels observed in apple jelly, especially the thick apple jelly. The relative amounts contributed by any other source are impossible to ascertain. Still, some conclusions are possible. As an example, thin apple jelly tends to be lower in sodium. This probably means it has had less exposure to water-adsorbing filters. Hence, it also has little or no thickener so its viscosity is low.”