Does the change to JP-8 fuel solve the Air Force fire concerns?
Recent articles in the Air Force Times discussed the change from JP-4 to JP-8 fuel. As one of the major proponents for the change and the co-author of the following document citing many of the safety reasons for the change, I would like to shed additional light on this subject. In the report (AFAPL-TR-74-71) titled, "Assessment of JP-8 as a Replacement Fuel for the Air Force Standard Jet Fuel JP-4," June 1975, factors including fuel properties, aircraft vulnerability, aircraft crash fires, fuel system and aircraft performance, fuel handling, maintenance, and environmental impact were addressed related to the fuel change. By the mid 1990s most Air Force bases changed over to JP-8 with resulting reductions in aircraft mishaps and damage caused by fuel combustion as the initial cause or as a secondary effect.
Obviously under the right circumstances all fuel burns; and we must not become complacent as aircraft fires and explosions still occur. A perfect example is the TWA-800 accident (17 July1996) in which 230 people perished as a result of an in-flight explosion of the aircraft's center wing tank. While the tank contained only unusable fuel, it was in fact the vapors from Jet A (a fuel which exhibits very similar physical properties and combustion characteristics as JP-8) which exploded in the empty portion of the tank known as the "ullage." Whatever ignited the fuel vapor/air mixture may never be determined, however, the fact that a catastrophic explosion occurred with Jet A fuel is significant in that a similar set of circumstances can occur again and cause the same tragic results.
To reinforce the issue of the explosion hazard, consider the NTSB's (National Transportation Safety Board) "Abstract of Final Report as of 23 Aug. 2000" conclusions:
6. The fuel/air vapor in the ullage of the TWA flight 800 center wing tank was flammable at the time of the accident.
7. A fuel/air explosion in the center wing tank of TWA flight 800 would have been capable of generating sufficient internal pressure to break apart the tank.
And new safety recommendation:
1. Examine manufacturers' design practices with regard to bonding of components inside fuel tanks and require changes in those practices, as necessary, to eliminate potential ignition hazards.
Previous NTSB recommendations to the FAA (Federal Aviation Administration) with regard to TWA-800 included:
A-96-174 and -175 -- Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks.
and numerous other recommendations related to aircraft wiring.
While it is true that JP-8 fuels generate fewer vapors than JP-4 fuels, it is possible given the right conditions, that there are enough explosive vapors in a nearly empty fuel tank with JP-8 fuel to permit ignition that leads to a catastrophic explosion. Perhaps a brief discussion of basic properties of the four major jet fuels might be useful in providing understanding of their relative fire hazards:
JP-4 fuel, also known as Jet B, is a blend of gasoline and kerosene. Its vapor pressure at 100° F must be between 2-3 psi to reduce boil-off and vapor lock problems. Its freeze point is -77° F and its flash point temperature around zero F, although flash point temperature is not a specification requirement. JP-4 was the U.S. Air Force's primary jet fuel from 1951 to 1995. In the mid 1980s an anti-static additive was added to JP-4 for fire safety reasons.
JP-8 was first introduced at NATO bases in 1978 and is currently the U.S. Air Force's primary fuel. JP-8 is very similar to Jet A-1. JP-8, however, contains an icing inhibitor, corrosion/lubricity enhancer, and anti-static additive. Conversion to JP-8 was virtually complete in 1995 and was accomplished for fire safety and combat survivability reasons.
Jet A and Jet A-1 are the two fuels used by commercial airlines since 1950 and both fuels have a 100° F (minimum) flash point temperature for safety reasons. Jet A has a freeze point of -40° F whereas Jet A-1 has a freeze point of -53° F. For this reason Jet A is more available and therefore more widely used. The commercial fuels in the United States are not required to have the anti-static additive and generally do not incorporate it.
JP-5 fuel, used from 1952 to the present by the Navy, has a 140° F (minimum) flash point temperature. This kerosene fuel is currently the U.S. Navy's primary fuel and was developed mainly due to fire safety concerns aboard ships. This fuel has a freeze point temperature of -51 F; JP-5 does not have the anti-static additive.
Flash point temperature is the lowest temperature where a fuel will give off sufficient vapors for ignition under ambient conditions. It is an estimate of the lower flammability limit. Flash point temperature is used by the National Fire Protection Association (NFPA), and in the Hazardous Substance Classification Act to differentiate the relative fire hazard of fluids. The higher the flash point temperature, the less hazardous the fluid. All of the above fuels have a minimum auto ignition temperature in the 435°-450° F range and therefore this is not a significant measure of the relative fire hazard.
An American Society of Testing and Materials (ASTM) test method is used as follows to determine the flash point temperature: The test fuel is placed in a container and slowly and uniformly heated with its temperature monitored. Periodically as the temperature is increased, an ignition source is inserted into the container and this is repeated until the lowest temperature where a flash (light) occurs is identified. This temperature is defined as the flash point temperature. Since this ASTM test is conducted at ambient conditions, the amount of oxygen present is fixed and the ignition source is specified. As the pressure in the fuel tank is reduced during aircraft ascent, the effective flash point temperature is lowered. In the case of TWA-800 at 13,800 ft., the effective flash point of the fuel was lowered by about 15° F. Some fuel at sea level may even be non-flammable but at altitude may become flammable.
As discussed at the recent International Aircraft Fire Protection and Mishap Investigation Course in Dayton, Ohio, JP-8 at a specific temperature above its flash point temperature is just as flammable as JP-4 at the same delta temperature above its flash point. This was the TWA-800 plight. The safety advantage for JP-8 over JP-4 is that the ambient temperature and fuel operational temperatures are generally below the flash point of JP-8.
Another factor that must be considered is that of fuel spray or mist. An object such as a projectile, compressor blade or crash debris impacting on a fuel tank or line can generate a flammable fuel spray or mist even if the fuel temperature is below its flash point temperature. Aircraft generated fuel slosh and vibration can also produce a flammable spray. A fuel thought to be non-flammable may be rendered flammable due to dynamic non-equilibrium conditions.
In the future, aircraft engines will become more efficient. This will be accomplished in part by higher engine operating temperatures. More fuel-efficient engines will result in less fuel flow for component cooling. At the same time the demand for cooling of electronic components on both military and commercial aircraft will increase. The result will be higher fuel temperatures with an increased probability of a lower volatility fuel such as JP-8 becoming flammable.
In conclusion, the Air Force change from JP-4 to JP-8 was a positive step for aircraft safety but not to the point that fuel fire/explosion safety features such as anti-static additive (ASA), fuel tank inerting, explosion suppressant foam, bonding and grounding, and the complete elimination of ignition sources in and around fuel systems can be abandoned. Both the Air Force and commercial aircraft operators would prefer a fuel like JP-5 with a flash point above 140° F, but JP-5 fuel availability and cost is prohibitive. Although Navy aircraft currently operate on JP-5 fuel, the Navy still expends significant effort addressing aircraft fire issues.
The motivation for the change to JP-8 fuel was to save lives and equipment while accomplishing the military mission in an effective manner. However, fire and explosion safety issues still remain which must be confronted.
Robert G. Clodfelter
6718 Pinewood Place
Dayton, Ohio
45459
937-435-8778
Mr. Robert G. Clodfelter is President of AFP Associates Inc. and has more than 37 years of aircraft and spacecraft fire protection and fuel system experience. Prior to forming AFP in 1990, Mr. Clodfelter was associated with the Air Force and Northrop Aircraft Inc. He has his MSME from Ohio State University and is a Registered Professional Engineer. He was Chief of the Air Force Aero Propulsion Laboratory's Fire Protection Branch with responsibility for all of the USAF aircraft fire and explosion research and development programs and related flight safety, system support, and survivability/vulnerability efforts. Mr. Clodfelter has provided on site fire pattern investigation support to many aircraft mishap boards and also has conducted System Safety Engineering Analysis (SSEA) of virtually all Air Force aircraft including Air Force One (The President's 747) and many Navy aircraft. He has published numerous technical documents and has developed a world wide reputation as an expert in the aircraft fire protection field. He was the major driving force behind the development of the On Board Inert Gas Generating System (OBIGGS) for aircraft fuel tank explosion protection. Mr. Clodfelter, was a major factor in the Air Force converting from JP-4 to JP-8 fuel and from MIL-H-5606 to MIL-H-83282 hydraulic fluid for fire safety reasons.
He recently participated in the National Research Council's workshop on "Fuels with Improved Fire Safety" and the FAA/SAE's "Transport Fuel Flammability Conference." He has recently investigated the TWA 800 and ValuJet 592 accidents and assisted the Air Force/Boeing in the design of the AirBorne Laser (ABL) aircraft fire protection system. Mr. Clodfelter is the coordinator and core lecturer for the International Aircraft Fire Protection and Mishap Investigation Course held annually in Dayton, Ohio.