Hi, max. fuel temperature in the AFM or our aircrafts (Citation CJ1+/CJ2+) is described as +57 degrees Celsius. Given the fact that the flashing point of Jet A1 is at +38 degrees, I was wondering why max. fuel temperature is not also set at +38.
The flash point is the lowest temperature where the liquid gives off vapours that can be ignited by an ignition source. So even if the fuel temperature is above the flash point, it will not ignite by itself. That won't occur until above 200 degrees Celsius.
"… max. fuel temperature in the AFM … +57 degrees Celsius. … the flashing point of Jet A1 is at +38 degrees, … why max. fuel temperature is not also set at +38 [?] ...
See below: flash point temperatures not a reliable guide, certificated ... based on the assumption that fuel tanks are considered to always be flammable
From NTSB's report on the ullage deflagration aboard TWA800 /17Jul96, AAR-00/03, PDF
"1.16.5 Jet A Fuel Vapor and Fuel Tank Research and Testing"
The FAA has certificated commercial jet airplanes based on the assumption that fuel tanks are considered to always be flammable and has required manufacturers to demonstrate that any energy inside the tank would be below the MIE for Jet A vapors to ensure safe operation of the airplane. However, at the time of this accident, specific information about fuel tank flammability and Jet A thermochemistry was largely unavailable. The research conducted during this investigation enabled analysis of the flammability of aircraft fuel tanks and provided a scientific basis for evaluating methods of reducing fuel tank flammability ...
From AAR pg 126:
Examination of the temperature data collected during the emulation flight test indicated that the highest ullage temperature measured within the CWT was 145° F and that it occurred in the left mid bay just before the airplane began to taxi for takeoff. Examination of the temperature data also indicated that the highest ullage temperature measured at 13,700 feet msl was 127° F …
From AAR pg 132:
"… flammability research focused on defining the conditions at which Jet A fuel vapor was found to be flammable. A precision test fixture was developed to generate fuel vapors over a large range of temperature and pressure conditions, and an electronic spark system was developed to ignite the vapors while accurately measuring the ignition energy supplied. Hundreds of tests were performed to determine the flammability limits of the fuel vapor, as a function of ignition energy, fuel temperature, pressure (to simulate altitude), fuel mass loading, and fuel weathering….
… explosion dynamics experts from CIT and the Safety Board determined the following: The flammability limits of Jet A fuel are variable and depend (at least) on ignition energy level, temperature, pressure, and mass loading. The magnitude of the ignition energy of the fuel vapor for the accident airplane's conditions (50 gallons of Jet A fuel in the CWT at a pressure equivalent to 13,800 feet msl) is estimated to vary from 0.5 J at 104° F to less than 0.5 mJ at 122° F…. [Joules to milliJoules]
From AAR pg 133:
Ignition of Jet A vapors (at sea level) can occur at temperatures significantly below their flash point temperatures. A survey of Jet A fuels used in the current research indicated that fuel/air mass … range from 0.0354 to 0.0488, which indicates that the flash point temperature of the fuel would be higher than its temperature at the LFL. Therefore, flash point temperatures are not a reliable guide for assessing the explosion hazards of Jet A fuels. Flammability tests in vessels or tubes with the appropriate mass loading factor, ignition source, test and analysis procedures, and instrumentation must be used to determine flammability limits."
[LFL= lower flammability limit]
The flash point is the lowest temperature at which the fuel vapor can burn in the presence of a continuous ignition source. If the ignition source is removed, the fuel stops burning.
The fire point is the lowest temperature at which the fuel vapor will continue to burn after the ignition source is removed...much higher than the flash point.
Both of these are specification standards; they are used to characterize a fuel's chemistry and behavior, not necessarily as specific safety thresholds. However, a higher flash point is used to distinguish a greater safety factor in storage and fuel delivery. The US Navy uses JP-5 for shipborne aviation because it has a flash point of around 62C.
Neither of these temperatures is related to the auto-ignition temperature, at which the fuel will spontaneously ignite and burn. This is generally much higher than the fire point.
One other lesson for line pilots is that the hazard is NOT the liquid fuel in your hotest tank (full of liquid is the safest): the hazard is the "ullage" -- the fuel-air vapor, especially in the near-empty tank (just above the Boeing's Heat Exchangers).
Can you specify that? You mean that above the max. fuel temp. the heat exchange fuel/oil doesn´t work properly?
Yes, think about it. If you had 65 degree fuel (granted, hard to get to but for an example) there is not going to be much cooling of the oil. Sixty five would be the lowest temperature but there is not a 100 % transfer from the oil to the fuel so the oil temperature will remain higher.