Fuel Temperatures
Greetings!
It has been my observation that the fuel on my company 777-200ER a/c will cool to a temperature equivalent to TAT or 1-2 degrees cooler. I attribute this difference to the location of the TAT observation, which is just below the Captain side window, and the actual temperature of the boundary layer air. The increased local air velocity obviously results in a temperature 1-2 degrees less than TAT.
TAT is a great predictor of future fuel temp. Given enough time exposure, the fuel will cool to TAT or 1-2 degrees below. Having said this, flight during daylight hours will retard the cooling rate, and night flight will facilitate the cooling rate.
Attempts to warm the fuel by increasing Mach are inefficient, and you must first overcome the thermal inertia of cooling before fuel warming can occur, in the meantime your increased fuel burn exhausts contingency fuel. Once the fuel begins to warm, the rate will only be about 1/2 degree per hour, so it is best to avoid cold fuel in the first place.
In the USA we are fueled with Jet A, which has a Fuel Freeze spec of -40c. Elsewhere in the world Jet A-1 is standard which has a Fuel Freeze spec of -47c. Obviously Jet A-1 is preferable if flying in a very cold airmass region.
Dispatchers should be careful to closely analyze forecast airmass temperatures, and when -65c SAT or below is forecast, great care should be exercised as to routes and altitudes for flight planning.
Crews are well advised to track fuel temp, perhaps on your Airep form, thus permitting a high level of fuel temp awareness. Employ efforts to avoid cold fuel EARLY, for the reasons described above.
Cheers,
777 Guy (KDFW) Texas, USA