B737 Fuel Temperature Guage

[POOKEY]

IS THE TEMP INDICATOR A TRUE INDICATION OF THE FUEL TEMP ?
I WOULD SUSPECT THAT FUEL IN THE MAIN TANKS IS WARMER THAN THAT IN THE CENTRE TANK DUE TO AERODYNAMIC HEATING, OR WOULD THE CENTRE TANK BE WARMER THAN THE MAIN TANKS BECUASE OF THE A/C PACKS LOCATION IN RELATION TO THE CTR TANK ?

[LEM]

Hi :yuk: , both of the factors you mention are certainly true, but it's a bit difficult to establish which has the greater influence over the fuel temp. Maybe some engineer can answer that.
Anyways, they put a temp. probe in the left tank because that's supposed to be the coldest, due to the smaller A Hyd System heat exchanger in it.
Not a bad question, and I also wonder if the temperature difference between the two main tanks can be really significant (personally don't think so).

[Laina Reeves]

What does the flight crew use the fuel temp gage for? If the temp is out of limits (too low or too high temperature of the fuel) does it result in restricting the flight until the temp is within limits?

[BOAC]

LR - two reasons come to mind straightaway:
NB Assuming the gauge is accurate!!

1) There is a lower limit on the 'freeze point' of aviation fuel (typically -47deg C) and this could (rarely) be a problem.

2) It is a good clue as to whether the wings will accumate ice on a turn-round due to condensation on a cold-soaked surface

I guess on Concorde (RIP) there was a heating consideration?

[idg]

Just heard about a presentation by Airbus re a software flight planning tool that predicts fuel temps in the tanks of the 340-500/600 for the trans polar routes. This can be built in to the CFP to give an accurate prediction of where the tank temps will be at their coldest and how to get around the problem.

Apparently going from HKG/SIN/DXB etc to the USA is no problem because the fuel being used is Jet A1, but on the return leg with Jet A in the tanks there can be some restrictions. Interestingly one technique (as I understand it) is to keep some of the fuel in the centre (warmer..cos it has not been exposed to the cold outside air) tanks and then transfer it to the coldest wing tanks...typically the outboard mains...to keep them above the freeze point.

Another technique being used is to freeze an actual fuel sample of the fuel loaded and then ACARS the crew after departure with the actual freeze point which can be significantly different from spec so the crew can take appropriate action.

Apparently sometimes if the fuel were to get real cold, descending to try and get a higher OAT and thus more skin friction heating is not going to work because of a the cold air 'bubble' that is found in the middle altitudes over the poles.

So yes cold fuel is an issue in today's environment but only for the extra long range types.

[Laina Reeves]

Thank you, both of you, for your replies. That helps my understanding. Do you know if too high a fuel temp is also considered a problem, say, sitting on the ground in any hot climate? Could it prevent dispatching the aircraft until the OAT cools down?

[Herc Jerk]

+49C is the limitation for the 73NG.

The fuel temp gauge may be inop for dispatch provided TAT (or RAT) is substituted for fuel temp and therefore fuel temp limitations.

So IMHO if the fuel is hotter than +49C then its a bloody good day for the beach...

HJ