That's correct, Speedy, we regularly run up against fuel temperature limits on the SYD-JNB route, depending on just how far south we fly. The B744 operations manual requires us to maintain a fuel temperature at least 3 degrees above the applicable fuel freeze point, and at -37C (3C above the -40C freeze point of Jet-A), an EICAS advisory 'FUEL TEMP LOW' is displayed.
Most of the ports we fly into refuel us with Jet-A1, which contains an icing inhibitor and has a lower freeze point of -47C. As such, provided the aeroplane has refuelled with Jet-A1, we can disregard the EICAS advisory and allow the fuel temperature to further decrease to -44C. However, the USA almost exclusively uses Jet-A (-40C freeze), and so if the aeroplane has refuelled in the US within the last 3 sectors, we need to consider all of our fuel as Jet A, and respect its higher freeze point.
The primary determinant of our fuel temperature is the TAT (total air temperature), which includes the friction heating effect of the airflow over the aeroplane. The fuel temperature will converge with the TAT over time, generally at a rate of 3deg/hr (up to 12 deg in 'extreme' conditions). As such, to manage our fuel temperature requires us to manage our TAT. We can increase the TAT by:
- climbing or descending into a warmer air mass
- re-routing to a warmer air mass
- increasing our Mach number (each M0.01 increase = ~0.5-0.7C TAT increase)
Obviously all well and good when fuel is surplus, but on long, limiting sectors such as SYD-JNB, there is little leeway in which to descend into warmer air (and away from optimum altitudes), or to re-route.
Another element on those long, southerly flights is the fact that the OAT can approach the limit of the aeroplane's 'environmental envelope' (around -70C at cruise altitudes), also requiring a descent or re-routing to warmer air.
As for the message described by Grahame, we don't send them from the flight deck.
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