Fuel is managed differently
on the 747-400, 777, and MD-11, but in all cases, the wing main
fuel tanks are the last to deplete. On some models, fuel in tanks
with high surface-to-volume ratios is held until near the end of
a flight. Whether a tank is full or partially depleted of fuel alters
the rate at which the fuel temperature changes.
During long-range operations
at high altitudes, fuel tank temperatures can approach the freezing
point of fuel. On long flights, the fuel temperature tends to adjust
to the temperature of the aerodynamic boundary layer over the wing
skin. This boundary layer temperature is slightly lower than the
TAT because theoretical TAT is not achieved. Initially, TAT is much
lower than the fuel probe temperature because of the thermal lag
of the fuel. Thermal analysis of the 747-400, 777, and MD-11 airplanes
shows that the fuel tank temperature is driven more by TAT than
and procedures with low fuel temperatures.
In flight, a temperature differential must be maintained between
the observed temperature indication and the freezing point of the
fuel. For the 747-400, 777, and MD 11, the observed fuel temperature
must remain at least 3°C above the specified freezing point.
(The actual fuel freezing point may be used if known.)
When fuel temperature
decreases to 3°C above the freezing point, a message of FUEL
TEMP LOW displays in the 747-400 and 777 flight decks; the message
FUEL TEMP LO is displayed in the MD-11 flight deck. If this condition
is reached, the flight crew must take action, as described below,
to increase the TAT to avoid further fuel cooling.