airbusbatics,
Good questions!
The Aircraft Operating Manual that I use has slightly different
limits than the ones you quote, but for the moment let's disregard
that.
The Fuel Tank High Temperature 'caution' comes on when certain
actions need to be taken in an attempt to reduce the temperature of
the fuel. Some of these actions are positive and some are conditional.
For the A319/A320 there is a caution that states "This caution may
spuriously trigger due to interference from the communications
equipment. Therefore the flight crew should wait two minutes while
the fuel temperature is updated. After two minutes, if the ECAM
caution has not disappeared, the flight crew must apply the following
procedure." The A321 does not have that caution.
On all models the ECAM starts off with a positive action. The GALLEY
power supply is turned off in an attempt to reduce the generator
loads thereby reducing the heat emitted by the offending IDG and
thereby reducing the temperature of the fuel returning to the tanks.
The expanded ECAM then offers a conditional statement ... "on the
ground" or "in flight". Let's deal with the "in flight" situation as
it has the potential to be more serious.
On all models the first action is to disconnect the autothrust and
adjust the thrust lever to increase the fuel flow through the IDG oil
heat exchanger and decrease the temperature of the fuel returning to
the outer cell (A319/A320) or the wing tank (A321).
The second step is conditional and is actioned if the fuel temperature
rises above 65c in the outer cell, or 57c in the inner cell in the
A319/A320 or above 57c in the wing tank of the A321. That step is to
start the APU, if available, so as to allow the IDG on the affected
side to be disconnected.
The third step, which is also conditional, is to disconnect the
affected side IDG, but only if the opposite generator is available.
The first caution comes on at the lower temperatures to alert you
that certain items need to be accomplished in an attempt to keep the
fuel temperature from rising over the next limit which requires even
more action.
If the first actions are successful then the later actions will not
have to be accomplished. And please note the conditonal statement
regarding the diconnectiong of the offending IDG ... the opposite
generator must be available. Airbus always wants you on at least two
generators.