Not knowing the details of the Trent/777 installation -
The HP fuel pump is typically sized to pump fuel for the SL takeoff condition, which means it is oversized in cruise, and VERY oversized in descent. It being a positive-displacement pump, the excess fuel must be routed somewhere, and depending on where the excess is dumped, it can either create an unwanted thermal runaway, or else solve an overcooled condition.
Or did I read that the RR HP pump is a variable-displacement type, in which case the above is null and void?
