gerryfoley,
I understand that the engines were idling when a command to increase thrust produced a brief increase followed by reduction in both engines.
Not quite. According to the AAIB, BA38 had complied with BA stabilised approach criteria, i.e. it was in the landing config. by 1,000'AAL with approach power set. As there was some wind that day, the power would have been varying around the normal datum during gusts and lulls; at around 700' one engine stopped responding to commands and ran down to a low EPR, followed seven seconds later by the other one.
Has it been reported if the engines were reduced to idle followed by a successful command for increased thrust at any earlier point in the descent? Or was each thrust reduction to a low level without in intervening idle period?
Following the normal LHR speed control involves staged reductions from 220Kts-->180Kts-->160Kts-->Vapp. The last two or three reductions usually occur after being established on the GS and for a 777 on an average day you need to have the engines at idle plus some drag from gear or speedbrake to make the change happen. I don't think the actual thrust levels during the intermediate/final approach have been reported but I would be surprised if the engines weren't at flight idle for a significant time before the power came up prior to the 1,000' approach 'gate'.
777fly,
...There is no need to reproduce the accident flight profile, as it has been flown many times without incident already.
Without
incident, defined as a discovered anomaly, yes. What we don't know is how many times there has been a restriction in the fuel supply system that has gone unnoticed because it has not been severe enough to affect the engines.
If it was, in fact, quite common, how would we know about it? Imagine a blockage forming that only became evident over 90% of rated power: how often do we use full TOGA power after starting the descent? Not very often, IMHO. If the restriction reduced the delivery capacity of the fuel system from 200% of maximum to 110% it would remain undiscoverable by normal means but still exist...
