To M.Mouse:
The final report will show whether any statement I have made is right or wrong.
Fair enough, patience is a virtue.
To FullWings:
About the only time you'd think of retarding a thrust lever when you're undershooting, in an attempt to 'cure' an engine problem, would be if it was surging badly - and this is not the case with BA38.
Hindsight has 20/20 vision.
In the case of BA038, the crew were confronted with non responsive engines. They had no time to accurately analyse their engine problems. According to the AAIB reports there were no alerts or EICAS procedures to assist the crew. Could very well be that they perceived engine surging to be their problem at hand at the time.
. . .the only thing you can influence is the mode they operate in - the RR Trent has a 'hard alternate' mode you can select from the flight deck that references N1 instead of EPR and removes some of the thrust (overboost) protection.
I am aware of how the system works, including the RR configuration.
In order to select the EEC in the 'hard alternate' mode the pilot has to retard the thrust levers before manually selecting the guarded EEC switch to the 'hard alternate mode' (manual selection is referred to as the 'hard alternate' mode, automatic selection is referred to as the 'soft alternate' mode).
Retarding the thrust levers before selecting the 'hard alternate' mode is necessary because there is no thrust limit protection in the alternate modes (in case of the BA038 situation the thrust levers would have been fire-walled). If the crew had decided to select the 'hard alternate' mode (regardless of an EEC fault or not) they would have retarded the thrust levers before doing so.
To do this, the action is generally triggered by an EEC fault which initially results in a (automatic) 'soft alternate' mode and an EICAS advisory message/procedure. A 'hard alternate' mode selection is the next step in order to acquire a [boxed] N1 reference indication on the affected engine.
Since the AAIB reports explain there is no evidence of EEC faults, hence no alerts such as EEC ALTN light or EICAS messages, logically there would have been no reason for the crew to act and select the 'hard alternate' mode. The only remaining scenario would have been if the crew decided to select the 'hard alternate' mode at their own discretion in an attempt to "wake up" the engines as a last resort. In this scenario they could have retarded the thrust levers to idle although it would not have been necessary to retard the thrust levers that far back.
I stress this probably did not occur or the AAIB would have mentioned it in one of their released bulletins but it could have been a scenario as a last resort attempt to avert premature contact with terra firma in that final minute. If it did occur, it could have resulted in engines reverting to minimum idle if flaps were retracted to less than landing position and EAI not ON. But based on the information that M. Mouse provided (he must have first hand information to back his claim), minimum idle can apparently be illiminated as a factor.
The only way I know to 'reset' the Trent engine is to momentarily put the fuel control switch into cutoff and hope that it will accelerate up again; even the manual says it could take a loooonnng time. Not a technique I'd apply at 700'.
It would have been risky to cycle the spar valves (by selecting fuel control switches from run-to cutoff-to run) which could result in jammed spar valve actuators which would certainly have resulted in fuel flow restrictions.
And it helped, did it?
No but in the BA038 scenario, if this action took place, it didn't either.
Regards,
Green-dot