Originally Posted by
pineteam
In managed descent, FMA is showing Thrust idle, if I go and pull OPN DES, the idle thrust will still be exactly the same
....
Right?
Cheers pin, well, no. Even if that is what the FCOM says, that is a bit of a
magenta moment.
The idle thrust is checked on E/WD EIS N1 indication where in-between it needs to say IDLE, on the old CFM the N1 is around 28-34 %. On the other hand what you normally get using DES if the FMS is loaded with winds properly, is a profile that is deliberately flatter (sooner TOD) than the gliding perfection. Once the "idle" VNAV path is attained, A/THR will indeed show THR IDLE but the N1 typically only reduce to 40-42%, for vertical rate around -2800 fpm.
Only when pulling OP DES as you suggest, the A/THR indication stays the same except now the idle command is finally sent to the FADECs, N1 drops and the airplane dives for -3200 / -4000 fpm in the Mach segment.
Guess this fills the blanks.
My belief is it is a feature not a bug, in line with the strategic logic pointed out in the post above. The ADOP seem to reduce the magintude of this behaviour, and its a fancy buzzword for "perhaps we had been overly conservative with the margins".
Remeber when early on type I could get aggravated a lot by not getting IDLE thrust when asked for
and indicated on the FMA. That would had been CFM/DAC, perhaps on the other installations it was not that pronounced. Surely I learned not to care that much about the concept.
Yet it still upsets me when above the profile in DES and reaching (but not yet) the fast edge of the managed speed range, the HAL would not set IDLE but insist on keeping those 38-42 N1.
Whenever the colleague in such situation reaches for the speedbrake - a sensible choice if only looking at the PFD - well, what do I say. Eversince The Burn I am not allowed the
Su’Kal scream.