B744 Thrust Mode At Altitude Capture
Thread Starter
Joined: Feb 2004
Posts: 1,410
Likes: 37
From: Australia
B744 Thrust Mode At Altitude Capture
Scenario: 744 takes off without VNAV armed. At 1000', the A/P is engaged (in current TO mode). The aircraft reaches, say 5000' (MCP altitude).
Is there an automatic A/T mode change? e.g. SPD... or...
Does the A/T remain in HOLD or THR REF at ALT capture/hold (until, say, a flap limit is reached or does it overspeed?)
Thanks.
Rgds.
NSEU
Is there an automatic A/T mode change? e.g. SPD... or...
Does the A/T remain in HOLD or THR REF at ALT capture/hold (until, say, a flap limit is reached or does it overspeed?)
Thanks.
Rgds.
NSEU
Joined: Jun 2002
Posts: 50
Likes: 0
From: home
My understanding is that when the TOGA is pushed for T/O (prior to 50 kts) without VNAV armed the following events occur:
1. The thrust levers advance to the selected T/O thrust (THR REF) and the vertical mode will be TO/GA.
2. At 65 kts the autothrottle annunciates HOLD
3. At 400' the autothrottle again indicates THR REF
4. At the selected altitude, the vertical mode changes to ALT but the A/T remains in THR REF and the aircraft accelerates up to the flap placard speed or VMO as applicable.
Therefore SPD mode needs to be selected or alternatively, select FLCH SPD while still climbing.
Regards
csd
1. The thrust levers advance to the selected T/O thrust (THR REF) and the vertical mode will be TO/GA.
2. At 65 kts the autothrottle annunciates HOLD
3. At 400' the autothrottle again indicates THR REF
4. At the selected altitude, the vertical mode changes to ALT but the A/T remains in THR REF and the aircraft accelerates up to the flap placard speed or VMO as applicable.
Therefore SPD mode needs to be selected or alternatively, select FLCH SPD while still climbing.
Regards
csd
Joined: Apr 2003
Posts: 49
Likes: 0
From: UK
Agree with CSD........
The pitch mode for takeoff is initially TOGA. If VNAV is not armed then TOGA will remain the pitch mode until altitude capture indicated by ALT.
The A/T mode for takeoff is THR REF followed by HOLD. I think that without VNAV armed it will remain in HOLD even through 400'.
At altitude capture, the autothrottle will remain in HOLD and the aircraft will accelerate until a placard speed limit is reached.
To prevent this another A/T mode needs to be selected i.e. SPD if the altitude has already been captured of FLCH (which is a combined pitch/power mode) if still climbing.
The pitch mode for takeoff is initially TOGA. If VNAV is not armed then TOGA will remain the pitch mode until altitude capture indicated by ALT.
The A/T mode for takeoff is THR REF followed by HOLD. I think that without VNAV armed it will remain in HOLD even through 400'.
At altitude capture, the autothrottle will remain in HOLD and the aircraft will accelerate until a placard speed limit is reached.
To prevent this another A/T mode needs to be selected i.e. SPD if the altitude has already been captured of FLCH (which is a combined pitch/power mode) if still climbing.
Joined: Feb 2000
Posts: 1,993
Likes: 512
From: UK
We had a look at this in the sim some years ago and (allowing for 'wrinkly' memory lapses) I think the CSD version is correct.
Just one 'picky' point, the acceleration will be to 5 knots below configuration limiting..... it most certainly will not overspeed.
Irrelevant, maybe, but without VNAV being armed one supposes that FLCH will be engaged at the assigned thrust reduction point.
Just one 'picky' point, the acceleration will be to 5 knots below configuration limiting..... it most certainly will not overspeed.
Irrelevant, maybe, but without VNAV being armed one supposes that FLCH will be engaged at the assigned thrust reduction point.
Joined: Feb 2000
Posts: 1,993
Likes: 512
From: UK
Reasonably certain that the A/T FMA will remain at THR REF.
It's worth bearing in mind that the THR REF annunciation indicates the POTENTIAL thrust setting as distinct from a set value. Thrust can be set to any value up to, and including, the specific Reference Thrust, which could be, for example, TO1, CLB, CLB1 etc. What will be set, or adjusted to, will be dictated by the safe operating envelope. That reference value will alter with the various stages of flight and dependent on initial set-up inputs. What is active at any time is what appears at the top of the lower EICAS display.
In parallel with the above, it is worth noting that at least one, very expensive, computer-based training programme DOES NOT reflect real-life FMA's for non-procedural, ad hoc scenarios.
It's worth bearing in mind that the THR REF annunciation indicates the POTENTIAL thrust setting as distinct from a set value. Thrust can be set to any value up to, and including, the specific Reference Thrust, which could be, for example, TO1, CLB, CLB1 etc. What will be set, or adjusted to, will be dictated by the safe operating envelope. That reference value will alter with the various stages of flight and dependent on initial set-up inputs. What is active at any time is what appears at the top of the lower EICAS display.
In parallel with the above, it is worth noting that at least one, very expensive, computer-based training programme DOES NOT reflect real-life FMA's for non-procedural, ad hoc scenarios.
Joined: Jun 2002
Posts: 50
Likes: 0
From: home
Cornish Jack
I would have thought that it would have gone to THR. I've always thought that THR REF indicated that the thrust rating, as indicated on the upper EICAS, has been commanded.
This is just an educated guess and I may well be wrong though!
Regards
csd
I would have thought that it would have gone to THR. I've always thought that THR REF indicated that the thrust rating, as indicated on the upper EICAS, has been commanded.
This is just an educated guess and I may well be wrong though!
Regards
csd
Joined: Feb 2000
Posts: 1,993
Likes: 512
From: UK
AFAIK, the THR annunciation is only a product of the THRUST button first press during Go-Around. This changes the thrust mode from THR REF to THR and commands a RoC of 2000'/min. The differentiation is this specifying a rate of climb. THR REF isn't interested in RoC - just configuration limits and FMC programming.
In the scenario mooted, the non-standard configuration (no VNAV, no pilot input etc.) doesn't have a practical application.
In the scenario mooted, the non-standard configuration (no VNAV, no pilot input etc.) doesn't have a practical application.
