747-400 autopilot flight level change
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In broad terms it'll maintain whatever speed its been told to do and vary the rate of climb/descent accordingly. Taking power off in the climb leads to a reduced rate of climb, adding power in the descent reduces the rate of descent.
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Join Date: Mar 2004
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If it's the same as the 757, FLCH engages in FLCH SPD. The pitch controls the speed, the AT does some clever maths and works out whether it needs a bootload of thrust (for a big climb) or a tiny bit (for a 1000' climb etc.).
If you move the throttle, the mode changes to THRHLD SPD, and the AT "lets go" of the thrust levers. You are then (as mentioned) controlling the ROC/ROD
NW3
If you move the throttle, the mode changes to THRHLD SPD, and the AT "lets go" of the thrust levers. You are then (as mentioned) controlling the ROC/ROD
NW3
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Unless there are other minor differences on type, and assuming AP is on:
1. FLCH in B744 will eventually result in idle power descend.
2. The pitch controls the speed and the AT is at IDLE then HOLD.
3. AT in HOLD mode means the pilot can adjust the throttles freely.
Therefore increasing the thrust will add power to the descend and the AP will react by reducing the pitch and thus the rate of descend.
RC 123
1. FLCH in B744 will eventually result in idle power descend.
2. The pitch controls the speed and the AT is at IDLE then HOLD.
3. AT in HOLD mode means the pilot can adjust the throttles freely.
Therefore increasing the thrust will add power to the descend and the AP will react by reducing the pitch and thus the rate of descend.
RC 123
RC123
Not quite!! You won't ever see IDLE in FLCH!
IDLE, when annunciated, is a specific FMC computed power value which normally equates to the throttles closed position - it is a function of VNAV.
In FLCH, when descending, the annunciations are THR, while the throttles are closing, then HOLD, which allows you to manually intervene, if required.
Not quite!! You won't ever see IDLE in FLCH!
IDLE, when annunciated, is a specific FMC computed power value which normally equates to the throttles closed position - it is a function of VNAV.
In FLCH, when descending, the annunciations are THR, while the throttles are closing, then HOLD, which allows you to manually intervene, if required.
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I believe that selecting FLCH will also make the 744 try to make the new altitude in 2 minutes minimum, so if you only need to descend 1000 feet, it will initially give you thrust to descend 500 fpm, then go to hold mode again to give you control of the descent rate again.
The '2 minute' function is part of the pitch/power program and, obviously, can't hack it for the large changes. What it does do, however, is to continuously compute the discrepancy and adjust as necessary, when able. The usefulness ..? gives you an asymptotic capture - non-overshoot. in other words.
