B757 Descent Planning
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B757 Descent Planning
Further to the thread on landing a 757 recently posted on the main forum, what tricks do 757 drivers have on descent planning. As a recently qualified 757 Pilot, I still find the plannning of the descent the most tricky bit. I obviously program the FMC with all the Info, ie. speed/height constraints but not long into the descent the FMC produces a 'Drag Required' message. Any suggestions would be welcomed.
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Assuming that the 757 Flight Management Computer functions similarly to the 737, this may be of help. For a VNAV descent, the FMC assumes that at it's calculated TOD, the throttles, sorry, Thrust Levers, will be closed and the aircraft will descend continuously at the specified airspeed until reaching any preset speed/altitude constraints. If the W/V isn't as predicted, this can upset profile maintenance. Within certain limits, the FMC will command the AP to vary airspeed to maintain the required profile, but if, for example, it is unable to re-acquire when too high even with increased airspeed, then that blessed DRAG warning appears. HOWEVER, far more importantly, if ATC don't give you continuous descent, then that carefully calculated profile is buggered, big time! How often do you level off whilst frequency is changed and further descent sought?
Seems to me that whilst the VNAV profile is good for economical descent guidance, it's far better to control it with V/S and LVL/CHG and keep just a little bit in hand for the inevitable ATC interruption to descent. Happiness is found beneath the glideslope.
Seems to me that whilst the VNAV profile is good for economical descent guidance, it's far better to control it with V/S and LVL/CHG and keep just a little bit in hand for the inevitable ATC interruption to descent. Happiness is found beneath the glideslope.
Let the FMC do its thing, but monitor it using 3 X the height + 10 miles for slowing down; this is particularly useful when ATC hold the descent at intermediate levels. Timely use of the girly lever and speed management will hold the profile.
be careful out there.
be careful out there.
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That used to happen to me a lot, but I find that if you start the descent slight early you rarely get the DRAG REQUIRED message. About 5 miles before predicted TOD go to DESCENT page, push DESCEND NOW and EXEC. It will descent at about 1000fpm until it reaches the calculated path when it will go into IDLE and fly down the slope. It seems that if it intercepts the path in level flight it will overfly it a little which eventually cases the drag required situation. However, as was mentioned before ATC can spoil your day and unpredicted winds can but the winds seem to be much less of a problem doing it this way. And yes, keep the descent calculations going in your head at the same time, it helps a lot.
Hope that helps.
Lazlo
Hope that helps.
Lazlo
