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Tony747-400

Question on VNAV Descent Speed

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Hi Guys. I am descending at a speed of 267kt (VNAV descent speed) in VNAV to a waypoint that has a height restriction of 4000ft, I am then asked by ATC to level off at say 8000ft,my speed starts to decrease to minium clean speed approx 200kts. When I am able to continue the descent, the speed then increases back to the FMC descent speed of 267kts. If you are stepped down to different levels by ATC is this the correct behaviour of the aircraft to keep slowing down then speeding up? I have looked in the manuals cant seem to see anything about it. Thanks.Tony


Tony Simpson

 

FLYING FROM EGKK, The worlds busiest single runway Airport.

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Yes. Descending in VNAV PATH mode will cause the aircraft to slow to below the planned speed in order to try and keep the right energy level for when you are cleared lower. Descending in VNAV SPEED mode would, however try to keep the target speed using thrust. I'm assuming you were in PATH mode. Sometimes even doing this won't be enough - ie. if ATC keeps you at an intermediate altitude for a long time on the way down, the aircraft will ultimately have to add thrust (and therefore energy) to stay at minimum clean speed. Which will mean a hectic dive back down to the profile, a speed up to above target speed, and a 'Drag Required' FMC message when you do finally get clearance. Again, it's important to differentiate between a VNAV PATH descent (the aircraft will pitch to maintain a pre-computed path), and VNAV SPEED mode (the aircraft will hold the target speed in the FMC with pitch). Note: I've seen some variations of the above on different individual aircraft (company option?) And the -300/400 seems to do this more consistently than the -800. But as far as I know, the above should be correct.


Simon Holderness

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Hi Simon. Thanks for the reply Yes I was in VNAV PATH mode not SPEED. Always been more of the tricky subjects for me VNAV Descent,but getting my head around it. Thanks for the detailed description. Tony


Tony Simpson

 

FLYING FROM EGKK, The worlds busiest single runway Airport.

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Hi Guys. I am descending at a speed of 267kt (VNAV descent speed) in VNAV to a waypoint that has a height restriction of 4000ft, I am then asked by ATC to level off at say 8000ft,my speed starts to decrease to minium clean speed approx 200kts. When I am able to continue the descent, the speed then increases back to the FMC descent speed of 267kts. If you are stepped down to different levels by ATC is this the correct behaviour of the aircraft to keep slowing down then speeding up? I have looked in the manuals cant seem to see anything about it. Thanks.Tony
You should never be above 250kts when you are at or below 10,000msl. so 267kts becomes irrelevant at 8,000. Mitch Bowman

Mitch Brown

Private Pilot | Aerospace Engineering Major

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You should never be above 250kts when you are at or below 10,000msl. so 267kts becomes irrelevant at 8,000. Mitch Bowman
Never say never. Outside the U.S. FAR 91.117 does not apply. Flights can (and are) cleared to higher speeds under 10,000 in different airspaces. Also, within FAA regulated space, part (d) of the aforementioned FAR can be actioned : (d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.

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Never say never. Outside the U.S. FAR 91.117 does not apply. Flights can (and are) cleared to higher speeds under 10,000 in different airspaces. Also, within FAA regulated space, part (d) of the aforementioned FAR can be actioned : (d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.
I do all my flying in the US so i didn't even think twice about that

Mitch Brown

Private Pilot | Aerospace Engineering Major

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