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Sekkha

Vnav Early Descent within 50 NM of TOD - 744

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Hi all,

I was wondering if I just remember wrongly or if it is not working correctly:

When on a Vnav Early Descent (started within 50NM of TOD)  upon capturing the Idle descent path the AP pitch mode changes to VNAV PATH as expected. However the Thrust mode stays in HOLD letting the speed build up. I was expecting an automatic switch to IDLE followed by HOLD.  I saw that 2 times now since the latest update....So not sure, if this is supposed to be like this ?

thanks for any clarification

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I also experienced the same scenario on one of my first flights in the B744 after the update.  I normally set my descent at M.86/330 but soon watched as the plane accelerated toward M.90 while on VNAV profile descent.  The same cause:  the throttles did not reduce to idle.  I had to manually intervene to reduce pitch and add spoilers to get back to target speed.  After that intervention, I re-engaged VNAV and everything behaved normally for the rest of the descent.  But I did not have that issue on the last flight and everything was set up the same.  The throttles reduced to idle as expected without incident. So...?

Craig


Craig Williams

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Verify your throttle option is set to over ride in hold only.  I always manually move my throttles to idle before TOD.  I've flown early descent with the 748 several times and never had a problem so I am suspecting your throttles are involved.


Dan Downs KCRP

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7 minutes ago, downscc said:

Verify your throttle option is set to over ride in hold only.  I always manually move my throttles to idle before TOD.  I've flown early descent with the 748 several times and never had a problem so I am suspecting your throttles are involved.

Not the -8.....its about the 744

My throttles are Ok. When reaching TOD in cruise thrust goes to IDLE as it should. Not in early descent however....

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2 minutes ago, Sekkha said:

Not the -8.....its about the 744

My throttles are Ok. When reaching TOD in cruise thrust goes to IDLE as it should. Not in early descent however....

During early descent you will still have enough thrust to maintain about a 1000 fpm descent until intercepting the VNAV profile.  I am probably not understanding your problem statement... you said the thrust mode remains in hold but I'm not sure of when this is.  What is the thrust mode while in early descent?  Does it change to hold when intercepting the VNAV path?  Are you saying the throttles do not retard on reaching the VNAV path?  Granted, I don't recall doing an early descent in the 744 and there is a difference in how each handles VNAV descents.


Dan Downs KCRP

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sekkha,

I would like you to check again the following please:

See if on early DES after you pushed the ALT knob that the airplanes autothrottles go into throttle hold (THR HOLD)
mode on reaching the 1,250 fpm descent rate to permit the pilot to readjust the throttles to change the rate of descent. On intersecting the original descent path, the vertical guidance function captures the computed vertical path for the selected mode of descent and continues the descent in VNAV PTH.

PMDG on the 400 did a legacy FMC (today you can find 2 FMCs on the 400, legacy and NG (new generation).

It should work as advertised, also check the FCOM VOL2 DESCEND portion.

If is not working as described something  definitely went wrong.

I cannot check anything before tomorrow night, but there are lots of people that can check this issue very easy not so complicated.   

Edited by killthespam

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50 minutes ago, downscc said:

 I am probably not understanding your problem statement... you said the thrust mode remains in hold but I'm not sure of when this is.  What is the thrust mode while in early descent?  Does it change to hold when intercepting the VNAV path?  Are you saying the throttles do not retard on reaching the VNAV path?  Granted, I don't recall doing an early descent in the 744 and there is a difference in how each handles VNAV descents.

When starting the early descent within 50NM of TOD thrust mode switches from SPD to THR followed by HOLD providing exactly what you said, a reduced vertical speed descent. But when intercepting the Idle descent path it does not switch to IDLE as I am expecting (and what it does when reaching the TOD while in cruise) . It remains in HOLD while pitch mode changes from VNAV SPEED to VNAV PATH. So speeds build up unless you retard the throttles manually..... 

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26 minutes ago, killthespam said:

sekkha,

I would like you to check again the following please:

See if on early DES after you pushed the ALT knob that the airplanes autothrottles go into throttle hold (THR HOLD)
mode on reaching the 1,250 fpm descent rate to permit the pilot to readjust the throttles to change the rate of descent. On intersecting the original descent path, the vertical guidance function captures the computed vertical path for the selected mode of descent and continues the descent in VNAV PTH.

PMDG on the 400 did a legacy FMC (today you can find 2 FMCs on the 400, legacy and NG (new generation).

It should work as advertised, also check the FCOM VOL2 DESCEND portion.

If is not working as described something  definitely went wrong.

I cannot check anything before tomorrow night, but there are lots of people that can check this issue very easy not so complicated.   

Alex, all that works as expected.... But my point is unanswered.  Shouldnt  thrust mode switch to IDLE followed by HOLD upon capturing the computed idle descent path ?? The aircraft captures this path correctly which is announced by the change to VNAV PATH..

If my memory doesnt fail (what could of course be:)  before the update it did this automatic switch...And I did dozens of  early descent. I nearly always do that ....

 

Edited by Sekkha

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33 minutes ago, Sekkha said:

But when intercepting the Idle descent path it does not switch to IDLE as I am expecting (and what it does when reaching the TOD while in cruise) . It remains in HOLD while pitch mode changes from VNAV SPEED to VNAV PATH. So speeds build up unless you retard the throttles manually

That is a good find.... you are correct in that it should retard the throttles when intercepting the VNAV path.  Please create a trouble ticket on this so it gets in the bug tracking system.


Dan Downs KCRP

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1 hour ago, downscc said:

That is a good find.... you are correct in that it should retard the throttles when intercepting the VNAV path.  Please create a trouble ticket on this so it gets in the bug tracking system.

thanks for confirming that

ticket filed

Edited by Sekkha

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3 hours ago, Sekkha said:

  Shouldnt  thrust mode switch to IDLE followed by HOLD upon capturing the computed idle descent path ??

Yes!

I understand you already filed the problem?

Something strange happened during update, not the first time. No matter how much attention is dedicated sometimes some issue will pop up.

Unfortunately every developer will have this problem.

Keep up the good work.


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