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VNAV - Descent with below altitude constraint waypoint

Started by Cbf, Sat, 4 Mar 2017 21:12

Cbf

Hi Hardy,
When there is a waypoint with altitude below constraint during descent, ATC (and CPDLC) robot seems to consider such waypoint as an above constraint. It sometimes lead to be well above the VNAV trajectory. To avoid this, I cancel the below constraint manually in the FMC but I think that it is not the way to proceed...
Tango24 topic, a few weeks ago, seems to point similiar configuration.
Thank's for the huge job (PSX & upgrade).
Cbf

Hardy Heinlin

Hi Cbf,

this shouldn't happen if you use PSX 10.1.1-beta11 or higher.

I just checked a CPDLC descent clearance while the active constraint was FL220AFL250B. It cleared me to FL240 -- that's perfectly within the constraint window.


Regards,

|-|ardy

Cbf

Hi Hardy,
My version was beta 10.1.1-Beta26. When I perform a flight, I always record a situ 20NM from TOD in order to fly again the end of the flight  ;).
This flight was LHBP to LROP, STAR TOSVI 3X - ILS 26R, options LNAV CRZ & LNAV APP checked. During this STAR, TOSVI constraint is FL170B. UVALU constraint, located 12NM after, is FL070A. I will fly it again. I just have a question. If I load the situ, does "Talk to us" option uncheck/check modify ATC robot scenario (descent clearance...)?
Thanks for support and software continuous enhancement :)
Cbf

Hardy Heinlin

I misunderstood your question. You mean just "B", not "AB" ("Tango24 topic" was about AB).

Voice-ATC has always a mix of random elements and route dependent elements, no matter if you cycle it off/on.


|-|ardy

Cbf

Hardy,
Nearly TOD, ATC cleared me to descent down to FL170. As next constraints were FL170B/FL070A, I expected a clearance lower than FL170.
Cbf

Hardy Heinlin

Just for your info: The active constraint is not always the target constraint. If there is a free corridor of ca. 3° to the next constraint, i.e. if the active constraint doesn't intersect this corridor, then the target altitude is the next constraint, not the active one displayed on the VNAV DES page.

Anyway, if ATC keeps you high, you need to apply speedbrakes thereafter. This may happen in real life as well.


Regards,

|-|ardy (having a break now after 2 months of non-stop work)

Cbf

Hardy,
Thanks for information.
Do not forget to arm speed break and set Autobreak to max ;)
Cbf

tango4

Quote from: Hardy Heinlin on Mon,  6 Mar 2017 18:46
Anyway, if ATC keeps you high, you need to apply speedbrakes thereafter. This may happen in real life as well.


NEVER. ATC is always spot on. And I also hear that air traffic controllers are all nice and handsome.
And if someone tells you otherwise, he is probably a pilot...