Speed constraint ignored on departure

[Chris Kilroy]

Just departed PANC. Runway 33, NOEND FOUR departure. There's a MAX 230KT restriction at LIFFE which was indicated correctly in the box as 230/3000A. As soon as flaps were cleaned up enough to allow it, the speed bug happily jumped up to 263KT prior to reaching EGKAJ. Had to speed intervene so as not to blow through the speed restriction.

Flew out of VHHH on the previous flight on the OCEAN2B departure (which also has a couple of speed constraints) and all were obeyed normally.

Running the latest PSX 10.76.

https://flightaware.com/resources/airport/ANC/DP/NOEND+FOUR+(RNAV)/pdf

[Hardy Heinlin]

Are you flying from PANC to PANC? If so, enter a different DEST airport just for a test and check again. EGKAJ should then have 230 in small font.

[Chris Kilroy]

Are you flying from PANC to PANC? If so, enter a different DEST airport just for a test and check again. EGKAJ should then have 230 in small font.

Nope, sorry. PANC to KPDX. I did notice as the issue was occurring that EGKAJ had 263 in small font followed by 230/3000A (in big font, of course) at LIFFE.

NOEND4 NOEND LAIRE BKA PRYCE TOU KEIKO BTG

[Hardy Heinlin]

Do you have a situ file?

[Chris Kilroy]

Do you have a situ file?

Just tried to replicate it after landing in Portland and can't make it happen again. EGKAJ still shows 263/ in small text on the ground but once airborne and in the left turn when EGKAJ becomes active it changes to 230/ and the speed limit is properly applied. Must have been a one-off. Thanks, Hardy.

[Hardy Heinlin]

Actually, the DEST itself is not relevant; the problem occurs when the ratio between total distance and CRZ ALT is too small. When a certain ratio is exceeded, a waypoint speed limit may lie beyond the predicted CLB phase; and when it lies within the CRZ phase, it will be ignored as usual. I guess I'll have to tune the factors a little bit. Automatic flight phase detection is always a tricky task, also in the real FMC. Increasing the CLB phase limits a little bit may also increase the risk of interpreting DES phase constraints as CLB constraints. And increasing the DES phase limits a little bit may also increase the risk of interpreting CLB phase constraints as DES constraints ...

(When you fly a SID with an open end for training without any reasonable arrival legs, and the problem occurs, then just add some legs or decrease the CRZ ALT.)

The point is that the FMC first calculates an approximate descend path backwards from the last waypoint or DEST airport. When this approximation is too conservative (too shallow) and the CRZ ALT is too high, it will intersect the predicted CLB path. And waypoint speed limits beyond this intersection won't be interpreted as CLB speed limits.


|-|ardy

[Hardy Heinlin]

The above problem is now solved in PSX 10.77:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy