VNAV Stuck in DESCENT

[EGT]

Hi,

This is the 2nd/3rd consecutive flight without a restart. I diverted from the 1st flight due to fuel (ooops) and re-programmed the FMS with a new dest/route while airborne. All seemed to go well. It was direct to the field, with a visual runway centerline extension. The rest of the LEGS page was cleared out.

Aircraft was landed, re-fuelled, and re-programmed with a new route (after reset by switching the databases). PFD still showed the altitude constraint from the last arrival. Unable to clear.

Programmed the new route and added a custom PBD waypoint at LSK2L. I ended up with a waypoint sequencing problem (it wouldn't insert correctly) but managed to clear it and sent the FMS direct to the new waypoint (it was first on the route).

PFD now showed this altitude constraint as 1200 in magenta. It initially wanted to DESCEND when this altitude constraint was first added, and it set all subsequent waypoints to 1200 ft also (instead of computing a climb profile).

I changed it /1200A and all seemed well...

I took off and started to climb. Aircraft performance was such that the visual circuit of the valley was not required so proceeded direct on course (departure of out of BGBW RW24).

I went to engage VNAV but it said 112 kts, not the expected 250/280 kts of the climb profile.

Please see the current state of the FMS. Aircraft is currently in cruise at FL410 at M0.82.

https://imgur.com/a/Ejy63MN

Edited January 13, 2022 by EGT

[skiselkov]

Can you describe exact sequence of edits you did to set up the flight and the sequencing problem in more detail? I suspect the PBD waypoint got interpreted as a descent constraint (simply adding "1200A" isn't going to necessarily make it a climb constraint, the "^" or "v" arrow above that number is what determines the class of constraint). You can toggle the climb/descent constraint flag on a custom waypoint by putting "D" or "C" into the right side next to the altitude.

To elaborate a little bit, I suspect you had no SID, and only a single PBD waypoint that you inserted as a descent constraint (which is the default). Consequently, the FMC thinks the rest of the route to the next MISSED APPR will be a descent path.

Edited January 13, 2022 by skiselkov

[skiselkov]

Incorrectly inserted manual descent constraint at the start of the flight plan, the airplane is not allowed to climb after passing a descent constraint:

After inputting "C" on LSK 2R, the FMC correctly computes a continuation of the climb:

[EGT]

2 hours ago, skiselkov said:

Can you describe exact sequence of edits you did to set up the flight and the sequencing problem in more detail? I suspect the PBD waypoint got interpreted as a descent constraint (simply adding "1200A" isn't going to necessarily make it a climb constraint, the "^" or "v" arrow above that number is what determines the class of constraint). You can toggle the climb/descent constraint flag on a custom waypoint by putting "D" or "C" into the right side next to the altitude.

To elaborate a little bit, I suspect you had no SID, and only a single PBD waypoint that you inserted as a descent constraint (which is the default). Consequently, the FMC thinks the rest of the route to the next MISSED APPR will be a descent path.

Good to know!

Correct - no SID (there isn't one) but due to terrain there is a required visual climb around the valley until > 1200 ft. I entered a PBD waypoint for the waypoint at which you're supposed to climb above 1200 ft before continuing on course, and entered /1200 for the altitude there.

I was unaware of the climb/descent constraint logic or the fact it treats it as descent by default! Good to know.

Thanks!!!

Edited January 13, 2022 by EGT