Hi all,

 

Don't know what happened... Prepared for a routine flight. Everything set up as usual. FMC and flightplan set. Performance initialization complete.

 

Both flight directors on, A/T on, 35000 selected in the MCP, no altitude restrictions in the FMC. Takeoff as usual, no issues. After passing 10,000 I turned off the landing lights, runway turnoff lights and switched off the seatbelt sign. I then "left" the plane climb on its own in VNAV and LNAV mode, and I went for a lunch.

 

When I returned, the plane was flying level at FL280. No altitude restriction in the FMC, no nothing. The altitude selector was set to 35000. Why did it stop climbing at FL280, I have no idea. VNAV and LNAV were still on, they didn't disconnect. I then clicked the Altitude Intervention button, but the FMC responded "Unable Next Altitude" - don't really understand why.

 

The flight was from LPPD to LHBP as per the below flightplan:

 

LPPD BAVAS 4119N BANAL ARMED UM430 RETEN 4410N UN866 KORUL UN460 LEBRI OLERO UM616 DIDRU KUKOR UM729 TUROM UZ613 INTEG UL603 OBEDI UL605 STEIN UP978 VEBOS LHBP

 

When I returned to the "cockpit" the plane was approaching the the waypoing 4119N where it should have already been at FL350...

 

The payload was around 90% and I loaded 16496 kg of fuel. Weight is the only thing I can imagine to be a reason why it stopped climbing. Does anyone know what really happened?

 

The takeoff weight was approximately 74500 kg. But I did flights similar in both distance and payload and never experienced anything like this.

What was the outside air temp?

6K feet 9K feet 12K feet 18K feet 24K feet 30K feet 34K feet 39K feet 44K feet 49K feet

 

BAVAS

300@24(5) 301@24(2) 302@24(-6) 304@31(-19) 304@34(-32) 303@35(-47) 293@35(-52) 285@35(-56) 279@29(-53) 260@17(-58)

 

4119N

357@16(4) 359@16(2) 005@19(-9) 355@23(-22) 349@24(-35) 346@25(-48) 338@23(-51) 342@22(-52) 340@14(-51) 001@05(-54)

 

BANAL

206@07(4) 204@07(1) 207@07(-10) 211@10(-25) 232@13(-38) 235@21(-49) 236@23(-48) 240@18(-47) 241@16(-49) 236@10(-53)

 

Ok, so the OAT was something betwen -35 and -48, let's say -41.5 or so.

There have been incidents where this happened due to the outside temp in the sim being wrong. No matter what the calculations say, if the sim gets it wrong, you won't be climbing.

I can accept this as a reason as I see no other reason that might have caused the plane stop climbing. The strange thing is that it leveled at exactly 28K feet, not 27645 or 28517, but FL280. Just to add, however, when I switched to VS mode - leaving VNAV - and commanded to climb further, the NGX did climb.

I can accept this as a reason as I see no other reason that might have caused the plane stop climbing. The strange thing is that it leveled at exactly 28K feet, not 27645 or 28517, but FL280. Just to add, however, when I switched to VS mode - leaving VNAV - and commanded to climb further, the NGX did climb.

 

I've had the same thing happen a few times. It's very sporadic and I can't reproduce it reliably. In all cases though, the climb leveled off in VNAV prior to T/C, but always on a round number (28K, 29K, etc). If I go to FLCH or VS and start climbing again, then switch to VNAV, it continues the climb appropriately to my MCP altitude. Strange.

I've had the same thing happen a few times. It's very sporadic and I can't reproduce it reliably. In all cases though, the climb leveled off in VNAV prior to T/C, but always on a round number (28K, 29K, etc). If I go to FLCH or VS and start climbing again, then switch to VNAV, it continues the climb appropriately to my MCP altitude. Strange.

 

Good to know I am not the only one. This occassion was the first time for me though.

If you guys come up with a reproducible way to cause this, let us know and we'll look into it. A single random occurring is a needle-in-a-haystack for debugging though.

If you guys come up with a reproducible way to cause this, let us know and we'll look into it. A single random occurring is a needle-in-a-haystack for debugging though.

 

"A needle-in-a-haystack"

Well said, it is, indeed:-)

 

I don't know if it matters but I was using Active Sky Evolution for this particular flight and ASE was set to feed VATSIM weather. Unfortunately I cannot even describe how things led to leveling out at FL280 because I was not in front of my PC. But, even when pressing the Altitute Intervention button, the plane refused to climb, and the FMC sent the Unable Next Altitude message. The only way I could climb further was to select VS mode (and probably Flight Level Change could have worked too).

Hi,

 

I've had this several times where VNAV does not seem to be following the vertical profile with no altitude restrictions and a higher altitude set in the main window. I usually solve this by going into V/S mode or ALT HOLD for a sec and then straight back to VNAV. That seems to be a work around for me. It doesn't happen to me all the time though.

 

Ceri

did you notice what pitch mode (FMA) was showing on the PFD?

did you notice what pitch mode (FMA) was showing on the PFD?

 

I didn't, unfortunately:-(