Hi, in the last month, in many flights, I suffered of a sudden loss of altitude in cruise phase with PMDG B737NG.

For exemple, today I was flying from YPKU to YBMA at FL370 in VNAV PATH, and after an hour or more of quiet cruise, the aircraft suddenly started a fall at 5000 ft/m and more till reaching around FL320... The required altitude was showing FL370, the CMDA remained engaged an during the fall the FD was showing a required pitch up not followed by any autopilot action.
When the aircraft stopped the descent I tried to regaine the correct altitude using FLCH mode, and once in cruise level I inserted the ALTITUDE HOLD mode, but after some minutes, another dive at 6000 ft/m started. The aircraft loose thousend of feet anytime without trying to correct (the engines were at 50% N1) and ther were no failures or AP disconnections, neither changes in the required performance or altitudes... 

Any idea?

Thanks

Mauro

Clear air turbulence.

 

If it were simulated. 😉

4 hours ago, zetato said:

Hi, in the last month, in many flights, I suffered of a sudden loss of altitude in cruise phase with PMDG B737NG.

For exemple, today I was flying from YPKU to YBMA at FL370 in VNAV PATH, and after an hour or more of quiet cruise, the aircraft suddenly started a fall at 5000 ft/m and more till reaching around FL320... The required altitude was showing FL370, the CMDA remained engaged an during the fall the FD was showing a required pitch up not followed by any autopilot action.
When the aircraft stopped the descent I tried to regaine the correct altitude using FLCH mode, and once in cruise level I inserted the ALTITUDE HOLD mode, but after some minutes, another dive at 6000 ft/m started. The aircraft loose thousend of feet anytime without trying to correct (the engines were at 50% N1) and ther were no failures or AP disconnections, neither changes in the required performance or altitudes... 

Any idea?

Thanks

Mauro

What was the OAT?

it's a shame to discuss 2020 issues on the last Holy night before 2024.

We all uninstalled msfs 2020 already! 

Edited November 19, 20241 yr by Vitold69

"For exemple, today I was flying from YPKU to YBMA at FL370 in VNAV PATH, and after an hour or more of quiet cruise, the aircraft suddenly started a fall at 5000 ft/m and more till reaching around FL320.."

Do you always see the problem when nearing TOD?

What is your MCP altitude set at? The cruise level or a lower level ready for descent? I'm trying to establish the first /primary point of broken logic. Also is the CMD ap light still lit and what is showing on your pfd before and during this 'uncommanded' descent? 

VNAV SPD? VNAV PTH? 

6 hours ago, Vitold69 said:

it's a shame to discuss 2020 issues on the last Holy night before 2024.

We all uninstalled msfs 2020 already! 

😆

14 hours ago, zetato said:

Hi, in the last month, in many flights, I suffered of a sudden loss of altitude in cruise phase with PMDG B737NG.

For exemple, today I was flying from YPKU to YBMA at FL370 in VNAV PATH, and after an hour or more of quiet cruise, the aircraft suddenly started a fall at 5000 ft/m and more till reaching around FL320... The required altitude was showing FL370, the CMDA remained engaged an during the fall the FD was showing a required pitch up not followed by any autopilot action.
When the aircraft stopped the descent I tried to regaine the correct altitude using FLCH mode, and once in cruise level I inserted the ALTITUDE HOLD mode, but after some minutes, another dive at 6000 ft/m started. The aircraft loose thousend of feet anytime without trying to correct (the engines were at 50% N1) and ther were no failures or AP disconnections, neither changes in the required performance or altitudes... 

Any idea?

Thanks

Mauro

Interestingly, as @Farlis mentioned, I don’t recall simulating a CAT (Clear Air Turbulence). It seems more likely to be an autothrottle (A/T) issue that led to a stall. Regardless, you should receive some sort of warning in such cases. In a situation like this, the autopilot (AP) will disconnect, making all other information (FD, VNAV, LNAV, etc.) irrelevant.

Out of curiosity, did you notice the speed tape indication?

 

It appears to me that this situation may resemble a high-altitude stall, compounded by several bugs in the PMDG 737 simulation. Additionally, the 50% N1 indication didn’t change, suggesting that the autothrottle wasn’t engaged. 

 

Just a reminder: in situations like this, don’t expect the airplane to correct itself. You are the one flying the aircraft; you are in control, not the other way around. Take control.

 

Thanks to all,

@Farlisarlis I though to that too, but this is started to happen in the last couple of months, and I always used real weather also in the past years.

@fppilotppilot I did not check the OAT, will do at the next occurence.

@Vitold69itold69 I know, but my 20 Mb/s connection for the moment does not consent the bandwith required for MSFS 2024.

@vonmaronmar not sure, but this time I was not so far from TOLD, less then 100 NM.

@sloppysmusicloppysmusic, MCP ALT was set at 37000 ft, CMDA was engaged and LIT and i had the issue first in VNAV PATH and later in ALT HOLD mode.

@LRBS AT and AP remained engaged, and after the dive the plane regained the altitude itself. The speed tape was showing overspeed (don't remember the exact value). I know I had the control, I left the plane go itself just to checking how it was acting and to see if some system was showing failures, but apparently all was as expected except the attitude.

Mauro

 

Next time it happens Pause the sim and try to get a few screenshots of MCP, PFD, ND and CDU (legs).

 

Which PMDG 737 were you flying?

Edited November 19, 20241 yr by vonmar

41 minutes ago, zetato said:

@LRBS AT and AP remained engaged, and after the dive the plane regained the altitude itself. The speed tape was showing overspeed (don't remember the exact value). I know I had the control, I left the plane go itself just to checking how it was acting and to see if some system was showing failures, but apparently all was as expected except the attitude.

Mauro

 

I see the PMDG 737 has been known for this problem for a long time in different situations. There is not much information about how it happened, but losing 5,000 ft at a rate of descent (ROD) of 5,000 ft/min, as you describe it, is not normal, especially since N1 was at 50%. Did this happen instantly? If you can remember the sequence of events, that will help a lot. At the same time, don't get worked up because this is a known issue for them. 

I've had this happen before and it's usually coincidental to running weather settings that update at a set frequency rather than "live." If you're at cruise speed with an 80kt headwind and suddenly the weather changes so you have a significant tail wind, you will fall out of the sky until you've built enough airspeed to compensate.

14 hours ago, Vitold69 said:

 

We all uninstalled msfs 2020 already and will soon flood both forums making threads crying about how broken MSFS 2024 is! 

Fixed!😛

14 hours ago, vonmar said:

Next time it happens Pause the sim and try to get a few screenshots of MCP, PFD, ND and CDU (legs).

 

Which PMDG 737 were you flying?

I'll do for sure, thank you.

I'm flying the B738.

Mauro

13 hours ago, LRBS said:

I see the PMDG 737 has been known for this problem for a long time in different situations. There is not much information about how it happened, but losing 5,000 ft at a rate of descent (ROD) of 5,000 ft/min, as you describe it, is not normal, especially since N1 was at 50%. Did this happen instantly? If you can remember the sequence of events, that will help a lot. At the same time, don't get worked up because this is a known issue for them. 

Some time the plane starts to loose a couple of hundred of feet but then it's able to regain the altitude and I consider this accettable and maybe due to the weather.

But sometimes like the other day it starts to put the nose down and in a few seconds it falls down in a very steep uncontrolled descent. At a certain point it stops, prtially regaining altitude but then starts to go down again. I tried to climb at the required cruise altitude using the FLCH mode, but sometimes it's able to complete the cliomb, sometimes not. The strange thing is that often the engines does not respond to the altitude loss, simply leaving the airplane go down...

Mauro