Aircraft spun into the ground while I was away for 10 minutes

[robertispas]

Hey guys,

I departed on a flight about 2 hours ago and stayed at the computer for most of it. I went away for 10 minutes and I returned to some messages from friends telling me that I essentially crashed. When I looked at the sim, I was in the ocean..

I looked at Volanta to see what the flight path was, and apparently the plane went up, then veered right and descended until it crashed. I'm not entirely sure what happened, but from the looks of it, it seems like an autopilot disconnect? I've had a few of those on departure a couple of days ago, but no issues after that. I'll leave the screenshot from Volanta as a reference.

So my question is, what could have happened? My controls were UNTOUCHED, no weather addon, real time/weather, nothing really that could impact the flight.. Is there a way to the logs or something similar?

[rjb4000]

I’ve had this happen as well. The issue was that VNAV and managed speed were engaged during the cruise and the cruise speed was selected as LRC. At some point the FMS went nuts and computed the LRC speed as Mach 0.4, which causes the power levers to idle and the speed to drop and the autopilot disconnects so the airplane… spirals slowly into the ground.
 

My solution until a fix is implemented is to manually select speed (cyan indication vs. magenta) if I need to leave the flight deck, as well as always saving the airframe state before walking away. 
 

You can confirm this is the cause by looking at the FDR report for your flight in the XPlane outputs folder. Check out the selected speed column and see if you find a super low speed right before the spiral. 

Edited April 30, 2022 by rjb4000

[robertispas]

Good to know it wasn't just me! I remember before leaving I had the speed selected in the MCP with MACH .80 in blue (so not managed I guess?) and NAV and ALT selected too!

In the previous cases where I had autopilot disconnects, I never lost speed, and it wasn't the case here either. I'll attach my FDR CSV as well! Funnily enough both engines were at 97.6 N1, everything was normal except PilotAPDisc and CopilotApDisc which are both 1 (assuming that means disconnected)...

FDR-1.csv

[rjb4000]

Check out row number 5,517 in your output file (I find it easy to highlight the row so it's easy to refer to) - this is the last recording where the Mach selector is at M0.8 (shown in column PX - FCC1MachSel). After that row was written, the mach selector was changed to M0.68 and the rest is history! The engine speed was reduced and the actual speed bled off until the autopilot couldn't hold altitude anymore and was disengaged. 

It also looks like you were slowly climbing to reach FL370 maybe in FLC mode? When the mach selector was changed from .8 to .68, your vertical speed jumped up to 3700 fpm in the climb before the airplane leveled and speed continued to bleed. 

I'm not sure why the mach speed selector would have changed without your input, but that would be the root cause of the crash. 

Edited May 2, 2022 by rjb4000

[FYG001]

They are still looking for staff at the NTSB Rich! 

[ois650]

Afternoon, 

If I recall correctly the mach rolling back in managed mode is a problem with the real box. Recommend that you use selected speed in cruise. 

KR

[robertispas]

On 5/2/2022 at 4:43 AM, rjb4000 said:

Check out row number 5,517 in your output file (I find it easy to highlight the row so it's easy to refer to) - this is the last recording where the Mach selector is at M0.8 (shown in column PX - FCC1MachSel). After that row was written, the mach selector was changed to M0.68 and the rest is history! The engine speed was reduced and the actual speed bled off until the autopilot couldn't hold altitude anymore and was disengaged. 

It also looks like you were slowly climbing to reach FL370 maybe in FLC mode? When the mach selector was changed from .8 to .68, your vertical speed jumped up to 3700 fpm in the climb before the airplane leveled and speed continued to bleed. 

I'm not sure why the mach speed selector would have changed without your input, but that would be the root cause of the crash. 

Yep, I remember climbing to 370 before I left, but I essentially remember leveling so not entirely sure the the speed changed! In any case, a very good explanation! Thank you for this!