VNAV initiates descent at TOD even though MCP ALT not changed to lower.

[Alpha Floor]

Hello, I noticed this issue yesterday and I am pretty sure it must be a bug, please correct me if wrong.

I am flying with AP engaged, VNAV/LNAV and FL300. The aircraft reaches the TOD but altitude is not changed from FL300. FMC is asking to change it (normal behaviour). But once the aircraft hits the TOD the speed bug goes back to 0 (or infinite, don't know exactly), and the aircraft starts to descend. This caught me by surprise, tried to stop it by pushing ALT HOLD which didn't work. Had to revert to V/S and command a climb.

This video shows this behaviour between 1:40:14 and 1:41:15 (please disregard the audio). Aircraft starts to descend at 1:40:40.

https://youtu.be/sXsVnTKGW-g?t=6014

Edited April 21, 2020 by Alpha Floor

[Litjan]

Hi Jaime,

I have to check this behaviour again - what you see (initially) is quite normal - the aircraft will start to reduce speed to "trade energy". In other words, it assumes (if you have not reset the MCP as requested) that you need to maintain the current altitude for whatever reasone (usually ATC, some traffic below) but would later on like to regain the pre-calculated descent path. So it "helps" by reducing speed, which you can then later regain (by descending more steeply) and therefore have an easier time re-intercepting your calculated descent path.

The minimum speed commanded should be green dot (minimum clean), though.

Looking at your video it looks like the speed bug goes to maximum speed (and the aircraft accelerates), though. This would clearly be a bug!

Thanks for bringing this to my attention,

Jan

[Alpha Floor]

Thanks for the quick respond Jan! 

I think I didn't give the AP enough time to develop that descent, stopped it pretty quickly. That initial V/S drop to about -500 fpm was only temporary, would it have been cancelled out if I had let it do its thing? The FMA was still in "ALT HLD" mode so why does it go down?

 

[Litjan]

It should not go down, and if you look at your video, it didn´t - it stayed at the flight level and increased speed (that was wrong). I think you maybe pushed on your yoke by accident, the autopilot went to CWS pitch for a second. Pushing on the yoke while the autopilot flies will actually change the pitch a little bit...

I just tested the behaviour here on my end and it worked as it should. The speed cursor drives down slowly, while the VNAV PTH changes to ALT HOLD. Energy trading.

Not sure what happened during your flight, some weird combination of values, maybe...

Cheers, Jan

 

[Alpha Floor]

Hello Jan, I am also paying close attention to the sequence with the sound muted. I can assure you I didn't touch the controls as it is a stick standing on the desk (not a yoke in front of me which I could have pushed with my body).

1:40:14 - TOD reached and speed bug jumps to "infinity" at the same time as VNAV PATH goes to ALT HOLD. Engines start to spool up.

1:40:41 - Aircraft starts to descend with VS going down to 500 fpm. This is where I notice something is not right.

1:40:43 - The first time I hit ALT HLD, speed window opens to M .40, SPD engages in the MCP and MCP SPD announced on FMA. The ALT HLD deselect light does not light up on the MCP though.

1:40:44 - The second time I hit ALT HLD, pitch channel is disengaged, ALT HOLD disappears from FMA and CWS P appears under CMD. This is what one expects to happen when you hit the button with ALT HLD engaged except the deselect light should have been on (light showing the mode can be deselected).

1:40:46 - I hit V/S and from here on behaviour is normal.
 

I'm intrigued now as to what the cause of this could be! 

Prost!

Edited April 21, 2020 by Alpha Floor

[Litjan]

Yes, that is essentially what I see, too. I didn´t see your mouse when you clicked the the ALT HOLD, but if you did it, then the behaviour is of course correct.

The two things that are not right are:

1.) Orange speed cursor moving "up" to maximum speed (it should move down slowly in relation to "VNAV PATH ERROR" shown on the vertical deviation bar).

2.) Aircraft descending while in ALT HOLD. It looks like it is "trying" to follow the VNAV PTH, but it should not at all while in ALT HOLD.

It could very well be some variable going to an extreme - this is the type of bug that is hard to troubleshoot because you often have to have the "exactly right" conditions to see it. I invite you to watch this behaviour and see if you can repeat it consistently.

Of course you did the right thing - when the automation *** up, reduce the level of automation or even revert to manual flight. We are Boeing pilots, not Airbus ;-)

Happy flying, Jan

 

Edited April 22, 2020 by Litjan

[Alpha Floor]

4 hours ago, Litjan said:

We are Boeing pilots, not Airbus ;-)

Happy flying, Jan

 

Amen to that; for now at least, let's see where I can find a paying flying job after Covid, I think I'd even accept an Airbus xD