Bug: Plane descends then starts climbing again ignoring MCP

[devgrp 1,233]

Ok can someone please tell me what the heck is going on. As you can see from the video. The plane is in flch mode descending to 8000ft with 250kts on the mcp, then it throttles up an starts overspeeding

 

 

Thanks

 

Pierre W

[Budbud 227]

Hi,

 

Two questions:

_ What option have you chosen in the FMC regarding the throttle overriding (Never/always/only in hold mode)?

_ How was your throttle during the descent? (ex full forward)?

 

Because to me it could be an interference from your throttle causing the plane to keep thrust forward while it should be idle.

[devgrp 1,233]

The throttle was about 50% forward. I don't think the throttle position is the problem because it doesn't happen all the time. I usually leave my throttle where it is when I hit toga to take of and it stays like that the entire flight until I land.

 

I can remember for sure but I think throttle I overriding is on hold. Never really messed with that setting. It's just strange because that's how I usually descend when I vector myself to the airport and it has happened twice now. The plane will be descending and then all of a sudden the engines will spool up and it starts clmbing and over speeding

[FLEX1978 373]

It's been months since I have flown the 777.  Why is the FMA displaying THR? It should be HOLD in FLCH in a decent.

[Mike777 163]

Rob is right that the throttle should be in HOLD or IDLE, not THRUST on a descent, especially when you are already exceeding the MCP set speed window speed.  Wondering if VNAV thinks you are still in climb phase -- click on VNAV on the CDU screen and see what it shows.  Also, is it possible the next waypoint in the flight plan somehow has a higher altitude?  The A/T is acting like it is set for a climb, not a descent.

 

Part of what's happening is that, when the aircraft overspeeds, it is pitching up and the VNAV mode is being temporarily overridden because the speed protection routine has kicked in.  But that doesn't explain why the A/T is in THRUST in the first place.

 

I don't think the joystick could be causing the A/T to go into THRUST mode; even if the joystick is set to override always, it doesn't change the A/T mode, and when you stop moving the joystick throttle, it should go back to the speed the system is commanding (except in HOLD mode).

 

Mike

[devgrp 1,233]

The thing is I descended from 16000ft to 8000 using flch and the throttle was in hold mode. When  I got down to 8000ft I was told to slow to 210kts so I hit the speed brakes and thats when the engines powered back up completely ignoring the mcp. Even if there were higher alt in the FP it shouldnt have affected it because lnav and vnav was off.  I was flying in heading mode with flch

 

Pierre

[FLEX1978 373]

Can you reproduce the issue?  A video showing the problem before speed protection kicking in would help.

 

EDIT - Guy's the logic is working correctly according to the FCOM.

 

In FLCH descent  -  Mode reverts to THR. If the throttle is at idle the mode reverts to IDLE and then HOLD.

 

I think the problem could be if the users hardware is not fully at idle, the system can remain in THR

 

I should probably start studying Beoing philosophy again, it's been a while :/

 

Regards

[Brazen 5]

That cyan line around the N1 indications, isn't that the hardware power position? It's showing almost full forward.

[voske 34]

 

 

That cyan line around the N1 indications, isn't that the hardware power position?

 

It is.

[devgrp 1,233]

That cyan line around the N1 indications, isn't that the hardware power position? It's showing almost full forward.

No if the throttle was full forward you would not see the cyan. The throttle was all the way back. The cyan is showing where the physical throttle postion is in relation to the virtual throttle position. I posted this on the pmdg FB group and another member Alex Farmer from Goldstar Textures said the same thing happened to him. 

 

I dont know if I can reproduce if I tried because it doesnt happen all the time. Its random. I've had it happen twice now. I just completed a flight and descended the same way and it didnt happen

 

Pierre

[kevinh 409]

I think the problem could be if the users hardware is not fully at idle, the system can remain in THR

 

This is the issue. With the 777 have to remember to put your hardware throttles to idle before commencing descent.

[Brazen 5]

The cyan is showing where the physical throttle postion is in relation to the virtual throttle position.

 

*bends head in shame*

[devgrp 1,233]

This is the issue. With the 777 have to remember to put your hardware throttles to idle before commencing descent.

Thats definately not it because like I said, this is how I usally descend when I vector myself and when I fly on vatsim and I've never had to move my physical throttle before descending. I stays at about 55% when I hit toga upon takeoff and stay like that the entire flight. The only time I bring the throttle back is when I land for the reverse thrust

[FLEX1978 373]

Thats definately not it because like I said, this is how I usally descend when I vector myself and when I fly on vatsim and I've never had to move my physical throttle before descending. I stays at about 55% when I hit toga upon takeoff and stay like that the entire flight. The only time I bring the throttle back is when I land for the reverse thrust

 

And that is likely causing your issue, any hardware spike may affect the system reducing to idle and going remaining in HOLD.

[devgrp 1,233]

And that is likely causing your issue, any hardware spike may affect the system reducing to idle and going remaining in HOLD.

If that was the case it would happen all the time and it doesnt. It has happened on 2 random occasions and the fact that someone else had this issue leads me to believe its a bug.

 

've flown on vatsim for hundreds of hrs and always use flch to descend and I never touched my throttle. This happens randomly. I dont know why but it does. Its like the lnav bug where the plane will just stop following the fp and do its own thing

 

Pierre