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Bug: Plane descends then starts climbing again ignoring MCP

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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

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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.

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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

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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.

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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

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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

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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

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

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

 

It is.

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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

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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.

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

 

*bends head in shame*

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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

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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.

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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

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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

I suggest you try it anyway just to eliminate it as a cause. On my 777 any time I have problems with descent it's because I forgot to retard the throttles at TOD. The reason to do it is exactly the same reason they recommend you advance your throttles after selecting TOGA.

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Is it possible that your speed got too low since you put on the speed brakes, and speed protection has already kicked in before the start of the recording?

 

Also, you were aiming for 210 knots but MCP speed is set to 250 -- is it  possible that when the aircraft got well below 250 (due to speed brakes) the A/T overreacted?

 

Just some additional thoughts.

 

Mike

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Is it possible that your speed got too low since you put on the speed brakes

 

I don't so how that would cause the FMA to display THR?  I only skimmed the FCOM, however it seems to be pretty clear.  With FLCH selected in decent you will see THR if your throttle sends a signal that it is not at idle.   Obviously this is not an issue on the real machine since the throttle is back driven.

 

 I have seen this kind of issue (usually random throttle spikes) plenty of times doing tech support.  

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I don't so how that would cause the FMA to display THR? I only skimmed the FCOM, however it seems to be pretty clear. With FLCH selected in decent you will see THR if your throttle sends a signal that it is not at idle. Obviously this is not an issue on the real machine since the throttle is back driven.

 

OK, I did a little experimenting.  During a descent, when I press FLCH, initial A/T mode is THR.  However it does not shoot up, but maintains appropriate thrust to maintain the set speed during descent.  If I move my joystick throttle up, N1 increases, but then it goes back to the A/T/FLCH commanded thrust.  This is what it should do.  My joystick throttle is set to override always.   A little surprised that FLCH starts in THR mode during a descent, but in fact this works fine.  So unless Pierre's joystick throttle is sending repeated spikes, I don't see how that can be causing the problem.

 

I couldn't get it to go into IDLE during my descent experiment -- it goes straight to HOLD, where it behaves as it should -- it maintains whatever thrust I set, which may reduce the descent rate if the aircraft starts to exceed target MCP speed.

 

Regarding stall protection or speed-below-MCP-target, I'm really not clear on what mode the A/T should enter.

 

Mike

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A little surprised that FLCH starts in THR mode during a descent, but in fact this works fine. 

You should not be surprised.....FLCH is a smart mode.

 

If you make a small climb or descend it will not add full thrust or idle thrust.

Instead it will add/reduce thrust a little bit in order to be at the target altitude after about 2 minutes. If the altitude change is large enough then the AT mode will set IDLE or CLB thrust.

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

Yes it does so randomly because sometimes your hardware sends a signal (or spyke) and sometimes it does not.

50-50.

Mine do the same if I have override-allways or in HOLD only set.

 

Rather then saying this cant be it......you should at least give it a try......otherwise why ask others for help?

Also, what is the point anyway of having hardware throttles at 50% during an idle descend??.....you feel better that way?

 

Move your physicall throttle to idle BEFORE starting the descend and see if that helps!

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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.

Ever thought that you hard ware throttles could have a intermitted problem where it works fine one flight and not  the other  this is why its random

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The first thing I do after hitting TOGA is to reduce physical throttle back to idle. It stays there for the rest of the flight, unless I need to fine tune by adding a little thrust while in hold mode.

 

I see no reason to leave your physical throttle at 50%, there's no point. It's contrary to advice that has always been given, in order to avoid spikes.

 

Spikes can be random, as Rob said, try it and find out. If it's not that, at least you have eliminated a variable.

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