Bug: Plane descends then starts climbing again ignoring MCP

[kevinh 409]

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.

[Mike777 163]

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

[FLEX1978 373]

 

 

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.  

[Mike777 163]

 

 

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

[777simmer 117]

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!

[pete_auau 1,154]

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

[martin-w 3,234]

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.