Nose-down trim inhibited with F/D on?

[Nick L 0]

I've been doing a few practice flights with the 773ER to improve my hand-flying skills. While doing this, I came across an oddity in the way trim responds with the F/D on versus with it off.

 

Since I only fly out and back, I set the altitude (both the FMC cruise altitude and the MCP altitude) to only 8000 feet or so. While climbing out, I tried to level off early at about 6000 feet. I noticed that, if I had turned the F/Ds on before takeoff, the aircraft would not respond to nose-down trim at all. The only way I could hold altitude was to push forward on the stick. Once I reached 8000 feet and the pitch mode changed away from "TO/GA", I could trim the airplane normally.

 

I tried another climb-out and level-off without turning on the F/Ds before takeoff, and in this case the airplane responded to trim normally. Is this normal? I noticed in the "Introduction" document that there is a reference to "hands-off speed stability *or* path hold modes" (emphasis mine) in the section describing the control column null zone. Is what I am experiencing with the F/Ds on the "path hold" mode? I tried digging around in the FCOM but couldn't find anything (though I certainly may have missed it).

 

Thanks in advance for your help!

[777simmer 117]

No not normal.

 

FD has nothing to do with what you do with your yoke or trim switches.

I have not noticed this behavior.

(make sure you uave the AP off!)

 

 

When you trim nose down while climbing the aircrafts nose should drop.

The airplane does not necessarilly have to level off though.

It could climb shallower, or stay pretty much level or even dive down.

All any airplane would do is drop the nose to achieve the higher in trim airspeed.

The more you trim down the more the nose should drop.

 

But leveling off with the trim switches just screws up your in trim situation!

A level off should be done with the yoke only and does not require any trim input at all!

[Mike777 163]

 

 

While climbing out, I tried to level off early at about 6000 feet. I noticed that, if I had turned the F/Ds on before takeoff, the aircraft would not respond to nose-down trim at all. The only way I could hold altitude was to push forward on the stick. Once I reached 8000 feet and the pitch mode changed away from "TO/GA", I could trim the airplane normally.

 

 

Hi, Nicholas,

 

I think your issue may have to do with being in TOGA.  In that mode (or in any Thrust Ref mode, which gives you high constant thrust) you are accelerating if you push the nose down.  As I understand it, if you trim the nose down you will accelerate more.  As an aircraft accelerates its nose tends to go up.  So you never reach a stable trim situation -- the more you trim nose down, the faster it goes, the more it pitches nose up, the more you have to trim nose down.  When your aircraft went out of TOGA at the target altitude the autothrottle then adjusted thrust to maintain a set speed, so once you trimmed for that speed it remained in trim.

 

I THINK that it won't stay in TOGA with the FDs off, but not sure about that.  I do know that with the FD on, VNAV set on the MCP, but autopilot OFF, the aircraft follows the thrust changes according to the settings in the FMC, including TOGA, CLB and then SPD when you reach the target altitude. 

 

Mike

[Mike777 163]

OK, some experimentation:

 

FD on or off does NOT prevent TOGA mode. 

 

With the autopilot off I can trim for pitch quite well, FD on or off.  But with the autothrottle in THR REF -- either TOGA or CLB -- the plane accelerates and it is necessary to keep trimming down (as speed increases) to prevent the nose from pitching up.

 

But I think the key question is what your thrust mode is.  I am not sure if the O.P. means TOGA thrust, TOGA pitch or TOGA roll mode.   When you say you are in TOGA mode, which TOGA do you mean?  What is your thrust mode?

 

Mike