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 Post subject: Pitch Trim warning on GFC 700
PostPosted: 30 Jul 2010, 22:18 
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Has any had a red PTRIM warning annunciate on their PFD? I was flying a practice coupled ILS today and was slightly above GS in NAV mode. I punched the APR button and put the gear down and approach flaps to join the GS from above. I immediately got a PTRIM warning. The trim seemed fine, i.e. no runaway condition. The light stayed on, so I disconnected the AP and hand flew without incident. When I later turned on the AP, all was well without again seeing that PTRIM annunciation. Any thoughts? Did it have something to do with the fact that I was above GS when I engaged APR mode, or is there really something wrong that I should have checked out? Really appreciate any thoughts as well as anyone else's prior experience with this.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 00:09 
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You were pushing against the yoke and the autopilot will not join the gs from above, it is not progrmmed to do this, the trim system will try to run against the forces you are putting on it and will cause a pitch trim runaway warning.

If this situation happens again you should either press and hold the cws button and maneuver the airplane as you see fit onto the gs and loc then release the cws button, it may not switch back into the apr mode or follow the gs at this point. You may have to hand fly it.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 07:12 
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Thank you Charles. Actually, I wasn't touching the yoke when I got the PTRIM warning. I just hit the APR button and put the gear and flaps down. However, I think that the warning was it's way of telling me that it won't capture the GS from above. Hence, no real probelm with the AP. Thanks again.

Rick Newman


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 12:44 
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The warning is telling you that if you release the autopilot when you see "PTRIM", you are in an out-of-trim situation, and you may have a handful. The servos are strong enough to be able to control the "out of trim" pressures, but as soon as you release the autopilot, it's up to your arm muscles to control the elevator pressures. It can mean that the trim can't keep up with momentary turbulence or may be telling you of something more serious. You certainly wouldn't want to be at MAP, release the autopilot for a go-around, and find you have full up elevator pressures to compete with.

At a minimum, the warning is telling you the trim can't keep up with the servos. I suppose if the autopilot is trying to fly a glideslope well below it's path, it runs out of trim for a few moments. I would want to be sure because it's not something you want to live with long. If you see the warning with any frequency, you are putting unecessary pressure and wear on your servos and risking a potential control issue.

My warning was a result of the manual trim switch on my yoke getting loose on it's shaft, causing the trim to operate intermittently. I ended up replacing my trim switch and I've never seen the warning since.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 14:05 
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Thanks for that insight. I'l need to keep playing with it and make sure that the situation is not repeating itself.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 14:24 
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Will the GFC capture if you are above the GS?


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Jul 2010, 14:27 
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I don't think that it will. I have to play with that some more. I'm thinking that that was why I got the warning, essentially the AP telling me that I was asking it to do something that it couldn't do.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 02 Aug 2010, 14:52 
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I don't think the GFC700 will capture the GS from above. I did take two surprises, flying ILSs, to learn that one has to capture the GS from below. Then, just perfect.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 02 Aug 2010, 17:27 
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I doubt the pitch trim warning has anything to do with being above the GS.

I don't know the 700 but normally this kind of warning comes from the trim servo running for more than X seconds continuously.

The GS is always intercepted from below. One tracks the localiser, in level flight, and intercepts the GS from below.

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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 30 Oct 2015, 14:47 
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On the same topic but a different issue: my AP told me that my GFC700 elevator trim is checking out at 27lbs vs 41-47lbs per specs and wants to charge 3 hours to adjust the servo. Has anyone had this issue? Thanks.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 30 Oct 2015, 16:27 
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It's not adjusting the servo, its adjusting the servo mount where the slip clutch is. It has to be carefully removed noting position of the bridle clamps, direction of wind and blocking ball position. You then have to set up the clutch in the garmin fixture with a torque wrench and reinstall everything and check bridle tension and travels to make sure nothing interferes.


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 31 Oct 2015, 19:42 
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Username Protected wrote:
On the same topic but a different issue: my AP told me that my GFC700 elevator trim is checking out at 27lbs vs 41-47lbs per specs and wants to charge 3 hours to adjust the servo. Has anyone had this issue? Thanks.


Hello David,
my apologies for not understanding completely when you PM'd me. if this is the case and one of the servo tests are indicating a low function if this is the GSM 85 it can only be adjusted with the servo test set as it's been outlined. this does require removal of the part from the airplane, adjustment and then re-installation which will take some time.

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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 01 Nov 2015, 00:42 
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Username Protected wrote:
Has any had a red PTRIM warning annunciate on their PFD? I was flying a practice coupled ILS today and was slightly above GS in NAV mode. I punched the APR button and put the gear down and approach flaps to join the GS from above. I immediately got a PTRIM warning. The trim seemed fine, i.e. no runaway condition. The light stayed on, so I disconnected the AP and hand flew without incident. When I later turned on the AP, all was well without again seeing that PTRIM annunciation. Any thoughts? Did it have something to do with the fact that I was above GS when I engaged APR mode, or is there really something wrong that I should have checked out? Really appreciate any thoughts as well as anyone else's prior experience with this.

Richard,

You did the right thing "By the book" in disengaging the AP and flying by hand.

I wonder if you really had a real problem at all. Could it be just a timing issue being that you had just activated the APR mode having passed thru the glidepath then changed your pitch attitude by lowering the gear and dropping the flaps. The AP sensor finding itself too long at max down position trying to capture the GP below you, triggering the alert annunciation. In hindsight the simplest fix was likely to engage the APR mode earlier.

If a similar issue ever occurs again consider not disengaging the AP to hand fly as that's a lot of work. Simply engage another pitch mode like Nose-DN/Nose-Up or VS to keep yourself on the GP. You can always hand fly and remember the Flight Director is your friend.

IMHO, as the problem has not recurred and removing the servos is indeed expensive, I might not jump to fix something that may not be broke. But whatever you do, keep us posted!

Mark

PS: I once had a similar AP rudder annunciation appear. Moving my lead foot off the pedal helped :D


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 01 Nov 2015, 04:46 
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The PTRM warning is there to let you know that the autopilot servo is being asked for,too much torque

In this case it was your elevator pitch servo

This happens when too much trim change occurs in a short period

You can recreate this by flying a coupled approach with VNAV , the type does not matter

Then selecting full flaps when below 100 knots, in the white tape zone

The AP trim servo has a hard time keeping up and you get this indication

The system is working fine, it's just letting you know you asked it to do something outside its programming

I hopee this helps


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 Post subject: Re: Pitch Trim warning on GFC 700
PostPosted: 01 Nov 2015, 15:54 
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Username Protected wrote:
I punched the APR button and put the gear down and approach flaps to join the GS from above. I immediately got a PTRIM warning.


A additional question,
How long was the time between putting the gear down and selecting Appr-flaps ?

It could be that the AP is correcting , pitch up for the additional drag for the gear (incl trim) if at the same time you select Appr-Flaps what should be compensated with a nose down drive on the pitch servo.
I think when the AP senses opposite pitch-trim drive(up) from pitch-servo drive(dwn) will give a Pitch-trim warning

just my two cents

grtz Eric


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