GFC 500 installation – pitch trim change & aggressive IAS climb

[Bartman]

This is exactly why I own maintenance manual, illustrated parts catalog, research and interact on mooneyspace, and participate in my own maintenance. I’m a physician but have a mechanical background, and the thought processes for differential diagnosis and treatment of illness are the same as mechanical issues. I am blessed to have a great IA that I work with and he appreciates all off the research and information I bring to the table before anybody touches my airplane, including him. For example, about 15 years ago we removed the trim jack screw and sent to LASAR for service. All of the information was in the MM and had been discussed here on Mooneyspace, and we avoided this exact issue during installation and return to service. I am very thankful for this community !

As a couple of members stated above, this should be entered as a service difficulty report. 

[DC_Brasil]

Very interesting (negatively, of course) to see many similar accounts. Indeed, there seems to be a repeating pattern here.

 

15 hours ago, Bartman said:

This is exactly why I own maintenance manual, illustrated parts catalog, research and interact on mooneyspace, and participate in my own maintenance. 

Unfortunately,  Garmin doesn't publicly offer installation manuals or STC supplements unless you're an authorized dealer. So we actually have no way of knowing in advance what and how things will be done to get minimally involved. 

[PT20J]

30 minutes ago, DC_Brasil said:

Very interesting (negatively, of course) to see many similar accounts. Indeed, there seems to be a repeating pattern here.

 

Unfortunately,  Garmin doesn't publicly offer installation manuals or STC supplements unless you're an authorized dealer. So we actually have no way of knowing in advance what and how things will be done to get minimally involved. 

In this case, it would not help. The installation information does not describe the need to maintain trim position integrity after the installation. 

[blaine beaven]

I feel lucky because I didn’t have these issues post GFC500 install, but great point about knowing where to expect your elevator in relation to trim indication, and looking hard at that post maintenance 

[Bartman]

12 hours ago, PT20J said:

In this case, it would not help. The installation information does not describe the need to maintain trim position integrity after the installation. 

If I have work done that involved the elevator trim or another critical system I would absolutely research.  The Mooney trim system is unique and I would absolutely provide crucial information including maintenance procedures to the shop performing the work. I use “teach back” all the time and I would make sure the shop can verbalize the importance, and would specifically ask about the install and rigging on pickup.  I would also be aware of potential incorrect rigging of the trim system and know how to test it during preflight to ensure travel to the stops in both directions and confirm the indicator is working properly too. 

I’m not trying to make myself look better than anyone else. My brain just thinks risk mitigation and I’m simply putting the things I have learned from experience and on Mooneyspace into practice. 

[unicom]

 

3 hours ago, Bartman said:

 I would also be aware of potential incorrect rigging of the trim system and know how to test it during preflight to ensure travel to the stops in both directions and confirm the indicator is working properly too. 

That’s something that really surprised me and which needs clarification. Although the tail was not in the correct position after the installation of the GFC500, I could still reach both the full-up and full-down trim positions. How is this technically possible ? Don't the upper and lower stops correspond to physical stops ? In other words, if the tail is already pitched nose-down even when the trim is in the neutral position, does that mean that when I move the trim to full down, the tail could go even lower than it normally should when properly rigged ?

This is the picture of the tail with the trim in the take off band (before proper rigging).

Edited November 8 by unicom

[PT20J]

3 hours ago, unicom said:

 

That’s something that really surprised me and which needs clarification. Although the tail was not in the correct position after the installation of the GFC500, I could still reach both the full-up and full-down trim positions. How is this technically possible ? Don't the upper and lower stops correspond to physical stops ? In other words, if the tail is already pitched nose-down even when the trim is in the neutral position, does that mean that when I move the trim to full down, the tail could go even lower than it normally should when properly rigged ?

This is the picture of the tail with the trim in the take off band (before proper rigging).

The front trim gearbox where the trim wheel is attached drives a leadscrew that turns the torque tube that runs to the jackscrew in the tail. This leadscrew has the up and down limit stops attached to it and the trim indicator is driven by a Bowden cable that attaches to a nut located on the leadscrew between the stops. Thus, the trim indicator indicates the position of the nut between the two stops. It is possible to adjust the cable length to the indicator at this nut.

If the trim torque tube is disconnected from the aft jackscrew and rotated before reconnecting, the stops and trim indicator will no longer agree with the correct incidence of the tail. If things are way out of whack, I suppose the tail itself would impose limits on trim movement since the hinge at the bottom can only extend so far and the jackscrew itself will have limits of fore/aft movement. The picture appears to show nearly full nose down trim and I'm surprised that if this did indeed correspond to a takeoff trim setting that that there would be enough movement left in the system to move the trim to both stops, but I'm not doubting you since I've never tried it. 

[Bartman]

@unicom I am traveling, have not read that section of the manual in several years, and cannot answer your question. I am sure one of our more knowledgeable members can answer. I can say that my 1977 tail looks like the second picture posted by @DC_Brasil  Mine does not look like yours with the indicator in the takeoff position  

When we rebuilt my jack screw I had that section of the manual marked, opened, and ready for the IA to discuss. We also made 3 reference marks on the tail feathers with magic marker on a piece of tape. One full up, one full down, and one at my normal takeoff position. Everything lined up after repairs, the indicator was in the right position, and I had confidence that it was done right. 

[unicom]

7 minutes ago, Bartman said:

Mine does not look like yours with the indicator in the takeoff position  

And that is a good thing, because this is precisely why the tail of my aircraft needs urgently to be re-rigged !

[unicom]

16 minutes ago, PT20J said:

The picture appears to show nearly full nose down trim and I'm surprised that if this did indeed correspond to a takeoff trim setting that that there would be enough movement left in the system to move the trim to both stops, but I'm not doubting you since I've never tried it

If the mechanic hasn’t made the adjustments yet, I’ll try to take some more photos with the tail in full trim down and full trim up positions. It’ll be interesting to see the difference.

[PT20J]

18 minutes ago, unicom said:

 

If the mechanic hasn’t made the adjustments yet, I’ll try to take some more photos with the tail in full trim down and full trim up positions. It’ll be interesting to see the difference.

That would indeed be interesting. Just for comparison, I made some measurements at the tail of my 1994 M20J some time ago:

With takeoff trim, the elevator is aligned with the stabilizer.

With full down trim, the elevator is deflected down ten degrees and the fin rotates back 4 deg measured at the leading edge

With full up trim, the elevator is deflected up ten degrees and the fin rotates forward 2 deg measured at the leading edge

The gap between the tailcone and the empennage measured at the bottom below the hinge is:

4.5" - takeoff trim

3.5" - full down trim

5.25" - Full up trim

[DC_Brasil]

10 hours ago, Bartman said:

If I have work done that involved the elevator trim or another critical system I would absolutely research. 

I believe that's the correct approach to take. However,  as I explained before, I bought the Mooney and took it straight to the avionics shop (my first experience as an owner).

I approached it as I would a very well recommended doctor for a surgery: I thought I needed not to know the tecnical details about the procedure to be executed. Certainly now, I see it is very different with aircraft maintenance. 

[Flyler]

19 hours ago, DC_Brasil said:

I believe that's the correct approach to take. However,  as I explained before, I bought the Mooney and took it straight to the avionics shop (my first experience as an owner).

I approached it as I would a very well recommended doctor for a surgery: I thought I needed not to know the tecnical details about the procedure to be executed. Certainly now, I see it is very different with aircraft maintenance. 

I had this exact same mentality when I bought my first plane.  I thought I knew just enough to be dangerous, and thought I would be better off taking the plane to someone of great experience.  The plane lasted 4 landings out of the shop before the gear collapsed and was totalled.  Next plane I bought, it went to a very well known MSC and had $28k put into it.  I did a preflight and noticed a bolt missing from one of the control horns on right side elevator.  I looked through my pictures of the plane that I took before the annual/prebuy and zoomed in- bolt is still missing.  I don't blame the shop for this- they did not touch the elevator, but they did miss it.  It's an old machine, it could use many sets of eyes on it.

Your life depends on this stuff, double-check everything.  No decent A&P should be offended by that.  If they are, go somewhere else, they have the wrong mindset.

I found this thread because it appears my plane has some misalignment with the elevator as well.  Not nearly as extreme as the pictures above, though.  Number one thing to keep in mind is that these planes come with their own Bible, it is the Service Manual.  It solves many mysteries.  You should look there first.  If something doesn't match the service manual, it should have a very good reason and I would be suspicious of it.

Edit: just re-read this and sounds all doom-and-gloom. I have since found a great, possibly the greatest, Mooney A&P who has spent lots of time with me learning the plane. We are able to check each other's work (and when I check his work there are ample air quotes on that statement) and feel much more comfortable.  I am sure that the previous shops I took the plane to would have appreciated "some" of my own involvement as well.  Maybe. I'm not sure. I am a better airplane mommy now.

[Will.iam]

After reading these issues, I’m going to mark the limits and take off zones of my Mooney now while it is in rig and nothing has been done to it. Airbus has markings that we check on every preflight to verify the horizontal stab has returned to 0 marking which the computer does after every landing. With having markings on my Mooney it would be another easy check that I remembered to reset my trim for T/O when I got out of the plane no guessing that looks about right.

[Flyler]

@unicom thanks for making this post.  I saw your picture and thought "mine looks sort of like that, I wonder if it is out of rig" and brought it to the attention of my local A&P.  He's sort of a wizard.  Anyhow, my plane has electric trim and we figured it had been disconnected and moved at some point too.  Rob worked his magic and gave me a good lesson on Mooney tail sections.  We took the plane for a flight, and ended up doing 8 landings as it just felt sooo much better!  Much more docile and predictable.  Setting trim to take off position makes a whole lot more sense now.

Before:

 

This isn't the true "after" picture but is about what it looks like now:

 

The elevator doesn't always end up in the exact same spot, so these pictures are far from precise.  You can tell just by putting a bit of pressure on the yoke- it has some "stiction" so it'll stay put in a small range.  Either way, it is reassuring that there's enough visual indication that *hopefully* pilots will notice this on preflight, if it is indeed off after having work done.

[cliffy]

As I mentioned here before- we could probably find at least 25% of the flying Mooneys out of rig either in flight controls or landing gear. 

[takair]

What is hard to see in Tyler’s picture is that the stab rigging was off by over 2 degrees. He never had full up trim which was actually closer to TO trim. Needless to say takeoff trim was very heavy and it was very difficult to get enough trim to land consistently. The elevator position varies depending on the vintage of Mooney.  Older ones have only the bungee springs that yield a more upward position in takeoff. This J has a down bias spring, on top of the bungee springs, that provides constant down pressure. Full rigging requires removal of that spring and then reinstallation. Later models removed the bungee and have other mechanisms including bob weights. It is super critical that shops understand the  mechanism.  Just putting index marks on the torque tube when taking it apart to do work on the trim servo or tail is not good enough. The trim stops and trim indication are in front of the plane. As shown in this thread, it can be miss indexed to a hazardous level. Any post flights should be treated with caution. I wish the FAA still had the old service difficulty system that most mechanics read. They still have it, but it is not highly visible like it used to be. Thanks again for the kind words Tyler, but it is your integration into the entire ownership process that resulted in finding this. I feel the previous owner suffered through a poorly rigged airplane. 

Edited 8 hours ago by takair
Clarification on bungee springs