GFC 500 installation – pitch trim change & aggressive IAS climb

[unicom]

Hi everyone,

I recently completed a full avionics upgrade on my M20J (I’ll post photos in the next few days), and I have a quick question for those who’ve also installed the Garmin GFC 500.

Since the installation, I’ve noticed a significant change in trim behavior. My airplane now feels much more nose-heavy — I need a lot more nose-up trim for both takeoff and landing. I’m actually well above the “takeoff band” on the trim indicator, not within the usual range shown on the instrument.

Has anyone else observed this after installing the GFC 500? Was this expected or addressed during your setup/calibration?

Also, during IAS climbs, the autopilot seems quite aggressive in pitch — it can initially raise the nose up to about 15°, then lower it again, producing a kind of oscillation until the target IAS is reached.

Is this typical behavior, or should I be looking into a gain adjustment or a servo calibration?

 

Photo below: Take off with 2 people (420 lb) in the front seats.

 

Photo below: Established on a 3° glideslope at 75kt

Thanks in advance for your insights.

Philippe

Edited November 5 by unicom

[DC_Brasil]

This has happened to me!

When the shop installs the pitch and pitch trim servos they have to disconnect the pivoting empennage. I don't know the exact procedure, but I infer that when they connect it again, if they haven't properly marked all the previous positions of each connection, there is a risk they will not return the linckages to the exact position they were before. Then, they will need to redo all tail flight control rigging.

In my case, the first test flight had a very pronounced nose down tendency and I had to use almost full nose-up trim to land the plane. After the flight I noticed hw much the elevator was drooping down and compared it to older pictures.

I live in Brazil, where Mooneys are rare birds, so the shop that installed my GFC500 had only previously worked on one Mooney ever. They had to get my regular Mooney A&P to travel there and redo all tail connections and F/CTL rigging.

This is the elevator after the test flight:

 

And this was the elevator before installation:

 

Hope it helps.

[Yetti]

It could be as easy as realigning the indicators.   Or may need to put the linkages back to where they were before the install.   An easy check would be to run flaps and trim end to end and see if they bottom out on either side. 

[PT20J]

During installation of the trim servo, the shaft between the front and rear trim gearboxes has to be disconnected in the tail. If the shaft is rotated while disconnected, it causes this problem. The trim system needs to be re-rigged per the Service Manual. BTW, the easiest way to avoid this is to run the trim all the way to one of the stops before disconnecting the shaft and make sure its all the way at the same stop before reconnecting. The stops are at the front gearbox. 

[Ragsf15e]

In addition to the rigging issues mentioned above, the indicator is adjustable.  I doubt it was messed with in this case, however, if you check the empennage and trim rigging and the indicator is still off, it can be adjusted in the belly.

[Aerodon]

There was a Bonanza that crashed after a GFC500 A/P install - on a test flight with the technician - because of an incorrectly reinstalled trim system.  There was another Beech where the avionics shop made swiss cheese out of the main spar web - leading to a lawsuit.

@dc_brasil pictures and description above are troubling - no need to remove too much, but it looks like the elevator spring was not installed correctly.

@unicom - the service manual is pretty good at describing the adjustment of the stops, adjustment of the trim indicator, and required horizontal stabilator.

@pt20J has got the right solution - if you mark everything before you take it apart and put it back together again the same way you should have no issues.  

You have to 'split' the torque tube behind the avionics tray to get the new trim gear on the shaft.

My local avionics guy subs the servo installation out to a properly qualified maintenance / structures shop.  You need quite a high avionics installation to justify a decent in-house solution.

 

Aerodon

 

 

 

 

 

 

[haymak3r]

Whoever did your install definitely didn't do the trim system good enough. There are 2 pins with cotter keys on each end of the trim shaft. You have to remove all of them, and slide the shaft out of the join so you can get the old gear out, and the poly bushing, new gear, and new trim bracket on to the trim shaft. If you don't mark the shaft to joint sections, that could be WAY out of whack now. I would not fly the plane again until you correct this. You could run in to a flight control issue... That is concerning if they did the work without doing something as simple as marking with a sharpie the original locations of that shaft. 

I also suggest you take the panels off by the tail section where the links are at and check the pitch servo cable mount. It's two aluminum pieces that sandwiches the end of the pitch tube and there are 3 mounting tabs of various lengths that also get bolted in between the aluminum pieces. This is what the cable attaches to. In my install, all of that was pretty close to the bulkhead, and so if you don't align things up properly, that mounting point could potentially hit the bulkhead, which could stop you from having pitch control. 

You can't cut corners here. I feel that there is some real danger present back there and if this wasn't done right, the consequences could be pretty severe....

[PeytonM]

Happened to me, too. Had to have it re-rigged. Test flight was exciting!

[KSMooniac]

Errors like this kill people!  Read the manuals, follow the procedures, use the travel/rigging boards, and double-check everything!

[ArtVandelay]

Happened to me, too. Had to have it re-rigged. Test flight was exciting!

Exact same thing happened to me.

[Echo]

This is VERY CONCERNING.  This thread should be bolded and pinned.  Someone is going to die from this!  Let's NOT have that happen.

Great information shared that seems to be a repeating installation problem.

[pirate]

Happened to me too, test flight out of RFD was a real eye opener.
I had another shop at DKB inspect everything, corrected errors and re set trim by the book. 
Works rock solid since. 

[KSMooniac]

46 minutes ago, ArtVandelay said:

Exact same thing happened to me.

Not to belittle any poster here, but I presume pretty much all of us here are owners and have familiarity with our birds.  If you see elevators out of whack to this extent right out of maintenance, that should be a glaring alarm bell that things are not right, and no flight should be attempted!

[EricJ]

This is the sort of thing that can be caught in preflight inspection, especially a post- maintenance preflight inspection.   With the trim indicator on Takeoff, the elevator on a J model (or most pre-K models,  I think), should be reasonably level,  not way down like in the above pics.   It's not a bad thing to check on every preflight.  

[ArtVandelay]

Not to belittle any poster here, but I presume pretty much all of us here are owners and have familiarity with our birds.  If you see elevators out of whack to this extent right out of maintenance, that should be a glaring alarm bell that things are not right, and no flight should be attempted!

Yeah I caught it during preflight but I was referring to the elevator position.

What I didn’t catch was the configuration of the AHRS module was configured backwards, so good thing I was VFR. It was a short test flight.

[DC_Brasil]

8 hours ago, EricJ said:

This is the sort of thing that can be caught in preflight inspection, especially a post- maintenance preflight inspection.   With the trim indicator on Takeoff, the elevator on a J model (or most pre-K models,  I think), should be reasonably level,  not way down like in the above pics.   It's not a bad thing to check on every preflight.  

I bought my Mooney and it went straight to the avionics shop. Was not familiar enough to notice the unusual elevator position on the test flight 

Completely unrelated, but just as curiosity: on that test flight I had a blown main tire during landing roll, so it was a very interesting morning   Luckily I was in very focused and prepared mental state for the test flight. 

[Fly Boomer]

1 hour ago, DC_Brasil said:

Luckily I was in very focused and prepared mental state for the test flight. 

Glad everything worked out.  Things can get overwhelming when more than one issue crops up at the same time.

[unicom]

14 hours ago, KSMooniac said:

Not to belittle any poster here, but I presume pretty much all of us here are owners and have familiarity with our birds.  If you see elevators out of whack to this extent right out of maintenance, that should be a glaring alarm bell that things are not right, and no flight should be attempted!

You’re absolutely right — that was exactly my thought when I saw my airplane in the hangar. I remember thinking the tail looked low, but then I completely forgot about it, maybe reassured by the fact that a test pilot had flown the aircraft after the re-instrumentation work. It wasn’t until the takeoff roll that I realized the airplane was totally out of trim. And yet, with over a thousand hours on my own aircraft and ten thousand more on others, I still got caught like a beginner. Just goes to show — as they say, if there’s doubt, there’s no doubt.

I’d like to take this opportunity to thank all of you for the feedback you share — it’s always incredibly insightful and truly helps make our flying safer. I appreciate it all the more since I don’t contribute very often, as I often feel technically unqualified to give meaningful answers.

[Yetti]

This concerns me about our current batch of mechanics.    When I was a wee pup I was taught to mark things that rotated before they were disassembled.   Mark covers before you took them off.   And or roll things against the stop.  Now days I add taking pictures with the cell phone. I guess common work practices are not very common anymore.   

[LANCECASPER]

2 hours ago, unicom said:

as I often feel technically unqualified to give meaningful answers.

Like most of us on here . . lol. 

But you opened up a conversation on a subject I had no idea was happening out there. Really good to know this and pay attention to what is happening when you have an autopilot install or even any work done on the flight controls.

[PT20J]

It's fairly easy to find gross trim setting errors on preflight of a J or before by noting the elevator position with the trim set for takeoff. However, K's and later have a different trim system and the elevator rests full trailing edge down on the ground because of the bob weight. You should still be able to ground check it by rolling the trim wheel from stop to stop and noting that the trim position indicator moves fully in each direction. Since this problem is pretty serious and seems fairly common, someone who had the issue should file a SDR with the FAA so it can post an alert.

[Ragsf15e]

13 minutes ago, LANCECASPER said:

Like most of us on here . . lol. 

But you opened up a conversation on a subject I had no idea was happening out there. Really good to know this and pay attention to what is happening when you have an autopilot install or even any work done on the flight controls.

Surprisingly, this same thing happens in other ways too, even at highly experienced MSCs.  I once had a trim rod replaced in my F because it was getting cut at a bulkhead pass through.  Long annual, lots of work done, so I was doing a very thorough preflight.  During the preflight I looked at the lower empennage where it moves with the trim and noticed the paint was pretty messed up.  I thought about it some more and realized that the cockpit trim indicator was in the middle, but the empennage was set to very nose high trim which was “uncovering” a big section of paint I didn’t normally see there.  I mentioned it to the shop manager and he was like, oh yeah, the trim could be way off now, let us know and we’ll make an adjustment after you fly it.  Well holy crap, were you going to mention that?!  Thank goodness I was prepared for it because centering the trim indicator resulted in a very aggressive nose up during takeoff.

[EricJ]

5 minutes ago, Ragsf15e said:

Surprisingly, this same thing happens in other ways too, even at highly experienced MSCs.  I once had a trim rod replaced in my F because it was getting cut at a bulkhead pass through.  Long annual, lots of work done, so I was doing a very thorough preflight.  During the preflight I looked at the lower empennage where it moves with the trim and noticed the paint was pretty messed up.  I thought about it some more and realized that the cockpit trim indicator was in the middle, but the empennage was set to very nose high trim which was “uncovering” a big section of paint I didn’t normally see there.  I mentioned it to the shop manager and he was like, oh yeah, the trim could be way off now, let us know and we’ll make an adjustment after you fly it.  Well holy crap, were you going to mention that?!  Thank goodness I was prepared for it because centering the trim indicator resulted in a very aggressive nose up during takeoff.

Ideally it shouldn't be a "fly it first and let us know" kinda thing, imho.   
 

[hubcap]

I had exactly the same experience after my avionics upgrade which included the GFC 500. Fortunately for me I was prepared because of a discussion on Mooneyspace. As soon as I was airborne I knew it was off, but I wasn’t surprised. 

[Slick Nick]

How the hell are these installs getting out of the shop in the first place?? Some very concerning stuff come to light in this thread.