GFC500 IAS Climb oscillations: post gain addendum

[201Steve]

I know there’s a million page thread out there on this, but since Garmin released the gain addendum to supposedly correct this issue, what’s everyone’s result?

I have found very little change. Everything works great on my AP except climbing through turbulence in ias mode. It will chase the speed by 10 knots. If bumpy, I initially climb in VSP. 
 

I had my avionics guys update the gain ad per the Garmin SB. No noticeable change. 

[PT20J]

I have the original gain settings. It holds airspeed well in smooth air. It chases it a little in turbulence, but I have a hard time holding a constant airspeed in turbulence manually. I use PIT if it’s bumpy. Tightening the pitch servo bridal cables to max spec helped. 

[mhrivnak]

My understanding is that it's common for the GFC 500 to do a poor job of maintaining IAS during a climb in turbulence, even on other airframe types. For example my flying club has 5 Piper Warriors and sees that same issue. IMO, pitch mode is the right tool for the job in that scenario. I like to climb at around 100kts IAS in my J, so in turbulence I'll establish that, and then turn on pitch hold. Then just keep an eye on your IAS to keep it in a desirable range, and adjust pitch if needed. It's a much more comfortable ride that way.

Even if the GFC 500 could do a perfect job of maintaining IAS in turbulence, it would still be doing significant pitch changes, and I think I'd prefer to just maintain pitch and accept the fluctuations in IAS.

[Pinecone]

My K had pitch oscillations with the original gain settings.  My shop did the SB gain setting change and that solved the issue.

But I started having some unexplained auto pilot disconnects (others have reported this also).  So my shop thought maybe go back to the original gain settings.  But they found that Garmin had changed the settings in the STC, so they put those in and so far, minimal pitch oscillations (1 - 2 knots) and no disconnects.

So it appears that there are 3 sets of gain settings.  The Original STC ones. The SB ones.  And the New STC ones.

[201Steve]

4 hours ago, Pinecone said:

 

So it appears that there are 3 sets of gain settings.  The Original STC ones. The SB ones.  And the New STC ones.

For crying out loud!!!!

[201Steve]

7 hours ago, PT20J said:

I use PIT if it’s bumpy.

What’s your method for this? Set up to desired speed manually, and then just click on the AP without any preselections, then select the desired horizontal source?

pitch vertical and selected horizontal?

(I wish pitch could be selected with its own button like any of the other settings)

[Emmet]

5 hours ago, Pinecone said:

My K had pitch oscillations with the original gain settings.  My shop did the SB gain setting change and that solved the issue.

But I started having some unexplained auto pilot disconnects (others have reported this also).  So my shop thought maybe go back to the original gain settings.  But they found that Garmin had changed the settings in the STC, so they put those in and so far, minimal pitch oscillations (1 - 2 knots) and no disconnects.

So it appears that there are 3 sets of gain settings.  The Original STC ones. The SB ones.  And the New STC ones.

Can you post the new STC settings?

[PT20J]

39 minutes ago, 201Steve said:

What’s your method for this? Set up to desired speed manually, and then just click on the AP without any preselections, then select the desired horizontal source?

pitch vertical and selected horizontal?

(I wish pitch could be selected with its own button like any of the other settings)

PIT is the default setting if no other vertical modes are selected. When the AP is first engaged it will hold the current pitch attitude unless you select another vertical mode. You can use the thumbwheel to adjust the attitude. If another vertical mode is selected, cancelling it will revert the AP to PIT.

[Pinecone]

14 hours ago, Emmet said:

Can you post the new STC settings?

They are available to Garmin dealers.  My dealer may give them to ME, but I would not distribute them

[William Munney]

So far, so good on mine.  Anytime you are in turbulence the AP will chase VS or IAS climb modes.  Better off turning it off and holding a pitch attitude and accepting changes in IAS or VS.

[201Steve]

@William Munney I like your clean panel. Not overfilled w gadgetry. Not sure how I feel about the offset screen but I see what you’re doing. 

[William Munney]

3 hours ago, 201Steve said:

@William Munney I like your clean panel. Not overfilled w gadgetry. Not sure how I feel about the offset screen but I see what you’re doing. 

Thank you.  I like it as well.  It was done that way when i bought the airplane and that was my first impression as well.  However, when the G500 TXi is in multi-function mode, as seen here, the PFD is centered on the yoke.  Most the time I am in multi-function mode looking at the moving map or weather or expanded engine instruments or the flight plan or approach charts etc…  When expanded you don’t notice anything.  It’s a clean, brand new interior and panel.  The transponder and audio panel are remote and controlled through the GTN750.

The GI275 is bright and packed with information.  I honestly wish it was just a bit larger.  The GFC500 is a giant leap ahead of the the King KFC150 which I replaced.  The Flight Mode Annunciators (scoreboard) on the PFD and GI275 along with VNAV and ESP make it a true pro autopilot.

Edited March 20 by William Munney

[Rick Junkin]

On 3/19/2025 at 8:23 AM, Pinecone said:

They are available to Garmin dealers.  My dealer may give them to ME, but I would not distribute them

What's the revision number on your GFC500 Gain Addendum?

[William Munney]

On 3/18/2025 at 5:08 PM, 201Steve said:

What’s your method for this? Set up to desired speed manually, and then just click on the AP without any preselections, then select the desired horizontal source?

pitch vertical and selected horizontal?

(I wish pitch could be selected with its own button like any of the other settings)

The users manual for the GFC 500 gives you all the information but not the actual method of operation.  It’s all in there somewhere but not obvious in some places.

One way to look at it is that the selections on the GFC500 for Roll/ AP mode/ Pitch are what you would like the autopilot and FD to do.  The FMA’s and the FD are telling you what the AP will actually do NOW and in the future (Alt Armed, VNAV armed etc).  The Flight Mode Annunciators and FD are the key to understanding the autopilot.  This will probably be new  for first time users of this autopilot.  It operates more closely to what you would find in a Boeing than what you would find in past GA autopilots.

In general, don’t use the FD if you are not using the AP.  It’s added work to make the FD do what you are currently doing with your hand flying.  And, its dangerous to develop habits that ignore the FD.  There are plenty of times I do not use the AP.  I like hand flying.  Your choice.  But, it’s going to be hard to beat this AP/FD on an instrument approach.  The only time I use “mixed automation” (AP=off and FD=ON) is on instrument approaches when i am hand flying.  In that situation its nice to have to autopilot chiming in saying “this is what i would do” through the FD commands.

General recommended AP use:

On Takeoff:  Select the FD on and push TO/GA to give the FD pre-programmed roll and pitch commands.

At a safe height:  Select a roll mode (NAV/ HDG) and verify with the FMA.

Then, select a Pitch Mode: (PIT/ VS/ IAS) and verify with the FMA.

The FD should now be pretty close to your actual pitch and roll.

Now, select AP and verify with the FMA.

Summary:  The FMA’s and the FD position are the windows to the soul of the AP.

 

[Pinecone]

5 hours ago, Rick Junkin said:

What's the revision number on your GFC500 Gain Addendum?

No idea.

How can I find it?

We redid the gain in September or so.  It was fine in October going to/from the TX PPP.

[Rick Junkin]

53 minutes ago, Pinecone said:

No idea.

How can I find it?

We redid the gain in September or so.  It was fine in October going to/from the TX PPP.

It’s on the cover page of the addendum itself. You’ll probably need to ask your avionics shop which one they referenced. The latest one I’ve seen for the M20J/K is Rev 5 that came out in February of 2024.

[Pinecone]

2 hours ago, Rick Junkin said:

It’s on the cover page of the addendum itself. You’ll probably need to ask your avionics shop which one they referenced. The latest one I’ve seen for the M20J/K is Rev 5 that came out in February of 2024.

They finished the job in late May, so it would have been that Rev.

But it was redone in September, IIRC.   They pull the latest from Garmin dealer website each time.

[PT20J]

The latest addendum didn’t change the primary settings. It did provide some optional settings to be used if “pitch or pitch trim is overreactive.” So, most likely an installer would start with the standard settings and only use the optional settings if unable to resolve pitch issues otherwise. My understanding is that while the autopilot is primarily attitude based, it does have a rate component to improve dynamic response and apparently this causes problems in some airframes. The changes dial back, or perhaps eliminate, the rate component. 

[Pinecone]

When my shop went to reset my gain settings back to the STC settings, they found that those settings had changed from what they had originally set.

Just going by what they told me.  But the settings that are in it now do not pitch oscillate and do not have random AP disconnects.

[Emmet]

On 3/19/2025 at 2:23 PM, Pinecone said:

They are available to Garmin dealers.  My dealer may give them to ME, but I would not distribute them

They can simply be looked up in the G3X - so nothing to hide or be aware of …

[NickG]

I use the VS mode to set a climb rate and then adjust it to get the approximate airspeed I want. THEN I activate IAS mode. Works well. As stated, it’s a little unreliable in turbulence. If I don’t set it first using VS mode, it oscillates a lot trying to get the right speed.

[PT20J]

Try hand flying a constant airspeed climb in turbulence and compare to the autopilot.