GI275 pitch signal to KAP150 Autopilot - Pitch Porpoising

[spayne59]

Hi fellow Mooniacs

Looking for advice from anyone with a KAP150 autopilot in their J model who has replaced their AI with a Garmin GI275.

As a project with the local Garmin dealer,  we decided to remove the original vacuum powered KG258 AI and KI107 DG, and replace them with a pair of GI275 instruments, one as the ADI and one as the HSI, and remove all of the vacuum system while we were at it.  Very happy with the functionality and appearance of these new instruments.  They are fully reversionary and battery backed for 60 mins; and also the ADI emulates the analogue pitch and roll signals of the KG258 that drive the autopilot.  

We have a KAP150 autopilot which has performed flawlessly in the past. It has pitch, roll and trim servos controlled by the KC191 flight computer.  It is not the A/P version that drives a KI256 indicator with Flight Director.

From a roll/heading perspective, the system works perfectly.  Set the bug on a heading and the autopilot rolls the aircraft and then levels off to fly that heading beautifully.  The GI275 also includes GNSS and if you select it, the aircraft follows the GPS flight plan like a go kart on a race track.  Our GPS source is a GTN650, btw.

But…the problem is pitch control.  Whenever the autopilot is engaged (even just in wing leveller/pitch control mode) prior to engaging either HDG or ALT modes, we get “pitch porpoising” happening.  Clearly this is being driven by the autopilot via the pitch servo (I have confirmed it is not the trim servo doing this).  We have removed and cleaned/serviced the pitch servo in case it was the cause – but it was not.

The GI275 has analogue pitch and roll signal emulation to the KC191, so it behaves like the KG258 that we removed.  The output is 50 mVac per degree of pitch to the KC191.

Given that the GI275 analogue output pitch signal level has been confirmed as very like that from the previous KG258, the only difference I can see is the response time of the signal – and that potentially this is upsetting the control loop tuning to the extent that oscillation starts.  The new GI is presumably much more responsive at detecting and reporting pitch changes than the KG258 it replaces.  It is asking for pitch adjustments faster than the A/P can respond?

When engaging the A/P, the oscillation starts and is of mild amplitude but at a frequency I estimate between 0.5 and 1 Hz.  It is not even detectable in turbulence, but in clear air the pitch stability between A/P Off and A/P On is very noticeable.  It is mildly nauseating to passengers.  It is particularly nauseating to the owners considering the amount we just invested, and that this has resulted in a problem we never had before!

We are now at the point where the local Garmin agent are pretty much out of ideas, and Garmin are saying “you need to ask King” and King are saying “the A/P is fine, you need to ask Garmin”.  Or, of course,  we could just spend another $30k and install a Garmin digital autopilot ☹

Recognising that there are a lot of KAP150 autopilots in M20Js, and hearing of good sales of GI275s to the GA market, I really hope to hear of others that have seen this problem, and pray that someone has a (preferably easy and cheap) solution to it!

[LANCECASPER]

On 1/31/2022 at 7:43 AM, spayne59 said:

Hi fellow Mooniacs

Looking for advice from anyone with a KAP150 autopilot in their J model who has replaced their AI with a Garmin GI275.

As a project with the local Garmin dealer,  we decided to remove the original vacuum powered KG258 AI and KI107 DG, and replace them with a pair of GI275 instruments, one as the ADI and one as the HSI, and remove all of the vacuum system while we were at it.  Very happy with the functionality and appearance of these new instruments.  They are fully reversionary and battery backed for 60 mins; and also the ADI emulates the analogue pitch and roll signals of the KG258 that drive the autopilot.  

We have a KAP150 autopilot which has performed flawlessly in the past. It has pitch, roll and trim servos controlled by the KC191 flight computer.  It is not the A/P version that drives a KI256 indicator with Flight Director.

From a roll/heading perspective, the system works perfectly.  Set the bug on a heading and the autopilot rolls the aircraft and then levels off to fly that heading beautifully.  The GI275 also includes GNSS and if you select it, the aircraft follows the GPS flight plan like a go kart on a race track.  Our GPS source is a GTN650, btw.

But…the problem is pitch control.  Whenever the autopilot is engaged (even just in wing leveller/pitch control mode) prior to engaging either HDG or ALT modes, we get “pitch porpoising” happening.  Clearly this is being driven by the autopilot via the pitch servo (I have confirmed it is not the trim servo doing this).  We have removed and cleaned/serviced the pitch servo in case it was the cause – but it was not.

The GI275 has analogue pitch and roll signal emulation to the KC191, so it behaves like the KG258 that we removed.  The output is 50 mVac per degree of pitch to the KC191.

Given that the GI275 analogue output pitch signal level has been confirmed as very like that from the previous KG258, the only difference I can see is the response time of the signal – and that potentially this is upsetting the control loop tuning to the extent that oscillation starts.  The new GI is presumably much more responsive at detecting and reporting pitch changes than the KG258 it replaces.  It is asking for pitch adjustments faster than the A/P can respond?

When engaging the A/P, the oscillation starts and is of mild amplitude but at a frequency I estimate between 0.5 and 1 Hz.  It is not even detectable in turbulence, but in clear air the pitch stability between A/P Off and A/P On is very noticeable.  It is mildly nauseating to passengers.  It is particularly nauseating to the owners considering the amount we just invested, and that this has resulted in a problem we never had before!

We are now at the point where the local Garmin agent are pretty much out of ideas, and Garmin are saying “you need to ask King” and King are saying “the A/P is fine, you need to ask Garmin”.  Or, of course,  we could just spend another $30k and install a Garmin digital autopilot ☹

Recognising that there are a lot of KAP150 autopilots in M20Js, and hearing of good sales of GI275s to the GA market, I really hope to hear of others that have seen this problem, and pray that someone has a (preferably easy and cheap) solution to it!

The same thing is likely to happen if you install a new or rebuilt KG258 (King Attitude Indicator) in your panel. (Or a new or rebuilt KI-256 if you have a KFC150)

The autopilot needs to be aligned to the new attitude indicator. A shop who was a King dealer, back when they had dealers, will have a 150 breakout box (KTS-158), which they plug everything into and align it so it doesn't porpoise.

I had an Aspen installed in Bravo I owned with a KFC150 and when it would level off at full power as I was getting things set up for cruise it would porpoise a little. My plan was to take it to a shop that had the ability to align it, but I never did since once I pulled back power for cruise I didn't get any porpoising so the settings must have been close. 

Also it wouldn't hurt to lube the control yoke joints with silicone.

I am just an owner, so the real experts can correct my mistakes and give you the real advice. @Jake@BevanAviation  @Bob Weber

[spayne59]

thanks, I'll ask about that

 

[Jake@BevanAviation]

@spayne59

A 1hz oscillation can also be a product of a bad tach generator inside the pitch servo.  Without a negative feedback from the tach generator the servo will run at full speed all the time.  This ends up causing over corrections that lead into a constant oscillation.  It is also possible if the gyro alignment procedure was not done that is outlined in the Garmin IM it could cause issues.  If the pitch gain is incorrect it can cause the A/P system to react to pitch changes incorrectly.

Ask your shop if they did the gyro alignment procedure outlined in the Garmin IM (5.7.2.2).  If this has not been done, it must be done before anything else is looked at.  If they have access to the KTS158 flight line test set the process is very simple.  If they don't they will need to build a harness per the IM.  That is where I would start.

Edited January 31, 2022 by Jake@BevanAviation

[Stan]

Have/had the same issue, though intermittent.  GI275's replaced a KI252A and a KI258 on a KAP150 back in June. 

Mine did and periodically does the same, however, as with Lancecasper: "when it would level off at full power as I was getting things set up for cruise it would porpoise a little. (Even to the point of thinking of) ... taking it back to a shop that had the ability to align it, but I never did since once I pulled back power for cruise I didn't get any porpoising so the settings must have been close."

Stan

[flyboy0681]

43 minutes ago, Stan said:

Have/had the same issue, though intermittent.  GI275's replaced a KI252A and a KI258 on a KAP150 back in June. 

Mine did and periodically does the same, however, as with Lancecasper: "when it would level off at full power as I was getting things set up for cruise it would porpoise a little. (Even to the point of thinking of) ... taking it back to a shop that had the ability to align it, but I never did since once I pulled back power for cruise I didn't get any porpoising so the settings must have been close."

Stan

Same here. Sometimes I'd be flying along minding my own business in perfectly smooth air and it begins a very mild porpoise. When this occurs, which isn't very often, I disconnect the A/P and a few seconds later re-engage it, where it continues to fly smoothly. Weird.

[MarcJohnson]

I recently had a Legacy G500 installed into my K-Model.  I kept the KAP150 Autopilot since it was working flawlessly prior to going into the avionics shop.  Now I have this dramatic pitch up trim roll when I engage the Autopilot.  They are shooting in the dark to try and figure out the issue.  Even sitting on the ground, the electric pitch trim trims up until it hits the end and it still sounds like the system would continue to roll pitch up if it didn't hit a hard stop.  The thought now is that the "Trim Sense" might have gone bad in the "3.5 months" of this install.  Taking any and all ideas of testing/trouble shooting this issue.  Tired of this in and out of the shop to only go home with no improvements.  

Thank you for any information and advice.

 

Marc

[flyboy0681]

23 minutes ago, MarcJohnson said:

  The thought now is that the "Trim Sense" might have gone bad in the "3.5 months" of this install. 

 

FWIW, my personality type doesn't believe in coincidences.

[MarcJohnson]

Can't say that I understand your response.  Wish to elaborate?

[flyboy0681]

4 hours ago, MarcJohnson said:

Can't say that I understand your response.  Wish to elaborate?

Some of my posts are getting crossed here.. What I meant by the comment is that I find it hard to believe that any component would go bad by itself after the G500 was installed and not be related to the G500.

[carusoam]

Sounds like a question for Jake @  Bevan….

Remove the spaces from his screen name and invite him to the conversation if you want….

Go MS!

Best regards,

-a-

[Jake@BevanAviation]

@carusoam had a good discussion with the aircraft owner and the shop.  Gave them some things to look at now that we know why the aircraft is trimming so much.

[MarcJohnson]

4 hours ago, flyboy0681 said:

Some of my posts are getting crossed here.. What I meant by the comment is that I find it hard to believe that any component would go bad by itself after the G500 was installed and not be related to the G500.

That makes sense. I completely agree with you.  I think it’s crossed wires and we are chasing tail until we get back to the installation. Autopilots and it’s components don’t go bad sitting on the ground during an install of something different. 

[flyboy0681]

20 minutes ago, MarcJohnson said:

That makes sense. I completely agree with you.  I think it’s crossed wires and we are chasing tail until we get back to the installation. Autopilots and it’s components don’t go bad sitting on the ground during an install of something different. 

Exactly. My KAP 150 is over 40 years old and working perfectly.

[Chessieretriever]

It sounds like you need to slightly reduce the gain in the proportional part of the P&ID control loop (Proportional, Integral and Derivative) on the AP to correct for the slightly lower hysteresis of the new attitude indicator. I am not sure how comfortable avionics shops are at making these adjustments (typically they rely on factory test equipment that 'does this for them' as mentioned above).

[MarcJohnson]

Hi there Chessieretriever.

Thank you very much for your reply.  I sent your post to my Avionics guy and their reply was that the can definitely take a look at the P&ID.   HMMMMM????!!!!!  I don't know either way.  I think I will have to get to someone with "test equipment' or something because the AP worked before the avionics upgrade.  They can't figure it out.  And I am sure if the plane isn't in front of them, they are busy doing future money making tasks.

I will continue the struggle,

Stay well,

Marc 

Edited April 10, 2022 by MarcJohnson

[MarcJohnson]

On 4/1/2022 at 6:44 PM, flyboy0681 said:

Exactly. My KAP 150 is over 40 years old and working perfectly.

Well, at the end of the day, it was ONE WIRE/PIN, off by one slot/location.  We are good to go save some slight left/right oscillations at cruise with AP engaged. I can add a little counter pressure and get it to shop for a short time.  But after time, the oscillations start up again.  Shop is thinking it simply needs a little gain adjustment.  The fact that I can use the autopilot with confidence makes me very happy.  I now have Pitch/Roll hold with AP pressed.  I now have Altitude Pre-select and VS control.  Just a few more squawks to go.   Thanks for all the input.

[Belisle]

I am having the same problem. After engaging the AP, the plane will start to porpoise after 15 minutes of flight or after effecting a change in direction. Disengaging the AP stops the porpoising but starts a 3-500 fpm climb. Manually trimming the plane using the control wheel restores level flight for another 15 minutes. Pitch and trim gyros were removed pitch overhauled and trim cleaned and re-calibrated. Did not solve the problem. Garmin says problem with AP, maintenance shops says no.  Any one have experience fixing this? (201j, GI 275, GTN650xi, KAP150) 

[Pinecone]

My KAP-150 will occasionally start a pitch oscillation, I found that tapping the CWS button stops it.  Sometimes I have to do this twice.