KAP 150 Alt hold

[Wingover]

I am curious where does the KAP AP get its alt source (GPS, G5 or transponder baro?)? Went flying today and noticed the alt hold wasn’t working. Everything else was working fine (heading bug, electric trim etc). I am wondering if something got lose in the connections. The AP pass the preflight test so it all seems fine.
 

[Fly Boomer]

Probably from your encoding altimeter.  KEA-130a.

[Wingover]

3 minutes ago, Fly Boomer said:

Probably from your encoding altimeter.  KEA-130a.

I will have to check if mine are (I have two altimeters) are encoding or not.

[LANCECASPER]

If you have altitude pre-select you most likely have a KEA-130A. It will say “Encoding” on the face of the altimeter. If you don’t have pre-select then it’s probably coming from your altitude encoder (box). 

But that may not be why it’s not holding altitude. It could be the board inside of the computer.

[Wingover]

11 minutes ago, LANCECASPER said:

If you have altitude pre-select you most likely have a KEA-130A. It will say “Encoding” on the face of the altimeter. If you don’t have pre-select then it’s probably coming from your altitude encoder (box). 

But that may not be why it’s not holding altitude. It could be the board inside of the computer.

I don't have pre select but the interesting thing is that I got a message on my Garmin GPS "Pressure altitude source inop or connection lost” so I thought it could be related.  I have a G5 and GTX330 with Alt so I was wondering if it was getting the alt source from one of them.

[Ragsf15e]

This could be an interesting one… technically, the G5 can only provide heading to non garmin AP.  However, depending on when the G5 was installed, it was legal to use it as the altitude encoding source, so possibly that altitude is fed to the AP for altitude hold?

Another interesting tidbit… my G5s were set up as the encoder and I was occasionally getting an encoder failure on my transponder.  Recycling the transponder off/on would fix it.  It took a long time to get the straight answer (and they were still shady), but garmin acknowledged a software issue with the G5s acting as an encoder and offered to send me their little external encoder that attaches to the transponder for free.  

[Wingover]

24 minutes ago, Ragsf15e said:

This could be an interesting one… technically, the G5 can only provide heading to non garmin AP.  However, depending on when the G5 was installed, it was legal to use it as the altitude encoding source, so possibly that altitude is fed to the AP for altitude hold?

Another interesting tidbit… my G5s were set up as the encoder and I was occasionally getting an encoder failure on my transponder.  Recycling the transponder off/on would fix it.  It took a long time to get the straight answer (and they were still shady), but garmin acknowledged a software issue with the G5s acting as an encoder and offered to send me their little external encoder that attaches to the transponder for free.  

It was installed in 2007 as far as I can tell.

[LANCECASPER]

1 hour ago, Wingover said:

It was installed in 2007 as far as I can tell.

G5 didn't come out until 2016

[Jake@BevanAviation]

Altitude hold information comes from the transducer that is internal to the KC192/KC191.  Can you give more detail on what issue you experienced with alt hold in flight?

[Wingover]

7 hours ago, Jake@BevanAviation said:

Altitude hold information comes from the transducer that is internal to the KC192/KC191.  Can you give more detail on what issue you experienced with alt hold in flight?

It doesn’t hold the altitude when I press the alt hold button.  Used to hold it well when pressed.

[Wingover]

7 hours ago, LANCECASPER said:

G5 didn't come out until 2016

Meant to say 2017, not 2007

[Jake@BevanAviation]

@Wingover A slow drift, a oscillation, hard pitch up or down?  If you have a video of the issue maybe that would help.  Most common issue for a slow drift would be servo startup voltage, a 1-2 hz oscillation would be failed tach generator, a hard pitch up or down would be failed transistor drive.

[Wingover]

@Jake@BevanAviation thank you for the explanation.  It just stays at whatever attitude the plane is prior to pressing the alt hold button.  I was climbing at 600fpm and when I decided to level off, I pressed it but noticed that the plane was still climbing.  After that I tried a few more time while fairly level but it was like hand flying, didn't oscillate or hard pitch.

What do you mean by "tach generator"?  the same one that is on the engine for the RPM gauge??  

[Jake@BevanAviation]

@Wingover There is a small dc motor that is attached to the drive motor inside the servo assembly.  It provides negative feedback to the computer to tell it the direction of rotation and how fast.  If the tach feedback motor is inop the servo will end up over driving causing a 1-2hz oscillation.  Sounds like you might have a pitch servo issue.  If part of the transistor bridge fails it will cause a hard pitch over or no drive in one direction.  You can test this by using the UP/DOWN rocker in the computer to change the pitch attitude.  You should be able to set a climb or descent attitude and the computer hold that attitude.  When activating alt hold the computer takes the attitude information and the output of the altitude transducer to maintain a selected altitude.

[Wingover]

@Jake@BevanAviation I get it now.  So the rocker switch (up/down) on the AP works, also the electric trim works (although it obviously disconnects the AP).  I guess the trim servo works?  Also the ground test pass so the AP thinks it is all okay.  I don’t get the slow oscillation, it’s just like you never pressed the alt hold.  I have to hand fly (push/pull the yoke) to keep altitude.

Any idea where to go from here?

[Jake@BevanAviation]

I would check the output of the transducer inside the flight computer if the aircraft was here.  You can remove the static line to the flight computer and attach a hose to the unit. When applying pressure or vacuum to the hose the computer should respond in the opposite direction.  For example with a level attitude source or the ADI unplugged.  If you applied pressure to the static line the computer should want to climb to get back to target altitude.  If you create a vacuum the system should want to descend to get back to target altitude.  You can do this with your mouth as the system is very sensitive to pressure changes. This would test to see that the transducer assembly is functioning.  

NOTE: there is a interlock circuit in the computer that if there is an issue with alt hold it will not allow you to select the mode and you will not get the annunciation of ALT on the bezel.