KAP-150 QUESTION

[Fly Boomer]

Question for any of you who have ever flown behind a KAP-150.  I'm creating a PFT (Pre-Flight Test) checklist from the Pilot's Guide, and getting behavior that I don't understand.

K-model sitting in the hangar.

GPU plugged in; Master on; Radio Master on; Elevator Trim on; Standby Vacuum Pump on; GNS-430 on; give the gyros a few minutes to stabilize.

Tap the Test button, all annunciators on both the KC-191 and KAS-297b light up; flashes and beeps five times; AP annunciator blinks; Trim annunciator goes off.  AFAIK all these automatic gyrations are expected.

MET (Manual Electric Trim) on yoke:  Left side up and down, wheel doesn't move BUT I can't overpower the MET using the wheel.  Right side up and down, wheel doesn't move, and turns freely.  Wheel does not turn when holding AP DIS/TRM INT button on yoke.  Again, AFAIK all is well (except for being able to overpower the MET when left side of trim button is used).

Turn on AP ENG button on KC-191 (pitch hold/roll level mode), and box the controls to assure I can overpower autopilot -- all okay.

Add HDG on the KC-191, and the yoke turns left and right to follow the Heading bug on the KI-525 HSI -- all okay.

Now, after about 5 seconds, the trim wheel starts creeping slowly nose up.  Takes at least a minute to get all the way nose up.  I'm not sure what it's headed toward, but in flight this would not work.

Also, if I use the Up/Dn rocker on the KC-191, nothing happens for 3-5 seconds, and then the trim starts to move.  If I take my finger off the rocker, the trim CONTINUES for at least another five seconds, or until the trim is against the stop either up or down.  Again, in flight this won't work.

Sorry for the long-winded explanation, but I'm hoping someone familiar with this autopilot can suggest something to look at.

[midlifeflyer]

19 minutes ago, Fly Boomer said:

Now, after about 5 seconds, the trim wheel starts creeping slowly nose up.  Takes at least a minute to get all the way nose up.  I'm not sure what it's headed toward, but in flight this would not work.

Just to clarify, you are saying when you engage MET, (1) it takes 3-5 seconds to do anything and (2) once it starts, it won't stop?

The only time I've seen something similar, it's been with a KAP 140 and it was a servo issue. I've flown with the KAP 150 but have not come across this, so I'm not sure how similar they are.

As an aside, I no longer do AP overpowering tests beyond holding the yoke in place for the brief time it takes to disconnect. I picked up the rationale from the more recent Garmins. In the case of real runaway trim it will fight you and make the out of trim condition worse,  and I can't help but think that overpowering when not necessary can't be all that good for the system.

[Rick Junkin]

I'm no autopilot expert, but I did operate a KAP 150 for a number of years. As I recall the pitch behavior on mine seemed a little wonky (technical term) if I played around with it on the ground as you've described but it worked fine in the air.

For my preflight checks I pulled the yoke back about half way, approximately to the cruise position, and engaged to A/P to confirm it would initially hold it there. I'd then hit the trim switches on the yoke to disengage, re-select A/P and HDG and twizzle (another tech term) the HDG bug back and forth to confirm the yoke moved appropriately, and use the big red button to disengage.

Other than running the trim both directions manually and with the yoke switches, that was the only ground check for pitch that I did. With the airplane sitting on the gear in it's classic Mooney nose-high attitude the A/P is going to command  pitch changes it thinks are needed to maintain level "flight", and that's what I attributed the A/P pitch inputs on the ground to. Using the rocker switch on the KC-191 with only A/P selected is telling the A/P to change the pitch up or down, so since the airplane isn't moving in pitch I think (?) the computer will continue to command a pitch change in the direction you told it to go until it hits the stop. Is that what you're seeing?

Bad on me, I didn't check the left/right trim switches independently, or exercise the panel rocker switch as part of my checks. I trusted the computer self-test caught everything I didn't physically manipulate. Hopefully one of our King A/P gurus will be by shortly to provide you with a more complete answer and assessment of what you've reported. The delays and trim wheel behavior you describe don't sound familiar.

[Fly Boomer]

1 hour ago, midlifeflyer said:

Just to clarify, you are saying when you engage MET, (1) it takes 3-5 seconds to do anything and (2) once it starts, it won't stop?

I meant to say I checked that the autopilot would respond to changes in the the heading bug, and after that, I'm reading my checklist for the next step, and the trim wheel starts moving -- apparently uncommanded.  Not fast, but I can't figure out where it thinks it's going.

[Fly Boomer]

17 minutes ago, Rick Junkin said:

Using the rocker switch on the KC-191 with only A/P selected is telling the A/P to change the pitch up or down, so since the airplane isn't moving in pitch I think (?) the computer will continue to command a pitch change in the direction you told it to go until it hits the stop. Is that what you're seeing?

That sounds right.  Every time I run my checklist, my notes get better, so I'll have to verify.

[Fly Boomer]

1 hour ago, midlifeflyer said:

Just to clarify, you are saying when you engage MET, (1) it takes 3-5 seconds to do anything and (2) once it starts, it won't stop?

When I use the rocker on the autopilot head to change pitch, it seems slow to react -- both to start, and to stop once started.  I feel like I'm chasing it up and down.