Autopilot question

M

mwkramer

Filing Flight Plan
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Apr 27, 2011
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7
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West Central Wisconsin
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Display name:
MikeK
Hi first post here. I'm a low hour pilot just 115 hours or so and only hour 5 with the KAP 140 w/Alt in this 182.

On a recent flight from KLXT to KCID I got to altitude straight and level and on course. I engaged the KAP 140 in nav (its coupled to a Garmin 750) mode with alt holding. After twenty minutes of normal flight we begin a slow right hand turn. I kill the ap and put us back on course and start the ap again. No problems for a few minutes then it drifts slow to the left then slow to the right back on course. Another ten minutes goes by and it starts the drift to right again. This time it doesn't stop and I let it go to see what its up to. After we where off course by 20 miles I reset it again and put us back on course. In two subsequent flights I had similar issues and resolved by manually resetting and voila everything worked out in the end.

Any ideas whats going on? Is this just normal KAP 140 wonkyness?

Thanks for any help you can offer!
 
Are you sure it was the KAP 140? Not the G750? Want to make sure the autopilot is getting the right instructions.


I used to fly with a KAP 140, did not have any real trouble. When flying though turbulence I always kept it off.
I used it only on long trips, when I'd be flying straight and level for about an hour. Sometimes I remember it had trouble keeping altitude, I'd have to tell it to climb or decent 200ft. So I can not say that it always worked perfect, but it did work.
 
Doesn't sound right.

Troubleshooting - does it behave the same way in HDG mode (which would indicate a problem with the output of the autopilot or servos if it does, and a problem with the input side if it doesn't).

I've seen behavior like this in a system where there were separate NAV and GPS and the pilot was following the GPS and the autopilot was following the VOR, but that shouldn't be possible with something like the 750, where I expect whatever you've selected to drive the CDI (GPS or VLOC) should drive the autopilot too.
 
Are you sure it was the KAP 140? Not the G750? Want to make sure the autopilot is getting the right instructions.
Click to expand...

It did occur to me that it could be the Garmin and while I can't be positive it would seem odd that the course would pick up again after I reset the KAP 140 and not the Garmin. There was no turbulence and I was at 6500' with a 7 knot headwind component.
 
Troubleshooting - does it behave the same way in HDG mode (which would indicate a problem with the output of the autopilot or servos if it does, and a problem with the input side if it doesn't).
Click to expand...

I will try that on the next cross country. I am sure that the 750 was feeding it GPS information rather than RNAV I double checked that.
 
The failure mode mentioned by TMetzinger is worth testing for.

The old Piper that I rent has that symptom. It has one-axis autopilot, ancient compared to the KAP-140, and it can track a heading in the HDG mode, but not a navigation input like the VOR or GPS. If I select one of those inputs on the autopilot, the plane just goes into a gradual turn, without any regard to the navigation input.

Do any autopilots have warning indicators to show that they are not receiving a valid input?
 
Bigger question - has it worked properly in the past, by your experience?

How recent is the 750 install (pretty recent I bet since that's a new unit)?
 
Bigger question - has it worked properly in the past, by your experience?
How recent is the 750 install (pretty recent I bet since that's a new unit)?
Click to expand...

Your right it is a recent install. It has however worked without issue on two other trips with me. One quite short(less than thirty minutes) and one from KLUM to KLXT (over three hours).
 
OK , assuming HDG works, then my guess is that either:

Something changed in the setup of the G750 so that it's no longer putting out the signals to the KAP140 in the proper format. This is likely to be something that's configured in the G750 software by an avionics tech.
A connection between the G750 and the KAP140 is loose/broken so that the signal's not getting there correctly (first thing I'd check, these kinds of gremlins are common after installation).
The KAP140 has a problem on the input side - either it's no longer configured for the type of signal it's getting, or it's broken and not interpreting it right.

You have given me a really good idea for future autopilot designs though... it should be possible to have more troubleshooting built into the unit, so an operator or tech can be told by the unit what it "sees" at the inputs from various sensors and equipment.

Please keep us posted with what you find.
 
TMetzinger said:
OK , assuming HDG works, then my guess is that either:

Something changed in the setup of the G750 so that it's no longer putting out the signals to the KAP140 in the proper format. This is likely to be something that's configured in the G750 software by an avionics tech.
A connection between the G750 and the KAP140 is loose/broken so that the signal's not getting there correctly (first thing I'd check, these kinds of gremlins are common after installation).
The KAP140 has a problem on the input side - either it's no longer configured for the type of signal it's getting, or it's broken and not interpreting it right.
Click to expand...

The KAP140 autopilot uses the L/R analog CDI signal and it in most installations these signals are wired in parallel to the CDI and the autopilot input. There isn't any configuration required. If the installation has a glass panel, such as an Aspen, then ARINC 429 digital data is used to interface to the PFD and the PFD or associated electronics generates the autopilot signals.
 
mwkramer said:
Hi first post here. I'm a low hour pilot just 115 hours or so and only hour 5 with the KAP 140 w/Alt in this 182.

On a recent flight from KLXT to KCID I got to altitude straight and level and on course. I engaged the KAP 140 in nav (its coupled to a Garmin 750) mode with alt holding. After twenty minutes of normal flight we begin a slow right hand turn. I kill the ap and put us back on course and start the ap again. No problems for a few minutes then it drifts slow to the left then slow to the right back on course. Another ten minutes goes by and it starts the drift to right again. This time it doesn't stop and I let it go to see what its up to. After we where off course by 20 miles I reset it again and put us back on course. In two subsequent flights I had similar issues and resolved by manually resetting and voila everything worked out in the end.

Any ideas whats going on? Is this just normal KAP 140 wonkyness?

Thanks for any help you can offer!
Click to expand...

Mike,

Can you describe the rest of the equipment in the aircraft, do you have a DG or HSI or a glass panel such as an Aspen PFD? If you have a DG, did you align the heading bug to the selected course as is required? The KAP140 has annunciators that will sequence in a specific manner, so it is always helpful when diagnosing problems in the system to be specific, such as I did this, it annunciated that and then annunciated this. There are three modes of navigation that use the same interface wiring and you can check their operation to isolate one or more mode is working or not. The three modes are NAV, APR, and BC.
 
John Collins said:
The KAP140 autopilot uses the L/R analog CDI signal and it in most installations these signals are wired in parallel to the CDI and the autopilot input. There isn't any configuration required. If the installation has a glass panel, such as an Aspen, then ARINC 429 digital data is used to interface to the PFD and the PFD or associated electronics generates the autopilot signals.
Click to expand...

In which case, assuming no glass, I still suspect a cabling problem. We don't know for sure if the CDI is working correctly either.

John, does one need to set the heading bug on the DG or the course on the CDI? I seem to remember both options depending on the autopilot when driving the CDI from the GPS, but it's been a while.
 
I was bopping along and the autopilot kept making S-turns rather than flying a straight course. As soon as I disengaged the autopilot I knew what the problem was. One tip tank was full and the other empty (due to a quirk in low throttle setting fuel flow).
 
John it has an HSI and the Garmin 750 other than that its a pretty standard Cessna Nav II package This is what I remember:
1)Set baro on Altimeter/KAP 140/Garmin 750 (reset each time Flight Following updates me)
2)Set my taget altitude on KAP 140
3)Enter destination or waypoint on Garmin get initial on course heading and set it on the HSI double check that the 750 is in GPS Nav mode not Radio navigation
4)Set my heading bug to my crosswind direction
5)Taxi and take off and ask the Garmin to give me a new direct heading put that on the HSI
6)at altitude and on course with power settings set engage the ap Press NAV button the NAV arm is briefly displayed

Under every situation so far the ap does what I expect for at least a significant portion of the time up to the entire flight. Long enough at least where I believe it was operating as expected and not just dumb luck that it was tracking the course I wanted.

I was bopping along and the autopilot kept making S-turns rather than flying a straight course. As soon as I disengaged the autopilot I knew what the problem was. One tip tank was full and the other empty (due to a quirk in low throttle setting fuel flow).
Click to expand...

I thought of that. The KAP 140 wants the tanks to be within 80lbs of each other. I always fly with my fuel selector on both but the tanks do not draw evenly on the gauges. Sticking the tanks proves that (once on the ground) the tanks are within 2-3 gallons of each other. Its hard to be more accurate than that with the fuel stick I have. Is it possible that I have a bigger fuel system issue that is only there in flight but works itself out on the ground? With all the retrimming this plane requires I could be trimming out a slightly heavy wing with the rudder and not even know it?
 
So a quick follow up. It turns out that I do have a fuel flow issue that has been getting worse. The right tank will not drain on until the left is below 50%. If I select the right tank fuel flows normally. Since I started managing for this problem I have had no further AP issues.

Now how do I fix my fuel flow problem. Two different mechanics have pronounced it fixed after messing with my venting. No luck sigh. Anybody know a fuel flow expert in the Minnesota Wisconsin Iowa norther Illinois area?
 
mwkramer said:
So a quick follow up. It turns out that I do have a fuel flow issue that has been getting worse. The right tank will not drain on until the left is below 50%. If I select the right tank fuel flows normally. Since I started managing for this problem I have had no further AP issues.

Now how do I fix my fuel flow problem. Two different mechanics have pronounced it fixed after messing with my venting. No luck sigh. Anybody know a fuel flow expert in the Minnesota Wisconsin Iowa norther Illinois area?
Click to expand...

Is the ball centered in flight? If not, the AP will tend to fly with one wing lower than the other and fuel will tend to flow that way. Properly adjusting the rudder trim will tend to fix that. You may want try trimming to the left so it puts your right wing a bit lower and see if the flow changes. That should show a slight slip to the right.
 
I rudder trim religiously. I've been reading quite a bit about this issue with 182's apparently its common. With no one true "fix". I would still like to talk to a mechanic that is familiar with Cessna fuel systems.
 
Call John Frank at Cessna Pilot's Association (and join) and then go read their tech pub on it.

You could spend half a lifetime getting a 182 to feed fuel evenly.

Even setting to Left or Right on the fuel-selector doesn't work in later models because there are so many cross vents between the tanks added to try to "fix" the problem.

An easier solution... If you can't fix it during fill-up and you have to launch with one wing heavy, put the fat guy on the other side. It'll take less rudder trim in cruise that way.

Okay I'm only half-joking. There are some real problems that can be fixed to make a 182 feed "close to even" but it's a miracle to find a 182 that really does always feed evenly. You make sure the vents are working properly, the left-side vent under the wing is positioned properly (again, CPA has the Cessna factory details on that, and it's measured in hundredths of an inch, which is rediculous) and you get it "close enough" and forget about it.

And if the aircraft is significantly out of rig (again, CPA has not only a tech pub but a certification program for mechanics for that), you'll be chasing your tail. (Literally, kinda. ;) )
 
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