KAP140 Autopilot Weirdness

My autopilot has started fighting with me, and I'm afraid it's going to
win.

It's a 3-axis KAP140 in a Skyhawk SP. What it's doing is ... well, it
just starts turning.

After about an hour, it goes into a turn, unannounced. There doesn't
seem to be anything instigating it. It's usually to the left, and
always a "standard" turn.

It ignores the heading bug in HDG mode. It ignores the GPS or VOR head
in NAV mode. It also ignores the ROL mode, and the APR mode - yikes!

I don't know how far it would turn if I left it alone, because the few
times it's happened so far, I've been going somewhere.

Is this a common failure? Does anyone know what might be causing this?
Is my autopilot trying to kill me?

I just went through the exact same behavior with my King autopilot. Assuming
the flight director responds to heading bug input, then the most likely
source is the roll servo. There are four drive transistors that can burn
out, and if you are lucky this is the only problem. However, the servo motor
can also be bad, and this can be expensive as well as time consuming if the
replacement is not available. It could also be a combination of both, as was
my case.

I have the old motor sitting in front of me now- it is amazingly small
considering the forces required, and also amazingly expensive ($1,500+). I
haven't gotten my bill yet, but I anticipate it'll be in the range of
$2,5000-3,000.

Good luck.

"Brien K. Meehan" wrote:

It's a 3-axis

Really?

"Dan Luke" wrote in message
...

"Brien K. Meehan" wrote:

It's a 3-axis

Really?
Two regular axises...axeses...whatever, and an evil axis.

On 8 Sep 2006 23:51:56 -0700, "Brien K. Meehan"
wrote:

My autopilot has started fighting with me, and I'm afraid it's going to
win.

It's a 3-axis KAP140 in a Skyhawk SP. What it's doing is ... well, it
just starts turning.

After about an hour, it goes into a turn, unannounced. There doesn't
seem to be anything instigating it. It's usually to the left, and
always a "standard" turn.

It ignores the heading bug in HDG mode. It ignores the GPS or VOR head
in NAV mode. It also ignores the ROL mode, and the APR mode - yikes!

I don't know how far it would turn if I left it alone, because the few
times it's happened so far, I've been going somewhere.

Is this a common failure? Does anyone know what might be causing this?
Is my autopilot trying to kill me?

Unfortunately, it's quite common. Usually ends up being the roll
servo.

On Sat, 09 Sep 2006 11:06:07 GMT, "Viperdoc"
wrote:

I just went through the exact same behavior with my King autopilot. Assuming
the flight director responds to heading bug input, then the most likely
source is the roll servo. There are four drive transistors that can burn
out, and if you are lucky this is the only problem. However, the servo motor
can also be bad, and this can be expensive as well as time consuming if the
replacement is not available. It could also be a combination of both, as was
my case.

Skyhawks don't have a flight director

At least in mine, the roll servo is one large assembly which has the
motor and all the electronics, you can't just swap out any
subcomponants - the entire unit is replaced. At least it has a gear
at the end so you don't need to deal with any of the rigging to
replace it, pop the old one out, stick the new one in, test and return
to service.

Interesting... I thought that the KAP140 was a 2-axis AP...

It sounds like your roll-servo is going south... Not bad to replace
because you just swap the old one out and the new one in..

Good luck!!

Jon Kraus
'79 Mooney 201
4443H @ UMP

Brien K. Meehan wrote:
My autopilot has started fighting with me, and I'm afraid it's going to
win.

It's a 3-axis KAP140 in a Skyhawk SP. What it's doing is ... well, it
just starts turning.

After about an hour, it goes into a turn, unannounced. There doesn't
seem to be anything instigating it. It's usually to the left, and
always a "standard" turn.

It ignores the heading bug in HDG mode. It ignores the GPS or VOR head
in NAV mode. It also ignores the ROL mode, and the APR mode - yikes!

I don't know how far it would turn if I left it alone, because the few
times it's happened so far, I've been going somewhere.

Is this a common failure? Does anyone know what might be causing this?
Is my autopilot trying to kill me?

Yea... Gotta have someway to combat that awful Skyhawk yaw..... ;-)

Dan Luke wrote:

"Brien K. Meehan" wrote:


It's a 3-axis


Really?

I realize most Skyhawks aren't going to have a FD, but it still sounds like
a roll servo as the source of the problem, unless there's a problem getting
the heading input to the servo.

"Jon Kraus" wrote in message
...
Yea... Gotta have someway to combat that awful Skyhawk yaw..... ;-)

Dan Luke wrote:

"Brien K. Meehan" wrote:


It's a 3-axis


Really?


I am sure it is just 2 Axis with alt Hold !!!!!