Autopilot Query

[Wayne Pearce]

G'day to everyone, its winter over here now damn stinking cold.

I have a '73G normal with 400 series autopilot the other day I went flying, tried the autopilot and when turned on with or without alt hold it immediately pitched down, did it several times until I gave up

Have'nt done any research yet but I thought someone may have had previous experience and possibly shed some immediate light on the matter

Best regards ........... Wayne

[hharney]

THERE IS A PITCH CONTROL AND A TRIM CONTROL. DID YOU ADJUST THESE TO COMPENSATE FOR THE PITCH DOWN?

IN MY 337 WHEN USING THE 400 AP I HAVE TO ROLL THE PITCH AND THE TRIM CONTROL (ON THE AP UNIT) TOWARD THE NOSE UP POSITION FOR LEVEL FLIGHT

[WebMaster]

On mine I have to roll the trim down, but once there, it is fine.
If you are getting excessive movement, it may be a problem not with the A/P but with the AI, which is frequently used for attitude control

[bede1@msn.com]

I have found that (after reading the service manual) that if you pull the bank knob out and then turn the autopilot on. Then manually adjust both pitch and bank for level flight. At that time you can engage the nav section by pushing the bank knob back in. This is what is recommended in the manuals and seems to work on mine as I had the same problem until I went thru the service manual.

[Jim Rainer]

Wayne - you mention 400 "series" autopilots. There is the straight 400; the 400A and the 400B. They are not alike even though numbered the same. I have a 400A in a 1976 337G and when you engage the autopilot on heading or with the knob pulled out, the autopilot adjusts to the current pitch of the plane - synchonizes itself, in effect. The altitude hold will not engage until the pitch has settled down. There should be no pitch up or down when engaging the 400A - it should maintain whatever pitch is there and once synchronized you can control the pitch with the wheel. Not too far back, one of the participants of this site made availablbe a copy of the 400A manual.

I don't know about the straight 400 nor the 400B.

Jim Rainer

[Pete Somers]

Hi Wayne

If the A/P pitches the airplane down all the time, it sounds like you have either a A/H that is unserviceable or the plug has dropped off. Or the pitch servo has a couple of transistors U/S in it.
If you have a 400A, remove the controller from the center console, at the back you will find a switch marked gyro in/out, switch it to out and center the elevators, engage the autopilot and the pitch wheel should run, it should stop in almost the center position, now you can adjust the pitch using the wheel, make sure the pitch runs in both directions.

If the pitch still runs down you have a servo U/S.
If you can adjust the pitch ok, the A/H is U/S.

REMEMBER TO PUT THE GYRO SWITCH BACK IN!!!!!

Let me know?

Pete Somers

[Bob Cook]

If you have the 400 series with Alt hold then you have a trim servo.

To test ...neutralize the yoke on the ground (centre) and turn on autpilot. you should be able to adjust the roll servo with the knob (roll) and see the yoke deflect in both directions (roll).

If you put force on the yoke in or out you should see the trim servo move the trim wheel on the instrument panel. You should be able to position the yoke to stop the movement in either direction (neutral).

The electric trim should be working with the AP off. If the above test works then you probably have a problem with the barometric sensor in the ap, a disconnected static line (or leak), or you have an IA cable lose or disconnected. Next step is to go for the switch on the back of the control head.

Did you check your alt static knob to make sure it is in?
When is last time you had a static check done. Some of the tubing is getting old and creating problems, especially some of the older fittings.

The above static test is recommended prior to each flight as a preflight test.

If the trim servo does not work then you probably have a failed trim servo..

hope this helps

bob

[Wayne Pearce]

Thanks guys for all the help, unfortunately wont be able to get to my baby for 3 or 4 weeks then intend to go thru it with a dose because we are taking 8000 nm trip from the bottom of Australia to the top and back again, I will gety back with a reply after that.


Regards ........... Wayne