Auto Pilot Fails When Cold

[Dale Campbell]

I have a 337H with a 400B auto pilot. Every time I go up & temp. is below 15 degrees F, the auto pilot fails after about 1/2 hour. I just went up this week-end, after doing pre-heat of engines & got the plane all warmed up in hanger, I went flying, it was 5 degree F . Every thing work fine for 1/2 hour in the air. Than the alt. hold went crazy. We dove at 1000 feet/min, than climb same rate, than off to the left. I grabbed the yoke & disengaged the auto pilot. But it would not release. I had to over power the auto pilot than it finally released. This is nerve racking for passengers when you don't expect it. Anyone have a answer?
Dale Campbell

[Bob Cook]

Dale

Problem with autopilot is probably the servo's. If we are dealing with the pitch servo then it is probably the pitch trim servo. Either the clutch is slipping or the motor cannot overcome the stiff grease in the servo. There is a proceedure in the manual which covers lubrication and checking of the clutch tension. Wrong type of grease or OLD grease can cause this problem.

1(Does the autopilot disconnect with the sw on the control colomn?
2)does the main power switch (rocker) drop out at anytime?
3)do you see any movement of the pitch trim after it starts to act up?
4)does the rest of the AP work okay? (roll axis)?

I am suspecting the pitch trim servo and not the elevator servo. This is the one in the pax side rear boom.

If you do not have a manual I can email you the proceedure to clean and adjust by your mechanic.

fyi

bob

[Kevin McDole]

Dale,

This is a good lesson for all. If you're going to use your autopilot, you need to know several ways to immediately disengage it *BLINDFOLDED*.

Here's my list:

1) Yoke disconnect switch.
2) The autopilot on/off switch
3) The circuit breakers (can you find them at ngith without a flashlight?)
4) The master avionics switch
5) The aircraft master switch

For the circuit breakers, you can either have your shop move them to a convenient memorable location (like last switches on the right on the bottom row), or there are plastic collars you can place on the breakers to make them easy to find by touch.

While we're at it, making the gear pump motor breaker easy to find is also a good idea.

[MikeZ]

Dale: I had the same problem for years and years with my 400 series autopilot (not cold weather, but intermittent HARD wingover). Passengers LOVED it! (just kidding).

I would have another odd problem. Cold start on the ramp, it would run the aileron servo hard left. I'd resist it (get George in a head lock) until it just sighed and let go. Then I'd fly and it would work most of the time, till it didn't.

Eventually I got sick of putting money into it and got an S-Tec 55. If you are going to keep the plane, I recommend biting the bullet and getting rid of the old autopilot. Once it gets to the point that you are having that kind of problem, its not worth the aggravation.

Regards,


mike zinn

[Bob Cook]

The autopilot, or any other piece of equipement, needs maintenance/ to be serviced once in a while. Probably the AP is the last to ever see a screwdriver or lubricant. These are electromechanical (electronic and mechanical) devices that should have servicing done and that includes the stec55 after awhile.

The 400b is a great autopilot and is not just a roll-rate device. You shouldn't throw out the baby with the bathwater just because it decides it needs some help.

If you cannot find some reliable shop to maintain it and you psych'd up to spend money then that is reason enough to change... I guess. The 55 is not the same level of sophistication as the 400 as it only acts AFTER a change in direction occurs.

maybe all the 737's built in the early 60's should get stec's as well. BTW the DC8 and 9s are still running around with their original APs.

mtsho (my not so humble opinion ) <G>

Kevin......

If the AP did NOT disengage then there is something seriously wrong as the power to the unit is fully disconnected when the front rocker switch (relay) is off or disconnected.

secondly, It you had to overide the controls with the AUTOPILOT OFF then for some strange reason the electromagnetic clutch did not disengage on the servo! This is rather strange. This does not make sense. The AP must not have been disengaged. The switch on the control colomn is in series with the rocker switch hold down coil (relay) so energizing this switch would drop ALL power to the AP. It does not make sense ... logically.

Bob

[Dale Campbell]

First I would like to thank all that helped with problem. Second I would have replaced unit with S-TEC if I knew of all the problems I was going to have. I have invested over $6000 in the last 2 years triing to make this unit work right. I have sent this unit back to Auto Pilot Central 7 times in 2 years. They replaced many components & it still fails.

[Bob Cook]

Dale AND OTHERS.

The autopilot is a device that is integral (married) to the aircraft. Pulling out the autopilot and sending it off does NOT necessarily fix anything. There are servos, computer, control head, switching, gyros all a part of the SYSTEM including aircraft rigging. Are you going to send all this stuff in as well?

This is quoted directly from the cessna 400 autopilot manual

Periodic preventative maintenance inspections reduce possibility of system failures....... Perform following every ANNUAL inspection or every 100 hrs of operation whichever comes first and also after REPLACMENT of any defective parts.

Comments: bet you did not send in any servos......

Navomatic 400 units

1) check connectors etc
2) inspect ALL interconnecting wires for abrasion etc.
3) inspect all vacuum connections (vertical hold)
4) inspect mounting hardware
5) inspect ELECTRICAL CLUTCHES and ACTUATORS for dirt or improper operation
6) Check TORQUE on mechanical slip clutches and actuators.
7) check computer amplifier STATIC AIR INLET connector.

RIGGING
a) inspect sprockets and chains for wear. Clean and lubricate as specified n in para 2-17
b) check mechanical connections between RIGGING HARDWARE andCABLES for security and signs of wear.
c) Check tension of aileron, elevator and actuator cables. It is essential that all acbles are kept tight for proper operation of the autopilot. See service manual for the particular aircraft involved. or control cable tension values.
d) check aileron and elevator control systems for binding and roughness of operation.

LUBRICATION
The acutators and drive chain should be lubricated with aeroshell no 7 grease at the annual inspection or after every 100 hors of operation whichever comes first. Before applying new lubricant to any surface, REMOVE OLD LUBRICANT with approved cleaning solvent. Note: You should clean and lubricate the servos (internally) on a periodic basis as well.

SUMMARY

Since the A&P must go the the autopilot service manual for this information the work NEVER GETS DONE!. When is the last time the above proceedures were ever carried out on your aircraft?

You cannot send your arm in to a doctor to see if you have colon cancer anymore than you can send the computer in to diagnose servo problems.

IF YOU WANT YOUR AUTOPILOT (any) TO OPERATE AT TOP PERFORMANCE then you need to adhere to the above AND take the aircraft to a suitable shop that can inspect, diagnose, and repair your aircraft and problem. REPEAT..TAKE THE AIRCRAFT TO A SUITABLE SHOP! Sending any parts in to be checked is no insurance that the autopilot is going to work. Read paragraph one.

Putting in a new autopilot will solve the problems for a few years until the lack of maintenance problem rears it's ugly head...
AGAIN.

The autopilot is an INTEGRAL PART OF THE AIRCRAFT AND SHOULD BE SERVICED AS SUCH... ask alaskan airlines regarding proceedures.

THERE IS NO EXCUSE FOR PROPER MAINTENANCE ON ANY AIRCRAFT OR ANY SYSTEM ON THAT AIRCRAFT.

My 400B works great and I do TAKE it to a good shop bi-annually.

fyi

Bob

[Dale Campbell]

Thanks Bob, for input to my problem. I called Auto pilot Central &
they told me to call a shop in Reading, Pa. that know the Cessna 400 auto pilots. I just called Reading & got a appointment for diagnostics 2nd week in Feb. I plan on printing your reply and taking it to the shop with me. Thanks again. Dale Campbell

[Bob Cook]

Dale

Glad to hear you are taking it to a shop! There are a number of excellent shops that are still very familiar with the 400B. It was used in all the 400 series aircraft incl. the 414.

Make an accurate list of the problems and insure they understand.

Secondly it is worth a test flight with them to check the roll limits, capture and GS lock capability. IT is capable of fully coupled approaches. If you have the flight director then have them set up the command bars. You should be able to intercept the loc at 45 degrees and fly a full approach to the MAP (hands off), hit the go-around button and see the command bars jump to 15 deg , uncouple, pitch up and fly the outbound. If not ... then don't leave until it does.

Test fly and make sure it does a max rate one either left or right before you get there. This way they can eliminate messing around with the AI /GYRO.

Make sure they take out the gyros and do a clean and relubricate all 3 servos.

One last point. Have them replace the small blue connector on the flux gate compass which has been prone to corrossion due to dissimilar metals. The new ones today have gold plated pins that will not corrode. At least have them check it. it is outboard on the port wing. It is a bit tricky to get at. It can cause havoc and cost time and money to find.

Half the problem is pilots go in and say "it doesn't work". That fine ... if you want to spend lots of money and waste technicians time.

Never send out a computer for repair without insuring it for the cost of a new autopilot. Losing it can be a disaster!

http://www.avionicswest.com/articles..._autopilot.htm

is worth reading.

Bob

[Dale Campbell]

Thanks again Bob, I printed your last reply to take with me also.
I hope the guy at Reading can do all we ask of him. He was recommended by Randy at Auto-Pilot Central. The radio guy at O & N Aviation was not to good the last 7 times I took it there. Dale Campbell

[MikeZ]

Good luck Dale. I feel bad about the $6000. But for the $, I agree with Bob, nothing wrong with the 400 AP if it works.....

[bede1@msn.com]

Dale,

Just last week had same problem in my P337. Took it to shop and they found small leak in pitot sytem which only showed up when cold. (really cold). It was in the auto piloy pressure swith. After they replaced it I flew back home and problem was gone.

Bob

[Dale Campbell]

They trouble shot the system and determined the voltages not right from computer. The computer assembly was removed & sent to Auto Pilot central. I have not heard back yet as to what they found. Dale