Autopilot hot to the touch

[r0ckst4r]

1995 M20R with KFC150 autopilot

Took a long, almost 4 hour flight today.   About 3 hours in I noticed some ups and downs with the pitch.   I went to touch the autopilot and it was very hot.  Much hotter than it should be.

Now it was between -5 and 0 C at altitude for most of the flight so the heat was on the whole time and coming from the lower vent.  I disengaged autopilot and hand flew for a while but it did not seem to significantly cool the unit.  I then pulled the defrost lever which changes the heat from the lower foot vent to the dashboard vent aimed at the windshield.  This does not heat the cabin as well but it also significantly reduced the temperature of the autopilot unit to its normal operating temp (by feel).  I re-activated it and it seemed to function normally for the rest of the flight.

My questions is this.  Is it possible that I have some sort of leak in the ductwork blasting heat against my radio stacks?  I have a DME and radio above the KFC-150 that were also hot to the touch.  The KFC-150 autopilot is on the bottom of the stack.  It would maybe explain why switch the vent cooled the electronics.

Or.....

I've read about the cooling fan for the electronics failing and overheating the stacks.  Maybe it was just the fact that switching the vent didn't heat the cabin as well and it cooled off just because the ambient air was cooler?   Is there a simple way to test these hypotheses?

[redbaron1982]

In my J, the KFC150 has always tended to run on the hotter side. Because of that, I had some issues with the KFC computer. While I was getting it fixed, I checked the electronic cooling fan and discovered it wasn't working, so I had that repaired as well. Now, when I turn on the master switch, I can clearly hear the electronic fan running.

In your case, while on the ground with the engine not running, I'd try pulling the electronic fan breaker and listening to see if you hear it spin down. If you don't hear anything when you pull the breaker, that would suggest the fan may not be working.

[LANCECASPER]

57 minutes ago, r0ckst4r said:

1995 M20R with KFC150 autopilot

Took a long, almost 4 hour flight today.   About 3 hours in I noticed some ups and downs with the pitch.   I went to touch the autopilot and it was very hot.  Much hotter than it should be.

Now it was between -5 and 0 C at altitude for most of the flight so the heat was on the whole time and coming from the lower vent.  I disengaged autopilot and hand flew for a while but it did not seem to significantly cool the unit.  I then pulled the defrost lever which changes the heat from the lower foot vent to the dashboard vent aimed at the windshield.  This does not heat the cabin as well but it also significantly reduced the temperature of the autopilot unit to its normal operating temp (by feel).  I re-activated it and it seemed to function normally for the rest of the flight.

My questions is this.  Is it possible that I have some sort of leak in the ductwork blasting heat against my radio stacks?  I have a DME and radio above the KFC-150 that were also hot to the touch.  The KFC-150 autopilot is on the bottom of the stack.  It would maybe explain why switch the vent cooled the electronics.

Or.....

I've read about the cooling fan for the electronics failing and overheating the stacks.  Maybe it was just the fact that switching the vent didn't heat the cabin as well and it cooled off just because the ambient air was cooler?   Is there a simple way to test these hypotheses?

On those models there was an avionics fan. That's the first place I'd look

 

 

 

 

[r0ckst4r]

The avionics fan has its own circuit breaker?   Is it activated on the Master or Radio Master switch?

[IvanP]

Avionics fan on my Bravo is activated by Radio Master. My guess woudl the that Ovations have similar setup. Not sure if there is a separate breaker for it, but you should be able to hear the fan when you turn the radios on with the engine off. These fans are not very quiet. 

[PT20J]

Check the circuit breaker panel -- my 94 M20J had a breaker for the avionics blower. There should be a duct from the blower to the back of the rack for the KFC 150 flight computer. I would check the operation of the blower and the integrity of the duct.

[r0ckst4r]

So the blower works fine.   I tried to follow the ducts but it's kind of a jumble back there.   However,  the floor heater duct runs directly under my autopilot.   I tried to check the integrity but its hard to feel an air leak until the blower is on.   I will check this next flight but it seems to be the culprit.  It would explain why diverting hot air to the defroster ducts which are no where near the autopilot significant cooled it.

[LANCECASPER]

1 hour ago, r0ckst4r said:

So the blower works fine.   I tried to follow the ducts but it's kind of a jumble back there.   However,  the floor heater duct runs directly under my autopilot.   I tried to check the integrity but its hard to feel an air leak until the blower is on.   I will check this next flight but it seems to be the culprit.  It would explain why diverting hot air to the defroster ducts which are no where near the autopilot significant cooled it.

If your heater hoses are original they are probably in need of replacement. They fall apart over the years. They were probably SCAT, but if you're going to go to all of the work of crawling around back there, replace them with SCEET with finished ends and they will last as long as the airplane. @Gee Bee Aeroproducts make some nice ones

[Yetti]

sorry obtrusive thoughts won.

[Yetti]

On 12/22/2025 at 4:20 PM, r0ckst4r said:

1995 M20R with KFC150 autopilot

Took a long, almost 4 hour flight today.   About 3 hours in I noticed some ups and downs with the pitch.   I went to touch the autopilot and it was very hot.  Much hotter than it should be.

Now it was between -5 and 0 C at altitude for most of the flight so the heat was on the whole time and coming from the lower vent.  I disengaged autopilot and hand flew for a while but it did not seem to significantly cool the unit.  I then pulled the defrost lever which changes the heat from the lower foot vent to the dashboard vent aimed at the windshield.  This does not heat the cabin as well but it also significantly reduced the temperature of the autopilot unit to its normal operating temp (by feel).  I re-activated it and it seemed to function normally for the rest of the flight.

My questions is this.  Is it possible that I have some sort of leak in the ductwork blasting heat against my radio stacks?  I have a DME and radio above the KFC-150 that were also hot to the touch.  The KFC-150 autopilot is on the bottom of the stack.  It would maybe explain why switch the vent cooled the electronics.

Or.....

I've read about the cooling fan for the electronics failing and overheating the stacks.  Maybe it was just the fact that switching the vent didn't heat the cabin as well and it cooled off just because the ambient air was cooler?   Is there a simple way to test these hypotheses?

First look at the Service manual for the routing of the heat hoses.   As said above the black hose goes bad and needs to be replaced. The heated air just makes it go bad.   Even the small hose that is used on the radio cooling fan goes bad.   I would start with the radio cooling hose and work from there. Could be as easy as one of the defroster hoses fell off. In the older Mooney's the defroster was active at all times.   Not sure how the Ovation works. 

[Slick Nick]

57 minutes ago, Yetti said:

First look at the Service manual for the routing of the heat hoses.   As said above the black hose goes bad and needs to be replaced. The heated air just makes it go bad.   Even the small hose that is used on the radio cooling fan goes bad.   I would start with the radio cooling hose and work from there. Could be as easy as one of the defroster hoses fell off. In the older Mooney's the defroster was active at all times.   Not sure how the Ovation works. 

Depends if you have the optional defrost blower or not. If you do, pulling the defrost on trips a microswitch on the defrost manifold which turns on the blower fan. The fan itself if I remember is just another Bendix KA33 avionics fan. This allows you to have defrost airflow on the ground. 

[PT20J]

The black ducting behind the instrument panel for heating and ventilation is CAT which is single-ply neoprene impregnated fiberglass. It is good for low temperature applications. SCAT is the same thing except it is silicone impregnated for use forward of the firewall. SCEET is double wall SCAT and is more durable, but it is less flexible. CAT (or SCAT) is fine for the interior air ducts. SCAT and SCEET are rated for up to 500 deg F and don't require fire sleeve. The ducts from the avionics blower to the avionics trays are smaller maybe 3/4 or 1 inch in diameter. I don't know what material was used for these.

[r0ckst4r]

1 hour ago, Slick Nick said:

Depends if you have the optional defrost blower or not. If you do, pulling the defrost on trips a microswitch on the defrost manifold which turns on the blower fan. The fan itself if I remember is just another Bendix KA33 avionics fan. This allows you to have defrost airflow on the ground. 

I believe I do have to optional defrost fan.   When I pull the defrost another fan comes on and the air is diverted up to the defrost vents and away from the floor vent.   When I have some more time to work on this I'll put up some pictures of where the hoses are going.  I can give a description that there is a hose that comes from the firewall to a metal box which then has a hose that goes down to the foot vent and another up to the defrost vent. the diverter and fan I assume are in that metal box.   The avionics fan is buried behind everything in a less than accessible location as many of you know.   Fortunately for me it currently works.

[Slick Nick]

3 hours ago, PT20J said:

The black ducting behind the instrument panel for heating and ventilation is CAT which is single-ply neoprene impregnated fiberglass. It is good for low temperature applications. SCAT is the same thing except it is silicone impregnated for use forward of the firewall. SCEET is double wall SCAT and is more durable, but it is less flexible. CAT (or SCAT) is fine for the interior air ducts. SCAT and SCEET are rated for up to 500 deg F and don't require fire sleeve. The ducts from the avionics blower to the avionics trays are smaller maybe 3/4 or 1 inch in diameter. I don't know what material was used for these.

They are 5/8". Depending on the avionics configuration, 3 were the standard black rubber/nylon fiber, wire reinforced hose included with the KA33 blower kit. Two were made from a thin plastic, and were corrugated "accordion" style in order to make the tight bend from the upper two outlets on the KA33. The plastic hoses were flimsy and did not last long. If you're able to eliminate them in your installation in favor of the standard rubber hoses, it's a good idea. 

Standard 5/8" vacuum caps can be used to plug the unused outlets on the KA33. (There are 6 outlets total.) This will increase the efficiency of your blower immensely. 

It was also standard practice at the factory to leave 1 or 2 extra hoses open and uncapped, pointed at the back of the avionics rack behind the panel. This increased air exchange for the passively cooled units in the panel. This air is exhausted through circular vents in the top of the glareshield, pointed up toward the windshield. 

[Slick Nick]

3 hours ago, r0ckst4r said:

I believe I do have to optional defrost fan.   When I pull the defrost another fan comes on and the air is diverted up to the defrost vents and away from the floor vent.   When I have some more time to work on this I'll put up some pictures of where the hoses are going.  I can give a description that there is a hose that comes from the firewall to a metal box which then has a hose that goes down to the foot vent and another up to the defrost vent. the diverter and fan I assume are in that metal box.   The avionics fan is buried behind everything in a less than accessible location as many of you know.   Fortunately for me it currently works.

The hose for the defrost vent comes right off of the air mix manifold on my MSE. I do not have the optional defrost blower. Technically, the defrost air is "always on" (I have confirmed this) but flow can be increased by pulling the defrost knob, which diverts some air away from the main outlet and small pilot's foot outlet, back up into the defrost ducting. It's not the most elegant system, but it seems to work. 

[r0ckst4r]

So I just took a another 3 hour 30 minute flight from CT to SC.  I only used the defrost for heat.   The auto pilot did not get hot and functioned normally.  I tried to feel around for air leaks after switching to the floor heat vent but could not feel any but holy hell do those pipes/hoses get hot.   It may have just been the ambient heat back there for the long flight (the autopilot is the lowest in the stack and thus closest to the pipes) but it's never happened before at least not that I've noticed and I've had the plane 6 years now with plenty of winter flying.   I'll still try and do some more inspection now that it's in the hangar but still a little bit of a mystery 

[Slick Nick]

Yes they do get quite toasty. I’ve flown my plane down to -41*C and still had good heat in the cabin in flight. There is no shortage of cabin heat in the Mooney. Perhaps the downfall is that it all comes out of a single outlet in the cabin. The fuel selector is on the edge of melting, meanwhile the rear seat passengers are calling for a bit more heat. 

[Gee Bee Aeroproducts]

On 12/24/2025 at 11:37 AM, PT20J said:

The black ducting behind the instrument panel for heating and ventilation is CAT which is single-ply neoprene impregnated fiberglass. It is good for low temperature applications. SCAT is the same thing except it is silicone impregnated for use forward of the firewall. SCEET is double wall SCAT and is more durable, but it is less flexible. CAT (or SCAT) is fine for the interior air ducts. SCAT and SCEET are rated for up to 500 deg F and don't require fire sleeve. The ducts from the avionics blower to the avionics trays are smaller maybe 3/4 or 1 inch in diameter. I don't know what material was used for these.

Not my sceet as we use a custom weave in our fiberglass 

My sceet is like others scat

no compromise