Interesting Prop Heat Issue

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    My prop heat has had a few gremlins over the past year. First it went out completely. Of course, it chose to do this as I was climbing through a 5,000 ft thick moderate icing layer in Newfoundland. Fortunately the boots worked, and I just kept the props at 2700 RPM, which worked well enough. 🙂

    That turned out to be a burned out wire in the controller. Controller fixed, prop heat works, back on with life.

    Then the prop heat had gremlin number two. Turn on prop heat, breaker pops. Fly another 30 minutes, try again, works just fine. Hmm. Problem wouldn’t show up with any regularity.

    Yesterday we had the spinner off the right engine to dynamically balance the prop on that side (left side will be another day and I’ll report on the total effect when done). While the spinner was off, we noticed that one of the wire bundles for the prop heat had rubbed against the Adel clamp, gone through one of the wires, and was shorting out intermittently. This makes sense. So, I need to order a new set of wires. The Adel clamp was also a DG6 (3/8″) which seemed to be a bit loose on more than one of the wire bundles, which is probably why it had slipped through in the first place. If you look at the wire bundle, the sheathing has a retention feature that works with the Adel clamp. So I think I might order some 5/16″ clamps for when it’s fixed.

    Thought I’d share in case someone else has the same issue at some point.


    Thanks for sharing. Probably when it was popping the breaker it was arcing the wire bundle to the frame, and probably burning the wire strand so that it did not make contact the next time you tried it? How many strands did you have left? 🙂

    A good reminder to check for rubbing, interference after any maint too. A 2nd set (or third) of eyes is invaluable in catching stuff. Also, some things do not show up until hrs are flown.



    There’s enough strands that it does still work when vibrated correctly, as verified by both the ammeter and my hands. 🙂

    But yes, it’s just one of those very interesting failures. We caught it just by luck. Which reminds me, now I need to look up that part number…


      Ted, why not just splice the wire with a butt splice? Then paint the splice ends with a waterproof sealer like the goo used to dip tool handles in.


      Dick, I considered that as well. The location of the rub on the wire would make that difficult, and the sheathing is formed so that it will be held in location without slipping in the Adel clamps. I may still do that, depends on how much I can buy the replacement harness.


        My ammeter is pegged all the way to the right when the prop heat is turned on.

        The mechanic has verified that the prop boots are working properly. To try and isolate the problem, he has by passed the timer and showed me some other stuff he did.
        He went over the wiring schematic with me and showed me how the system works and all of the items he checked. He has verified that he system is using between 13-14 amps.

        He ordered an o/h ammeter and it acts the same way as the old one. My mechanic is stumped. Any ideas?


        Did you check the shunt? I wouldn’t worry about it – if it was really a problem, it’d be blowing the breaker.


          I know of a pilot who turned on his prop heat while in ice, the prop ammeter went to full deflection and stayed there even after turning off the prop heat switch. I forget if he pulled the breaker. This was in a C414 at 9,000′. He then noticed smoke from the instrument panel where the ammeter was located. Smoke also came out from under the floor as well as from the switch panel. He declared an emergency, and began an emergency descent. Lots of smoke filled the cockpit and the doctor in the back of the plane thought that the pilot was turning purple so he opened the cabin door during the descent causing it to depart from the airplane. They got below the cloud deck at 900′ and landed at a nearby airport. This 1971 airplane was totaled due to the significant burning, and welding, of many wire bundles along with other issues.


          Shut off entire electrical system (alternators and battery) if you have an electrical fire. IPad works well for navigation. (He may have done this but the system may have stayed on due to welding of electrical wires/components.) The fire fighters had to cut into the wing to disconnect the battery.

          Open outflow valve by selecting depressurize in order to remove smoke from a pressurized cabin.

          Fly the airplane. This pilot landed safely.

          Land as soon as possible – field, road, airport, whichever is closest.

          Safe flying,


          I’m in the middle of a panel upgrade and we found significant evidence of overheating, melting, and possible burning on the wires going to the prop ammeter.


            My only double alternator failure occurred immediately after I turned on the prop heat. The prop ammeter quickly went to a full deflection then to zero. At the same time both alternator inop annunciators illuminated followed by the low voltage annunciator. The timing was perfect – I had just briefed the pilots, during our pre flight briefing, on how to handle a single or dual alternator failure.

            If this happens and it can be fixed in flight, then the fix is easy. There are only four things you can do to get the alternators working again. See if you can remember these four things before reading the answer!

            1. Check the CBs and reset after a three minute cooling period.
            2. Turn the alternator switches off then on.
            3. Replace the alternator field fuses if they had blown.
            4. Turn on the emergency alternator field switch.

            Of course, this does not have to be memorized because it is all spelled out in the emergency checklist.

            Number 2 fixed our problem.

            Safe flying,


              My mechanic threw his hands up, he can’t fix it!

              I think I’ll take the plane to Tom’s at Long Beach. Any other recommendations for a Cessna shop in the Southern California area?

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