Hey, That Carb Heat’s Not Right


I was thinking the other day that I’ve come to view social media as sort of like four busy Unicom frequencies continuously splattering each other until the occasional legible word breaks through. Then I realized I’ve always thought that, just more so now. But the legible word did break through last month on our YouTube channel.

A few days after I posted the engine failure video, this comment appeared: “At 13;45 your view of the carb heat control is totally incorrect, the airplane you filmed that on should be corrected. The cable clamp bolt in the arm of the butterfly shaft should rotate, the one you showed does not, and clearly the cable is bending as it actuated the arm. The wire is clamped thru the hole in the CLAMP only.” This got my attention because, well, it’s my Cub he’s talking about.

His description is mangled, but he correctly identified a problem I missed and may have caused. The Cub’s carb heat knob is probably a choke cable from a 36 Dodge—the trim control is the window crank—and the cable snakes down to an arm on the carb heat box. Some time ago—quite a while, I think—I started noticing that the carb knob wouldn’t seat correctly, as if the heat flapper wasn’t closing entirely.

I chalked this up to an old, creaky airplane showing its age, but I uncowled the engine and assured myself that despite the cable oddity, the heat was opening and closing fully. Following the sharp-eyed viewer’s comment, I uncowled the thing again and had someone work the knob while I observed. You can see what’s happening in the video; the cable binds slightly and acts like a weak spring, pushing the knob off the fully closed position while the flapper remains shut, as it’s supposed to.

That cable has a little jog in it that passes through a hole in the bolt attached to the actuating arm. The self-locking nut just snugs it up, but doesn’t really lock the cable in place.  A little shimming with lean washers freed up the bolt to rotate and now the cable is bind free and works like it used to.

I probably caused this myself when I removed the carburetor chasing a fuel leak. I didn’t notice the error when I reassembled it and neither did the IA signing off the annual. It’s kind of a subtle thing and you have to know what you’re looking for and at. I do and I missed it.

It reminded me of something I was told when I visited the Rotax factory to see how the engines are made. These days, engines are assembled with a lot of electronic oversight. Torque settings for major fasteners like rod caps and main bearings are done with electronic torque wrenches and the data is stored for posterity. But there are a lot of hand assembly steps that aren’t backstopped this way and are subject to what the factories sometimes call “four eyes.” That means one assembler does the work, a second comes along with a checklist and assures it was done correctly. They still occasionally miss things, but not much.

In my case, I had benefit of more than 2000 eyes, there being more than 1000 comments on that video. But only one pair caught the problem. But that was all it took to alert me to fix it.     

Other AVwebflash Articles


  1. This reminds me of “Cunningham’s Law”, named after computer programmer and inventor of the first wiki, Ward Cunningham.

    Cunningham’s Law states “the best way to get the right answer on the internet is not to ask a question; it’s to post the wrong answer.”

    • I taught A&P for a local Jr. College for a couple of years during my time out at Edwards AFB, Kirk. I lived in fear that I’d stand in front of a couple dozen ‘smart’ AF mechanics who were just waiting for me to make a mistake and attack me. After being bitten a few times, I learned that I better prepare well and make sure I knew what I was talking about before I flapped my jaws. That’s where I picked up the habit of being honest and saying, “I don’t know … but let’s look it up together.” I didn’t fill in the gaps with BS; that’s why when I know I’m correct and someone still challenges me, it peeps me off. That period made a better mechanic outta me. If you want to learn something you already know better … teach it.

  2. For all …

    See AC43.13-1B Acceptable Methods, Techniques and Practices, para 7-64 (a) and (b):

    (a) DO NOT use self-locking nuts on parts subject to rotation
    (b) Self locking castellated nuts with cotter pins or lockwire may be used in any system.

    • I’ll inform Univair of your concern. They sell the approved part with…a locking nut.

      • The AC43.13-1B is only applicable if no OEM data…see the letter on page one of AC43.13-1B.
        Cessna does some really non AC43.13 stuff, too!

        • Agreed, however, if you want a fail safe connection … a nylon insert nut ain’t it no matter what the IPB might show. I’d be surprised if there is a clear IPB for it, too.

        • Don’t know if an equivalent CAR reg exists, but design and construction FARs 23.603(a)(3) and 23.607(b) would likely prohibit this installation in newer aircraft. Mostly because the heat of the engine compartment can adversely affect the locking feature of the nut.

          That being said, I’ve seen lots of nyloc nuts in engine compartments on certified airplanes. Usually on less critical items such as wiring harness tie downs. So it apparently isn’t a universally recognized no-no.

    • In addition, my understanding is any self-locking nuts on the engine side of the firewall should be metal, not nylon.

      • You are correct, Kirk. Para. 7-64(e) says, in part, “Metal locknuts are primarily used in high temperature areas.” Section (f) says, in part, “Fiber or nylon self-locking nuts are not installed in areas where temperatures exceed 250 °F. After the nut has been tightened, make sure the bolt or stud has at least one thread showing past the nut.” I do not know of any reference that dictates ALL nuts on the engine side of the firewall should be metal but it is considered good practice for anything important or structural and I make it MY business to do just that.

        PB: Take a look at Spruce’s website. Plug in “Bug Nut” and you’ll see several interesting variations to allow rotation of that attach point while securely capturing the heat cable. I had this exact problem when I installed a brand new OEM carb heat box on my Cessna … actuation of the heat control arm took too much pressure; I couldn’t go back to cold air once activated hot. I cannot opine what Univair is up to but IMHO, if they spec’ed out a nylon nut … it ain’t right for all the above reasons. It’s your airplane and your carcass … do what you will … just don’t become one of your own statistics testing out the RTB 180 deg maneuver.

        This issue hits close to home for me. Many moons ago when I was taking my A&P Practical, the DME purposely snuck a nylon nut onto a the rotating arm of a Cessna’s mixture control wanting to know if I’d find it. ONLY because I had gotten a pre-warning did I know he’d do that but … the learning point stays with me to this day. In fact, there IS an AD Note to make sure the locking nut on the mixture arm of a end of a carburetor cable should have a castle nut and cotter key vs. a nylon locking nut because the boys at Cessna shipped new airplanes that way.

    • Aircraft that are engineered and built using AC 43.13 Acceptable Methods, Techniques, & Practices are proven to be very reliable. Many Homebuilt Aircraft engineers try to reinvent the wheel making the ‘Annual Condition Inspection’ challenging. Causing the A&P to scratch their head quite a bit. Certified Aircraft engineers rarely deviate from AC 43.13 Acceptable Methods, Techniques, & Practices which makes ‘Annual Conformity Inspections’ much more straight forward. That makes the repairs straight forward. Besides, your taxes have already paid for a ‘FREE digital copy’ of Advisory Circular 43.13-1B and 2B:

  3. …oh, and so long as we’re all playing arm-chair editors, change the time-stamp in the video link to t=825s, not 835, to show the relevant section.

  4. Lot of problems here on many levels, there is a major problem if Paul is not an A&P and performed the work. If a pilot wants to work on his or her airplane without being an A&P, they should perform the work under the supervision of an A&P.

    • I didn’t want to say that but … you’re mostly correct, Victor. There IS a list of “preventative maintenance” items a pilot or operator CAN legally perform and some of ’em aren’t exactly minor. R&R of a carb heat box isn’t on that list. Still, there is a list. When done, they require a logbook entry by the pilot. And, I question how an IA could do an annual on an airplane and not have a helper inside the cockpit running the controls to notice the attach point isn’t turning? When I help owners do maintenance, I’m either there or I’m not signing it off and I won’t get involved. Being an A&P assumes you’re already good with a wrench but involves so much more administrative and logistical issues, too.

      • When the engine in that Cub was due for overhaul, I removed it. When it came back, I installed it. But in your self-righteousness, I suppose it would never occur to you to ask if I had A&P oversight, which I did.

        And Victor, in your world, few of us could afford to defray the cost of owning airplanes because we couldn’t do our own maintenance. Little wonder the industry is shrinking.

        • I agree 100% about dealing with flying costs and you did say an IA looked it over so I didn’t ask. In fact, when I decided to take up flying >50 years ago, I decided that I couldn’t afford to do so unless I could mostly do my own maintenance. So I went to school, et al, and achieved an A&P; took me 6 years. It’s saved me a ton of $$ plus I get both familiarity with my machines while getting better work which I wouldn’t be able to otherwise afford. MY quality system is that I walk away from critical tasks and come back later or — preferably — another day and look again or have another look at it. ALL controls to an engine need to be checked for correct operation and most checklists have a statement to that effect, as well. (You did use a checklist and/or approved data while you were saving money, right?) I was trying to send everyone to school on the subject.

          You wrote that, “I didn’t notice the error when I reassembled it and neither did the IA signing off the annual. It’s kind of a subtle thing and you have to know what you’re looking for and at. I do and I missed it.” WHAT! He did too! Apparently, because you (and your IA) missed it, it’s OK because you knew about it? I zero’ed right in on that statement because it typifies what most owners say. And, it’s the reasons many A&P/IA’s won’t go near an airplane they know an owner has been fiddling with on their own. That’s also why I wrote that owners shouldn’t be checking or tightening up cylinder attach nuts or thru-bolts last week, too.

          Now lets transpose that sort of thinking to the Martha Lunken story. SHE too knew better but spontaneously flew under a bridge anyhow. She — like you — admitted it, said she knew better but the FAA decided to come down heavy on her by way of its draconian Compliance and Enforcement Program. Many of the readers here thought she could or should get jail time. See the problem? We’d likely agree that there has to be some middle ground both on maintenance and with punishment for a singular lapse of good aviation judgement. Apparently, the FAA does not. If the following occurred and the FAA found out … your IA would lose his ticket. I guarantee it.

          You have a partner in that airplane. How would you feel if he crashed and the NTSB investigation revealed that the engine flamed out because the engine experienced carb ice and the pilot was unable to apply carb heat because the attach nut got loose or fell off when he tried to apply heat because the nylon lock function failed? I HAVE twice experienced the near heart attack feeling when an airplane I’ve had my hands on subsequently had an incident or crashed. In both cases, the issues were elsewhere but — up front — I didn’t know. It, too, guides many of the actions I take … especially on OPA — other people’s airplanes.

          I wasn’t trying to point my finger OR criticize; I was trying to help school folks. I’m certain that’s what drove you to self-confess your errant action?

        • As mentioned previously Paul, you can become an A&P/IA with a 3 week course for the J3. A description of the course would make great press for AVweb as well. Then you can school the back seat mechanics on these forum with impunity. Any interest ? -=Marc

    • Princess Auto stores in western Canada sell metal signs for your garage etc.

      One says “If it ain’t tight it ain’t right.”

      I’m remiss in not hanging that on the door of my garage.

      (I do check tightness of engine oil drain plug and filter at least once after running my vehicle for a couple of days, having made sure it was tight when I changed oil.
      Decades ago I had an oil leak after a shop changed the oil on my Chevrolet 350, turned out they’d failed to take the old o-ring off before re-installing the cannister with a new o-ring (they said). (The beast used a cannister with filter in it, held on with long bolt through middle IIRC, not screw-on cartridge of later decades.)
      I’m amazed at the number of vehicles that have substantive oil leaks, judging by fresh slicks in parking lots.

  5. When I was a youth, I once had use of my brother’s ’68 Firebird for a few months. I thought it would be a friendly gesture to change the oil, which I did.

    I took it for a quick test spin, and all was well till I was almost home and the oil lamp came on.

    I coasted into my driveway, to be greeted by a six foot oil slick in the garage. I’d neglected to remove the old gasket before I screwed on the new filter/gasket combo.

  6. Great story Paul and a lesson to us all. It takes a rare spirit to share his work and findings for all to see and benefit in the insalubrious swamps of social media, but that’s where the folks hang out. For every case someone told me ‘he’ll get violated for this or revoked for that by the FAA’ I’ve seen it happen once in the case of an engine that literally came apart following a field reman that was pointedly undocumented by the IA. He lost his IA for a while and that was the extent of it. Keep up the good work and let the naysayers argue among themselves.

  7. The problem with bashing people when they make a mistake is that they tend to hide future mistakes. Not the case with Mr. Paul B here, but hiding mistakes helps no one.

    Instead, we should be celebrating that a sharp-eyed reader caught the mistake and others learned from it (I’m certain there’s a bunch of people that learned something new but are unwilling to admit ignorance and be publicly shamed).

    • I definitely learned several new things in this article. And chatted yesterday with an IA on my field who confirmed that an IA generally looking over one’s work followed by a sign-off is nothing of concern to the FAA. The signature transfers the liability, which is of great concern An LSRM/IA myself I’ll be taking the A&P practical this summer, which will allow me provide IA services to E-AB and LSA aircraft. Paul did you know that your J3 qualifies and an LSA and the you could become and A&P/IA for that plane in a 3 week course ?

      • Sorry for the typos, I’m in between sets on my workout and not wearing the glasses required on my PPL.