Today as I was engaging the carb head on my '64 172E, I heard a slight "click" as I was pulling the cable out, then after landing I was unable to push it back in. It'll go in a little way, then you can hear a definite hard stop which appears to be right behind the panel (like the cable itself is binding up right behind the panel). I could manually move one of the controls on the airbox, but not the other, so it seems like only the 1 cable is stuck. Anyone have this happen, and if so, how did you fix it?
stuck carb heat control
- Thread starter RickC
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It seems you may have the somewhat uncommon “ 1 knob - 2 wires “ system.
My guess is one f the wires broke. Let’s hope MacFarland makes them . Saved one for years but I’m not sure of current hiding place.
If you do have the dual system note it has 2 flappers. This is to allow heat from the muffler providing Carb Heat and pass it through the Airbox and provide additional cabin heat.
The flapper with the vertical hinge ( no shaft) has a tendency to have the hinge fail. It would be prudent to verify all is well when addressing the problem. You don’t want parts floating in the Induction System!
My guess is one f the wires broke. Let’s hope MacFarland makes them . Saved one for years but I’m not sure of current hiding place.
If you do have the dual system note it has 2 flappers. This is to allow heat from the muffler providing Carb Heat and pass it through the Airbox and provide additional cabin heat.
The flapper with the vertical hinge ( no shaft) has a tendency to have the hinge fail. It would be prudent to verify all is well when addressing the problem. You don’t want parts floating in the Induction System!
Correct, it's 1 cable that splits into 2 behind the panel, with 2 separate cables going to the airbox. The one that is stuck is the one with the vertical hinge. I should be able to get a good look at it by removing the lower cowling. If it's not something easy and obvious I'll have the shop dig into it. But I always like to check for low-hanging fruit first. Thanks.
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Domenick
Attention: My experience with airboxes is restricted to Piper Cherokees. 
Pull the cowling. Check for cable slippage at the airbox. Also, check the airbox flapper for full movement and condition of the flapper seal. While you are at it, lube up anything that turns, slides, or spins.
Pull the cowling. Check for cable slippage at the airbox. Also, check the airbox flapper for full movement and condition of the flapper seal. While you are at it, lube up anything that turns, slides, or spins.
Dan Thomas
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Be careful with the lube. Oil attracts dust and forms a sticky sludge that makes things much worse. There are places for it, and places where dry lubes such as graphite or dry silicone are preferable. The Lycoming starter hassles are an example of pilots and mechanics making life miserable for themselves by using oil on it. And oil isn't just oil. There's the right stuff specified by the manufacturers in their manuals, and there's the wrong stuff that dries out and seizes everything.Attention: My experience with airboxes is restricted to Piper Cherokees.
Pull the cowling. Check for cable slippage at the airbox. Also, check the airbox flapper for full movement and condition of the flapper seal. While you are at it, lube up anything that turns, slides, or spins.
Carb heat controls are not owner-maintenance items. Not preventive maintenance. It's easy to get controls messed up and misrigged, resulting in an accident. https://www.ecfr.gov/current/title-14/part-43/appendix-Appendix A to Part 43
I don't intend on trying to fix anything myself. But I'd to see what is going on down there. I'll probably pull the cowl, take a look, then call the shop. Thanks
I hear ya. I only use WD-40 for cleaning stuff. Great for lots of things, but I always use something else when a lubricant is what is called for. The shop checked it out and said the airbox is ok. Of course, this is the most expensive carb heat cable on the planet ($400+ with shipping). Just thankful this plane gives me very few problems (knock wood).
FYI: while the Coleal letter is brought into these prevent mx discussions, the context of the letter is toward transport aircraft. However, several portions (below) of that LOI have been used to illustrate how the lists in 43 Appx A are used on Part 91 aircraft. Just as with major repairs and alterations you apply the Part 1.1 definition to those lists and "expand" those specific categories provided the task you perform can "fit" into that listed category.
Nowhere does 43 Appx A(c) mention engine controls so that would exclude the carb heat control. One can't make up their own category. However, a pilot could lubricate that control as prevent mx per A(c)(6) provided he had a proper ref. per Part 43.13. This is how all those Appx A (a, b, c) lists have been "managed" by the FAA in most cases since before the Coleal letter was published. I made this topic a priority for my owner-assist customers so that they could fully appreciate the amount of work they could legally perform and sign-off on their aircraft.
"As with the other paragraphs of Appendix A (i.e., on major repairs and major alterations), the lists are better viewed as examples of the tasks in each category-they cannot be considered all-inclusive"
Nowhere does 43 Appx A(c) mention engine controls so that would exclude the carb heat control. One can't make up their own category. However, a pilot could lubricate that control as prevent mx per A(c)(6) provided he had a proper ref. per Part 43.13. This is how all those Appx A (a, b, c) lists have been "managed" by the FAA in most cases since before the Coleal letter was published. I made this topic a priority for my owner-assist customers so that they could fully appreciate the amount of work they could legally perform and sign-off on their aircraft.
"As with the other paragraphs of Appendix A (i.e., on major repairs and major alterations), the lists are better viewed as examples of the tasks in each category-they cannot be considered all-inclusive"
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Dan Thomas
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The Canadian regs are a bit more specific. https://tc.canada.ca/en/corporate-s...ntary-work-canadian-aviation-regulations-cars
In there we see:
(28) checking and adjusting air pressure in helicopter floats and aircraft tires except on aircraft operated under CAR 705.
CAR 705 is airline operations.
The idea is that the person doing this stuff, especially if it's not on a small private aircraft, has been trained for it.
Flight and engine controls are addressed in another section of the regs, and are considered serious enough that any "disturbance" to them requires the signatures of two qualified people in the maintenance records, verifying that they have been connected and secured and routed properly and move things in the right direction. It sure isn't preventive maintenance here, and I'd be surprised if any FAA folks considered it so.
In there we see:
(28) checking and adjusting air pressure in helicopter floats and aircraft tires except on aircraft operated under CAR 705.
CAR 705 is airline operations.
The idea is that the person doing this stuff, especially if it's not on a small private aircraft, has been trained for it.
Flight and engine controls are addressed in another section of the regs, and are considered serious enough that any "disturbance" to them requires the signatures of two qualified people in the maintenance records, verifying that they have been connected and secured and routed properly and move things in the right direction. It sure isn't preventive maintenance here, and I'd be surprised if any FAA folks considered it so.