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I understand this is to “check our engine is still operating at idle”, but can someone explain this more in depth? Like why wouldn’t it?

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    $\begingroup$ Out of curiosity, which model C172 is this? I've never heard of a carb check at idle before now and I didn't find it in the POHs I have. The only carb check I can see is done during the run-up checks at 1700RPM. But there are lots of different models and it might be specific to a certain engine or whatever. $\endgroup$ – Pondlife Oct 26 at 15:46
  • $\begingroup$ Both p and m. That is the last check in the run up. $\endgroup$ – leha007 Oct 27 at 16:03
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Because, in engaging the carburetor heat, you are decreasing the density of the air entering the engine with the same fuel flow, thereby richening the mixture. At idle power, if the throttle idle set screw is not properly adjusted, there is a risk of an engine failure from too rich a mixture entering the engine. The idle power carb heat check is a good way to verify the engine won’t do this in flight.

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    $\begingroup$ It also verifies that the carb heat is working. If you pull the lever and the RPM doesn't drop, something is wrong. $\endgroup$ – Ron Beyer Oct 25 at 10:43
  • $\begingroup$ That's done at run up rpm. The part you check at idle is the rpm rise from slow idle when taking CH off. $\endgroup$ – John K Oct 25 at 14:12
  • $\begingroup$ @RonBeyer you would typically verify carburetor heat operation during the run up. This these here is just an addition to ensure that using carb heat at idle power won’t cause the engine to stall. $\endgroup$ – Carlo Felicione Nov 17 at 14:33
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Carlo Felicione has given a great explanation of why carb heat at idle would need to be checked. The incoming air heated by the exhaust manifold would be less dense than the incoming air without carb heat. If the fuel mixture is too rich, it may cause the engine to quit. Some may call it flooding the engine.

The most common time for this to be an issue is on final approach. In a Cessna 172, you set the mixture to rich (or best power) to prepare for a possible go around. You would set carb heat to hot in order to avoid carb ice during low power settings. This would be a bad time for engine failure. You are very low to the ground and at a very low airspeed. If the prop stops, you may not have enough airspeed over the prop to continue to windmill it for a restart. And, you may have very little time or mental bandwidth to kick over the starter.

While I have not had this happen on final in real life, I have seen videos of Cessna props stopping when the throttle was brought back to idle in flight. I have also had a 172N prop stop after landing at a field elevation of about 500 feet with carb heat on and mixture full forward. The same N model had no problems for the A&P check. But, the prop stopped later that same week during the run-up at about 500 feet of field elevation.

It performed perfectly during the higher run-up RPM carb heat check. It had a consistent drop in RPMs when carb heat was applied with a consistent return to RPMs when the carb heat was turned off. Idle setting was checked and functioned well at about 700 or so RPMs. The engine quit as soon as carb heat was applied at 700 RPMs. Obviously, that was the end of our flying day. And, I have never skipped the carb heat idle check after that.

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  • $\begingroup$ Can also second your story. Our aircraft (Not a Cessna) did not have the idle check as part of the checks. What you described happened during a training flight, and the check was introduced by the flying school after $\endgroup$ – Dan Oct 26 at 11:17

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