Skyhawk/Cutlass Shutdown Procedures

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KennyFlys

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The CFI I flew with yesterday brought up something about setting the throttle at a specific RPM before pulling the mixture. I pulled back below a thousand as I had with nearly every other aircraft I've flown.

I looked at the manuals I have. Only the 172RG Cutlass specifies setting the throttle at 1000 RPM prior to pulling mixture.

172M, 172N and 172P manuals I have do not even list throttle in the shutdown checklist. I don't have a 172R manual. The 172S manual states "Idle." The R182 and 182T simply state throttle to "Idle."

David had heard different versions as well. "Idle" could mean 1000, 900, or even to its lowest point and still running, around 600 RPM.

Obviously, I'll pull the Cutlass to 1000 RPM as specified. But, what do you use when there is no specified speed during shutdown as is the case with every other manual I have? Is even the spec for the Cutlass appropriate?

:dunno:
 
KennyFlys said:
The CFI I flew with yesterday brought up something about setting the throttle at a specific RPM before pulling the mixture. I pulled back below a thousand as I had with nearly every other aircraft I've flown.

I looked at the manuals I have. Only the 172RG Cutlass specifies setting the throttle at 1000 RPM prior to pulling mixture.

172M, 172N and 172P manuals I have do not even list throttle in the shutdown checklist. I don't have a 172R manual. The 172S manual states "Idle." The R182 and 182T simply state throttle to "Idle."

David had heard different versions as well. "Idle" could mean 1000, 900, or even to its lowest point and still running, around 600 RPM.

Obviously, I'll pull the Cutlass to 1000 RPM as specified. But, what do you use when there is no specified speed during shutdown as is the case with every other manual I have? Is even the spec for the Cutlass appropriate?

:dunno:
Click to expand...

I doubt that there's much basis for the 1000 RPM, but doing this can eliminate some unnecessary "shaking" as the engine winds down and it can ensure a more rapid shutdown. When you pull the mixture, the flow to the engine doesn't completely stop immediately, it tapers off as the pressure to the injectors bleeds off. That means the mixture goes from "normal" to lean enough to slow the engine down but not stop it, to too lean to support any combustion, and it can take several seconds to get there. Starting from 1000 RPM might make it so that the mixture reaches the "too lean for any combustion" before the RPM drops to the point where any cylinders firing would cause excessive vibration at a frequency too low fo the engine vibration isolators to attenuate it much.

That said, I'm pretty sure those words in the POH are a recommendation, not a requirement.
 
Thanks Lance,

I dug up a 1974 Decathlon manual I had. It also makes references to reducing vibration as you described. I can see how that would reduce wear and tear on the engine mounting as well as other components attached to the engine. I'm sure that tubing assembly gets enough abuse from primary students.
 
The Seminoles I flew had the same recomendation...something above "throttle closed" when you shut down the engine. Only a handful of people ever obey it, in my experience, and it never made any sense until I read Lance's explanation...now I see the wisdom there.
 
Teller1900 said:
The Seminoles I flew had the same recomendation...something above "throttle closed" when you shut down the engine. Only a handful of people ever obey it, in my experience, and it never made any sense until I read Lance's explanation...now I see the wisdom there.
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An instructor taught me that for the Mooneys and Cherokee Six I flew, especially when doing a quick turn for fuel or lunch. If you remember not to touch that throttle when you come back out for the engine restart, she'll be PURRING after start, exactly on target RPM, right where you left it!
 
The other day, I was taxiing back at 600 RPM. Leaned to 1.9gph (it will quit at 1.8), the IO520 runs extremely smoothly there. It almost sounds like a turbine :)

That shutdown was amazingly smooth. Usually, I shut down from 1000 or so, too, and there's quite a bit of shaking. Pulling the mixture at 600 RPM didn't produce any shaking, interestingly enough.

-Felix
 
If you fly an injected IO-360, as I have in my Cardinal, it is much easier to hot start if you remember to set the rpm at 1000-1200 before you pull the mixture. When you start by up just turn the key, DON'T TOUCH ANYTHING, ease in the mixture as it begins to fire, and Bob's your Uncle, it starts first time every time.

I don't think it really matters in a carberated motor like on the Cutlass or most Skyhawks.
 
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JimNtexas said:
If you fly an injected IO-360, as I have in my Cardinal, it is much easier to hot start if you remember to set the rpm at 1000-1200 before you pull the mixture. When you start by up just turn the key, DON'T TOUCH ANYTHING, ease in the mixture as it begins to fire, and Bob's your Uncle, it starts first time every time.

I don't think it really matters in a carberated motor like on the Cutlass or most Skyhawks.
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That was the only reasoning I could think of as well.
 
Standard shut-down practice on every recip engine:
Most engines will "lope" (vibrate) at full idle. It is bad practice to allow this "loping" when operating at idle on the ground. It causes excess wear on the engine mounts and any parts that may be rubbing together as a result of this "pilot-induced" engine vibration.

It also increases the "incomplete combustion" that occurs when the mixture is too rich, increasing the chance of carbon crusted spark-plug tips which can cause a bad mag check.

The only time the engine should be at full idle is when you are braking to a stop. Once you have stopped, you should automatically bring the throttle up a bit to a smooth idle. This will occur around 1000 RPM, but it should be taugt to feel and hear the smooth spot above idle.

This is where you should pull the mixture if you are shutting down.

Baby your engine.
 
For engines like those in the mentioned aircraft, Lycoming recommends that before shutdown, you set the throttle to 1000-1200 RPM, lean to peak RPM (actually, you should have done this upon clearing the runway). When ready to shut down, they suggest 1000-1200 RPM and cutting the already-leaned mixture. See Lycoming SL's 185, 192, and 197 for more lead-reducing and leaning recommendations.

BTW, if you leave the thottle there after the engine stops and lock it, it will be perfectly set for your next start. Works good on your plane and fair in partnerships if everyone buys into it, but not much use on FBO/flight school planes.
 
JimNtexas said:
If you fly an injected IO-360, as I have in my Cardinal, it is much easier to hot start if you remember to set the rpm at 1000-1200 before you pull the mixture. When you start by up just turn the key, DON'T TOUCH ANYTHING, ease in the mixture as it begins to fire, and Bob's your Uncle, it starts first time every time.
Click to expand...

Yup, that's exactly what I was trying to describe above... the IO-360 in the Mooneys and the IO-540 in the Cherokee Six behaved exactly as you described. Smoothest hot starts I've ever had. Impresses the passengers, too, with such a smooth non-revving startup. :yes:
 
nosehair said:
The only time the engine should be at full idle is when you are braking to a stop. Once you have stopped, you should automatically bring the throttle up a bit to a smooth idle. This will occur around 1000 RPM, but it should be taugt to feel and hear the smooth spot above idle.
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Like I said, it depends on the engine. If you have closely matched FF to all cylinders, idle can be perfectly smooth. Most people are simply way too rich at idle.

There is no vibration whatsoever at idle (around 600 RPM) in mine, and in fact that's the only sensible taxi setting unless you want to ride your brakes....

-Felix
 
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