Shutting down C-172 with fuel shut-off valve?

Hippike

Hippike

Pre-takeoff checklist
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Hippike
Early morning pre-flight, noticed the tow bar was still attached to the nose gear. Hm… somebody last night left in a hurry…

Open the door, notice fuel selector in the BOTH position (my CFI taught me always to select left or right after shutdown).

Then I see the fuel shut off valve knob pulled all the way out. I figure the person before me shut the engine down not by pulling the mixture lever but by pulling out the shut off valve. Why would they do that? What’s the difference between shutting off the engine with the mixture lever vs. shut off valve knob. Does it affect the engine at all?
 
I only get to fly old C172s. Haven't flown newer ones. :(

Think the fuel shut off valve is on newer C172s. Little I read it's for emergency use, but I'm not entirely sure about that so don't take my word. As far as I know the mixture should be used to shut off the engine. As for the fuel selector, everywhere I've been it's left on BOTH. On level ramps mostly.
 
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i believe the checklist item for that year is pull mixture and that Card is right, it's an engine fire item to pull the fuel shutoff (might be in the shutdown checklist after mixture pull, but I can't recall as I also fly only old ones nowadays.

i would talk to the chief instructor, they should coach that pilot on proper storage procedures for the club
 
[NA] Had a dream the other night I was in a 172 at the end of a flight, pulled mixture to shut down and the prop kept turning. Tried the fuel shutoff, key to off, even master to off (it WAS a dream), no joy. Pulled harder on the mixture and the whole cable came out of the panel, engine just kept on turning. Then I woke up, and realized I was at my Uncle's house, in the spare bedroom with a ceiling fan mounted too close to the wall, which sounds like a Huey when it is running. :rolleyes: [/NA]
 
catmandu said:
[NA] Had a dream the other night I was in a 172 at the end of a flight, pulled mixture to shut down and the prop kept turning. Tried the fuel shutoff, key to off, even master to off (it WAS a dream), no joy. Pulled harder on the mixture and the whole cable came out of the panel, engine just kept on turning. Then I woke up, and realized I was at my Uncle's house, in the spare bedroom with a ceiling fan mounted too close to the wall, which sounds like a Huey when it is running. :rolleyes: [/NA]
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Like this?

 
SkyDog58 said:
When I was in the Army, I worked on them, rode in them, rappelled out of them, skydived out of them and went to aerial gunnery range in them. Plus I like their sound.
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That's what I figured. I was at Quonset airport in RI during the controller strike and there was an Air Cav group there flying Hueys. Amazing machines. They'd land on bunkers on the airport, we would actually run a traffic pattern to the pinnacle area where the bunkers were, and of course auto rotations on the runway. They even drop one of our controllers off at his parents house in Conn in their back yard! Good group.
 
Hippike said:
Early morning pre-flight, noticed the tow bar was still attached to the nose gear. Hm… somebody last night left in a hurry…

Open the door, notice fuel selector in the BOTH position (my CFI taught me always to select left or right after shutdown).

Then I see the fuel shut off valve knob pulled all the way out. I figure the person before me shut the engine down not by pulling the mixture lever but by pulling out the shut off valve. Why would they do that? What’s the difference between shutting off the engine with the mixture lever vs. shut off valve knob. Does it affect the engine at all?
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The mixture control shuts the fuel flow off at the fuel servo on the engine. The fuel shutoff valve shuts the fuel off at a valve aft of the firewall and is intended for use during emergencies or maintenance. The engine should be shut down with the mixture; it cuts the fuel off instantly and as the engine runs down it pumps air through itself so that no fuel vapors are left. Shutting down using the fuel valve can result in a more erratic shutdown as the engine-driven pump pulls a bit of fuel from the line between the shutoff valve and pump and you might just be left with some fuel vapor where you don't want it.

Besides, those cable-operated fuel valves tend to get a bit stiff as they age and the cable suffers if it's being used all the time and has to work hard.

The left-or-right tank thing when parked is based on the reality that high-wing Cessnas have a habit of crossflowing fuel through the system if the airplane isn't quite wings-level when parked. On Both, the fuel can flow from the higher tank into the lower, and if that lower tank is the left tank and there's enough fuel transferring across, it drips out the vent onto the pavement. Avgas is hard on pavement, it irritates airport authorities, it's expensive to buy. Some older 180s/185s had vents on both sides and a Both position on the selector; you could lose fuel out either side. When filling the tanks and the selector is on Both, you could end up with less than full fuel. Fill one side, put the cap on, and while you are moving the ladder and hose to the other side, fuel has been flowing across into the tank you're about to fill. You fill it, but the first tank is now a little low. The head pressure difference is now small enough that it isn't going to quickly flow back across and replace the fuel lost from the first tank while you are topping the second.
 
Only reason I could think would be a mx event where you want the fuel line to the engine to be dry.
 
SkyDog58 said:
My favorite helicopter.
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Mine too! Although I have no real experience in them like you do, apart from seeing one up close at an air show, but I just love their sound. I can listen to Huey sounds on YT for hours :D

Dan Thomas said:
Besides, those cable-operated fuel valves tend to get a bit stiff as they age
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So that's why I had so much difficulty pushing it back in, had to really lean into it...
 
The fuel shut off valve should be safety wired open so it won"t vibrate closed. The wire is the very thin type that can be broken if there is a fire.
 
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Maybe if the mixture wasn't working. Checklist and POH cover what should be done. If I were renting an airplane and the tow bar was still attached I'd expect all kinds of weird stuff.

Once I found some blue fingerprints on the aircraft binder. I reached in the side compartment to get the checklist and there was an exploding blue pen inside. I don't know how the person before me missed that, they had to have noticed, yet they still put the pen back in the side compartment. I think renting is what keeps me from becoming complacent during pre-fight.
 
Dan Thomas said:
When filling the tanks and the selector is on Both, you could end up with less than full fuel. Fill one side, put the cap on, and while you are moving the ladder and hose to the other side, fuel has been flowing across into the tank you're about to fill. You fill it, but the first tank is now a little low. The head pressure difference is now small enough that it isn't going to quickly flow back across and replace the fuel lost from the first tank while you are topping the second.
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Exactly, Cessna's refueling procedure is to fill left, fill right, then fill left again. Seen it many times in my old 172. If I'm going on a long XC (350+ nauticals) I tamp it down both sides - twice.
 
I shut off fuel & run the carb empty too - especially if my A/C is going to sit more than a few days. Make starting much easier with fresh fuel in the bowl.
 
Careful if you havent turned the selector in a very long time. Might cause it to start leaking around the valve.
 
comancheflyer30 said:
Careful if you havent turned the selector in a very long time. Might cause it to start leaking around the valve.
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A good sign that it needs work. Better to find it on the ground than in an emergency in flight.
 
The LEFT or RIGHT thing is an advice for fueling the plane. It's not a generic thing you have to do when shutting down.

Frankly, until they put the "one tank only above 5000'" AD out, I never touched the 172 fuel selector.
 
cgrab said:
The fuel shut off valve should bee safety wired open so it won"t vibrate closed. The wire is the very thin type that can be broken if there is a fire.
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no it should not! safety wire is meant to keep something from turning, period. breakaway wire is meant to hold something in place until you mean to turn it. you never use safety wire where breakaway wire is called for and vise versa. they are made from different metals.

bob
 
You've got a serious airworthiness problem if your valve will vibrate into some position other than where it was set.
Safety wiring it for whatever purpose is unairworthy.
 
flyingron said:
The LEFT or RIGHT thing is an advice for fueling the plane. It's not a generic thing you have to do when shutting down.

Frankly, until they put the "one tank only above 5000'" AD out, I never touched the 172 fuel selector.
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The what?
 
I was referring to the "one tank only above 5000' AD" comment.
 
unsafervguy said:
no it should not! safety wire is meant to keep something from turning, period. breakaway wire is meant to hold something in place until you mean to turn it. you never use safety wire where breakaway wire is called for and vise versa. they are made from different metals.

bob
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Thanks for the correction. That is what I meant to say.
 
flyingron said:
But even revised, it's wrong.
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No. It is right. Breakaway wire is commonly used to secure emergency systems against inadvertent operation and to indicate to a mechanic or subsequent pilot that the control has been moved. Copper wire is the usual stuff.

https://www.aopa.org/news-and-media/all-news/2005/july/pilot/airframe-and-powerplant-(6)

An excerpt:

One other type of wire may be seen on airplanes — soft copper wire (0.020-inch diameter only). Commonly called breakaway wire, it is used to prevent vibration problems with equipment that may be needed during abnormal operations. Breakaway wire is used for tasks such as securing fire-extinguisher safety pins, emergency landing-gear releases, escape-hatch actuating handles, and switch covers — it is always intended to be broken if the need arises.

I recently did an STCd installation of an IO-550 in a 180. Part of that STC was a separate fuel shutoff that used the copper wire to lock it open. I also used the copper on the Electras to lock the emergency brake control valve in the closed position. This is common aviation practice.
 
mscard88 said:
Ahh ok, older models of 172s thru K. Thanks for that. Ours are both P models.
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The was an SB and Service Kit to correct the situation on older models. It involved removing the fuel line between each tank, which ran from the tank and aft and down the aft doorpost, and installing a welded Y-line so that the lower arm of the Y connected to the fuel tank and the upper fan forward and up to the crossover vent line and was teed in there. The bottom of the Y connected to the rest of the system lower in the doorpost. The fix (the placard or the SK) was to avoid engine failure if a bubble (from fuel sloshing and unporting in the tank; it got worse with higher temperature or altitude) got into the vertical line down the doorpost; it could stay in one spot, trying to rise while fuel flowing downward held it there, and it would act as a major obstacle to the flow and kill the engine. The Y line bled the bubble back to the vent system as it started downward from the tank.

Single-tank operation doubled the flow rate and pulled the bubble through the system and vented it through the carb bowl vent.

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=30276
 
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It's right in that it is the definition of breakaway wire.
I was referring to that it is wrong to put it on a 172 fuel selector.
 
Cgrab did, that's what stasted this whole line of nonsense.

The fuel shut off valve should bee safety wired open so it won"t vibrate closed. The wire is the very thin type that can be broken if there is a fire.
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Neither safety wire nor break away wired should be used here. If the valve is going to "vibrate closed" there's something extremely unairworthy about it and wiring it into position is NOT the correct repair. Even if the thing is operating correctly, putting wire on it is inappropriate.
 
I took that to be referring to the fuel shutoff valve knob and not the fuel selector. Maybe I misread.
 
No, I have the breakaway wire on my fuel shut off valve on my Cardinal.
 
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