Most GA planes have a fuel shutoff valve which I don't quite understand the purpose of. Does it cut the fuel flow with the mixture. My theory is to prevent fuel flow at earlier stage for safety reasons?
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asked Jul 24, 2019 at 14:47
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3$\begingroup$ If you're changing the mixture, doesn't that indicate the fuel is reaching the carb? Typically you don't want residual fuel sitting in the carburetor as it can break down and gum it up. $\endgroup$– zymhanJul 24, 2019 at 15:10
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1$\begingroup$ Is your question inspired by the fact we don't usually have fuel cut-off valves in a car? (note to pedants: a manual one that can be operated directly by the driver). $\endgroup$– Oscar BravoJul 26, 2019 at 10:17
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1$\begingroup$ Some years ago, my car (Trabant 601s) had a manual fuel cutoff valve. Tank above the carburetor... you could pretty much not find any gas in the tank if you forgot to close it. You could as well find few liters in the engine. $\endgroup$– fraxinusAug 26, 2021 at 12:37
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$\begingroup$ @fraxinus: I've also owned motorcycles with fuel shutoff valves. Same reason: the tank is above the carbs, so if there's any leakage in the system, it could empty the tank. While the updraft carb of most GA planes probably wouldn't allow fuel into the engine, driving out to the ramp and finding your plane sitting in a big puddle of avgas could ruin your day :-) $\endgroup$– jamesqfAug 26, 2021 at 22:32
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$\begingroup$ Ah, of course, I forgot the puddle of 2-stroke oil (gas evaporated, oil stayed on the ground). Yes, Trabant car had 2 stroke engine. $\endgroup$– fraxinusAug 26, 2021 at 22:39
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4 Answers
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It's both a maintenance and safety feature.
You need a way to cut off fuel flow to the engine compartment, either to work on the engine, or because of a fire at the engine, or because you are doing a forced landing and it helps reduce the risk of your entire fuel contents seeping onto your hot engine if you bend things a bit putting it down and a fuel line gets broken.
So you will always have a shutoff valve, and it's always somewhere upstream of the firewall.
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Fuel shutoff valves are a hold over from the days of float style carburetors (and necessary on any float style carb system). With a float style carb when the fuel is on the bowl will fill up until the float floats up and shuts the flow off. Since gasoline becomes a vapor at ambient temperature the small pool in the carb will be constantly evaporating. As such the float will sink and refill the bowl. Given enough time this will drain the fuel tank. This vaporous fuel also fills the intake and can cause backfires or intake fires.
The mixture is adjusted post bowl and merely leans you to the point of cutout which may not truly be a full stoppage of fuel.
For fuel injected systems or systems that use some kind of blow through carb a shutoff valve is not strictly necessary for leak reasons but JohnK makes some good points on when its used for safety reasons. Some of these systems will also allow small paths for fuel to escape so a shutoff is often implemented to ensure total fuel stoppage.
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2$\begingroup$ Another factor here is that a lot of planes have the fuel tanks located above the carburetor. Obviously true of high-wing planes, but it's even true of many low-wing ones, since (unlike the downdraft carb used on most cars pre-EFI) the updraft carb of most aviation engines is located on the bottom of the engine. So even the smallest leak in the carb float could drain a bunch of gas. Many motorcycles have fuel shutoff valves for the same reason. $\endgroup$– jamesqfJul 25, 2019 at 4:57
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$\begingroup$ Based on experience in the automotive world, it may also act as a slight theft deterrent. $\endgroup$– ivanivanJul 25, 2019 at 14:27
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5$\begingroup$ Goodness, I hope nobody who can't identify a fuel shutoff valve is going around stealing GA planes. $\endgroup$– ChrisJul 25, 2019 at 16:57
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$\begingroup$ @Chris Especially when they are usually clearly labelled with something like FUEL SHUTOFF... $\endgroup$– userJul 25, 2019 at 20:32
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The less technical, but accurate, answer is to meet the certification requirements of the FARs. Specifically, FAR 23.2430 is Airworthiness Standards for Fuel systems. It reads in part: (a)Each fuel system must-...(5) "Provide a means to safely remove or isolate the fuel stored in the system from the airplane"
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$\begingroup$ Man that's an awfully broad standard. There must be more detailed requirements somewhere else. $\endgroup$– John KJul 25, 2019 at 3:13
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Here are two examples of high wing fuel systems. The left is a from a 1973 Cessna 177, the right from a later model Cessna 172. You can see in the 177 the fuel cutoff is earlier in the system, before the fuel system is pressurized, while in the later plane it is after the fuel pumps. Either way, fuel is cutoff after the header tank to limit the amount of fuel available in case of an engine fire that could result from a crash.
answered Jul 25, 2019 at 12:43