Why is there a fuel selector switch?

kicktireslightfires

Pre-takeoff checklist
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kicktireslightfires
This is probably going to sound dumb to you experienced pilots out there (I admit I'm going to ask a lot of dumb questions because that's how I learn), but why do most planes even have a fuel selector switch? Why don't both tanks merely automatically drain into the same place (to the engine)? Rather than the pilot needing to manually switch to the tank that has the most fuel?
 
For one thing, consider a high wing plane with two tanks interconnected at the engine. If the plane is parked off level gas will drain from the higher tank through the interconnection valve to the lower tank which, when it becomes full, will overflow thus pouring expensive gas onto the ground. Many Cessna aircraft have a "both" position in the fuel valve which will allow this but they also have positions for each tank to feed to the engine. Selecting one tank operation allows one to more accurately time the amount of fuel remaining under known power settings. An "off" position permits work on the carburettor/fuel pump, etc without having to drain the tanks.
 
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Lots of smaller high-wing airplanes don't have a selector. Just a shutoff valve. The lines are teed together into a single line to the valve. The valve is a legally-required thing so you can shut the fuel off in case of a massive leak or fire ahead of the firewall. The Cessna 140 and 150 are like this. The Citabrias are all like this.

But low-wing airplanes need a selector, and there is no "Both" position. If there was a Both, and one tank ran dry, the pump would happily suck air instead of fuel and the engine would quit. Fuel has to be lifted up out of a low-wing tank to the engine. Take a glass of water and two straws. Stick one straw in the water and the other outside the glass beside it, and put both in your mouth and suck and see if you get any water.
 
What if one tank gets contaminated, good to have the option to switch to another tank

Also, Cessna is basically just on off with a "both" switch, as was replied above

PS - Cirrus CEO at some point suggested auto switching tanks
 
For one thing, consider a high wing plane with two tanks interconnected at the engine. If the plane is parked off level gas will drain from the higher tank through the interconnection valve to the lower tank which, when it becomes full, will overflow thus pouring expensive gas onto the ground. Many Cessna aircraft have a "both" position in the fuel valve which will allow this but they also have positions for each tank to feed to the engine. Selecting one tank operation allows one to more accurately time the amount of fuel remaining under known power settings. An "off" position permits work on the carburettor/fuel pump, etc without having to drain the tanks.

Makes sense. Thank you! The overflowing when parked on off level ground makes sense. But there's got to be some kind of engineering solution to that. Just make the fuel cap water tight? And for working on the engine without draining the tanks, the a/p could either clamp the line or just install a manual fuel selector on the line right before it feeds into the engine that the a/p manually toggles before beginning work? I get things are the way they are, just seems pretty easy and inexpensive to fix those two things.
 
Lots of smaller high-wing airplanes don't have a selector. Just a shutoff valve. The lines are teed together into a single line to the valve. The valve is a legally-required thing so you can shut the fuel off in case of a massive leak or fire ahead of the firewall. The Cessna 140 and 150 are like this. The Citabrias are all like this.

But low-wing airplanes need a selector, and there is no "Both" position. If there was a Both, and one tank ran dry, the pump would happily suck air instead of fuel and the engine would quit. Fuel has to be lifted up out of a low-wing tank to the engine. Take a glass of water and two straws. Stick one straw in the water and the other outside the glass beside it, and put both in your mouth and suck and see if you get any water.

Definitely makes sense with low-wing and great analogy. I suppose once a tank runs dry an electronic valve could shut it off so the pump only sucks fuel from the tank with fuel remaining. Hey, I can dream.
 
What if one tank gets contaminated, good to have the option to switch to another tank

Also, Cessna is basically just on off with a "both" switch, as was replied above

PS - Cirrus CEO at some point suggested auto switching tanks

Interesting re Cirrus and auto-switching tanks! I really wish that was a thing. Do you know how long ago the CEO was talking about that?

Re tank contamination sounds more like a nice to have than an essential. You'd find out it was contaminated on the ground, right? Or how would you discover that while in-air? Engine quits and you switch tanks before restart?
 
Interesting re Cirrus and auto-switching tanks! I really wish that was a thing.
I want to say it was a year or two ago? It was after the CEO stepped down and the new guy came in.. I have no idea if it will become a reality but I think he was trying to demonstrate how there is still room for tech advancements

You'd find out it was contaminated on the ground, right? Or how would you discover that while in-air?
It's just one of those.. switch tanks during engine roughness or failure. Maybe if water got in through a cap?

There's certainly a ton of room for improvement overall in GA
 
Makes sense. Thanks!

How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.
 
Definitely makes sense with low-wing and great analogy. I suppose once a tank runs dry an electronic valve could shut it off so the pump only sucks fuel from the tank with fuel remaining. Hey, I can dream.

Too much automation is not necessarily a good thing. Remember the Korean pilots that lost the tail on a VFR landing? Or the Air France that pancaked into the Atlantic ocean because they stalled it at 30,000 feet? Or the Airliner that exploded in flight when one of the tanks was low on fuel? These are examples of pilots who became unable to fly when the automated systems became unavailable or unreliable. I'll stick with mechanical systems thank you.
 
Makes sense. Thanks!

How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.

I have a timer set on my efb that reminds me. Its nice because it automatically starts & stops when the airplane is moving. Before I used the efb, I would use the timer built into the navcom radio. Lots of GPS's also have a reminder function, alas not the ancient king in the archer I usually fly.

On a low wing airplane, the fuel pump has to lift the fuel. If the tanks were interconnected, and you had one wing lower than the other, the fuel would run to the lowest tank, while the pump would pull from the point of least resistance, the high tank. Probably wouldn't be an issue until you were low on fuel and got uncoordinated turning in the pattern because you were tired from the long flight.....and then the engine quits while you're low & slow....

An automatically switching valve is certainly a good idea, but it adds weight, cost, & complexity to do something that is very simple. Once you fly a plane for a while, switching tanks becomes a muscle memory item that takes zero brain power. If the tank switched automatically, you'd still have to monitor it to make sure it was working.
 
Makes sense. Thanks!

How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.
How do you remember to stop flying before you run out of gas with one tank? The answer is the same.
 
Makes sense. Thanks!

How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.

Both my old Cherokee and my Mooney are all mechanical, no wing leveler, you can feel when one wing is lighter by how much aileron pressure you have to hold. I like to be light on the controls so I get an automatic reminder to switch tanks based on the feel of the yoke.

The Cherokee was more sensitive and I tended to switch thanks about every 20 minutes. I had a west bend timer velcroed to the dash for that purpose and total time fuel burn. The Mooney doesn't have room for it so I set the dash clock for 12 o'clock at takeoff, and run the right tank when the minute hand is on the right and the left tank when the minute hand is on the left. If my trim is off by aileron feel I shorten or extend the time a little bit to get the feel I want. It works pretty well and becomes an automatic practice after a couple of XCs.

PS. 375 NM xc today with some headwind. 3 hrs in flight. It was smooth and flew hands off most of the flight. Used 24.5 gal and the tanks were only 1/2 gallon different on landing.
 
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Makes sense. Thank you! The overflowing when parked on off level ground makes sense. But there's got to be some kind of engineering solution to that. Just make the fuel cap water tight?

Fuel caps are indeed well sealed against leaks. A fuel tank requires a vent to equalize pressure when fuel is consumed. Otherwise your tanks would crumple and implode. So fuel will run out the vent if it is overfilled. You could put in a check valve in the vent but that's one more thing to fail and if it sticks, your tank or wing crumples when fuel is consumed. Being able to select a tank also allow isolation of a fuel issue in one tank, e.g. water or ice contamination.
 
Too much automation is not necessarily a good thing. Remember the Korean pilots that lost the tail on a VFR landing? Or the Air France that pancaked into the Atlantic ocean because they stalled it at 30,000 feet? Or the Airliner that exploded in flight when one of the tanks was low on fuel? These are examples of pilots who became unable to fly when the automated systems became unavailable or unreliable. I'll stick with mechanical systems thank you.

What about the millions of flights that went without incident?
 
Makes sense. Thanks!

How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.

I use a clock.

It's not difficult, and is part of good flight management. Something like first change around 1/2 hour point, and ever hour thereafter to keep fuel load balanced. If you keep a flight log, you can note tank changes at the appropriate projected navaid or location on your log as a reminder it should be done around then. If you fly a low wing long enough, it will be ingrained.

Fuel management is a critical safety of flight issue. If you are not regularly reviewing fuel situation during flight, you are being lax. If you are periodically reviewing fuel, you will stumble across tank changes.
 
Definitely makes sense with low-wing and great analogy. I suppose once a tank runs dry an electronic valve could shut it off so the pump only sucks fuel from the tank with fuel remaining. Hey, I can dream.

Buy a PC12. Its a low-wing and does the fuel switching for you.
 
How does everyone remember to switch tanks during a flight? Is there a technique for this? Does everyone have a timer that goes off or what? Seems like much too important of a thing to leave to chance or just normal memory.

There is also an audible warning built into every airplane.
 
And for working on the engine without draining the tanks, the a/p could either clamp the line or just install a manual fuel selector on the line right before it feeds into the engine that the a/p manually toggles before beginning work?
We're way ahead of you. Fuel shutoffs at the firewall have been a mandatory thing for, oh, about 80 years. They not only let the pilot shut the fuel off in case of fire or huge leak or an imminent forced landing, but the mechanic can shut it off so he can remove the fuel filters and carburetor and stuff like that.

Clamping a rubber line is a good way to start it failing. Damages it internally. Besides that, lines aft of the firewall are aluminum, not rubber, since rubber deteriorates and needs periodic replacing. Gets expensive inside the airplane. There are sometimes short sections of rubber line between tanks and the airframe to allow some movement, and they are often found old and cracking, and replacing them can be a pain. The flexible lines are usually found between the engine, which moves in its mounts, and the firewall. And they go bad in five or ten years. I've found those lines hard like wood, cracking, leaking, ready to just fall off. Big problem.

Automatic stuff presents more failure points. Electric or electronic stuff depends on the airplane's electrical system, which many here will tell you can fail at any old time. Automatic stuff is fine on the highway; not so much in airplanes, as a failure can mean an accident, so aircraft automated systems are of high quality and have some redundancy. All of that adds weight and cost, and flying is already expensive.

And automation dumbs down the pilot. ABS--antiskid brakes--are a case in point here. That ABS in your car is real nice and lets you drive safely. It also lends a sense of invincibility on a slippery road, and sooner or later you'll drive a little too fast or brake a little too late on a surface that is a little too slippery, and the ABS won't save you. Nice SUVs and 4x4s in the ditch are a common sight in Canada in the winter, while Grandpa in his old Chevy has slowed down, feels the road, and gets home for supper.
 
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The standard Navion configuration is just an on-off valve and it's a low wing. The two main wing tanks drain into a common accumulator and the engine pumps from that. For most low wings, it's to avoid drawing from an empty tank (ever try to suck from two straws, one that's in a glass of water and one that isn't?). You still have a cutoff for emergency purposes primarily.
 
An "off" position permits work on the carburettor/fuel pump, etc without having to drain the tanks.

Off also helps to stop fueling an engine fire, right? Wouldn't that what we call safety?

Also high wing or low, don't you check that there's fuel flow from Left and Right during taxi, and then assure you're on "desired tank" (low) or Both (high) before the runup?

If you don't, you should. It's important to confirm your feeding from each tank as directed by the fuel selector.
 
Off also helps to stop fueling an engine fire, right? Wouldn't that what we call safety?

Also high wing or low, don't you check that there's fuel flow from Left and Right during taxi, and then assure you're on "desired tank" (low) or Both (high) before the runup?

If you don't, you should. It's important to confirm your feeding from each tank as directed by the fuel selector.
It takes a long time for my lines to empty, and there is no way to know if you waited long enough. You might have waited just long enough from them to empty one clog and then switched, not having a clue.
 
Off also helps to stop fueling an engine fire, right? Wouldn't that what we call safety?

Also high wing or low, don't you check that there's fuel flow from Left and Right during taxi, and then assure you're on "desired tank" (low) or Both (high) before the runup?

If you don't, you should. It's important to confirm your feeding from each tank as directed by the fuel selector.

This seems uncertain, and potentially hazardous for a carbureted aircraft. It takes a little while for the carburetor bowl to empty (how long I don't know for sure at idle) so switching tanks on the ground to check fuel feed could involve a considerable, unspecified, delay. In the case of contamination, it might conceivably take some time for the contaminants to accumulate in the carburetor bowl to an extend to affect engine performance. Worst case scenario is that you switch to a contaminated or poorly feeding tank during the pre-flight check and discover it only after a delay during takeoff phase, instead of on the ground prior to departure.

My checklist includes selection of the fullest tank prior to start, and leaving it there during taxi and runup. The time required to do that is sufficient (I think) to assure that the fuel is flowing and not contaminated from the selected tank. Perhaps I might find that the other tank is somehow faulty when I first switch tanks 30 minutes after departure, but that would constitute an emergency for which I would still have sufficient fuel in the original tank to deal with. I'd rather do that than deal with a fuel interruption emergency during takeoff.

YMMV.
 
@Salty and @chemgeek - I guess I could do the engineering study of how long the fuel flow at idle takes to get to the carb or fuel spider to be precise.

BUT... start up uses a bit of fuel and while I'm sitting there waiting for the oil temp to come up takes a few minutes, as does completing my startup checklist and any pre-prep I want to do on my navigator, all that. If there was any issue with the "starting side" fuel line it should have it's chance to rear it's ugly head. Then while taxiing switching to the other tank (no boost pump) and getting to the run-up area takes another few minutes while the engine is not at idle, so some hesitation may be discovered, yah?

For the low wing I don't touch the fuel selector again and do the run-up and ignition check (I don't have mags) and that takes a couple minutes at a higher fuel flow. After the runup I leave the selector where it's at, wait my turn and launch. That's a bit of time for an issue to present itself, yah?

For the high wing the difference is that the startup on one side and switching while taxiing proves mostly the same things. Then going to Both for the runup confirms that as set the fuel is there at higher RPM, then continues the same... meaning don't touch it after the runup. Also in the high wing example the flow is assured and the only issue could be at the selector. But using this process consistently "exercises" the selector valve and reduces chances of issues there.

I did have an interesting "proof of concept" in an Arrow. I was getting checked out in at and my primary CFI was right next to me (literally). When I switched I turned the selector to off instead of the other tank. We were approaching the end of the taxiway (approx 4500' of 6000') and it got quiet. Pilot error for sure. Put it to the "desired tank" a little prime, and continued as per normal.

We all test the mag grounding before shutdown (if we have mags) right? But do we ever test fuel selector off?

Anyway... that's my procedure, it works for me, and as Chemgeek said, YMMV.
 
Good discussion.

Im in the camp of one tank startup-to-takeoff, and then I have a known good source if the engine falters when I switch. On a plane that normally runs in 'both', though I can see the value in making sure it runs on each tank individually, and as you mentioned exercising the selector.

Our planes are always full, so as long as one tank is working I've got 2 1/2 hours. I've never set them to 'off' except with the engine off practicing emergency procedures. Next time I go I think I'll try that and see how long it takes to shut off.
 
We're way ahead of you. Fuel shutoffs at the firewall have been a mandatory thing for, oh, about 80 years. They not only let the pilot shut the fuel off in case of fire or huge leak or an imminent forced landing, but the mechanic can shut it off so he can remove the fuel filters and carburetor and stuff like that.

Clamping a rubber line is a good way to start it failing. Damages it internally. Besides that, lines aft of the firewall are aluminum, not rubber, since rubber deteriorates and needs periodic replacing. Gets expensive inside the airplane. There are sometimes short sections of rubber line between tanks and the airframe to allow some movement, and they are often found old and cracking, and replacing them can be a pain. The flexible lines are usually found between the engine, which moves in its mounts, and the firewall. And they go bad in five or ten years. I've found those lines hard like wood, cracking, leaking, ready to just fall off. Big problem.

Automatic stuff presents more failure points. Electric or electronic stuff depends on the airplane's electrical system, which many here will tell you can fail at any old time. Automatic stuff is fine on the highway; not so much in airplanes, as a failure can mean an accident, so aircraft automated systems are of high quality and have some redundancy. All of that adds weight and cost, and flying is already expensive.

And automation dumbs down the pilot. ABS--antiskid brakes--are a case in point here. That ABS in your car is real nice and lets you drive safely. It also lends a sense of invincibility on a slippery road, and sooner or later you'll drive a little too fast or brake a little too late on a surface that is a little too slippery, and the ABS won't save you. Nice SUVs and 4x4s in the ditch are a common sight in Canada in the winter, while Grandpa in his old Chevy has slowed down, feels the road, and gets home for supper.

Good info across the board! Thank you!

I know cars though, so let me share some knowledge, too. ABS brakes have nothing to do with giving you better braking power whether slippery or not. In fact, braking distances are almost always longer with ABS because there are milliseconds when the brakes are not applied during every rotation of the wheel, as opposed to if you locked the brakes up you would be braking 100% for 100% of the time. The only purpose of ABS brakes is to allow you to steer around something while under hard braking. If you didn't have ABS and your brakes locked, you could not turn. With ABS, you can still steer and turn while threshold braking.
 
Off also helps to stop fueling an engine fire, right? Wouldn't that what we call safety?

Also high wing or low, don't you check that there's fuel flow from Left and Right during taxi, and then assure you're on "desired tank" (low) or Both (high) before the runup?

If you don't, you should. It's important to confirm your feeding from each tank as directed by the fuel selector.

It seems like there's loads of warnings about a plane engine catching fire and what to do if it catches fire. I've never had a car engine catch fire. Airplanes and everything in aviation is supposed to be built to a much higher quality and safety standard than automotive parts, so why are airplane engines catching fire and car engines not? I realize plenty of car engines do catch fire, but I've personally never experienced it and I run the cars I drive harder than 99.99% of drivers.
 
There is also an audible warning built into every airplane.

If you mean it gets quiet when the fuel stops flowing I'd agree. But... other than I have no clue what you mean. There is no "tank switch" alarm on the PA28 or PA32 series. Nor on any RV I've ridden in.

What's this statement based upon?
 
so why are airplane engines catching fire and car engines not?

Most of us are flying planes that were designed and built before we were born. (now then, my oldest planes were built when I was 9 yrs old, since I was hatched in 1966)

I think Ford Pintos used to catch fire a lot then, but I was too young to drive.

What age cars and tech are you comparing to our venerable Lycosaurauses and Continentals?
 
Most of us are flying planes that were designed and built before we were born. (now then, my oldest planes were built when I was 9 yrs old, since I was hatched in 1966)

I think Ford Pintos used to catch fire a lot then, but I was too young to drive.

What age cars and tech are you comparing to our venerable Lycosaurauses and Continentals?

That's what I thought and good to know. I rarely drive a car that's more than a few years old. I also wouldn't be too comfortable owning an airplane that's more than 10 years old. You can't trust the longevity of old things. Plenty of planes flying great that are 30, 40, 50, 60 years old. But you never know when something is going to snap on a machine that age.
 
I also wouldn't be too comfortable owning an airplane that's more than 10 years old.

That's an "interesting" view. I don't trust planes that haven't been flying longer than I have.

Oh... and the "new planes" still have they same Lycosauerses and Continentals under the cowling. Not a lot of changes in the piston single engine.
 
Like many other things in aircraft, redundancy. One tank could get contaminated.

On my Cessna 140, both tank gauges are labeled “No Takeoff” on the bottom 1/4 tank level of each tank. This is because it’s a taildragger and while the tail is low, the pickup can be uncovered. Being able to select tanks allows you to keep a fuller tank for takeoff or possible go around.
 
This seems uncertain, and potentially hazardous for a carbureted aircraft. It takes a little while for the carburetor bowl to empty (how long I don't know for sure at idle) so switching tanks on the ground to check fuel feed could involve a considerable, unspecified, delay. In the case of contamination, it might conceivably take some time for the contaminants to accumulate in the carburetor bowl to an extend to affect engine performance. Worst case scenario is that you switch to a contaminated or poorly feeding tank during the pre-flight check and discover it only after a delay during takeoff phase, instead of on the ground prior to departure.

There are three filters in a light airplane's fuel system. The first is a finger screen in the tank to keep larger crud from blocking the tank's outlet. The next one is the fuel strainer on the firewall, and it has a bowl to catch water and heavier contaminants by gravity, and it has a very fine screen to filter out particles and even water has a hard time getting past it when that screen is wet with fuel. The carburetor's (or injection servo) fuel inlet has a fine screen as well. There's a fourth screen in the fuel injection system's manifold.

So if you are draining the tank fuel sumps and fuel strainer before flight, you're removing the worst of the junk. If your mechanic is doing his job at annual or 100 hour inspections, he's taking that fuel strainer bowl off and cleaning the bowl and screen. At annual and 200-hour inspections that carb inlet strainer should be removed and cleaned, and after it's back in, the carb bowl drain plug should be removed and the fuel turned on briefly and the bowl flushed.

During warm-up or taxi is not the place to be checking for contaminants. Properly-maintained and preflighted airplanes won't have any contaminants. Junk in the carb bowl will kill you.

And yet----I lost count of the fuel strainers that hadn't been apart in many years. I had trouble getting them apart, being all seized shut. Finding the screens half-blocked with junk. It's a crime.
 
Airplanes and everything in aviation is supposed to be built to a much higher quality and safety standard than automotive parts, so why are airplane engines catching fire and car engines not?

Shoddy maintenance, mostly. High-quality stuff degrades. Nothing lasts forever. Like I said earlier, I've found fuel hoses in the engine compartment so old they're hard like wood. Their wire braid rusty and falling apart. Leaking fittings. Wrong fittings, sometimes, such as aluminum fittings where steel were called for. Aluminum fittings won't stand the constant vibration nearly as long as a steel fitting. Fuel strainers not apart in years so their O-ring seals are hard and rotten and cracking. Oil hoses old and ready to fail and squirt oil all over the hot engine and exhaust. Oil burns well, too. Engine hoses should be replaced every five years or so, and certainly when the engine is overhauled. So why did I sometimes find hoses with date tags showing that they were 40 years old?
 
Plenty of planes flying great that are 30, 40, 50, 60 years old. But you never know when something is going to snap on a machine that age.
Yes, you know when it will snap if the airplane is being inspected to the legal standards. That's why the government has those standards. There have been enough airplanes that failed in flight where investigators find stuff that should have been easily caught long before it failed altogether.
 
I also wouldn't be too comfortable owning an airplane that's more than 10 years old. You can't trust the longevity of old things.

You're scaring me here as my plane is eight years old and I'm not ready to build another one just yet. And I trust (pun intended) that you can trust the longevity of old things as I'm getting old and trusting the Lord for a little more longevity ... :)
 
I think Ford Pintos used to catch fire a lot then, but I was too young to drive.

What age cars and tech are you comparing to our venerable Lycosaurauses and Continentals?
Cars have had intertial fuel cutoffs since the 80's.
 
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