Fuel Selector

[troopernflight]

Simple question: I always operate with the fuel selector on "Both". I've never seen any reason why I would operate with left or right tank only. Why would you ever want to operate on only one tank? The operations manual says that you should only operate on one tank while at cruise. But it does not explain why you would need to operate on one tank. Is there any purpose for this, or have any of you ever utilized one tank selection for any reason? The only thing I could think of is that it may be used in an emergency, like if one tank became clogged. But with a vent running from one tank to the other, I didn't see how that would be an issue. Thanks.

 

[Roger, Roger]

Simple question: I always operate with the fuel selector on "Both". I've never seen any reason why I would operate with left or right tank only. Why would you ever want to operate on only one tank? The operations manual says that you should only operate on one tank while at cruise. But it does not explain why you would need to operate on one tank. Is there any purpose for this, or have any of you ever utilized one tank selection for any reason? The only thing I could think of is that it may be used in an emergency, like if one tank became clogged. But with a vent running from one tank to the other, I didn't see how that would be an issue. Thanks.

Most Cessna 172s will burn fuel a little quicker out of the left tank than the right. Some aircraft are worse than others. After 2 hours or so, it is a bit easier to handfly if you switch to the right tank to balance the fuel load.

 

[Blackhawk]

Minor point. It is not that the 100 series Cessnas burn more fuel from the left tank than the right tank. The "problem" is differential head pressure between the left and right tank caused by the L shaped fuel vent on the left tank; since the right tank also needs to be vented Cessna engineers took care of the problem by connecting the top of the right tank to the top of the left tank. Thus, in theory, equal pressure in each tank.
Of course nothing is ever that simple in aviation. Dihedral, while not great in Cessan wings is still there and places the fuel tank interconnect line below the fuel level when the tanks are full. When air enters the left tank to replace any fuel that has been used it will force fuel from the left tank to the right tank via the interconnect line replacing any fuel that has been used from the right tank. This will continue until the fuel level in the left tank drops below the level interconnect line- and may continue beyond that point due to fuel sloshing.
One of the misconceptions going around is that this should not happen due to the vented fuel caps. The vented fuel caps, however, do not provide constant venting for the fuel system. They only have secondary vents that normally remain closed and only open when there is a substantial difference between ambient pressure and the pressure of the fuel tanks.
Information provided is from the CPA (Cessna Pilot Association), website. There is a very good article there by John Frank. I would recommend any CFI who teaches in Cessna airplanes to become a member.

 

[ppragman]

Simple question: I always operate with the fuel selector on "Both". I've never seen any reason why I would operate with left or right tank only. Why would you ever want to operate on only one tank? The operations manual says that you should only operate on one tank while at cruise. But it does not explain why you would need to operate on one tank. Is there any purpose for this, or have any of you ever utilized one tank selection for any reason? The only thing I could think of is that it may be used in an emergency, like if one tank became clogged. But with a vent running from one tank to the other, I didn't see how that would be an issue. Thanks.

Think "aileron trim." If your load is more on one side of the airplane (fat uncle or fat pilot) you will notice a slight slight difference. Burn a little out of the heavy side, and you'll fly straight as an arrow.

 

[USMCmech]

Why would you ever want to operate on only one tank?

In the Caravan, we almost never have both fuel selectors on.

No matter how perfectly trimmed you think you have trimmed the airplane, one tank WILL feed faster than the other. Since you are burning 300 lbs per hour, this can create a serious fuel imbalance.

 

[sundog]

Is there any purpose for this, or have any of you ever utilized one tank selection for any reason? The only thing I could think of is that it may be used in an emergency,

some other situations:
you could have a load of water or rust or some other junk in one tank that you forgot to sump out,
or perhaps a fuel tank cap wasn't on properly and a good portion of fuel got sucked out of one side.

It is not a very likely of a scenario that one side would be bad and the other not, but it may give you enough to get to an airport.

 

[PGT]

It should NOT be on both after the last days flight as this will cause some evaporation.

For what it's worth, I flew a C152 for 2.5 and had 1 gal in left tank and 7 in the right.

 

[troopernflight]

It should NOT be on both after the last days flight as this will cause some evaporation.

For what it's worth, I flew a C152 for 2.5 and had 1 gal in left tank and 7 in the right.

I guess it depends quite a bit on what aircraft you are flying. I fly a 1981 172rg cutless. I've never had any fuel inbalance issues, no matter how long the flight. Or maybe I was compensating with the controls and didn't realize it.

 

[mshunter]

It should NOT be on both after the last days flight as this will cause some evaporation.

For what it's worth, I flew a C152 for 2.5 and had 1 gal in left tank and 7 in the right.

So the fuel will more likely evaporiate from the tanks if the fuel selector valve is left on both rather than left/right/off?

The way I understand the system is that whenever the airplane is parked, switch the tank to left/right/off to prevent fuel from going from the high wing to the low wing. It's an open system that will let the fuel stayto keep the fule level with the horizon. So if the wings are not level, the fuel will be (I hope this makes sense). Parked includes sitting on the ramp being fueled. How many times have you gone to check the fuel in a 172, taken the fuel cap off the low wing and had fuel poor out of it because the last guy didn't turn the fuel selector from the both position to any other position?

 

[twdeckard]

I've heard some folks suggest that it offers a wake-up call in a fuel management mistake. If you toggle left/right every half hour and actually run one dry at least you have 30 minutes to land.

For me thats like rolling your windows down when driving on the ice. If you think the ice isn't thick enough ...

 

[SpiraMirabilis]

In the Caravan, we almost never have both fuel selectors on.

No matter how perfectly trimmed you think you have trimmed the airplane, one tank WILL feed faster than the other. Since you are burning 300 lbs per hour, this can create a serious fuel imbalance.

And dear god don't forget to put them both on off when you park and secure. Well, you only make that mistake once. Or you come back with like 600lbs imbalanced and have to think up creative ways to fix it like turning the aircraft around 180 degrees so the the other tank is on the incline.

UPS Guy: 'Why're you parked backwards today?'
Me: 'Just mixing it up, you know.'
UPS Guy: 'I see.'

 

[minitour]

And dear god don't forget to put them both on off when you park and secure. Well, you only make that mistake once. Or you come back with like 600lbs imbalanced and have to think up creative ways to fix it...

Hey, I know I normally don't get gas but uh...can I get 80 gallons.....................in the left tank?

Single point FTW.
-mini

 

[Douglas]

\

The way I understand the system is that whenever the airplane is parked, switch the tank to left/right/off to prevent fuel from going from the high wing to the low wing. \

I still don't understand this. Maybe somebody can help me out. I hear people saying this all the time but in a 172 when you switch the selector to left or right is anything actually happening above your head that would prohibit the fuel from transferring from tank to tank. I don't think so, but again anybody in the know that can clear it up.

Edit: PGT I've never heard about that evaporation. Did you read that some where or did someone tell it to ya.

 

[Roger, Roger]

I still don't understand this. Maybe somebody can help me out. I hear people saying this all the time but in a 172 when you switch the selector to left or right is anything actually happening above your head that would prohibit the fuel from transferring from tank to tank. I don't think so, but again anybody in the know that can clear it up.

My understanding was always that the fuel was feeding through the selector to the other tank, not through the vent tube.

 

[Bernoulli Fan]

How many times have you gone to check the fuel in a 172, taken the fuel cap off the low wing and had fuel poor out of it because the last guy didn't turn the fuel selector from the both position to any other position?

And I thought it was just because the line guys fill the planes at night while listening to rap music.

 

[mshunter]

I still don't understand this. Maybe somebody can help me out. I hear people saying this all the time but in a 172 when you switch the selector to left or right is anything actually happening above your head that would prohibit the fuel from transferring from tank to tank. I don't think so, but again anybody in the know that can clear it up.

Edit: PGT I've never heard about that evaporation. Did you read that some where or did someone tell it to ya.

http://www.takeflightsandiego.com/documents/Cessna172NPOH.pdf

Look at the selector valve in the PDF on page 64. The fuel will try and stay at a constant height above the ground, and match the horizon. If you leave the selector valve in the both position, it allows the fuel to go from the high wing to the low wing, to make the fuel in both tanks level with the horizon. It's kinda like the ball in the TC. Park the airplane on an incline, and the ball is out of the cage. Same with the fuel. I hope I described it well enough.

 

[splash]

http://www.takeflightsandiego.com/documents/Cessna172NPOH.pdf

it allows the fuel to go from the high wing to the low wing, to make the fuel in both tanks level with the horizon.

I believe there is a check valve (a valve which allows free flow of fluid in one direction but no flow or restricted flow in the opposite direction) next to the fuel selector coming from both tanks. The 172 POH (p.65) only says under the note area that in flight if the wings are not level then this can happen because the pressure is greater on one side than the other forcing the fuel from the high wing to the "engine" to be burned and not the other tank with low pressure. It does not say the tanks can talk to each other through the selector valve.

However, if my understanding is not correct about what you are saying... if you are correct this would mean that having the selector on both with one tank completely empty and the other just topped off the next day you would find about a half tank in each. For me this is unheard of.

switched to both causing evaporation???? eh? again unheard of. How does it stop evaporation if switched to left/right/or off?

 

[splash]

How many times have you gone to check the fuel in a 172, taken the fuel cap off the low wing and had fuel poor out of it because the last guy didn't turn the fuel selector from the both position to any other position?

This is from the fuel getting hot from the sun and expanding in a topped off full tank or If filled on a level surface then moved to an unlevel surface the high wing will spill out of the fuel cap when opened.

 

[Roger, Roger]

I believe there is a check valve (a valve which allows free flow of fluid in one direction but no flow or restricted flow in the opposite direction) next to the fuel selector coming from both tanks.

Never heard of that before. I don't think that is correct.

 

[mshunter]

I believe there is a check valve (a valve which allows free flow of fluid in one direction but no flow or restricted flow in the opposite direction) next to the fuel selector coming from both tanks. The 172 POH (p.65) only says under the note area that in flight if the wings are not level then this can happen because the pressure is greater on one side than the other forcing the fuel from the high wing to the "engine" to be burned and not the other tank with low pressure. It does not say the tanks can talk to each other through the selector valve.

However, if my understanding is not correct about what you are saying... if you are correct this would mean that having the selector on both with one tank completely empty and the other just topped off the next day you would find about a half tank in each. For me this is unheard of.

Bolded part.... They both have the same volume. The pressure would be tha same.

Try it next time you fly a 172. Fly around on 1 tank, and come back to it in the morning and see what has happened to the level. You will be shocked, I promise.

This is from the fuel getting hot from the sun and expanding in a topped off full tank or If filled on a level surface then moved to an unlevel surface the high wing will spill out of the fuel cap when opened.

Fuel expands a minimal amount, negating this "theroy." And most planes sit after being fueled by a truck, because they are in a tie down spot. What about the plane that sits overnight (has happened to me), that was fueled, and now it's a cool morning. My student checks the fuel before the first flight, and spills a gallon on the ramp during his pre-flight?

If it's the same as a C-170 valve (I have rebuilt a leaky one) then there is no checkvalve system, just a few springs with detent balls, and some o-rings in an aluminum block.