Fuel selector Both or fullest

[piperboy84]

My POH states on take off and landing select "Fullest Tank or Both, I always select Both as it seems safer, is there a chance of engine feed interruption in the unlikely event one of the tanks running dry while set to Both in a gravity feed system? If not why would Fullest be the first choice in the POH as opposed to always using Both for TO and landing which seems like a more logical/safer option? (note: there are 3 inline pumps on the fuel system, 1 engine driven, 1 boost and 1 for vapor lock (there are additional 2 that are not relevant to this issue as they are xfer pumps)

[FlyingKiwi_73]

because you fly something with xfer pumps i feel deeply unqualified to comment....

but i will say some time ago i flew a 172S which for some odd reason wasn't taking fuel from both tanks evenly (it had the G1000 suite so i had visual and audible warnings) it was giving a low fuel indication on left tank but 3/4 full on the other, instead of taking BOTH on the return leg take off i went for FULLEST didn't fancy a cough from the engine climbing out with 4 pax on board...

Just me.. switched back to BOTH and monitored all the way home...

[Big Pistons Forever]

Piperboy what airplane are you flying ?

[Fostex]

instead of taking BOTH on the return leg take off i went for FULLEST didn't fancy a cough from the engine climbing out with 4 pax on board...

You are much more likely to get a cough on takeoff with a single tank selected in a 172 even if the unselected tank is empty. The POH recommendation for using both is there for many reasons, have a read of the POH and familiarise yourself with the fuel system.

[piperboy84]

BPF maule mx7 180a with the Peterson and maule Mogas stc which involves adding an electric pump to the supply

[thing]

but i will say some time ago i flew a 172S which for some odd reason wasn't taking fuel from both tanks evenly (it had the G1000 suite so i had visual and audible warnings)

I feel unqualified to comment now...

I have heard although it might be an old wive's tale but seems to be fairly well accepted amongst the experienced pilots at my club that the fuel vent on a typical Cessna can also when airborne increase the air pressure in the left tank making it feed quicker than the right.

I am prepared to be shot down in flames and retreat gracefully.

[Monocock]

I am prepared to be shot down in flames and retreat gracefully.

Don't worry, that only happens on the Flyer forum when one of the 6 self-appointed sky-gods deem a question too simple. :rolleyes:

I was always told that the fullest tank would, in the event of fuel pump failure, give a better head of pressure than a lesser full one. I suppose I've just believed that ever since!! How that works in a low wing set-up is beyond me though.

[thing]

How that works in a low wing set-up is beyond me though.

As long as the tanks were level then the fullest one would still have a greater head of pressure. You still have to pump it to the engine somehow though in a low wing...

We have a 152 (the unfeasibly light one...) that throughout flight drips fuel from the fuel vent, still haven't figured that one out either...

[Cobalt]

In a low wing set-up, the fuel gets "sucked" up by the fuel pump(s). Hence no "both" setting - try to suck up from an empty glass and a full glass simultaneously through a pair of straws, and you will only get air.

In a gravity fed system, there is a positive pressure in the fuel lines. That pressure does not really change that much, it comes mostly from the fuel in the line (a couple of feet above the engine) and not from the fuel in the tanks (a few inches in addition to that).

In general, it helps to put the fuel selector to a single tank

When refuelling. With it on "BOTH" the tanks are linked, and a small amount of fuel will flow across.
When parked on anything than level ground. Otherwise the lower wing tank will be VERY full, and if it is the side with the vent [if there is only one] you can lose a lot of fuel overnight
On some aircraft, the OFF position does that as well, on others it does not shut off the link between the tanks, so use whatever the POH says.

but i will say some time ago i flew a 172S which for some odd reason wasn't taking fuel from both tanks evenly (it had the G1000 suite so i had visual and audible warnings) it was giving a low fuel indication on left tank but 3/4 full on the other, instead of taking BOTH on the return leg take off i went for FULLEST didn't fancy a cough from the engine climbing out with 4 pax on board...

The most likely reason for this would be flying out of balance. Even a single degree of bank, barely noticeable on a murky day if you never look at the balance ball [or triangle in the G1000], can lead to significant fuel imbalance.

One degree of bank will make the outside end of the lower wing tank sit a couple of inches lower than the higher wing tank, and fuel will flow down... although empty on one and 3/4 full would be a bit extreme; 1/4 to 3/4 would not be unheard off for the less sensitive pilots. Especially when they fly uncoordinated steep turns or holds in an air exercise...

[thing]

I'll second that. When I was learning to fly I remember refuelling after a sortie and putting much more in one tank than the other. When I asked my instructor he simply replied 'Ball. Middle.Not'

[IFMU]

I vote both. Takes more mismanagement to run out of fuel.

I used to fly a 140 with no both position, and I used to keep one tank empty for weight reasons. The 140 I fly now has a both position.

Bryan

[mikehallam]

Some a/c, Rans S6 with two wing tanks have Left, Right & Off tap.
So the choice is easy, fullest tank.

Despite this separation the low wing overflow syndrome is still possible when parked left wing low - as in my shed - and Right selected. Because the pumped fuel return bleed is to the left tank, a surprising amount can drain from the 'top' right tank via the return bleed in a week parked.

I know !

mike hallam

[Big Pistons Forever]

BPF maule mx7 180a with the Peterson and maule Mogas stc which involves adding an electric pump to the supply

With a gravity fuel feed to a carburator I just can't see how the engine would not get fuel when set on both even if one tank was empty.

For fuel injected engines it is a different story as you must have a central collector tank in order to have a both setting even in high wing aircraft.

BTW there is a vent line on high wing Cessna's that runs across the top of the cabin from the right tank to the left tank. The left tank is then connected to the vent behind the left wing strut. Placing the fuel selector to left will not stop fuel moving from the right tank to the left tank via the vent line and then overboard out the vent, if the aircraft is parked on a slope with the left wing down.

[Big Pistons Forever]

We have a 152 (the unfeasibly light one...) that throughout flight drips fuel from the fuel vent, still haven't figured that one out either...

Not only are you almost certainly flying the aircraft overgross on most flights you are also flying it while it is unserviceable. There is no way it should be continually leaking fuel out the vent while in flight. There is definitely a problem.

[BackPacker]

For fuel injected engines it is a different story as you must have a central collector tank in order to have a both setting even in high wing aircraft.

The GA8 (IO-520) has a neat system where the two wing tanks feed to a collector tank via float valves in the collector tank. The float valve for the left tank is located on the right side of the collector tank and vice versa. If you happen to fly out of balance, or if the aircraft is loaded out of balance (fuel or cargo), it's the low tank which drains first, effectively restoring the balance all by itself. Furthermore, this system prevents fuel from flowing from one tank to another while refueling or parking on a slope. So there's no need for a L/R/B/O switch. There's just a firewall fuel-cutoff switch.

Placing the fuel selector to left will not stop fuel moving from the right tank to the left tank via the vent line and then overboard out the vent, if the aircraft is parked on a slope with the left wing down.

In the GA8 all tank vents come together in a plenum on top of the wing, and the exposed ends are so high that there's no chance at all that fuel drains from one tank to the other via the vent lines, even on extreme slopes.

All in all a clever and completely foolproof setup. The only mistake you can make is to fly deliberately out of balance, with the fullest tank on the high side, to restore the fuel balance that way. That will just work counterproductive.

[Shaggy Sheep Driver]

In a low wing set-up, the fuel gets "sucked" up by the fuel pump(s). Hence no "both" setting - try to suck up from an empty glass and a full glass simultaneously through a pair of straws, and you will only get air.

dH Chipmunk - low wing, fuel selector offers only 'on' or 'off' selection.

Whatever you do, if you change tanks before t/o give the engine time to stop (if it's going to) while you are still on the ground. Chap a couple of years ago who I knew didn't, and died following EFATO.

[Victorian]

Placing the fuel selector to left will not stop fuel moving from the right tank to the left tank via the vent line and then overboard out the vent, if the aircraft is parked on a slope with the left wing down.

Hmmm not sure about that BPF. I've observed the effects of

a) forgetting to select a single tank on parking (mucho fuel loss)
b) remembering (most of time after (a)), parked on a slope, no fuel loss.

All this aggravated by the FBO's habit of topping the tanks off at night.

But the 172R has a 'both' position. I believe some 172's don't so maybe they're different.

[dont overfil]

Victorian I agree. Handbook for the C182T suggests selecting right or left to prevent crossfeed when parked and it works.

It always uses more fuel from the left tank when both are selected. All the C172 and C182 I've flown have done that. Maybe it's my flying!:hmm:

D.O.

[Big Pistons Forever]

But the 172R has a 'both' position. I believe some 172's don't so maybe they're different.

The C172 R and S models have a fuel injected engine unlike all of the older fixed pitch prop C 172. It has a different fuel system with a collector tank under the floor ahead of the right front seat. It also has a separate fuel shut off valve as well as a tank selector

[riverrock83]

Yet the SA bulldog with an injected engine and low wing, has a both position (and left/right) yet doesn't have a collector tank to my knowledge.

When parking you set it to left to prevent crossfeed and so loosing the fuel out the vent...

Agree with the comment in that if you change your tanks on the ground then wait to see if the engine is going to stop. We startup on left. When warm enough to taxi we change to right, then change to both for power checks. I believe it takes 10 to 15 seconds at full power for fuel to get from a tank to the engine (judging by how long you can keep the plane inverted before the engine stops...).

I think a lot of this comes down to actually knowing your steed and not to take things for granted!

[FlyingKiwi_73]

Sorry i think we jacked this thread from the OP....however... i reported the fuel differences when i got back on the ground, The CFI and i dipped the tanks there was a 20 odd liter difference (both tanks had been equally filled as per the sheet when i took off) so there was a difference,.. he also suggested i was not flying in balance... i was slightly offended being an ex glider pilot 'n' all. :)

However i have gone and looked at the photo's of the trip... (my dad got very snap happy) and...yep the G1000 suite tells me what i need to know... 22 kt crosswind from....you guessed it...the left.... for most of the flight.

soo.... erm...not totally in balance then... even the GPS shows my crab...

[mikehallam]

Hang on a mo' Kiwi,

Shirley the a/c once cruising aloft doesn't know it's a cross wind ?

mike hallam.

[Gertrude the Wombat]

Shirley the a/c once cruising aloft doesn't know it's a cross wind ?

The G1000 bit of the aircraft does :)

[Crash one]

Gravity still works in a crosswind wing down crab dunnit?

[Fly-by-Wife]

Shirley the a/c

What a lovely name for an aeroplane, Mike.

Oh, did you mean surely?

FBW

[Fostex]

As Mike points out there is a difference between flying in a cross wind and flying out of balance.

Be careful what you trust off a G1000 in a 172 as well, it can only work on the sensor data it has available. In the case of the 172's float based fuel level sensors, that data is often a fairy tale.

[stevelup]

What a lovely name for an aeroplane, Mike.

Oh, did you mean surely?

Don't call me Shirley....

[Gertrude the Wombat]

Be careful what you trust off a G1000 in a 172 as well
Plus I asked the instructor exactly how it did the wind calculation. He thought it included an assumption that you were flying forwards, ie motion through the air was in the direction you were headed, ie there was no correction for flying out of balance.

If correct, that says to me that if you choose to fly out of balance you had better not take the G1000's calculation of wind very seriously. I haven't actually experimented by flying the 172 sideways on purpose and seeing how what the G1000 reports for wind changes.

[VP-F__]

Fuel tanks are not very deep, there cannot be a significant increase in pressure feed down the fuel line.

I always thought that you select the fullest tank if there is an imbalance simply to bring the aircraft back into balance, why select the other tank and make the balance issue worse?

On a 172 with L,R, Both and off settings the fuel will crossfeed in both and off but not in L or R. So if you are flying out of balance with Both selected then you will end up with an imbalance and as has already been mentioned if you park on a sideways slope then fuel will fill the lower tank if Both or Off is selected. Expensive if it is left wing down!

[Maoraigh1]

a crosswind wing down crab

Would only be on very short final - not on cruise.

[On-MarkBob]

Use the "Both" position for take-off and landing. In the cruise select the higher tank until the fuel is "balanced" then back to "both". On a long flight you might have to "balance" the fuel more than once. I use an egg timer to remind me to put it back to both.

However, If the lack of fuel is becoming an issue with both tanks below 1/4, it might be a good idea to select left or right tank. Run the engine on that tank until it starts to cough (run dry), immediately select the other tank and find somewhere to land!!!!

Bob.

[Shaggy Sheep Driver]

yep the G1000 suite tells me what i need to know... 22 kt crosswind from....you guessed it...the left.... for most of the flight.
soo.... erm...not totally in balance then... even the GPS shows my crab...

Maybe it also caused the G1000 to draw more from one of its batteries than the other.

I do hope you are pax, and not a pilot.