Fuel Tank systems on most GA aircraft...

[jcomm]

Hi,

most low wing GA prop aircraft are equipped with fuel selectors for "Left" / "Right" only, usually no "Both" choice, while high wing aircraft usually have the 3 options.

Is it a question of simplification, cost reduction due to the systems / pumps that would otherwise be required on low wing aircraft ?

Thanks for any hints…

[Pilot DAR]

The certification requirement for GA airplanes states that for there to be a "both" fuel selector position, it is required to interconnect the vent spaces of the left and right tanks with a vent tube with no low spot. The geometry of a low wing aircraft with dihedral makes it impossible to do this, so only "L" and "R" positions are permitted by design arrangement.

[jcomm]

Thank you for the prompt and rather informative answer Pilot DAR !

:ok:

[n5296s]

The geometry of a low wing aircraft with dihedral makes it impossible to do this
Thanks for the explanation of something I've often wondered about too. But... does this mean in effect a tube or hose joining the highest part of each tank? If so (or even if not) why is it harder with low wing than with high wing? (Written by a happy 182 pilot who has never had to worry about switching tanks!).

[tescoapp]

because you basically need a tube that remains higher than the wing tip because no dip is allowed. High wing you can just run a tube along the top of the fuel tank. Low wing, single wing its impossible. unless you connected something between the wing tip and the roof and back down the other side to the other wing tip.

[Pilot DAR]

More to the point, as low wing airplanes typically have noticeable dihedral, the vent space of the tank begins at the outer portion of the tank, and works it's way inboard as fuel is burned. To connect the vent spaces in the tanks at the outer tank end, it would be necessary to have the vent tube come across above the top surface of the wing. For all the high wing aircraft, there is less dihedral, so the vent space can be captured at the inboard end of the tank, and thereafter, connect the two tanks across the spar carrythrough, with a bit of a bend up in the vent line, so it will drain into either tank.

This characteristic is somewhat related to the requirement (by AD, for some Cessnas) for two venting fuel caps, as if the net line is plugged, and one cap does not vent, you cannot use the fuel in that tank. There is more to it than just that, but it gives you an idea. It is noteworthy, that the design standards (generally known as "part 23"), is a lot of hard learned wisdom, we don't always know what the lesson was, but we benefit from the learned outcome! Part 23 makes for an interesting read, if you wonder about aircraft design.

[Maoraigh1]

The proof was an RV built at Sligo with an unapproved "both" modification. It ended in the sea on its maiden flight, I think.

[meleagertoo]

Do I not recall the HASELL check for the Bulldog (low wing with dihedral) included the line under E for Engine;

Fuel Pumps on, Balanced, Sufficient, Feeding from Both

How does this fit the explanation given above?

[sharpend]

Do I not recall the HASELL check for the Bulldog (low wing with dihedral) included the line under E for Engine;

Fuel Pumps on, Balanced, Sufficient, Feeding from Both

How does this fit the explanation given above?

It does not. The Bulldog indeed has a Both position. The fuel selector in front of P1 (in an RAF Bulldog) has L, Both, R and Off. As a matter of interest, if you leave the selector is Both with the aircraft on a slope you will rise fuel being syphoned from one side to the other.

[meleagertoo]

My question was how can it have a "both" position given PilotDar's explanation that implies it isn't possible. Bulldogs are certified as civil aircraft so surelu must comply with that rule?

I don't recall whether we ever had cross-draining (not syphoning!!) on the Bulldog but then RAF aprons and hangar floors tended to be level.

[Genghis the Engineer]

It does not. The Bulldog indeed has a Both position. The fuel selector in front of P1 (in an RAF Bulldog) has L, Both, R and Off. As a matter of interest, if you leave the selector is Both with the aircraft on a slope you will rise fuel being syphoned from one side to the other.

The Bulldog was approved to an old UK Military standard, AvP970, which had different philosophies behind it compared to the current civil aircraft standard CS.23.

It does surprise me that CAA allowed transfer of ex-military Bulldogs onto CofA without modification of that valve, but they did - JAR-23 would have been extant at the time, and that certainly never permitted a "both" setting on a non-gravity pumped 2-tank system, any more than its successor CS.23 does.

G

[megan]

I know it's military, but mention it as an illustration of how fuel systems can be plumbed. The T-28 is low wing and has two fuel tanks in each wing, all tanks drain into a sump tank, Between the sump tank and engine is a fuel shut off handle labelled "ON" and "OFF". Fuel tanks in each wing has a vent line which terminates at its respective wing flap.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/452x568/w009_b11b3db2d730aabc53d3b15d9cf21786ba322bd6.jpg

[asyncio]

It does surprise me that CAA allowed transfer of ex-military Bulldogs onto CofA without modification of that valve, but they did.

I guess it's because of the commonality with Pups, which were never required to change either, so the Bulldogs just got grandfathered in.

Although personally, I always use left/right. For the simple reason that if there is a suspected fuel problem, I can just switch to the other tank, rather than having to make a 50/50 choice.

[Genghis the Engineer]

Hmm, might be a Section K compliant aeroplane then - I'd have to wade through, but it certainly hasn't got identical wording to CS.23.

G

[asyncio]

Yes, the Pup was originally certified under BCAR section K

[meleagertoo]

Kinda makes you wonder why the CAA waste their time and everyone else's money making rules like that whan there was evidently no problem to address in the first place. If all those (300+) Bulldogs that were flown so much more intensively and so much more enthusiastically than any civvy type never had a problem then this configuration of fuel system is surely about as sound as one can be, isn't it?
I wonder why they thought it 'needed' regulating?

[Pilot DAR]

Despite my desire to, I have never flown a Bulldog, and know little about them. However, I have found that the design standards are a gold mine of hard won wisdom, free for anyone to apply to their aircraft design. Note, that they are standards, not regulations. Therefore, if an eager aircraft designer would like to demonstrate that their aircraft design is entirely safe while not complying with the standard, they are free to do so, and if successful, will be awarded certification of that design. I have approved designs which were not in harmony with the design requirement, though it was not an easy task!

I believe that the airworthiness authorities are very diligent, and conservative when it comes to fuel system design. During research I did in the 1980's relating to accidents in Lycoming 320 powered aircraft (I was looking for a latent design shortcoming in the engine, I found that Canadian accidents in the 320 powered aircraft were nearly 50% fuel exhaustion, and 20% more crashing without engine power due to fuel shortage, while another tank contained fuel. (4% of crashes were maintenance related). So I can see why the FAA and other authorities focus on very conservative fuel system design.

For myself, I really like L/R only systems, it forces fuel management habits. My C 150 only has on/off, and I really considered modifying it to include a L/R selection as well - it just was not worth the effort for the small benefit it could bring. The more tanks, and the more control the pilot has over those tanks, the greater operational flexibility available, as well as abnormal situation management. Twice (C 177RG and C 206), I have been able to use the L/R to isolate a tank venting fuel to assure that I used what was left, in the venting tank, and not loose more fuel from the non venting tank via crossfeeding. One of my other aircraft is L/R only, with four tanks, During a nerve wracking flight with an unforecast headwind, and no alternate, I was able to run three tanks dry, and know what I had remaining in my last tank to complete the flight. Doing so got me safely to a runway with the least risk of running dry on one tank, while carrying fuel I had not used in other tanks.

[asyncio]

As I understand it, In all cases, if you have an empty tank selected, then the engine will suck air until you select a tank with fuel, and which point it must recover without an air lock developing.

The gotcha with both is that if one tank runs empty, then it can continue to suck air from the empty tank even if the other one is full. In which case you just do the same as in the L/R only aircraft, switch to the tank with fuel still in it.

I can only assume there were enough cases of people getting confused that it was decided that something must be done. I have seen that possibility mentioned in at least one Pup accident report.

[Discorde]

The Chipmunk (military and civil versions) had only an ON/OFF selector even though fuel was supplied by two wing tanks (one in each wing).

[Genghis the Engineer]

As I understand it, In all cases, if you have an empty tank selected, then the engine will suck air until you select a tank with fuel, and which point it must recover without an air lock developing.

The gotcha with both is that if one tank runs empty, then it can continue to suck air from the empty tank even if the other one is full. In which case you just do the same as in the L/R only aircraft, switch to the tank with fuel still in it.

I can only assume there were enough cases of people getting confused that it was decided that something must be done. I have seen that possibility mentioned in at least one Pup accident report.
That has always been my understanding of the reason for the rule in CS.23

G

[jimjim1]

T-28

It's amazing the difference a dash makes. T28 follows.

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/512x366/the_t28_super_heavy_tank_f975e7762649891efcad448b94478762097 9513e.jpg

T28 Super Heavy Tank in Aberdeen Proving Ground 1945

[asyncio]

Of course tanks have some less successful fuel system designs as well...

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/640x480/640px_tank_cruiser_mark_vi_a15_2c_crusader_iii_26_rotatraile r_284535976889_29_f3b48454818254e6c4b9a037aca369ee711be632.j pg

[Desert Flyer]

More to the point, as low wing airplanes typically have noticeable dihedral, the vent space of the tank begins at the outer portion of the tank, and works it's way inboard as fuel is burned. To connect the vent spaces in the tanks at the outer tank end, it would be necessary to have the vent tube come across above the top surface of the wing. For all the high wing aircraft, there is less dihedral, so the vent space can be captured at the inboard end of the tank, and thereafter, connect the two tanks across the spar carrythrough, with a bit of a bend up in the vent line, so it will drain into either tank.

This characteristic is somewhat related to the requirement (by AD, for some Cessnas) for two venting fuel caps, as if the net line is plugged, and one cap does not vent, you cannot use the fuel in that tank. There is more to it than just that, but it gives you an idea. It is noteworthy, that the design standards (generally known as "part 23"), is a lot of hard learned wisdom, we don't always know what the lesson was, but we benefit from the learned outcome! Part 23 makes for an interesting read, if you wonder about aircraft design.]

Pilot Dar, what you said above in a few posts meets the current FAA Part 23, but the old CAR 3 planes which were up to 1956 didn't have those same rules. For instance, the Aeronca Sedan, 15AC, had no interconnect across the fuselage to both tanks. Just a goofy vent system that had two 1/4" vent lines meet under the fuselage which would get filled with bugs and dirt thus leaving no vent. Then the bladders would collapse like a baby nurser bottle, and eventually the engine would quit for lack of fuel. Even though the fuel gauges showed near full as the gauges were a plastic tube connected to the top and bottom of the inboard side of the bladder. Lots of Sedan crashed because of this crazy system.

[Pilot DAR]

ARRANGEMENT

§ 3.430 Fuel system arrangement. Fuel systems shall be so arranged as to permit any one fuel pump to draw fuel from only one tank at a time. Gravity feed systems shall not supply fuel to any one engine from more than one tank at a time unless the tank air spaces are interconnected in such a manner as to assure that all interconnected tanks will feed equally. (See also§ 3.439.)

CAR 3.430 of March 13, 1956. The wording of CAR 3.430 of November 01, 1949 is identical. My bold.

I cannot explain why the Aeronca Sedan fuel system does not conform, I just read what the requirement says. 'Sounds as though you clearly understand the intent of the design requirement.

[27/09]

The certification requirement for GA airplanes states that for there to be a "both" fuel selector position, it is required to interconnect the vent spaces of the left and right tanks with a vent tube with no low spot. The geometry of a low wing aircraft with dihedral makes it impossible to do this, so only "L" and "R" positions are permitted by design arrangement.

It's not impossible to do in a low wing aircraft. I know of at least one low wing aircraft which operates with the selector effectively set to both (It only has an ON/OFF fuel valve) and has an interconnected vent tube.