Fuel Selectors...?
 

Here's one for the technically minded out there.

Why do training aircraft like PA38's have tank fuel selectors when others like the C152 have a basic on/off one. I don't need to know all about gravity feeds and fuel pumps though. This isn't a question about the good/bad points of the C152/PA38 either, merely why don't all aircraft just have on/off valves?

I can appreciate in larger multi-tank a/c that there have to be selectors from the weight and balance and fuel planning points of view but in training aircraft is seems like another potential problem.

If this is just for the "added realism" factor, then do you think/know whether it increases ful burn or not.

The fuel burn question is the one that's been bugging me all day...!

Ah, 10 years since Trevor Thom means it's all gone hazy!

Thanks, Dai.

Differences in design philosophy really.

Both Cessna and Piper fit wing tanks on both sides. Cessna (at least for the 150/152) then connect them to each other, have a single feed out of them, and treat the whole thing as a single tank.

Piper treat them as separate tanks, with a selector between them, one setting on the selector being off.

An alternative method you see on some high wing aeroplanes, is to have two tanks in the wings, feeding simultaneously a header tank behind the seats, and then feeding the engine from that. The homebuilt EasyRaider works that way.

I'm firmly of the view that a fuel system, so far as is reasonably possible, should be designed to give a single gauge and a simple on-off valve. Everything else should be down to plumbing and / or automation. Complications belong in the drawing office where we have all the time in the world to get it right, not in the cockpit, where we don't.

There is a further approach, which is used on US aircraft and UK military aircraft, but is banned on UK/European civil aircraft. That is with two tanks to have a L-R-Both selector. The reason it's not accepted in the UK, is that it will invariably drain one tank first, then suck air from that tank and cause an engine failure with stacks of fuel still on board.

There was also a 1960s Cessna with the fuel selector labelled "Left / Right / Both OFF". You guessed it, the OFF was right at the bottom and tended to get rubbed off by the pilots knee. Apparently quite a few pilots who didn't know the type well would select "Both", and cause an engine failure through fuel starvation.

G

The answer is because..... .And there is little technical justification for some of the rest!

Pipers - even the high wing ones usually have separate tank feeds - the technical justification is obvious when you're dealing with a low wing and surprisingly similar for the high wings - the right hand tank (usually) has more convoluted tubing runs and hence poorer fuel delivery and water seperation - hence the need to use the left tank in low fuel conditions.. .Cessna simply plumb to two tanks together - easy on/off cock and the fuel delivery is not a problem. Of course decanting from one tank to the other can be if the aircraft is parked on sloping ground.. .My Auster has both wing tanks plumbed together and then has the addition of non-return valves! (well built on a cost plus basis for the military so what do you expect!).

Interestingly even with one tank nearly empty and the other half full these valves do not offer enough resistance to starve the engine (its one of those things you try when you become aware of it - don't worry there was fuel in the belly tank!).

So as I started not a lot of logic and almost as much common practice or company policy. In most cases the fuel system is however too complicated and has rubbish gauges - I wonder if this is why people still run out of fuel?

In Beagle Pups and Bulldogs you can still select Left, Right or Both. To select "Off" in flight would require a cognitive error of even greater than average proportions, although no doubt it has been/will be done. Ordinarily you fly around with "Both" selected, aiming to keep the amount in each wing in balance. Every now and then you may have to adjust the balance by flying on L or R for a while, but quite often you can do the whole thing with "Both" selected....which is nice. Even nicer when you have one tank only and can switch the fuel on and leave it there until you've finished or are about to crash, as in a Tiger Moth.

Thanks for replying peeps, it's interesting.

But on comparable aircraft types with the same engines I feel, for some reason, that the fuel burn would be higher in the ones with a simple on/off although I really can't think why.

It's just one of those things that is really bugging me, because I'm convinced that I've heard something about this in the past maybe...?

Mind you, it is better to keep the two fuel tanks totally seperate, in the case of the Piper, so each tank can be isolated in case of fuel problems......plus its a useful reminder when to land and refuel, leave the selector on one tank, when the engine starts to cough, you know you have 20 odd gallons left..... :)

Daifly, Re your fuel burn question:. .Most aero engines still use carburettor's, although late model engines use fuel injection.. .I suspect you think the height of the fuel above the carburettor can effect the fuel flow through the carb, and therefore the fuel consumption. Not true. (It is true that the height above the carbutettor will increase the fuel pressure delivered to the carb marginally, but the volume of fuel available has no effect) The carburettor will control the amount of fuel entering the float chamber via a float controlled valve, the higher the level in the chamber the harder the valve presses aginst it's seat to prevent excess fuel entering the chamber. So the carb will deliver only the amout of fuel it is designed to. Remember, you the pilot, control the air fuel mixture! . .With fuel injected engines, the fuel is always delivered under pressure by pumps to the injection metering unit. The pumps are always capable of exceeding both the volume and pressure needs of the system, so pressure/volume is controled by a pressure relief valve. The excess fuel is returned to the fuel tank. . .White Bear.

Nice one White Bear - that was the answer I was looking for.

I think you focussed it in my mind. I was thinking that with two tanks feeding the engine the fuel pressure would be higher than if it is just one tank at a time.

The pressure would be higher but the carb would only deliver the same amount, so the fuel burn wouldn't be higher.

Thanks for taking the time to explain it - I can finally get some sleep now!!