Low wing fuel system vrs high wing

[Bernoulli]

Anyone know why the fuel selectors on single engine low wing airplanes never have a "BOTH" selection. It's always left or right which begs for human error. Just curious because high wing gravity fed planes have BOTH... why can't low wings have the selector bring both fuel tank lines into a manifold that is pulled by a machanical fuel pump / elec boost pump? Even new low wing singles have not come out with a both fuel selector. What is it about the low wing that restricts this capability. It can't be just that they are not gravity fed like high wing.... there must be some other design factor that limits this ability. Anyone who thinks they know please chime in...Thanks in advance for all constructive answers.

 

[gkrangers]

Good question.

 

[rfeathe1]

If a low wing aircraft had a "both" position, when a tank runs dry on "both" the pump would suck air from the empty tank.

 

[avbug]

My Dromader is low wing, and draws from both tanks.

 

[mtrv]

My Dromader is low wing, and draws from both tanks.

Whatever a Dromader is, does it have a header tank?

As a general rule with low wings, a both selector is not the norm. Things happen, such as sucking air as mentioned, or cross-feeding in an un-level airplane, that results in an overfull fuel tank that get's fuel vented overboard.

 

[avbug]

Whatever a dromader is? Good gravy.

Not really a header tank, though this varies from airplane to airplane. More of a collector or common point between the two feed-in lines.

A good rule of thumb with the airplane is don't go below 1/4 tanks, if there's any way of really identifying where that might be in the airplane, then drawing air from one tank or the other isn't usually an issue.

The type design did have several engine failures this summer from pilots who drew one tank or the other down below 1/4 on one side, and flamed out, resulting in several forced landings and a really spectacular long distance glide back to the airport, in one case.

In the event everything else goes wrong, there is a wobble pump which is supposed to supply the engine until the pilot's arm gives out. Of course, that doesn't leave him with a hand to fly with...

 

[freightdog227]

The commander 112 is a low wing and has a "both" position on the fuel selector. At least the one that I flew did.

 

[Bernoulli]

So the final answer for almost all low wing GA aircraft is because if one tank gets too low the pump will suck air? Good answer. Any other reasons. It just seems like the aircraft manufacturer would want to create the most safe aircraft possible... realizing that eventually human error will cause far more engine failures because pilots forget to switch tanks than with an aircraft that feeds off both at the same time. So if some low wing aircraft do have a "both" selector, why would companies like Cirrus who strive for safety (going so far as parachute protection and automatic prop adjustment) not create a fuel system with BOTH. I still think theres more to it. Thanks for the comments so far... keep em coming.

 

[StrykerFL]

Perhaps the engine driven fuel pump on most low wing single engine airplanes does not have enough suction to pull from both tanks.

To answer your question about the Cirrus:

The prop in the Cirrus is not so much automatic as it is mechanical. The prop govener control cable is connected directly to the throttle. There is no way to adjust the prop other than changing the power setting. It is designed more or less to reduce the workload on the pilot and make power adjustments easier. Most cruise settings in the Cirrus have the prop usually between 2400 and 2600 rpms. I'm not too sure about the safety aspect of it.

As for changing the tanks, there is a scheduler that can is set on both of the GARMIN 430's onboard to remind the pilot every 30 minutes to switch tanks. It is relatively idiot proof.

 

[Bernoulli]

Perhaps the engine driven fuel pump on most low wing single engine airplanes does not have enough suction to pull from both tanks.

Good point. engine driven fuel pump not strong enough to push or pull from both tanks and pump could suck air from a low tank. Those are all very good answers. I just thought of one too...possibly...Perhaps when you are not gravity feeding your fuel (high wing) but are drawing it, or sucking it from a low wing the quantities from either tank could be drawn or sucked unevenly resulting in the first good answer of drawing air from the empty tank. Any other ideas... keep em coming. Much appreciated.