Backcountry Pilot

[Zzz]

I'm looking for some operational philosophy on fuel sector valve design. As we all know, the majority of the Cessna fleet uses a valve that allows selection of left tank, right tank, or both tanks supplying the engine. Early Pipers had valves that only allowed left tank OR right tank to supply the engine.

I've read about pilots operating on one tank for an hour, then switching and running the other tank dry, then switch back as some sort of reserve management or load balancing technique. Maybe that's just because they didn't have a BOTH option. I have to admit, I've only ever put the selector on BOTH and just flown within the endurance limits of the aircraft. Fact is, my butt has much less endurance than any aircraft I've ever flown.

But for fuel system design considerations, I'm looking more closely at whether there are advantages to utilizing separate tanks independently in flight.

Is it a big deal to not have a BOTH selection?

Thoughts?

[contactflying offline]

I also kept Cessna fuel selectors on Both. The only real consideration for Left or Right or Off was when you had to park with one wing low and tanks full. Since I am a forgetful type person, I made sure to find a level spot to park.

I have worked with a lot of pilots over the years. Those who consistently used check lists carefully and thoroughly have had fewer incidents and accidents than I. I just recognize my limitations. Attempting to be organized and assuming that I am has just never worked for me. I kept the fuel on both even if the carb leaked a bit. I also avoided retractable geared airplanes.

[rw2 offline]

Is it a big deal to not have a BOTH selection?

Thoughts?

Big deal? No.

Nice convenience? Yeah.

You can always fly a plane with a "both" switch as if that setting didn't exist if you want. You can't fly a l/r switch like a both.

Tracking when to switch tanks by a clock or timer is just so much busy work for no added value IMO, so I'm almost always on both. The only exception is that very occasionally all the fuel seems to draw from one tank anyway, so I switch it to the other one for a while to balance things out.

[lesuther offline]

I think it is a carryover from the necessities of simple low wing fuel systems. However, aux tanks also generally have their own line ports/selection. Flints, on the other hand, require pumping to the mains.

[Zzz]

I should be more transparent about this: I'm designing a return fuel line system. Not really designing, but analyzing all the known techniques to see what I'm most comfortable with.

In a full return design, utilizing a duplex fuel valve, it's recommended to supply and return to one tank or the other, not both apparently. The reasoning behind this is that the balance of the return flow can't be relied upon with guaranteed precision. We've all seen the supply balance vary from left or right tanks when using the BOTH selection. When you're on a single tank, the return should match the supply less the fuel burned. If you're supplying from and returning to both tanks, it's conceivable that you could draw less from one tank and return more to it...eventually filling that tank in flight. Uncoordinated flight and or other flight dynamics can affect the flow through a tee.

I don't like the idea of swapping between tanks but with a full return system that doesn't utilize a header tank, this seems the only option.

Some of you will respond with "that sounds complex, just use a carbureted engine." That might be a truth, but I'm a scholar of airplane building now, and more specifically fuel injection systems, and I want information!

[fly offline]

Zane,

Have you purchased your fuel injection system yet? Bendix and AFP systems (without purge valve) don't require a fuel return.... Simplicate, and add lightness.

[Cub271 offline]

But for fuel system design considerations, I'm looking more closely at whether there are advantages to utilizing separate tanks independently in flight.

Being able to select left and right is needed for belly / reserve tank operation (if equipped) in most back country aircraft. The belly tank or in-cabin reserve tank is typically going to electrically pump into the left wing tank, which necessitates the ability to fly on left only.

Additionally, most every airplane has some 'unusable' fuel in each wing tank that can be accessed with a tank selector valve. Engineers and the FAA define 'unusable' fuel as the fuel that's sloshed to the outboard portion of the tank (away from the fuel pickups that are typically located on the inboard side of the tank), in an uncoordinated turn.

In a fuel emergency, you can take advantage of this knowledge. Select one tank, use a bit of aileron and opposite rudder to raise the wing of the tank selected, which will get more fuel to cover the pickups in the tank, and thus access this 'unusable' fuel to continue to fly until that tank is very nearly completely dry. Then repeat with the opposite tank. In some back country high wing aircraft you might access another 3-5 gallons of fuel with this method, which just might be the difference that prevents a forced landing in an unsuitable area.

The 'off' position is important to starve fuel to a fire in the engine compartment or possibly prevent a fire in a forced landing. As others have mentioned, it also prevents a fuel load imbalance when parked cross-ways on a slope.

[fly offline]

Zane,


Have you purchased your fuel injection system yet? If you haven't, you might want to consider Bendix and AFP systems (without purge valve). They don't require a fuel return.... Simplicate, and add lightness.

[Zzz]

But for fuel system design considerations, I'm looking more closely at whether there are advantages to utilizing separate tanks independently in flight.

Being able to select left and right is needed for belly / reserve tank operation (if equipped) in most back country aircraft. The belly tank or in-cabin reserve tank is typically going to electrically pump into the left wing tank, which necessitates the ability to fly on left only.

Additionally, most every airplane has some 'unusable' fuel in each wing tank that can be accessed with a tank selector valve. Engineers and the FAA define 'unusable' fuel as the fuel that's sloshed to the outboard portion of the tank (away from the fuel pickups that are typically located on the inboard side of the tank), in an uncoordinated turn.

In a fuel emergency, you can take advantage of this knowledge. Select one tank, use a bit of aileron and opposite rudder to raise the wing of the tank selected, which will get more fuel to cover the pickups in the tank, and thus access this 'unusable' fuel to continue to fly until that tank is very nearly completely dry. Then repeat with the opposite tank. In some back country high wing aircraft you might access another 3-5 gallons of fuel with this method, which just might be the difference that prevents a forced landing in an unsuitable area.

The 'off' position is important to starve fuel to a fire in the engine compartment or possibly prevent a fire in a forced landing. As others have mentioned, it also prevents a fuel load imbalance when parked cross-ways on a slope.

Hey, nice to hear from you Brad. Thanks for those points.

[Zzz]

Zane,


Have you purchased your fuel injection system yet? If you haven't, you might want to consider Bendix and AFP systems (without purge valve). They don't require a fuel return.... Simplicate, and add lightness.

I'm aware of that design and have talked to Don at AFP, even bought their installation manual. I'm researching all the considerations of plumbing required for EFII, which has distinct advantages.

[fly offline]

Zane,

Obviously it's your airplane, build it the way you want. Saying that, EFII adds a lot of complexity. Now you will need redundant electrical systems, two electric fuel pumps, return lines, duplex fuel selector, ect.

Personally, I'd prefer a less complex system for a backcountry airplane. I'd likely use a Bendix fuel injection system with magnetos and a wobble pump. Light and simple. No battery, no problem.

Making the decision is the hard part. No matter what you decide, building it will be easy!

[Zzz]

Yep, all factors. But that's not the information I'm after in this thread. I just want to get some thoughts on tank selection. Thanks.

[mtv offline]

Zane,

Obviously it's your airplane, build it the way you want. Saying that, EFII adds a lot of complexity. Now you will need redundant electrical systems, two electric fuel pumps, return lines, duplex fuel selector, ect.

Personally, I'd prefer a less complex system for a backcountry airplane. I'd likely use a Bendix fuel injection system with magnetos and a wobble pump. Light and simple. No battery, no problem.

Making the decision is the hard part. No matter what you decide, building it will be easy!

There's no reason I can think of that you'd need two electric pumps....typically, these installations have a mechanical pump and a backup electric pump. That's not a bad thing in any case. One pump and it fails, and you land.

I am one of those odd ball pilots who's actually had an engine driven fuel pump fail....in a 185, and about halfway across 40 miles of open water. Argue all you like about the wisdom of flying a single engine airplane over that much water ( I wouldn't do it at my age) but that back up pump saved my life that day.

Zane,

I'd go whichever way you feel most comfortable. If you're going to feed the return line into the left or right tank only (and I would), I'd choose a right/left/off selector.

I'm sorry, but fuel management in such a system isnt' that sophisticated that you can't handle it. Is a Both position handy? Sometimes it is. And, as a typical lazy Cessna pilot, I generally run those on Both. I've also run several thousand hours in airplanes where you have to actually choose where the gas is coming from.....it ain't rocket science. The Beaver I flew for many years had five fuel tanks......and I persevered nevertheless.

I have the utmost faith that you are bright and competent enough to be able to manage a right/left/off fuel selector system.

I mean, run it on left for an hour, switch to right for an hour, etc isn't that hard....to me. That said, have I run one out of gas inadvertently.....Hell no....I meant to.... . But, you learn to switch tanks pretty quick.

But, if you're weird for a "both" position on that selector, I sure wouldn't hold it against you.

MTV

[blackrock offline]

Just get a duplex valve so the return is to the same tank you draw from. Or am I missing the point of your question? Both is needed for Gravity feed, not mandatory with pumps.

[fly offline]

Zane,

To directly answer your question, with a duplex valve, it doesn't matter if you have a both position or not. It's personal preference. You may need to "tune" the fuel vents to equalize flow in the both position.

MTV,

I think you missed the "EFII" part of the conversation. Those don't use engine driven fuel pumps.

[Zzz]

Just get a duplex valve so the return is to the same tank you draw from. Or am I missing the point of your question? Both is needed for Gravity feed, not mandatory with pumps.

I'm studying return line designs. I found two duplex selector valves on the market, Andair and SPRL. Andair does not make a duplex valve with a BOTH position, SPRL does. However, the theory behind return fuel lines in pressurized fuel systems suggests that returning to both tanks simultaneously is a bad idea. It should be supply and return to one tank only.

I already have a nice Andair standard valve installed, I like their quality. I've heard a few reports that the SPRL are not as nice.

[mtv offline]

Zane,

To directly answer your question, with a duplex valve, it doesn't matter if you have a both position or not. It's personal preference. You may need to "tune" the fuel vents to equalize flow in the both position.

MTV,

I think you missed the "EFII" part of the conversation. Those don't use engine driven fuel pumps.

Is there some reason you couldn't use an engine driven fuel pump in that system? Electrons have been known to escape some of us.....myself included. I wouldn't want to fly an airplane that absolutely depends on electrons to stay airborne, personally. But, that's just me.

MTV

[Zzz]

Good question. It is true that the mfr of the EFII system calls for both primary and backup fuel pumps to be electric. I'm not sure why an engine driven pump wouldn't work, but my guess is that the ECU failover to the backup pump uses feedback from the electric primary pump.

The whole point of EFII is letting the software control the mixture and the timing advance. I'm not sure if there is a complete electrical loss failure mode for both fuel and ignition, but I have seen mention of running one magneto. The engine driven fuel pump combined with a full rich default would complete the non electrical failure mode.

SDS/Fly EFII's solution to redundancy is dual ECUs, basically two independent systems, but it all runs on electrons bumping each other.

[blackrock offline]

The supply pressures are higher with electrics than an engine driven diaphragm pump. Likely the mechanical pump isn't capable of meeting required supply pressure for EFII.

[Zzz]

The supply pressures are higher with electrics than an engine driven diaphragm pump. Likely the mechanical pump isn't capable of meeting required supply pressure for EFII.

Ahh. That's probably it. Thanks Mike.