Why do we switch tanks?

[schmookeeg]

Huh? I must have missed out on this bit of trivia, can you educate me? Are you saying the unusable quantity is in fact all useable after all?

dtuuri

Sorry for the disconnect between avatar and plane I was discussing.

In my Bonanza, yes. The placard indicates 37 usable per tank. My left in level cruise will flow ~39.7 on the totalizer, and the right will flow ~40.1. (obviously tested on different flights ). The gas pumps have shown appx that in the fill also, so it's not a calibration error, or at least they're within tenths of a gallon, not off by 10%

I believe the unusable limit is to prevent unporting during a sideslip. The fuel is quite accessible in straight/level flight.

I suppose using unusable fuel is illegal per the POH. Oh noes.

I don't do this in my Baron, which has the same amount of unusable fuel, for obvious reasons -- cross-feeding to scavenge fuel seems like extra work and 3 gallons is not a lot of extra juice to squeeze on a twin. I don't want air in my crossfeed lines.

 

[MachFly]

I really like this approach, but have to admit that the idea of running a tank completely dry, until the engine begins sputtering, makes me quite uncomfortable. Will the engine actually simply quit or will it first run rough for a few seconds ,giving the pilot enough time to switch tanks and to turn on the auxiliary fuel pump?

I was also wondering, whether dirt on the bottom of the tanks might get flushed into the system, clogging the fuel selector or the fuel filter!?

It won't just come to a complete stop right away. It will kinda like misfire a little and vibrations will significantly increase. You should have 1-3 seconds or so to switch tanks.

Yes all the crap at the bottom of the tank will go into the engine (or hopefully stopped by the filter) unless this procedure is done occasionally. So if the tank is never ran dry, I'd be concerned about doing it.

 

[FastEddieB]

Yes all the crap at the bottom of the tank will go into the engine (or hopefully stopped by the filter) unless this procedure is done occasionally. So if the tank is never ran dry, I'd be concerned about doing it.

I still can't quite picture that.

Any dirt sloshing around in the bottom of the tank will go into the pickup first, whether the tank is full, or nearly empty.

Right?

By what mechanism would it wait until the level is really low to do so?

 

[Stewartb]

Most tanks use finger screens at the outlets.

 

[German guy]

I still can't quite picture that.

Any dirt sloshing around in the bottom of the tank will go into the pickup first, whether the tank is full, or nearly empty.

Right?

By what mechanism would it wait until the level is really low to do so?

When I asked the question above, I had the flow of dirty water out of the kitchen sink in mind, after doing the dishes. It appears as some of the crap stays on the bottom of the sink, almost until all the water is gone and only then gets swept into the drain.

Thinking about it, I believe that the reason for this effect is that the lower the water level in the sink gets, the stronger the current in the remaining body of water, towards the drain, gets, in order to maintain the (roughly) same mass flow down the pipe. Then again, even though our planes are thirsty, the current down the fuel line does probably not even come close to the current in a kitchen sink.

 

[FlySince9]

Put straws in two cups fill one with water, leave the other one empty. Now try to get a drink of water with both straws in your mouth.

That's assuming you have to suck it out...Gravity fed is not a problem, Right? Oh, that's right... You guys swithcing tanks (usually) sit on the wing... Sucking is imperative!

 

[Pilawt]

Will the engine actually simply quit or will it first run rough for a few seconds ,giving the pilot enough time to switch tanks and to turn on the auxiliary fuel pump?

In my K35 Bonanza, the fuel flow needle would start to twitch, then a couple seconds later the engine will start to stumble.

It's an FAA certification standard that the fuel system be designed so that if the engine (normally aspirated) stops from fuel starvation in flight, it must be able to produce full power within ten seconds after fuel flow is restored. For turbocharged engines I think it's 75% power within ten seconds.

 

[eman1200]

Take trips short enough to only use fuel in one tank, then u never have to switch!

 

[FastEddieB]

When I asked the question above, I had the flow of dirty water out of the kitchen sink in mind, after doing the dishes. It appears as some of the crap stays on the bottom of the sink, almost until all the water is gone and only then gets swept into the drain.

Thinking about it, I believe that the reason for this effect is that the lower the water level in the sink gets, the stronger the current in the remaining body of water, towards the drain, gets, in order to maintain the (roughly) same mass flow down the pipe. Then again, even though our planes are thirsty, the current down the fuel line does probably not even come close to the current in a kitchen sink.

I'm thinking on that.

Still, in the sink analogy would not the major dose of dirt just go down the drain initially?

 

[Hank S]

Your kitchen sink is not bouncing through the air, and will never hit turbulence. Think instead like pouring muddy water out of a bottle, while shaking it around to keep it mixed up. Unless you only fly on dead calm days?

I switch tanks every hour. There's a clock in the yoke with a set of red hands that don't move; I set them at engine start and every time they match, I switch tanks. Also. Ames it handy to track flight time.

 

[Silvaire]

I wonder why we need to switch tanks at all. How is it not possible for fuel to be fed continuously from both tanks?

It is possible with a high wing but it requires a cross vent to equalize the head pressures between the two tanks. On airplanes without a cross vent there is no BOTH selection because a higher positive vent pressure on one tank will push the fuel to the opposite tank where it will vent overboard.

In aircraft that do have a cross vent and a BOTH selection the two separate tanks can, for all intent and purpose, be considered as one single tank.

 

[timwinters]

Your kitchen sink is not bouncing through the air, and will never hit turbulence. Think instead like pouring muddy water out of a bottle, while shaking it around to keep it mixed up. Unless you only fly on dead calm days?.

^^^this^^^

 

[Dan Thomas]

When I asked the question above, I had the flow of dirty water out of the kitchen sink in mind, after doing the dishes. It appears as some of the crap stays on the bottom of the sink, almost until all the water is gone and only then gets swept into the drain.

Thinking about it, I believe that the reason for this effect is that the lower the water level in the sink gets, the stronger the current in the remaining body of water, towards the drain, gets, in order to maintain the (roughly) same mass flow down the pipe. Then again, even though our planes are thirsty, the current down the fuel line does probably not even come close to the current in a kitchen sink.

But the tank's outlet to the system isn't at the bottom of the tank. It's slightly above it. The tank's sump drain is lower, and that's where the crud accumulates. As long as you faithfully sump that tank before flight, you have little risk of junk heading for the engine.

And that outlet being higher than the bottom is one reason that there's a certain amount of unusable fuel. The other reason has to do with the number of outlets; one somewhere between the front and back of the tank will retain more unusable fuel than a system having outlets at the front and back that run together at some lower point in the airframe.

Dan

 

[Dan Thomas]

Tank configuration has a lot to do with the need for tank switching. The Supercub with wing tanks that are between the front and rear spars and right in against the wing roots on a narrow airplane won't have the same imbalance problems that some aiplanes with their fuel stored ahead of the leading edge and reaching a long way out toward the tips will. The Cirrus had some issues with that; pilots would set the auropilot and forget to switch tanks, and the autopilot would feed in more and more aileron to keep the airplane level and the pilot didn't notice until the ailerons hit the stops and the airplane went into a bank that they could arrest only with rudder. But now, in the slip, the fuel in the full tank was away from the outlet port, so fixing the problem was impossible. I once worked on a Cirrus that had been 'chuted because of that.

And the Cessna ttX (400) has its fuel well out in those wings, away from the fuselage, probably for safety reasons. Switching gets more important.

Airplanes should have their wings on top, like birds.

Dan

 

[N918KT]

Just wondering, is there a setting on some low-wing single-engine airplanes with wing tanks to feed from both tanks instead of one wing tank at a time? I find there is a "both" setting on high-wing planes but not on low-wing planes. Why is that?

 

[MooneyDriver78]

Just wondering, is there a setting on some low-wing single-engine airplanes with wing tanks to feed from both tanks instead of one wing tank at a time? I find there is a "both" setting on high-wing planes but not on low-wing planes. Why is that?

High wings feed by gravity, low wings need to pump, if one side went dry, then the pump would start sucking air and you lose the engine despite still having fuel in other tank.

 

[RotorDude]

High wings feed by gravity, low wings need to pump, if one side went dry, then the pump would start sucking air and you lose the engine despite still having fuel in other tank.

You make it sound as if it's a law of nature.
But with simple engineering you could have a "smart pump" that would stop pumping when sucking air (and restart when sensing fluid again, in case of sloshing/unporting of minimal fuel).

 

[schmookeeg]

I believe Skippers have a "Both" for their low-winged fuel tanks. I recall a pretty decent sized interconnect joins the two tanks into a header arrangement.

 

[Hank S]

I switch tanks to stay in the air longer and fly further.

 

[MooneyDriver78]

You make it sound as if it's a law of nature.

But with simple engineering you could have a "smart pump" that would stop pumping when sucking air (and restart when sensing fluid again, in case of sloshing/unporting of minimal fuel).

Oh, there are ways around it, but that means extra weight or complexity, a center tank with transfer pumps on the wings would be another.

 

[bcool]

Just wondering, is there a setting on some low-wing single-engine airplanes with wing tanks to feed from both tanks instead of one wing tank at a time? I find there is a "both" setting on high-wing planes but not on low-wing planes. Why is that?

Low-wing Rockwell Commanders have a "Both" setting....

 

[ARFlyer]

When it really come down to it, I just plan my x/c at the highest fuel burn. When the clock hits 1 hour left I start looking for an airport nearby.

If I'm in a school aircraft I'll fuel every 2-2.5 hours. It adds a landing or two on long distance travel, but rules are rules.

 

[jesse]

When it really come down to it, I just plan my x/c at the highest fuel burn. When the clock hits 1 hour left I start looking for an airport nearby.

If I'm in a school aircraft I'll fuel every 2-2.5 hours. It adds a landing or two on long distance travel, but rules are rules.

Get into a slow enough airplane or a sparse enough area and 1 hour won't get you to an airport. A common issue in the Flybaby. Sometimes my range is about 50 miles with 1 hour

Better to just always be aware of how many minutes of fuel are remaining and how many airports you can make it to. I *really* do not like leaving myself with one airport as the only option. Might be closed or they might be out of fuel..or the WX might be crap, etc. I *try* to have enough to get to one airport and another airport after that (not always possible in slower planes).

Not very often I've seen an airport run out of fuel...but...I've seen it at least three or four times.

 

[Dan Thomas]

Oh, there are ways around it, but that means extra weight or complexity, a center tank with transfer pumps on the wings would be another.

If it's a gravity feed center tank, that tanks needs to be lower than the mains. In some airframes, that's difficult. You'd need a deeper belly on the fuselage. Transfer pumps in the wing tanks are fine as long as they work, as would be a smart pump that reliably shut off once it sucked air due to a dry tank rather than just a brief unporting.

All of it adds expense (more engineering, parts, manufacture and assembly) and liability (failure points and more insurance premiums for the manufacturer). Airplanes are already far too expensive and the manufacturers know that, so they're reluctant to add anything more to the costs.

And it all adds weight. Current certified airplanes tend to be a lot heavier than when they first started out, and heavy airplanes lose performance and utility.

Dan

 

[BGF_Yankee]

It is possible with a high wing but it requires a cross vent to equalize the head pressures between the two tanks. On airplanes without a cross vent there is no BOTH selection because a higher positive vent pressure on one tank will push the fuel to the opposite tank where it will vent overboard.

In aircraft that do have a cross vent and a BOTH selection the two separate tanks can, for all intent and purpose, be considered as one single tank.

Just because you have a BOTH setting doesn't mean that fuel comes out of both tanks at an even rate. When I had my C172M, fuel was constantly coming out of one faster than the other (can't remember which now), so I would switch to drain specifically from the tank with more fuel to maintain a balance. I don't think I could've ever got into a dangerous configuration/imbalance as it was never that great of a difference, but if I let it go too long, I could feel it since input was required to correct it. Now that I have a PA32R, I get the joy of being barked at as well by my GPS to remind me to switch between LEFT and RIGHT...no BOTH setting now

 

[ARFlyer]

Not very often I've seen an airport run out of fuel...but...I've seen it at least three or four times.

I've fueled at an aiport that had about 2" of fuel left in the tank. You can be darn sure we sumped the living crap out of the plane afterwords.

 

[jesse]

The real answer to this question is if the Cirrus displays a warning light when you're more than 9 gallons out you shouldn't let that warning light remain lit..Ignoring such things will bite you in the ass.

Others above said it's a limitation, I didn't check to see if that's true, but I assume it is. So why do you change tanks so often? Because it's a limitation of the airplane and because if you don't a warning light will aluminate and as a result shouldn't be ignored.

 

[jsstevens]

The DPE who gave me my private checkride suggested that I use the tank selector in the C-172 to manage fuel. His reasoning was being sure how much fuel you had and not guessing. He suggested switching every hour. It seemed (and seems) like a good idea, but I've never done it. I've also never come close to maximum endurance. My longest cross country flight was Orlando Executive (KORL) to Marathon Key (KMTH). And I had the plane fueled before heading back. I did make a couple of instrument training flights that were 4 hours and I was puckering a bit as we came back into Orlando, but we never used more than 30 gallons out of 40 usable.

The C-172C I fly from time to time has a placarded limitation about using both above 5000 feet MSL.

John

 

[Henning]

I've fueled at an aiport that had about 2" of fuel left in the tank. You can be darn sure we sumped the living crap out of the plane afterwords.

Why? He water sinks to the bottom, it doesn't matter how much fuel is in the tank, the water will always be in the same relationship to the pick up.

 

[FastEddieB]

Others above said it's a limitation, I didn't check to see if that's true, but I assume it is.

2003 SR22 POH.

So why do you change tanks so often? Because it's a limitation of the airplane and because if you don't a warning light will aluminate and as a result shouldn't be ignored.

No warning light. I think most Cirrus pilots program a "Switch Tanks" warning into their 430's (or whatever) for every 20 or 30 minutes.

 

[jesse]

2003 SR22 POH.



No warning light. I think most Cirrus pilots program a "Switch Tanks" warning into their 430's (or whatever) for every 20 or 30 minutes.

Ah ok, thanks for the correction, was just assuming it was there based on his post. Some types have it.

 

[timwinters]

Why? He water sinks to the bottom, it doesn't matter how much fuel is in the tank, the water will always be in the same relationship to the pick up.

There are lots of other contaminants out there besides water and in a stationary tank I'm confident that they tend to come out with the last of the fuel as the little remaining fuel runs horizontally across the bottom of the tank towards the outlet. This horizontal movement doesn't exist when the tank is more full.

Of course, the filter SHOULD catch it but I surely wouldn't second guess or chastise someone for sumping out of excess caution.

OTOH...I do not subscribe to the theory that this same low fuel contaminant pick up occurs in airplanes because these tanks are not stationary and fuel is sloshing around all the time during taxi and flight.

 

[MooneyDriver78]

If it's a gravity feed center tank, that tanks needs to be lower than the mains. In some airframes, that's difficult. You'd need a deeper belly on the fuselage. Transfer pumps in the wing tanks are fine as long as they work, as would be a smart pump that reliably shut off once it sucked air due to a dry tank rather than just a brief unporting.

All of it adds expense (more engineering, parts, manufacture and assembly) and liability (failure points and more insurance premiums for the manufacturer). Airplanes are already far too expensive and the manufacturers know that, so they're reluctant to add anything more to the costs.

And it all adds weight. Current certified airplanes tend to be a lot heavier than when they first started out, and heavy airplanes lose performance and utility.

Dan

Not to mention what it would add to the certification process, the testing that would be necessary, etc

 

[MooneyDriver78]

Do annuals require any testing of low fuel sensors or cleaning of the tank?

 

[schmookeeg]

There are lots of other contaminants out there besides water and in a stationary tank I'm confident that they tend to come out with the last of the fuel as the little remaining fuel runs horizontally across the bottom of the tank towards the outlet. This horizontal movement doesn't exist when the tank is more full.

Of course, the filter SHOULD catch it but I surely wouldn't second guess or chastise someone for sumping out of excess caution.

OTOH...I do not subscribe to the theory that this same low fuel contaminant pick up occurs in airplanes because these tanks are not stationary and fuel is sloshing around all the time during taxi and flight.

Our 12000 gallon avgas tank had the pickup 2" from the bottom of the tank. There was a pickup for a "water thief" that ran at the true low point, where our mechanics would find a ready source of solvent. There was appx 160 gallons of unusable 100LL due to the configuration.

 

[Sac Arrow]

Your kitchen sink is not bouncing through the air, and will never hit turbulence.

Obviously you do not live in California.

 

[German guy]

When it really come down to it, I just plan my x/c at the highest fuel burn. When the clock hits 1 hour left I start looking for an airport nearby.

If I'm in a school aircraft I'll fuel every 2-2.5 hours. It adds a landing or two on long distance travel, but rules are rules.

Frankly, I don’t see how the concept of running one tank empty in order to get an precise idea of the actual remaining fuel, and to have all of the remaining in one tank would interfere with the rules, as long as the difference doesn’t exceed the aircraft’s limitations.

I think that it actually increases the safety, as this strategy improves the awareness regarding the remaining fuel and as it also avoids the situation that tanks will have to be changed at a possibly very late phase of the flight.

I might be overly cautions, but I always end up with at least two hours of fuel left in the tanks, what makes long cross countries somewhat inefficient.

The reason is, that I plan for 1 – 1.5 hours of reserve, depending on the availability of fuel along the route, to deal with possibly necessary deviations or unexpectedly unfavorable wind conditions.

Roughly another hour however comes from my conservative assumption regarding fuel consumption and that I neither want to run a tank dry nor change from one almost empty tank to the other, which does also not have much fuel left in it. 1 hours worth of fuel in the Archer is just about 4.5 – 5 gal. per side. I would not feel comfortable to find myself on the final approach, wondering if I REALLY still have 4.5 gallons left in this tank or whether I should rather switch to the other tank, of which I am just as uncertain regarding the remaining fuel. As a result, I toss around almost 10 gal. of ‘unusable’ fuel. To have a reality check at a comfortable altitude, when one tank runs dry, and to have all the remaining fuel in the other tank, would make me a lot less nervous…

Even though there is no max. allowed difference between the Piper Archer’s fuel tanks, it is definitely noticeable, if one wing gets significantly lighter than the other. As my wife is quite a bit lighter than me, I would plan for having the remaining fuel towards the end of the flight on her side.

I thought about how to apply this to my next longer cross country flight and came up with this idea:

Taxi, takeoff + first 40 min. of flight, left --> 80 min. right --> Run left tank dry (approx. 100 min), take time --> All the remaining fuel is in the right tank, approx. 60 min. (still very conservative, as the other tank supplied the taxi, takeoff and climb to cruise altitude)

This strategy should provide me with a max. difference between the tanks of no more than 60 min. or about 10 gal. + maybe 4 – 5 gal reserve due to this conservative approach.

 

[kenjr]

In my Cherokee I have 4 tanks to think about. How I switch or balance depends on how long I'm going to be flying.

I always do start/taxi/takeoff on the right main and once it's half empty I start switching around...but I always come back to the right main at GUMPS because I know I've got at least an hour of fuel in there.

She can go about 6 hours on the 84 gallons in there...which is much longer than I or anyone else I know can stand to be cramped up in there so I never really worry about not having enough gas but I do like to make sure that I'm balancing the tanks.

 

[Henning]

There are lots of other contaminants out there besides water and in a stationary tank I'm confident that they tend to come out with the last of the fuel as the little remaining fuel runs horizontally across the bottom of the tank towards the outlet. This horizontal movement doesn't exist when the tank is more full.

Of course, the filter SHOULD catch it but I surely wouldn't second guess or chastise someone for sumping out of excess caution.

OTOH...I do not subscribe to the theory that this same low fuel contaminant pick up occurs in airplanes because these tanks are not stationary and fuel is sloshing around all the time during taxi and flight.

Not really. All the stuff you don't want is heavier than gas and will settle to the pick up sump. The load of fuel you don't want (without sampling) is the first load after a fill and the tank settled for a few hours. That is the dangerous load for contamination, not the last load. Fuel tanks don't work on a skimmer system.

 

[timwinters]

Not really. All the stuff you don't want is heavier than gas and will settle to the pick up sump. The load of fuel you don't want (without sampling) is the first load after a fill and the tank settled for a few hours. That is the dangerous load for contamination, not the last load. Fuel tanks don't work on a skimmer system.

Don't know what type of tank you're describing or you are familiar with but the two designs I've been associated with both set up horizontal flows across the floor when nearly empty similar to the aforementioned kitchen sink. The sh*t laying on the bottom of your kitchen sink as it's draining doesn't start moving towards the drain until the very end. Both tank designs I dealt with were similar..