Why do we switch tanks?

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
16,415
Display Name
Display name:
Sixer
In the TB9 we did it seemingly arbitrarily. MY CFI said every hour, the POH I think said every 15 min.

Honestly in that plane, it didn't take much of an imbalance and the plane started tipping toward the heavy wing so ya switched.

The Cirrus barks at me every 9 gallons to switch tanks. So far at the 9 gallon imbalance, the plane seems to be unaffected. It continues level.

As VFR pilot, I am trying to keep my eyes out the window as much as possible. So far, 100% of the time, I miss the flashing yellow [msg] that tells me to switch tanks.
CFI is very quick to point out those messages.

Why do I need to switch tanks every X gallons?
 
That reminds me I need to check and see if my selector valve works ... :lol:
 
SixPapaCharlie said:
So far, 100% of the time, I miss the flashing yellow [msg] that tells me to switch tanks.
CFI is very quick to point out those messages.

Why do I need to switch tanks every X gallons?
Click to expand...

I'll stare at the student or the screen if they don't select the msg when it's flashing.

In the Arrow I switch tanks every 10 gals or so. In the Maule I'll leave it on both unless it starts burning heavy on one side.

Mostly to keep the loading as evenly as possible. You may not notice the imbalance on your average flight.
 
Left side of the clock, left tank. Right side of the clock, right tank.
 
It's for fuel balance. Some airplanes are more finicky than others. In my old C model Mooney I'd takeoff on the left, switch at 30 minutes, the run one hour right, one hour left, etc. That way I was never more than 30 min out of balance.

If I was maximizing range, I'd then run the left dry (my takeoff tank) and use the total minutes out of it as my absolute max for the right (with reserves, of course.) Since the left had my takeoff and climb, it always left me a few gallons conservative in my estimates and never busted my reserve.
 
Do you feel the aircraft getting one wing heavy? If not then you don't really need to switch tanks. For example the Piper Cherokee POH recommends taking off on one tank, when you get to cruise switch to the other and burn it all off, then go back to the original tank.
Also consider the maximum fuel differential in the POH, if it's in there, there is probably a reason.
It's been a while since I flew a Cirrus, but I'm fairly curtain that there is a limitation for fuel imbalance.

Regarding you keeping your eyes outside, yes that's very good, however why can't you just time when you need to switch tanks? Just like you time the distance between waypoints or when you'll be running out of fuel.
I'm all for keeping the eyes outside, but once in a while you got to look inside to see your gauges. If you're missing the fuel imbalance annunciator then how do you know you won't miss the engine file annunciator or that your oil pressure has just dropped to zero?
That being said, spending 100% of the time staring outside is better than spending 100% of the time staring inside (which is what most pilots tend to do).
 
Last edited:
Believe it or not there are people out there who have the philosophy of running a tank completely dry before switching. Maybe the idea is that you then know exactly how much fuel you have left or maybe it's just for thrills but I don't buy it either way. First of all you should never get yourself into a situation where you are concerned whether or not you have half or a full gallon left.

Secondly, if for any reason the feed from the second tank should fail (perhaps it unknowingly had vented itself dry over the past two hours) you're gonna want to have the original to switch back to right?
 
Silvaire said:
Believe it or not there are people out there who have the philosophy of running a tank completely dry before switching. Maybe the idea is that you then know exactly how much fuel you have left or maybe it's just for thrills but I don't buy it either way. First of all you should never get yourself into a situation where you are concerned whether or not you have half or a full gallon left.
Click to expand...

I do this. I think working the fuel selector every 30 minutes is needless wear, but that's not why I run a tank dry. Laziness is 50% of it. Desire for uncomplicated landings is the other 50%. To wit:

The tank dry scenario isn't so I can scavenge those 3 lawyer-made unusable gallons and arrive at the airport on fumes -- it's so those "unusable gallons" effectively end up in the other tank, rather than sitting at the very bottom of an empty tank, stressing me out about their usability in, say, bumpy conditions, or in a Xwind where I may run the risk of unporting fuel on final by side-slipping.

If I arrive at an airport with 20 gallons remaining -- each of my tanks has 3 "unusable" per side.

I can arrive with 10 in each tank, "7 usable", for 14 gallons

Or I can arrive with 0 in one tank, and 20 in the other, "17 usable".

If I arrive with only 10 gallons total, it's more important. "2 usable per side" is terrible and will cause me anxiety. I'd rather have "7 usable" on one side. This requires running the other side dry at some point earlier in the flight.

It reduces my worry about "is that gas REALLY in there? Did my watch or fuel totalizer lie to me?"

Every plane is different, and I'm sure other types lend themselves to other ideas, this one is just mine.
 
schmookeeg said:
... it's so those "unusable gallons" effectively end up in the other tank, rather than sitting at the very bottom of an empty tank, stressing me out about their usability in, say, bumpy conditions, or in a Xwind where I may run the risk of unporting fuel on final by side-slipping...
Click to expand...

I'll have to admit, that's a good arguing point.
 
If for nothing else than a constant reminder of fuel management. The bane of the Cessna is the "Both" switch. When you're out of gas...you're really out of gas. When you are changing tanks, you're mindful of your fuel. Assuming the last time you switched tanks the engine was running fine, there's probably still fuel there is you have to switch back. Remember, fuel accuracy in light planes is dicey. The only requirement is that the gauges can't show fuel when the tank is empty and the reason for most fuel exhaustion accidents is the failure of the pilot to check the fuel before takeoff. Preflight, preflight, preflight. Don't treat your plane like your car...it's a lot less forgiving.
 
In the AA5 we had a fuel panel that after many attempts couldn't be fully sealed on the left tank. We would ideally run that one as close to dry as possible so when it went in the hangar there wasn't any fuel to seep out and waste.
 
schmookeeg said:
The tank dry scenario isn't so I can scavenge those 3 lawyer-made unusable gallons and arrive at the airport on fumes ...
Click to expand...
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? :dunno:

dtuuri
 
LDJones said:
It's for fuel balance. Some airplanes are more finicky than others. In my old C model Mooney I'd takeoff on the left, switch at 30 minutes, the run one hour right, one hour left, etc. That way I was never more than 30 min out of balance.

If I was maximizing range, I'd then run the left dry (my takeoff tank) and use the total minutes out of it as my absolute max for the right (with reserves, of course.) Since the left had my takeoff and climb, it always left me a few gallons conservative in my estimates and never busted my reserve.
Click to expand...

Good way to do it.


Your CFI should just let you go, once you run one dry or get a significant imbalance you won't forget again.
 
dtuuri said:
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? :dunno:

dtuuri
Click to expand...

at cruise altitude with plenty of fuel in the other tank is a good time to find out.
 
bartmc said:
at cruise altitude with plenty of fuel in the other tank is a good time to find out.
Click to expand...

So, he's gonna feed both engines in his avatar through the crossfeed for landing??

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

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.
 
Not all airplanes have a "both" capability. My plane does. When I built my Cub I added it there as well. I prefer both but have no issue with a plane that doesn't have it. Some planes, like a Cub, will specify to use one tank for takeoff and landing. Because of that guys may run the right tank dry to preserve fuel in the left tank. As for balance? I've never noticed it. I can add 20 gallons to one side and don't feel a heavy wing. I try to get them sort of similar but when I feel too lazy I add all I need to one side. No big deal. In a both system it'll equalize itself given some time.
 
The bane of the Cessna is the "Both" switch
Click to expand...

The beauty of the "both" selector is you minimize the potential of un-porting a tank and having the engine quit as a result.
 
Stewartb said:
The beauty of the "both" selector is you minimize the potential of un-porting a tank and having the engine quit as a result.
Click to expand...

A guy a few years back landed a 182 on the highway south of Hamilton Montana with a dead engine and an entire wing full of fuel.
 
dtuuri said:
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? :dunno:

dtuuri
Click to expand...

Yes, in my 182 there is 5 gallons "unusable" in each tank. I can get to 4.5 of that in one of the tanks if I "dry tank" in level flight. Effectively increasing my safety margin by over 20 minutes.

bartmc said:
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.
Click to expand...

That's a quite elagant and simple explanation. Another benefit of a high wing.
 
Usable fuel is by definition capable of sustaining maximum engine power in the most critical flight attitude. Unusable fuel is what's left once that line is crossed. The ability to use some or most of it in level flight is no surprise, you just can't depend on it for max performance ops. Some planes added header tanks to supply fuel to the engine while tanks may un-port during maneuvers.
 
timwinters said:
Yes, in my 182 there is 5 gallons "unusable" in each tank. I can get to 4.5 of that in one of the tanks if I "dry tank" in level flight. Effectively increasing my safety margin by over 20 minutes.



That's a quite elagant and simple explanation. Another benefit of a high wing.
Click to expand...

My 182 has only 4 gallons total unusable.
Flying on "Both" theoretically (per POH) requires periodically switching tanks in cruise, since over time one wing may feed more than the other, but I have discovered that keeping the rudder properly trimmed maintains an even feed, so no switching is required even for 6+ hr legs.
 
310 made it easy, start on mains for the first hour, switch to auxes and run them dry, them back to finish the mains.
 
bartmc said:
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.
Click to expand...

If you have 2 tanks that gravity fill into a lower central area w/ 1 straw or even 2 straws in that same area just incase one gets clogged, the problem is solved.
 
bartmc said:
A guy a few years back landed a 182 on the highway south of Hamilton Montana with a dead engine and an entire wing full of fuel.
Click to expand...
He never thought to switch to a single tank instead of leaving it on both?
 
Or to look at his fuel levels along the way? Or to use the required both position for takeoff, let alone landing? Or emergency procedures? There are a lot of ways to hose up a flight. Complacency is one of them. That much complacency may qualify for another label.
 
SixPapaCharlie said:
Why do I need to switch tanks every X gallons?
Click to expand...

I believe the 10 gal differential is found in "Limitations", so it's not optional.

I believe the general consensus is that it has to do with the autopilot.

Of course, a skinny pilot and a large passenger and just a rear seat passenger sitting in the right seat could possibly exceed the imbalance one might get from a 10+ gal difference, but with different arms it's hard to know.

To slightly reduce the switching, you could take off on one tank, burn 10 gals, switch, then burn 20 from the other side. And so on. But every 30 minutes worked well enough for me.
 
FastEddieB said:
I believe the 10 gal differential is found in "Limitations", so it's not optional.

I believe the general consensus is that it has to do with the autopilot.

Of course, a skinny pilot and a large passenger and just a rear seat passenger sitting in the right seat could possibly exceed the imbalance one might get from a 10+ gal difference, but with different arms it's hard to know.

To slightly reduce the switching, you could take off on one tank, burn 10 gals, switch, then burn 20 from the other side. And so on. But every 30 minutes worked well enough for me.
Click to expand...

I've stopped the fan on two occasions switching tanks. I hate it. But not enough to fly a high wing.
 
LDJones said:
[...] In my old C model Mooney I'd takeoff on the left, switch at 30 minutes, the run one hour right, one hour left, etc. That way I was never more than 30 min out of balance.

If I was maximizing range, I'd then run the left dry (my takeoff tank) and use the total minutes out of it as my absolute max for the right (with reserves, of course.) [...]
Click to expand...

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!? :dunno:
 
bartmc said:
I've stopped the fan on two occasions switching tanks. I hate it. But not enough to fly a high wing.
Click to expand...

Just once for me, climbing through 16,000'. Especially at altitude, it's a good idea to watch the fuel pressure like a hawk. Not sure the cause of my stoppage, and it came back up as soon as I switched back, but I vaguely recall thinking it was either vapor or an excessively rich mixture caused by the boost pump.

In any case I tended to delay the switching of tanks until I'm over or within easy gliding range of an airport. Always the paranoia that the selector could get stuck, the handle could snap off, whatever.
 
Last edited:
German guy said:
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!? :dunno:
Click to expand...

Since the pickup is near the bottom of the tank, wouldn't dirt in the bottom of the tank be the first thing to be pulled into the pickup anyway?

If there were to be a problem, I'd envision it to be detritus floating on the surface getting sucked in at the very end.

But no idea if it really works that way - it's just my mental image.
 
FastEddieB said:
Just once for me, climbing through 16,000'. Especially at altitude, it's a good idea to watch the fuel pressure like a hawk. Not sure the cause of my stoppage, and it came back up as soon as I switched back, but I vaguely recall thinking it was either vapor or and excessively rich mixture caused by the boost pump.

In any case I tended to delay the switching of tanks until I'm over or within easy gliding range of an airport. Always the paranoia that the selector could get stuck, the handle could snap off, whatever.
Click to expand...

First time in the cherokee, an upholstery screw jammed the selector. I was with a CFI. He took control of the plane and I told him , I'll either break the handle off or get it started again.

2nd time was leaving Liberal, KS for Wyoming. Was my first flight in the Bo. Was told the selector points towards the tank its on. I thought he meant the front of the selector, not the back.So, I proceeded to turn it to the off position.
 
You must log in or register to reply here.
Back
Top