Why do we switch fuel tanks before landing?

dcat127

dcat127

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The Piper checklists I'm familiar with have "fuel on fuller tank" in the before landing checklist. Of course if there is not adequate fuel in the tank you are on you need to switch. But if I have 10 gal in the left and 15 in the right, am I actually improving anything by switching? I feel that switching fuel tanks low to the ground is a somewhat risky endeavor. Should our checklist not say, "fuel on the appropriate tank for landing?"
 
Primarily you want to make sure you have an uninterrupted flow in case of a go around.

You shouldn't be that low when you make this change. This is an "in range" item, not something you do on final.

Of course, it woke me up fast when my wife turned the fuel selector to OFF rather than MAIN on downwind once. Fortunately, the engine restarts quickly when you put fuel back in it. (I won't mention the time I ran a tank dry in the middle of a procedure turn while doing my instrument training).
 
Switch it 10 miles out or not at all. I assume it's not near empty, but if so, why?

You're running on a known fuel source that isn't giving you trouble. Why would you try to switch away from it just to be on a tank with more fuel? They shouldn't be that far off and you've got 10 minutes of flying left? Don't change anything.
 
It might be written that way to make you think about it whike you're not task saturated.
Check whatever tank you're on, make sure you have enough fuel in it for what will happen next (approach, procedure turn, land, go around), switch if you don't thonk you'll have enough.
 
Great question

I think most flight manuals say fullest tank in the checklist. But switching in the pattern or worse, final could cause an engine shutdown due to no fuel. Example - I was flying a Cherokee years ago and switched tanks in the pattern (yup the CFI said to ). On short final I got to do a real engine out procedure. The selector was in the detent but not centered and it had burned through the last bit of fuel in the line until final. That was “interesting “ to say the least.

Now I do like @bflynn said, switch 10ish miles from the airport. I like to plan my bonanza fuel levels to switch when I start out of my cruise when possible, and that’s what the Bonanza society promotes too.
 
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If nothing else you're burning fuel from the heavy wing. (Most checklists just advise to use "fullest tank"). If you go around during some task saturation emergency the fuel tank selection is one less task to trip you up while you sort things out, and the all while the W&B is trending to a more neutral condition.

Like many checklist items it's a PRE-landing check list and failure to do it rarely has any real consequences.
 
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It's an oversimplification to say that one should switch tanks before landing. And it isn't always the same. It depends on the aircraft and the intent of the checklist writer. Balance and flow characteristics - especially for a balk - play a part, but do not play the same part in all designs.
 
Another point to consider, some aircraft have a minimum fuel in a tank for take off. Probably a good idea to have at least the much in the tank you are using for landing, in case of a go around.
 
With a little planning, switching tanks just before landing should be perfectly avoidable.

I cringe whenever I see pilots reach for the fuel selector handle the millisecond their GPS timer tells them to switch tanks, like their lives depend on immediate action. What's the hurry? In the pecking order of aviate, navigate, communicate, the routine switching between fuel tanks might be a distant fourth even behind communicate (unless you've run a tank dry).

Why is this important? The fuel selector is a mechanical valve, and like all things mechanical, it can break and fail. The valve can get stuck. The handle can break off. The whole assembly could potentially come apart. Does that happen often? Absolutely not, but why take chances and accept exposure to this (admittedly low) risk probability when the severity of the realized risk could be high?

In plain language: switch tanks when you are in gliding distance of an airport, or at least of survivable terrain in case of an emergency landing. With a little planning ahead, you can minimize the number of times you switch between tanks (certainly less often than every 30 minutes) and avoid switching at low altitude. All while arriving at your destination on the fullest tank.

- Martin
 
dcat127 said:
The Piper checklists I'm familiar with have "fuel on fuller tank" in the before landing checklist. Of course if there is not adequate fuel in the tank you are on you need to switch. But if I have 10 gal in the left and 15 in the right, am I actually improving anything by switching? I feel that switching fuel tanks low to the ground is a somewhat risky endeavor. Should our checklist not say, "fuel on the appropriate tank for landing?"
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As long as I'm absolutely certain there's plenty of fuel remaining, I don't switch tanks. Seems to introduce possible failures by messing with the sector.
 
JackL-3J said:
It's an oversimplification to say that one should switch tanks before landing. And it isn't always the same. It depends on the aircraft and the intent of the checklist writer. Balance and flow characteristics - especially for a balk - play a part, but do not play the same part in all designs.
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Yep, my aircraft says put the selector back on main (the aux tanks are approved only for level flight). We had at least one Navion crash when taking off on the tip tanks. I believe there are some Bonanzas that had similar issues.
 
flyingron said:
Yep, my aircraft says put the selector back on main (the aux tanks are approved only for level flight). We had at least one Navion crash when taking off on the tip tanks. I believe there are some Bonanzas that had similar issues.
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The 5- or 6- tank airplanes I’ve flown only allow takeoff and landing on one of them. My understanding is that it has to do, at least in part, with smaller fuel lines that can’t support takeoff power.
 
Martin Pauly said:
With a little planning, switching tanks just before landing should be perfectly avoidable.

I cringe whenever I see pilots reach for the fuel selector handle the millisecond their GPS timer tells them to switch tanks, like their lives depend on immediate action. What's the hurry? In the pecking order of aviate, navigate, communicate, the routine switching between fuel tanks might be a distant fourth even behind communicate (unless you've run a tank dry).

Why is this important? The fuel selector is a mechanical valve, and like all things mechanical, it can break and fail. The valve can get stuck. The handle can break off. The whole assembly could potentially come apart. Does that happen often? Absolutely not, but why take chances and accept exposure to this (admittedly low) risk probability when the severity of the realized risk could be high?

In plain language: switch tanks when you are in gliding distance of an airport, or at least of survivable terrain in case of an emergency landing. With a little planning ahead, you can minimize the number of times you switch between tanks (certainly less often than every 30 minutes) and avoid switching at low altitude. All while arriving at your destination on the fullest tank.

- Martin
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Exactly.

And every 30 minutes is excessive. The GFC-500 autopilot in my plane says maintain the fuel within 10 gallon difference between wings. But that means, first change is about 35 - 45 minutes for takeoff and climb and some cruise to burn 10 gallons. But the next change is 2 hours later (10 GPH), as you start 10 high on that tank, run to equal, then to 10 gallons low.

But if I am going a long trip, I try to have one tank near empty with the majority of the fuel in the other one. That way I can do let down, approach, missed approach, etc without worrying about having to switch again or actually running one dry.
 
Pinecone said:
Another point to consider, some aircraft have a minimum fuel in a tank for take off. Probably a good idea to have at least the much in the tank you are using for landing, in case of a go around.
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I was just about to post essentially the same thing.
 
It will be interesting to read the NTSB report on the Cherokee 6 that just crashed at the Pomona Speedway while attempting to land at Brackett. Based on all the absorbent material spread around the crash site, there was probably plenty of fuel, but not in the selected tank. 4 tanks on that vintage of Cherokee 6. The manual states that if fuel starvation occurs, it could take up to 6 seconds to return fuel to the engine after selecting a tank with fuel.
 
It’s not uncommon to slip the plane during landing, and a tank low on fuel may unport during uncoordinated flight. The fuller tank has less chance of unporting.

Does it matter when both tanks have a lot of fuel? No. But it’s a good habit.
 
MauleSkinner said:
The 5- or 6- tank airplanes I’ve flown only allow takeoff and landing on one of them. My understanding is that it has to do, at least in part, with smaller fuel lines that can’t support takeoff power.
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Flow is one issue, availability in all flight attitudes is another.
 
dcat127 said:
The Piper checklists I'm familiar with have "fuel on fuller tank" in the before landing checklist. Of course if there is not adequate fuel in the tank you are on you need to switch. But if I have 10 gal in the left and 15 in the right, am I actually improving anything by switching? I feel that switching fuel tanks low to the ground is a somewhat risky endeavor. Should our checklist not say, "fuel on the appropriate tank for landing?"
Click to expand...

Piper says tell to use the PROPER TANK on the Warriors and Archers and Proper tank (fullest) on the Arrows.

The APPROACH and Landing checklists provides the operations to be accomplished prior to landing. Where you perform these operations is your discretion. As an example, the approach and landing checklist states the seat backs should be erect. I doubt you are adjusting seat backs low to the ground.

Piper recommends for Archers and Warriors (page 4-22) to use one tank for 1 hour after takeoff; then the other tank for 2 hours; then back to the first tank, which will have 1-1/2 hour of fuel and the second tank will have 1/2 hour of fuel when the tanks were full on take off.

 
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I agree with everyone else, I don't switch fuel tanks in the pattern. I also don't switch tanks between leaving the ramp and initial climbout. I've seen some checklists phrase it "switch to a tank with fuel" which is slightly better. Really what this is all about is making sure you're not going to run out of gas with your currently selected tank.

Something nobody really mentioned is that if you inadvertently switch from a tank with fuel to one without, there probably won't be an immediate change. You have some quantity of fuel in the lines, in the gascolator, in the fuel bowl if you have a carburetor. How long will probably vary quite a bit between aircraft and power settings but you absolutely could switch to an empty tank before entering downwind and not have the engine stop until short final. Maybe even from farther out than that.

Likewise if you do switch to an empty tank(or put the selector between detents or "off" and have your engine quit it will take some time for the system to re-prime(meaning fuel getting from the tank to the engine). How long will depend on the aircraft but 20-30 seconds is not unheard of. Add in some time to process what's happening, time to figure out what to do and do it and you can absolutely end up making an unintentional power-off landing if that happens at pattern altitude.
 
There's perhaps something to be said for the practice of switching fuel tanks after take-off while still within gliding distance of the airfield, to ensure that you have access to both tanks. Not something I ordinarily do, I admit, but potentially something to consider prior to a longer cross country in which both tanks will be needed. If you do it as you're climbing out of the traffic pattern and away from the airport, it could serve as a check rather than making that first switch potentially over unfriendly terrain.

Personally, I picked up and like the clock-hand method, with left/right tank based on which side of my watch the big hand is pointing. Sure, it's a tank switch every 30 minutes but it serves as a nice reminder to periodically double check my fuel burn and endurance. On my longer cross countries where I'll be burning down to about 10 gallons left on board, I'll usually run one tank nearly dry to keep at least 10 gallons in a single tank for the approach, landing, and potential go-around.
 
AvNavCom said:
There's perhaps something to be said for the practice of switching fuel tanks after take-off while still within gliding distance of the airfield
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For years we always planned to switch tanks while we were over an airport. Of course, our planning tended to fail and we'd run the tank dry before we got to the field. But she always relights.
 
No reason to switch unless there is not enough fuel left to be certain of flying the approach and missed, if necessary, on the current tank. Switching tanks always introduces some small element of risk (e.g. failed fuel selector) and changes what is currently a properly functioning fuel source. If changing tanks seems advisable, I do it 10 minutes or so from the IAF if IFR, or 10 miles out from pattern entry if VFR, so there is time to switch back if there are issues and before things get busy.
 
cowman said:
I agree with everyone else, I don't switch fuel tanks in the pattern. I also don't switch tanks between leaving the ramp and initial climbout. I've seen some checklists phrase it "switch to a tank with fuel" which is slightly better. Really what this is all about is making sure you're not going to run out of gas with your currently selected tank.
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My POH checklist states to start on the lowest tank. Then to switch to the fullest tank for the runup (after taxiing) and take off.

This ensures that you can feed fuel from both tanks before you take off. If you don't switch tanks at some point before take off, you don't know if the other tank will feed or that it actually contains fuel.
 
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