Why do we switch fuel tanks before landing?

AvNavCom said:
After mulling over some of the posts, I think I'm going to start doing the same. Since I habitually have both tanks topped off at baseline, I'll probably start up on the "opposite clock" fuel-tank and switch to the other tank during run-up. After that, I'll do the switches on the hour and half-hour as normal. I understand the logic of a potential fuel-selector failure, but I rate it as unlikely enough that it's not going to a factor in my practice.
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If you’re going to switch tanks on the ground before takeoff, it might be good to know how long the engine runs on the ground before fuel from the newly-selected tank gets to the engine.
 
MauleSkinner said:
If you’re going to switch tanks on the ground before takeoff, it might be good to know how long the engine runs on the ground before fuel from the newly-selected tank gets to the engine.
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True. I can't imagine it's much more than 30 seconds, at most. Probably testable by switching the fuel-tank selector to OFF while at normal RPM (call it 1000 rpm). Perhaps I'll test that one day. I imagine my fuel-pressure reading on my EIS would also probably reflect any flow issue fairly promptly.

Practically, after using the first tank for taxi, making the switch to the other tank upon stopping for run-up should allow sufficient time to ensure the feed is working, especially since a run-up at 2000RPM incurs a reasonable fuel-flow demand. I certainly see your point that you wouldn't want to discover a fuel-flow problem during take-off!
 
I don't switch tanks based on time in cruise. Many autopilots have a 10 gallon differential limit on fuel imbalance, so I take off on tank 1. Switch when it is 10 gallons low. Then the next switch if after tank 2 is down 20 gallons (still only 10 gallons difference).

I may fudge that a bit to have most of the fuel in one tank for approach and landing. Better to have 15 in one tank than 7.5 in each.
 
Long Island crash about a year ago. Piper was out doin maneuvers, in base, lost its engine. One tank was almost full, the other bone dry. If he would have switched they wouldn’t have made the news.
 
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.

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.
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Some manufactures include switching tanks as part of the runup. What you are basically doing is confirming fuel flow from tank #1 during taxi and tank #2 during runup.
 
I'd never do that....not unless I know how much time it takes to run a line dry. And then we sit there and wait.
 
Checkout_my_Six said:
I'd never do that....not unless I know how much time it takes to run a line dry. And then we sit there and wait.
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If decide to omit checking the fuel pressure gauge as part of the runup checklist, you should just sit there until that time expires I suppose.
 
Switching tanks before landing means you have two available tanks that are known to be working.
 
AvNavCom said:
True. I can't imagine it's much more than 30 seconds, at most. Probably testable by switching the fuel-tank selector to OFF while at normal RPM (call it 1000 rpm). Perhaps I'll test that one day.
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That will only tell you how long the engine will run on what's in the float bowl since the fuel in the lines and gascolator won't reach the engine with the fuel valve off. That will probably be a good bit more than the contents of the float bowl.
 
Rgbeard said:
Does slipping a Lance count?
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dcat127 said:
I wonder how often people see the need to slip a Cherokee Six?
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I'd say it really depends on how long the runway is versus how much of it you want to use. If you are on the glide and aiming for the touchdown marks, you would be throwing away a significant length of runway, 500 or maybe 1000 feet. Using a slip after you have the runway made brings much of it back into use. I do it alot, both in my old Cherokee and in my Mooney now. I will say that some passengers can find a slip unsettling, so it is best to brief them before entering the pattern. I also practice slips and low speed maneuvers and use an LRI in the plane.
 
I don't do that unless the tank is insufficient
 
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