changing fuel tanks

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spiderweb

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Ben
I haven't got the PIM for the Socata, but I wanted to ask a question about this aircraft: does one need to turn on the fuel pump before switching tanks? Of all of the low-wings I have flown, this has always been the case, but I don't like to assume anything.
 
I never understood why the need to turn on the pumps. you are at altitude, and have the mixture set proprerly. If you switch tanks, the engine should continue to run smoothly.
 
since the socata is a low wing, putting the fuel pump on insures continuous fuel pressure in the event you get an air bubble during the switch. The engine driven pump will suck the bubble through the electrical pump, and then the electrical pump will push the bubble through the engine driven pump.

The description section (part 7) of my Trinidad POH says (page 7.37) "...a continuation of fuel flow must be assured as the new tank is being selected. When switching from one tank to the other, place the auxiliary fuel pump switch momentarily in the "ON" position until normal fuel flow has been restored."
 
I would always turn on the pump just in case some air got in the lines to help it get through quicker. But I even did that on high wings when I would switch to a left or right tank to balance the load if one was draining faster than the other in the both position
 
I can't speak for Socata's, but I imagine a low wing Mooney is similar. The POH says turn on the boost pump. I did that for awhile until on some forum or another I found out this really wasn't necessary. The fact of the matter is that the switch is expensive and the pump is very expensive. It doesn't make sense to wear them out unnecessarily.

So I tried it. Without the boost pump, my engine has never, ever had even the tiniest hiccup when switching tanks. Never.
 
OK. Thanks for the input. I have to follow the procedures in the POH and at the place where I'll be renting. I'll see what they say next week. BTW, if I am going to be flying this plane a lot, maybe I will just go ahead and get the comm. . . .
 
Lance F said:
I can't speak for Socata's, but I imagine a low wing Mooney is similar. The POH says turn on the boost pump. I did that for awhile until on some forum or another I found out this really wasn't necessary. The fact of the matter is that the switch is expensive and the pump is very expensive. It doesn't make sense to wear them out unnecessarily.

So I tried it. Without the boost pump, my engine has never, ever had even the tiniest hiccup when switching tanks. Never.
Click to expand...

When it happens the stumble will get your attention.

Don't ask me how I know.
 
Ed Guthrie said:
When it happens the stumble will get your attention.

Don't ask me how I know.
Click to expand...

how DO you know???
 
Ed Guthrie said:
When it happens the stumble will get your attention.

Don't ask me how I know.
Click to expand...

I believe the boost pump is required to be below the lowest fuel tank outlet and the engine driven pump is not. Neither pump is very good at pumping air/vapor although diaphram types will generally do an adequate job of that. Having a pump below the fuel supply allows gravity to provide the pressure necessary to get fuel to replace the air/vapor quickly without relying on the pump's ability to push air.

The most likely time to experience an unexpected "air bubble" in the fuel supply is when switching to a tank that has been run dry on a prior flight and currently has sufficient fuel in it. When the tank was run dry, the fuel lines from the tank to the selector were emptied of fuel and filled with air and/or fuel vapor. Subsequent filling of the tank may or may not cause the air/vapor to return to the tank and be replaced with liquid fuel (depends on the fuel line routing and diameter). When you select such a tank, the air will be drawn into the engine pump which will cavitate for a while (perhaps forever) and during that time there will be no significant amount of fuel coming out of the pump. On a carbureted engine the engine will continue to run for a while as the bowl is emptied of fuel but there's a good chance that it will quit running due to a lean mixture before the bowl empties. On a FI engine the stoppage will be immediate once the pump ceases to pressurize the mixture control unit.

Nearly the same thing happens when you actually run a tank dry, but the use of a boost pump is a bit more instinctive then and in any case the engine stoppage has already occurred so the boost is a cure rather than a preventative measure.
 
lancefisher said:
The most likely time to experience an unexpected "air bubble" in the fuel supply is when switching to a tank that has been run dry on a prior flight and currently has sufficient fuel in it. When the tank was run dry, the fuel lines from the tank to the selector were emptied of fuel and filled with air and/or fuel vapor. Subsequent filling of the tank may or may not cause the air/vapor to return to the tank and be replaced with liquid fuel (depends on the fuel line routing and diameter).
Click to expand...

And an easy way to fix that is to switch the tanks on the ground. allowing the engine to run for a few minutes on each tank.
 
Ben:

I use the boost pump when ever vapor lock could be an issue during the climb.

When switching tanks, if the engine dies and doesn't immediately restart you have no fuel pump; so, the boost pump (which is electric) is a good preventitive measure or be prepared to put it on quickly if the engine does stumble. Passengers may not appreciate the quiet glide while you fidget with where the boost pump is.

For many years I ran a tank dry in my A-36 on long trips. As has been pointed out, when I went to that tank again, there was often some air in the line. Either use it long enough on the ground to be confident any air is out, or don't got to it until you're well above the ground. There have been several incidents in Bonanzas and Barons where someone switched tanks just before takeoff and had an engine stumble badly or quit on departure.

Be safe.

Dave
 
I have intentionally run a tank dry to a. get an accurate fill volume or b. maximize endurance. That does cause a stumble and gets your attention! However, after filling I never had any stumble when switching to that tank. Different makes and models could certainly be different in this regard. OTOH I never switch tanks just before take off.

and to Pilawt's comment, yes I believe the mechanical pump would keep pumping away courtesy of the windmilling prop.
 
Michael said:
And an easy way to fix that is to switch the tanks on the ground. allowing the engine to run for a few minutes on each tank.
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Amen. My start up procedure uses one tank. My runup has me switch to the other tank. That way, I know there are no surprises waiting for me in the air.

I also do not use the pump when switching tanks. Haven't had a problem yet.
 
Dave Siciliano said:
There have been several incidents in Bonanzas and Barons where someone switched tanks just before takeoff and had an engine stumble badly or quit on departure.

Be safe.

Dave
Click to expand...

This worries me. There was an accident in the Trinidad we had here previously, and although the official report says the engine quit for unknown reasons, the unofficial word was that it was some sort of fuel mismanagement of the type you describe.

So, how does one avoid that? Switch tanks a few times on the ground?
 
wangmyers said:
This worries me. There was an accident in the Trinidad we had here previously, and although the official report says the engine quit for unknown reasons, the unofficial word was that it was some sort of fuel mismanagement of the type you describe.

So, how does one avoid that? Switch tanks a few times on the ground?
Click to expand...
I was taught to switch before the runup. That way, if there's any problem with that part of the fuel system it should manifest during runup. Otherwise if you switch just befpre t/o there could be a blockage or some other problem which might not show up at idle.
 
Pilawt said:
Wouldn't the engine-driven pump continue to operate so long as the prop is windmilling?

-- Pilawt
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I would think so, but wouldn't know what RPM the engine would be turning, or what flow the fuel pump would turn out. If you try it high up sometime, you will see the boost pump can make a noticable difference.

Dave
 
Book for my Commander doesn't require electric pump for tank change in-flight.... but... I watch the fuel pressure guage with an eagle-eye. About half the time during tank change it goes low enough to warrant a bump of the electric pump. After about 15-20 seconds, all is fine and I switch it off.

Unlike most low-wing planes, the Commander has a "both" position, which is called out for takeoff and climb. In cruise, I switch tanks every 30 minutes to ensure equal burn.
 
There is something like a tenth of a gallon (12 oz) in the typical piston single's fuel line path from tank to engine plus in many cases another few ounces in the gascolator. A 150 HP engine uses about 1.5 gallons per hour at idle or something like 3 oz per minute. To me that means you'd need to operate at idle on each tank for at least 5 minutes and maybe as long as 10 minutes to provide some margin. Even then, the low flow rates at idle might not dislodge a bubble or slug of water if there were any incentive for the contaminant to stay in place. For this reason alone I've never been fond of the idea of switching tanks prior to takeoff as a preventative measure. During runnup the fuel flow typically increases by a factor of 3 so even then you need to run at that power for at least 2-3 minutes on each tank selection (can you say overheat?) to be certain you have purged something in the lines.
 
Boy, I don't know if I want to get into this. But twice, once riding along in a Piper Tomahawk and once in a Warrior I saw the pilot switch tanks on the ground and the engine cough just before departure. In the Tomahawk the engine actually dies as we were starting to roll to take the rwy. Combination of taxi time and quick runup allowed just enough time for the line fuel to clear.
 
lancefisher said:
During runnup the fuel flow typically increases by a factor of 3 so even then you need to run at that power for at least 2-3 minutes on each tank selection (can you say overheat?) to be certain you have purged something in the lines.
Click to expand...

My only direct experience in this situation was in a PA-32 Saratoga. A screw had backed out of the shroud surrounding the tank selection switch, permitting the shroud to move, so that the thumb lock to prevent selecting "off" inadvertantly would not engage.

My runup checklist includes switch tanks just before the runup. I switched tanks (inadvertantly selecting "off") then advanced throttle looking for 2000 rpm but the engine died before it got there. Selected tank 2, restarted, tried it again - same story, the plane died within 15 seconds of selecting off.

Surely glad I did this on the ground! There is some wisdom in those checklists!

I taxi-ed back to the tiedown, suspecting water in the fuel lines to the left tank. At the tie down I repeated the runup and it again stalled. I then hit the belly sump in the PA-32, and climbed out expecting to see water on the ramp. It was dry!!! That got me nose first into the cockpit footwell to inspect the switch, and I found the problem.

Long story to say that I think your calculated estimate of 2-3 minutes is an overestimate. Try it. Shut off the fuel to one of your engines and see how long it will run during your normal run-up procedure.

-Skip
 
My Tiger's POH says to turn on the electric aux. fuel pump when switching tanks so that's what I do.
 
Well, this is also where one may operate much differently if they own an aircraft than if someone else is using it and the current operator doesn't know how it was run before.

I never fiddle with the tanks on my plane before departure. If I try the off feature, or swithch tanks it's well before departure. Where I'm based, there is at least a ten minute run time from start to departure (on a good day). Generally, what ever tank I land on is the one I depart on, but I always top the plane at my home field or leave enough fuel to depart on.

Had a partner in a Baron that this the Mains, auxillaries and cross-feed checks before each first flight of the day. We had an engine sputter a couple times and talked through this. We agreed we shouldn't do it before each first flight in the future, that once a month or so would do. All we were really checking was cross feed as we routinely switched from main to auxes in flight.

Best,

Dave
 
This is all sort of confusing. Would it be safe to start up and stay on one tank and stay on that tank until a half hour after departure? That's what I've always done. In addition, I don't change tanks until and unless I am within gliding distance of an airport.
 
wangmyers said:
This is all sort of confusing. Would it be safe to start up and stay on one tank and stay on that tank until a half hour after departure? That's what I've always done. In addition, I don't change tanks until and unless I am within gliding distance of an airport.
Click to expand...

Ben,

Here's my procedure, well actally its Ron's but he taught it to me. The Tiger has 51 gallons useable and it burns 10 GPH. I always start out on the left tank and taxi, takeoff, cruise for 1/2 hour. At this point (1/2 hour into flight) I switch to the right tank for one hour, then back to the left for one hour. I keep doing the every hour tank switch until I land or have to refuel. I have 2.5 hours per tank and switch until I land with a minumum of one hour's fuel remaining. My typical legs are three hours so we usually have two hour of fuel remaining. Switching like this helps keep track of fuel in each wing and keeps the plane balanced.
 
Anthony said:
Ben,

Here's my procedure, well actally its Ron's but he taught it to me. The Tiger has 51 gallons useable and it burns 10 GPH. I always start out on the left tank and taxi, takeoff, cruise for 1/2 hour. At this point (1/2 hour into flight) I switch to the right tank for one hour, then back to the left for one hour. I keep doing the every hour tank switch until I land or have to refuel. I have 2.5 hours per tank and switch until I land with a minumum of one hour's fuel remaining. My typical legs are three hours so we usually have two hour of fuel remaining. Switching like this helps keep track of fuel in each wing and keeps the plane balanced.
Click to expand...
Thanks--that is what I do in low wings.
 
wangmyers said:
This is all sort of confusing. Would it be safe to start up and stay on one tank and stay on that tank until a half hour after departure? That's what I've always done. In addition, I don't change tanks until and unless I am within gliding distance of an airport.
Click to expand...

The first part is my normal procedure and it sounds like that's what Lance and Dave also do. The second part I don't do...that's pretty restrictive for longer flights, and I've never had even a single hiccup switching tanks without the boost pump (except when intentionally running one to empty).
 
In my Tiger, I start on one tank and then switch to the other before run-up. I figure I get enough ground idle on the first to ensure it's feeding and the run-up covers the other one.
 
Ben:

Very few times have I gotten a hiccup switching tanks, but it has occurred and normally after I burned a tank dry then, later switched over to that tank. Probably would have restarted without the boost pump, but I didn't wait to find out.

As Ron said, if you switch on the ground do it well before takeoff. In the air, wait until you're high enough to deal with the engine stumbling. If you have passengers aboard, a heads up won't hurt--act like it's normal if it stumbles and tell 'em it only happens the first time the tank is switched!!

As usual, you're thinking through things well, and that's very important. If you rent a plane, you just don't know what happened on the previous flight; so, do things accordingly.

Best,

Dave
 
Anthony said:
Here's my procedure, well actally its Ron's but he taught it to me. The Tiger has 51 gallons useable and it burns 10 GPH. I always start out on the left tank and taxi, takeoff, cruise for 1/2 hour. At this point (1/2 hour into flight) I switch to the right tank for one hour, then back to the left for one hour. I keep doing the every hour tank switch until I land or have to refuel. I have 2.5 hours per tank and switch until I land with a minumum of one hour's fuel remaining. My typical legs are three hours so we usually have two hour of fuel remaining. Switching like this helps keep track of fuel in each wing and keeps the plane balanced.
Click to expand...

That's pretty much the drill I follow for my Saratoga, too. If I have to switch tanks on the ground I'll do so either right before or after engine start. I will not switch them during or after the runup as I want to take off on a known good tank. Drives some CFI's nuts. "How do you know if the other one is good?" I don't <shrug>. Make that first switch near an airport or something soft and be ready to go back to the first tank.

Regards,
Joe
 
Anthony said:
Ben,

Here's my procedure, well actally its Ron's but he taught it to me. The Tiger has 51 gallons useable and it burns 10 GPH. I always start out on the left tank and taxi, takeoff, cruise for 1/2 hour. At this point (1/2 hour into flight) I switch to the right tank for one hour, then back to the left for one hour. I keep doing the every hour tank switch until I land or have to refuel. I have 2.5 hours per tank and switch until I land with a minumum of one hour's fuel remaining. My typical legs are three hours so we usually have two hour of fuel remaining. Switching like this helps keep track of fuel in each wing and keeps the plane balanced.
Click to expand...

I do a little different. I still have an analog closk in my instrument panel. If you mentally draw a line from 12 straight down to 6 I take off on the take where the minute has is at i.e. from 12 to 6 right tank, from 6 to 12 left tank. Then I swtich every half hour. The minute hand is alway pointing to what tank I should be on.
 
I taxi and do my runup on the tank that I will depart with.
That way, any water or sediment that may turn up, will show up during the taxi and/or runup. It seems to me that changing tanks on the ground any later than that on departure is riskier.

In the air, I don't use the Aux Pump (but would do so if my POH so indicated). Earlier advice about changing tanks when you have a landing area within glide is good advice. But remember the age old advice: If you changed something in flight and things go bad, then switch it back.

Lastly, I change tanks at 30 mins, then after each 1 hour.

Phil
N5722Q
 
wangmyers said:
This worries me. There was an accident in the Trinidad we had here previously, and although the official report says the engine quit for unknown reasons, the unofficial word was that it was some sort of fuel mismanagement of the type you describe.

So, how does one avoid that? Switch tanks a few times on the ground?
Click to expand...

Ben, I believe the problem in the crash you refer to is that the pilot didn't get the lever ALL the way over from one tank to another, essentially turning the fuel off or greatly reducing the potential fuel flow. Didn't discover it until shortly after takeoff, as there was enough fuel in the system for the runup and takeoff.

That was the impression I got from speaking with folks after the accident. I had been concerned there was a mechanical problem with the airplane, since I'd taken it to Florida and back about 4 days before the crash. I was relieved to hear the verdict of pilot error (though sad for the owner/pilot).

I was up in 169GT yesterday. My normal procedure is to start the plane on the lowest tank, taxi, switch tanks during taxi (with the fuel pump on), then conduct the runup, takeoff, and climb on the fullest tank.

ALL Socatas require the use of the fuel pump during tank switchover, whether they are carbureted like the TB9s I learned in or injected like the TB20 or 21.
 
Joe B said:
That's pretty much the drill I follow for my Saratoga, too. If I have to switch tanks on the ground I'll do so either right before or after engine start. I will not switch them during or after the runup as I want to take off on a known good tank.
Click to expand...


Yes, me too. If I switch tanks its before I start the plane so I have taxi and runup time to make sure I'm getting fuel. Here's why I'm so paranoid:

Before I had my own plane I rented an older Cherokee 180 with the fuel selector down by my lower left leg. Its not very visible and it was old so some of the markings were warn off. I switched to the left tank, or what I thought was the left tank, taxied and did my runup. During the runup the engine died. I looked at the fuel selector again and it was not fully on the left tank, just a little off. I reset the fuel selector, and started right up again. I did another runup and waited a while to make sure all was good. If I had skipped the runup the first time, the engine would have died on takeoff and I would have ended up in a swamp. There's 1001 ways to kill yourself in these things.
 
Ron Levy said:
In my Tiger, I start on one tank and then switch to the other before run-up. I figure I get enough ground idle on the first to ensure it's feeding and the run-up covers the other one.
Click to expand...

Yeahbut, when your fuel selector has four positions that could leave you on the ground for a long time.

Since I mainly use the main tanks I can follow your procedure.

I never thought of why my checklist has "Select tank" on startup, run-up, and departure. Now I know. I think I might switch tanks efore doing the run-up.

I've never been shy about keeping the fuel pump on, and turning it on when switching tanks, as per the POH and checklist. I'm still on my first nose landing light after 7 years so the fuel pump might hold up too.
 
TMetzinger said:
Ben, I believe the problem in the crash you refer to is that the pilot didn't get the lever ALL the way over from one tank to another, essentially turning the fuel off or greatly reducing the potential fuel flow. Didn't discover it until shortly after takeoff, as there was enough fuel in the system for the runup and takeoff.

That was the impression I got from speaking with folks after the accident. I had been concerned there was a mechanical problem with the airplane, since I'd taken it to Florida and back about 4 days before the crash. I was relieved to hear the verdict of pilot error (though sad for the owner/pilot).

I was up in 169GT yesterday. My normal procedure is to start the plane on the lowest tank, taxi, switch tanks during taxi (with the fuel pump on), then conduct the runup, takeoff, and climb on the fullest tank.

ALL Socatas require the use of the fuel pump during tank switchover, whether they are carbureted like the TB9s I learned in or injected like the TB20 or 21.
Click to expand...
Thanks. All of this makes sense, and it also jibes with the flight school's procedures guide for the bird.
 
Skip Miller said:
Long story to say that I think your calculated estimate of 2-3 minutes is an overestimate. Try it. Shut off the fuel to one of your engines and see how long it will run during your normal run-up procedure.

-Skip
Click to expand...

Turning the fuel selector off is a completely different situation than passing a slug of bad fuel/water/air through the lines. When the lines are full and you close the valve, the flow stops almost immediately but the fuel lines remain full of fuel. If you try this when the engine is running fast enough to take several seconds to stop turning, (or in the air if you're braver than me) you'll find that the fuel flow resumes and the engine begins running almost immediately when you turn the fuel back on.

This is all assuming you have a fuel injected engine. With a carburetor, the whole issue is confused/complicated by the filling/draining of the carb bowl.

If you want a true worst case test, you'd need to install a two way valve at the tank outlet which allows the fuel line to be fed with gasoline or air depending on the valve position. Start the engine with the valve on fuel and then switch to air and time how long the engine runs. It will be quite a while.
 
Taxi, run-up, and take off are all done on one tank for me. I don't use the electric fuel pump during the switching of the tanks.
 
The fuel selector valve on the IAR has LEFT, BOTH, RIGHT, OFF every 45° starting at the 7:30 position going clockwise to the 12:00 on the center console. Having OFF pointing straight ahead and BOTH pointing to the left tank is a bit counter-intuitive, but you get used to it.

After I had the valve refurbished I did a test run on the ramp to see if it completely shut off the fuel flow in the OFF position. The FI engine quit about 4 seconds after going to the OFF position.

The book says use the electric pump when switching tanks. I go 15 minutes on both after t/o then swap every 30 minutes. Otherwise the left tank empties first.
 
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