Why do you need to turn on fuel pump when switching tanks ?

[joegoersch]

The POH for many low wing aircraft (Mooneys included) state one should turn on fuel pump when switching tanks.  I can't think of any good reason for doing this.  Anyone have any thoughts ?  Nothing bad has happened when I have forgotten to do so..,

Thanks.

[MyNameIsNobody]

I never have in my vintage Mooney.

[joegoersch]

Thanks.

Thinking about this a little more, I could imagine that if you ran a tank dry and were switching tanks, this would make sense--i.e. to purge the air bubbles.  But that's not what I'm talking about.  I'm never run a tank dry...

[bonal]

I never run um dry and I don't switch on the pump but I do fixate on the pressure gauge and monitor pressure for a while.. I will never understand the wisdom of running a tank dry if you have a fuel problem on the new tank what then.

[carl]

agreed, just to purge air , when you are on a empty tank, I never do that and don't use the pump. 

 

I don't know why i use it on take off , well just in case the engine pump fails, that why i use it. 

[philipneeper]

I have an opinion on why to use a boost pump for this application. So, when you use a fuel boost pump on start up or on take off is no different than using the boost pump when you transfer tanks. The reason is partly purging air bubbles but also to have a constant state of pressure delivered to the engine driven pump. So, when you are flying along your engine driven pump is keeping a steady lets say 25 psi for math purposes. Well when you transfer takes you have a hick up in fuel flow and pressure fluctuation. Kinda like putting your hand on a running vacuum and removing. Now I'm sure that millisecond it takes to transfer tanks my not effect anything but its the redundancy, what if that moment when you switch tanks the diaphragm in your engine driven pump fails? your boost pump is designed to sustain engine performance. 

[PTK]

Because the Mooney fuel system is not gravity fed. 

You want to do everything you can to ensure continuity of fuel flow. You don't want to have fuel stop flowing. 

And because the POH says so.

 

[Mooneymite]

This is what my POH says.  It doesn't mention the fuel pump during tank switching, but I think it's generally a good idea.  I definitely disagree with the third sentence and do not operate this way.

 

 

FUEL MANAGEMENT
Proper fuel management during flight will help maintain
lateral trim and will also serve as a fuel quantity check.
After takeoff with both tanks full, use fuel from one tank
for one hour; then, switch to the other tank and note the
time. Use all the fuel from the second tank. The remaining
fuel endurance in the first tank can be calculated from
the time it took to deplete the second tank, less one hour.
You must remember, however, that this endurance calculating
procedure can be relied upon only if power and mixture
remain the same and an allowance is made for the extra
fuel used during climb. For estimation purposes, consider
fuel consumption during a full-power climb to be 40
percent higher than that of best-power cruise, and 50 percent
higher than that of best- economy cruise.
CAUTION: Do not allow the engine to lose power or
quit before switching fuel tanks. If a tank runs dry
and the engine quits, retard the throttle before restarting.
Restarting with an advanced throttle may
cause engine overspeeding that can lead to mechanical
malfunction.

[jetdriven]

 Now I'm sure that millisecond it takes to transfer tanks my not effect anything but its the redundancy, what if that moment when you switch tanks the diaphragm in your engine driven pump fails? your boost pump is designed to sustain engine performance. 

that's such minute possibility as to be disregarded.  The pump also has dual diaphragms and will continue to operate with one torn.  The fuel pump drain line will give clue to that. These diaphragm fuel pumps usually run to TBO and beyond. Perhaps it's the most reliable accessory on the rear of the engine.

I usually follow the POH unless I have a compelling reason to do otherwise.  For a short or confined area takeoff, of for one where are no suitable options for a forced landing, I'll use it for takeoff. Other than that, I won't.  But mooney says in case of power failure switch tanks and turn the boost pump on. Mine is the farthest right switch and is right near the throttle.  Be ready to instantly turn it on in case of engine power loss 

 

The biggest reason not to use the Dukes pump is economic. It costs 1400$ to overhaul it and it is not rated for continuous duty. When we first bought the plane my wife was out practicing landings. Pump on for takeoff and landing like the book says. After an hour  of this the pump seized.  Dukes wanted to investigate this, so we sent them the pump. I had to buy another new pump with core charge n the interim.  So it was 1000$ for the first pump, 1000$ for the second one, plus 800$ for a core charge. And I had to wait 4 weeks to get the original one I sent in overhauled, because no cores were available. So 2800$ and 4 weeks later I was flying. I simply won't use it unless I need to because of this. 

Edited September 5, 2015 by jetdriven

[DXB]

This is what my POH says.  It doesn't mention the fuel pump during tank switching, but I think it's generally a good idea.  I definitely disagree with the third sentence and do not operate this way.

 

 

FUEL MANAGEMENT
Proper fuel management during flight will help maintain
lateral trim and will also serve as a fuel quantity check.
After takeoff with both tanks full, use fuel from one tank
for one hour; then, switch to the other tank and note the
time. Use all the fuel from the second tank. The remaining
fuel endurance in the first tank can be calculated from
the time it took to deplete the second tank, less one hour.
You must remember, however, that this endurance calculating
procedure can be relied upon only if power and mixture
remain the same and an allowance is made for the extra
fuel used during climb. For estimation purposes, consider
fuel consumption during a full-power climb to be 40
percent higher than that of best-power cruise, and 50 percent
higher than that of best- economy cruise.
CAUTION: Do not allow the engine to lose power or
quit before switching fuel tanks. If a tank runs dry
and the engine quits, retard the throttle before restarting.
Restarting with an advanced throttle may
cause engine overspeeding that can lead to mechanical
malfunction.

The POH for my '68 C also says nothing on turning on the boost pump to change tanks, and I don't really grasp the rationale for it.   It would seem that fuel flow is momentarily disrupted regardless as you pass through the "off" position while fumbling with the awkwardly located selector.  Having said that, I turn on the pump every time anyway and stare nervously at the small fluctuation in fuel pressure as I turn it off.  Force of habit from training in a Piper.   I also don't think I have the guts to risk running a tank dry -  as noted above, it seems a little nutty that the POH wants you to do this deliberately -  and then tells you not to let the engine lose power?!   I guess this might be reasonable if you get yourself into a tight fuel situation and want to make sure you've used one tank fully while still at altitude before committing to the other one?

Edited September 5, 2015 by DXB

[Conrad]

Read the context, the rest of the paragraph tells you why they recommend running dry. But there's far better ways to judge your endurance now.

[DXB]

Read the context, the rest of the paragraph tells you why they recommend running dry. But there's far better ways to judge your endurance now.

I think I get the rationale for calculating and extracting maximum endurance, if one wants to use the full range of the aircraft. It just unsettled me to see it described as a standard procedure when I first read my POH.  I  haven't had need to go beyond 2.6 hrs yet, so I'm no hurry to try it, even though my plane didn't come with fuel flow.  

[Hank]

I also don't use my fuel pump when changing tanks, not even the one time I got distracted and ran the left tank dry. It hiccuped, I checked power setting, okay, hiccuped and coughed, I glanced down, saw I had missed the last change by 20 minutes and flipped the lever. She immediately ran smooth. Never thought about the pump.

[RobertGary1]

I find that when I don't use the pump the fuel pressure bounces slightly for a few minutes after switching. Running he pump prevents that.

 

-Robert 

[gsxrpilot]

I was taught and have understood the reason for running the electric pump on take-off is that if your mechanical pump goes out at V1 or anywhere between the ground and 500 to 1000 ft. you might not have time to diagnose the issue and switch on the electric pump.  But once at pattern altitude and any time above, I would never use the electric pump unless the mechanical pump has failed. I don't use the pump on landing either unless shooting an instrument approach.

[Yetti]

The thing I would think is the boost pump is below the fuel tanks so it is getting gravity fed and will get all the fuel.  The mechanical fuel pump is in a hot environment above the fuel tanks.   So the boost pump has the best fuel source.

[1964-M20E]

I use the boost pump on takeoff and sometimes when switching tanks.  I use electronic fuel flow to determine when I switch tanks..  10 gallons off first tank and up to 26 on second tank Irvin trying to extend range.  Only once I ran one tank dry on purpose

[RobertGary1]

The mechanical pump does get hot and maybe risks vaporizing fuel. As a result there is a cooling scat tube running down to it. Some people don't know what it's for and see a tube going "nowhere" and it ends up not pointing at the pump. 

Definatley take off and land with the boost. It would be a bad time for the diaphragm in the mechanical pump to die. 

-Robert

[kortopates]

It's really the type of engine installation that calls for use of the boost pump. It's not all money's at all, but all Lycoming Mooneys that use the Boost pump on takeoff, landing and changing tanks. Many of you say older 60's POH's don't mention using the boosts pump for changing tanks but it sure does in later years when the the POH's became much more thorough. But non of the Continental installations call for use of the boost pump except in an emergency and this has everything to do with how different the two fuel injection systems work. With the Lyc installation the boost will simply eliminate the fuel pressure oscillations and will not allow excess fuel to be pushed through the fuel controller but with the Continental you need two different circuits - high and low boost so you don't overly enrich the cyls with two much fuel.

I can't recall specifically if the advise to run a tank dry was ever put in the fuel injected POH's but keep mind running a tank dry on a carbureted mooney is a non-event and much different than doing it in a fuel injected Mooney. Delaying the tank switch in the FI mooney may surprise you as to how long it takes to restart. Try it in a Turbo charged installation and you may find yourself taking 30 seconds and have no luck at all till you descend to 12K! It's really important to understand the systems used in your Mooney from both a trouble shooting standpoint as well as giving you informed knowledge for how to best operate your mooney and avoid surprises.

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[xcrmckenna]

Can anyone explain why Mooneys don't have a "both" selection on the tank switch? I asked my instructor during my checkout in a 201 and he didn't know. I think it would solve a lot of debate when to switch tanks or to run a tank dry or not. Maybe give us more time to agree on LOP or ROP:)

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[Deb]

Can anyone explain why Mooneys don't have a "both" selection on the tank switch? I asked my instructor during my checkout in a 201 and he didn't know. I think it would solve a lot of debate when to switch tanks or to run a tank dry or not. Maybe give us more time to agree on LOP or ROP:)

From: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_ch14.pdf

Pump Feed Systems

Low- and mid-wing single reciprocating engine aircraft cannot utilize gravity-feed fuel systems because the fuel tanks are not located above the engine. Instead, one or more pumps are used to move the fuel from the tanks to the engine. A common fuel system of this type is shown in Figure 14-14. Each tank has a line from the screened outlet to a selector valve. However, fuel cannot be drawn from both tanks simultaneously; if the fuel is depleted in one tank, the pump would draw air from that tank instead of fuel from the full tank. 

[Guest]

The Beech Skipper also a low wing does not have a "Both" position it has an "On" and an "Off". Both tanks are connected together.

Clarence 

[joegoersch]

Wow !  Thanks for all the answers.  I didn't mean to get into the "run a tank dry" debate.  I don't do that or see any routine need to do it.  I was just saying that MIGHT be one case where you might need the boost pump.  I haven't really read any compelling reason to use the boost pump for the one second it takes to switch from left to right or vice versa.  

I appreciate Deb's explanation of why no BOTH setting.   (One nice tthing about high-wing planes !)

Thanks for all the thoughts.

[jlunseth]

In the 231 that I fly, the fuel pump is not switched on except for emergencies, so I don't use it when I switch tanks.  That said, I have a 930 that displays FP, and I see a small, brief drop in FP when I switch, say from 23 psi to 19.

[RobertGary1]

Swift is another low wing that feeds both tanks with no selector. It also only has one filler. You have to wait for the fuel to equalize as you fill it. You fill it, watch it drain down and repeat.