3487 Electric Fuel Pumps Running All The Time?

11-18-2008, 08:10 AM

Jim Small | 3487 Electric Fuel Pumps Running All The Time?


Is there any problem with running an electric fuel boost pump at all times during normal flight? It seems reasonable to me, but some of you have expressed other ideas. Is the issue that the pump may accumulate too many hours and then be prone to failure. Or is there something more sinister to watch out for? - Jim

11-19-2008, 08:11 AM

keithw | 3488

Hi Jim If your fuel system is set up right you should only use the pump for take off and landings. I have forgot to shut it off before for 15 20 min. after being airborne. Adverse effects on the pump? I dont know. I would not want to rely on a constant running fuel pump to keep my engine running. The engine driven pump should do the job. Why do you want to run it all the time????? Keith N80CC

11-19-2008, 07:08 PM

Robehouse | 3489

Hi Jim, I fly in the Desert Southwest. In addition to the engine driven fuel pump my Seawind (N521GR) has 2 electric pumps that feed the engine. The primary pump (low boost) used for take-off climb & landing is a large capacity ‘Mallory’ that pumps constantly when on. When switched 'on', the other pump (high boost) used for high altitude comes on only when the fuel pressure drops below approx. 15psi. This pump is switched ‘on’ anytime the primary pump is turned ‘off’. I incorporated this procedure after experiencing engine sputter one HOT day (3 years ago) while climbing through 7500’ and since then, (some 400hrs) have not experienced engine sputter or any other fuel problems. I didn’t build my plane however, I believe the fuel system was installed ‘by-the-book’ Jerry

12-02-2008, 07:39 PM

Leonpesche | 3495

Running at least one fuel pump all the time, is a warranty, keeping air out of the pre engine fuel pump lines. The pumps are maid for continuous running. Nearly all mishaps of the SW's were fuel starvation, this is normal due to the engine being about 3 feet higher as the fuel tanks. In all airline ACs operating without a fuel pump is a NO GO item. I think many of our SW accidents could have been avoided by this way of operation! Leon.

12-06-2008, 02:39 AM

Ed Lynch | 3496

Dear Jim, There is one significant downside to running two fuel pumps ALL the time, and that is you don't know when one pump has failed- only when BOTH pumps fail, and you then have no backup to switch on. So I feel the best procedure is to use the boost pump for takeoff and landing, and switch it off in between. The high altitude cruise pump comes on climbing above 8000 ft, otherwise off. Ed

12-07-2008, 02:41 AM

Leonpesche | 3497

Hello Jim, I have no problem to find out, if one or the other Fuel pump fails! 1rst; I hear them, they are very loud.(under the floor of the cabin. 2. I can see it on my Amp meter. 3. I have a fuel pressure indicator, where I can directly see the result of the pumps by the corresponding pressure, on the fuel pressure gauge! On top, in total I have three pumps: From each side a facet pump, and a main WELDON HP EL. FUEL PUMP. One Facet pump brings 3psi at the Eng. level. Two pumps bring 6 psi and the HP Weldon 25psi. All three together, give over 30psi. With the smallest leakage in your admission fuel line will fuelstarve your engine. My advise. But you decide what you like? Good luck,Leon

12-11-2008, 07:39 PM

Mike | Interesting reply from another forum (with permission)

12-09-2008, 06:21 PM

airguy Join Date: Oct 2006

Location: Midland, Tx

I have to plead ignorance of the specific definition of "Reid Vapor Pressure" - I don't know how they define that term, whether it is a true absolute vapor pressure at ambient temperature, or a pressure differential from ambient pressure at ambient temperature, etc etc.

Having said that, vapor pressure is driven by two factors - volatility and temperature. Gasoline is a mix of hydrocarbon components with different volatilities (boiling points). As the temp of the fuel increases, the components of the fuel with the lowest boiling points naturally want to vaporize and come out of the liquid state. At the molecular level, molecules of this component are constantly flying out of the liquid state and into the gas state, and vice versa. At some point you get an equal number of molecules joining back into the liquid as you have leaving it, and you've reached equilibrium at that temperature. The pressure of the vapor of that particular molecule is what we call "vapor pressure", and it is specific to that component, and at that temperature.

For fuels with multiple chemical components, each contributes slightly to the total vapor pressure. When this vapor pressure reaches (or exceeds) the pressure that the fuel is under (whether that's ambient or pressurized, doesn't matter) the fuel will be at it's boiling point and you will have a vapor bubble form in the fuel line or fuel container. Pumps don't "do" vapor very well, so this results in what we call vapor lock - the pump loses it's prime.

In aircraft (and cars) the most common source of vapor lock is heating the fuel. This can happen from heat soaking a carburetor on top of the engine block at shutdown (resulting in the famous "hot start" issue), or any other situation where the fuel picks up enough heat from ANY source to raise it's vapor pressure to the point where it is equal or greater than the fuel pressure. At this point you have fuel vapor, not fuel liquid, and you have a problem. Mechanical fuel pumps on the engine block operate at an elevated temperature due to heat soak from the engine, and they transfer this heat to the fuel flowing through the pump - you are essentially "cooling" the pump with fuel. When the fuel is flowing fast enough (full throttle or a good portion of it), the mass flow is high enough that the temperature increase in the fuel is not of much concern. The problem comes when you close the throttle (very low fuel flow) for a few minutes (think descent into pattern altitude from cruise), and then the same fuel quantity is exposed to the hot pump for a much longer period of time, raising its temperature to a point where it can easily result in vapor lock.

To totally avoid vapor lock, all you have to do (techically speaking) is keep the fuel pressure higher than it's maximum possible vapor pressure. You can accomplish that either by keeping the fuel pressure high, or the temperature low, or both. You could certainly pressurize the tanks, but you'll have more issues with leaks and stress on rivet lines that it is worth. Any type of pump arrangement that will increase the pressure of the fuel will raise the temperature the fuel is able to achieve without boiling - and any type of cooling or insulation in the FWF area will reduce the amount of pressure required in the fuel system to keep the fuel from boiling. Electric pumps are convenient because you can place them aft of the heat source (pulling cool fuel from the tank) and pressurize it all the way forward - at that point, you don't worry about fuel temps except under extreme circumstances with a carbed engine. An injected engine will operate at high enough fuel pressures that you won't see vapor bubbles in the pressurized fuel line, even under extended heat soak conditions. The primary function of a fuel "boost pump" on an aircraft is to temporarily increase the fuel pressure leading to the engine-driven fuel pump, this collapses the vapor bubble and allows the engine-driven pump to catch it's prime again, and continue pumping. If that hot fuel is driven out of the pump area before the boost pump is shut off, then you're good to go. If you shut off the boost pump while there is still hot fuel in the area of the engine-driven pump, you'll go right back into vapor lock as soon as the fuel pressure drops, as the fuel will immediately boil again at that point.


Greg Niehues

Midland, TX

VAF # 722, builder 91691, N16GN reserved, http://websites.expercraft.com/airguy/

12-13-2008, 02:01 PM

tseawind | Great explanation of Fuel Vapor Pressure

I had to read this several times to get a good understanding, but the fuel vapor pressure is accurate and on point here. Never quite understood it til now.

I am new to this forum, I hope to not tick anybody off, I am sure I will but, here goes, I deal mostly with "certificated" seaplanes (Old ones), as if there are many new ones!!!!!, for someone to say "nearly all mishaps of the Seawind are fuel starvation, this is NORMAL due to the engine being about 3 feet higher than the fuel tanks." This is NORMAL!!!!!!! Did I miss something here? I have flown many seaplanes over 25 years, with fuel as high as that and to run across a situation of fuel starvation has only happened once to me, and that was only because of a bad engine driven fuel and combined with a pressure carb that was at best partly functional.

A fuel boost pump is just that a boost pump, to get the fuel to move from the tank to the engine for start-up.......and yes, it is THE backup pump to an engine driven pump, this is what got me around the pattern after my engine quit. In my situation, I had never turned on the boost for takeoff, never needed it, but did have my finger near the switch at all times to flip it ON if needed, I NEEDED!!!!!! Since then, it goes on all the time for take-off until my first power reduction, then OFF, again, now with my finger on the switch and looking at the fuel pressure to see if it stabilizes!!!!!!!

For all big bore Lycoming engines, I believe that they are certified on an airframe with a backup pump. Most certified pumps are not continuous duty, I don't know anything about the automotive styles, or Mallory's, etc. I would never run one continuous anyways, remember it is you backup, and your engine driven pump should be doing its job, if not, you need to look there for a problem.

Jerry, the fuel pressures from a Bendix Fuel Injections system, on most IO-540's is around 18 -19 PSI on the low end and about 30 PSI on the high, if you are getting 15 PSI, you need to adjust your pressures on the engine driven pump, darn right it will quit on you. I see that you bought your Seawind from another builder, but if your procedures and system is by the book...... that's one hell of a book.

You said, "In addition to the engine driven fuel pump my Seawind (N521GR) has 2 electric pumps that feed the engine. The primary pump (low boost) used for take-off climb & landing is a large capacity ‘Mallory’ that pumps constantly when on. When switched 'on', the other pump (high boost) used for high altitude comes on only when the fuel pressure drops below approx. 15psi. This pump is switched ‘on’ anytime the primary pump is turned ‘off’." I didn’t build my plane however, I believe the fuel system was installed ‘by-the-book’ Jerry

I read your post about a dozen time to try and understand.......engine driven pump......, 2 elec pumps, ...... primary (low), t/o and ldgs is a large capacity (???) that I would hope would pump constantly where ON.......then...... the the other secondary (high boost) for high alt., and pressure below 15 PSI....and this switch is ON any time the primary(low boost) is OFF.......so you are running the HIGH boost all the time???? Again, trying to understand what you procedure is here. I have never seen an IO-540 system that works this way, again, I am throwing in the certified thing.

George's post is totally, totally correct, that's how a proper system should perform, it is not complicated, just to many people trying to be engineers. Because these are experimental, people believe that they have to reinvent the wheel, the wheel works fine, go to any certified aircraft with a big Lycoming and copy their fuel system, it works, it has for a heck of a long time and you know what............they don't have the fuel starvation problems that is deemed here as NORMAL. Again, my .02 worth.

I really hope this post helps more than it hurts, I'd rather hurt somebody's feelings than have it hurt your next of kin, to much of that has happened already. I am seat belted in my LazyBoy and await the fallout. Hank.

12-31-2008, 03:28 PM


Pumps, pumps & more pumps......

When flying the Cherokee 6 the poh wanted the aux pump to be used when taking off and landing. Ok so sometimes when I was ready to land an hour later...it was still on. Never had a problem in the 6 (same engine).

When re-building the fuel system I installed the Airflow Performance dual pump package.


When taking off and landing, one of the pumps is on. When I get to 3k ft If I wish I could turn it off and use the engine driven pump. I still see 24-30 psi before the spider. I have left it on for hours without any problems. I believe airflow mentions that the pump is rated for continuous duty.

These are some older pictures, but you get the idea....

PS - I use the mallory for cross filling......(not shown)

03-03-2011, 07:26 PM

gderamel | Fuel Pressure and Mallory Fuel Boost Pump Questions

Hi Everyone,

I have some questions regarding the Mallory Fuel Boost pump which doesn't seem to be answered in previous posts. I'm especially interested in Keith's 11/19/2008 post where he mentions that his engine driven fuel pump is able - independent of the Mallory - to handle fuel pressure in cruise flight conditions.

It seems - this winter - that I've had to increasingly use my Mallory fuel boost pump, even at low altitudes (~1,500') or else have my indicated fuel pressure dip (occasionally remain) in the 'red zone' (<14psi?).

Vice versa, this last summer, I only needed to turn on the Mallory above +5,000' (ish).

Is the increased necessity for the boost pump attributable to winter (cold) temps? Doesn't seem to make sense.

I should note, I keep the boost pump on for the remainder of these flights with no [apparent] problem.

Again, I read with interest from Keith's 11/19/2008 post where he states his engine driven fuel pump does the job without the help of the Mallory. For those with the standard fuel system (not you Mike!) is this typical? I thought I heard that because of the high engine (5' from the header tank) that boot pressure was an inherent issue with the Seawinds.

Or do I need to ask my mechanic to adjust the engine driven fuel pump? (I'm not an A&P, in case you couldn't tell). Should I be concerned that something ominous is about to happen to the engine driven pump -- go sayonara on me?!