AX-O

AX-O

Well Known Member
All,
saw a post today and it indicated the builder removed his mechanical fuel pump and just used his electric one. Is that safe? Would you need to add an additional electric fuel pump in case the first one fails? Does it need a secondary battery in order to have a better system in case of electrical failure?
 
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I have dual electric pumps in my -3. There is a back up battery and an isolated electrical system in the event I need to seperate the batteries. It is pretty rare that the Facet pump fails, but having two is only smart in my book.
 
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Do you always use the same electrical pump or do you switch between one and the other? How did you plug the hole where the mechanical fuel pump goes? Also, does your -3 have wing tanks or fuse tank? sorry for all the questions.
 
Redundancy...

In my Dakota Hawk, I used to have a single Facet electrical pump feeding my Continental C-85. On one occasion, on departure I lost engine power at about 300' agl due to fuel starvation. I pulled the throttle and commenced to land on the runway which was still in front of me.
5vspw4.jpg


After landing, I realized that my engine was still running (at idle)! Back at the hanger, troubleshooting revealed that my single electric fuel pump had failed prior to take-off, but I didn't realize it because my carb fuel bowl holds enough fuel for about 40 seconds of full power. Gravity drain from the high wing tanks is good for about 1700 RPM sustained power.

I determined that my plane needed at least two fuel pumps (electrical) - or - one fuel pump and fuel pressure indicator. Now, I take off with two pumps on, and fly with one pump on. If the single pump fails in flight, I'll know because my engine will lose power and I can swap to the standby.

Of course, all of this is based on a high wing airplane. I can throttle back to 1700 rpm and cruise for hours with NO fuel pump. The low wing RV's would be hard pressed to do the same!

After building my RV, I've learned a lot more about engine systems due to the excellent information available on VAF and various builder web pages. I've made several mods to my Dakota Hawk based on what I've learned here, and I consider both of my planes to be safer now.
 
I guess I look at it like this....if you're going to remove it then you better put in another electric. Unlike high wing airplanes, these won't run well/at all without some fuel pump running. Running with one single electric fuel pump in a low wing plane to me is like pulling off a mag and now running on a single electronic ignition with no electrical backup. The though of a single electric fuel pump to me on an RV in my opinion is frankly not smart and not safe.

Lastly, I wonder what benefit you really get. The mechanical pumps for the most part last longer than the engine does (of course things do fail) and that's where the electrical "backup" comes in. If you want to do it right, then it'll be like Larry said, extra battery and separate electrical system - which I guarantee will weigh more than the mechanical fuel pump and plunger. So, you're not saving weight and you're adding complexity. BTW, if you remove the fuel pump it's also a decent idea to remove the plunger too.

My 2 cents as usual.

Cheers,
Stein
 
AX-O said:
Do you always use the same electrical pump or do you switch between one and the other? How did you plug the hole where the mechanical fuel pump goes? Also, does your -3 have wing tanks or fuse tank? sorry for all the questions.
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No need to be sorry, better to know.

I switch back & forth about every month or so. They make blank plates for the hole, but mine did not have a hole. The 0-235 came out of a high wing Champ and it had no fuel pump at all, other than gravety. ;)
 
"Safety" is ensuring constant fuel flow - however you choose to do that. You can use seventeen hamster-powered pumps in any series/parallel combination you like, as long as you can guarantee constant, sufficient fuel flow. It's not about electric versus mechanical. Just make sure that the octane keeps meeting the spark.

Electric pumps require backup (whether a second battery, second pump, second alternator, etc) just like mechanical ones do. The fact that almost all mechanical pumps use an electrical one as a backup says something about the reliability of the electrical pumps, in my book. Choose your own method, but in the end, safety (in this conversation) means constant, consistent fuel flow, regardless of how you do it.
 
Back up fuel pump

I put a hand pump for aux. in my Sonerai years ago and it saved my bacon when I experienced vapor lock with MO gas.

RV-4
completed 7-07
350 hours
 
Conceptually you decide "Do I want an electrical bus dependent power plant?"

Yes / No.

If yes build the dc bus supply to be fault tolerant and have back up/isolation options. Guys have been doing this for a long time with dual lightspeed ignitions so it's not really a way out there idea.

Second you must assume the permutations of pump & bus failures. This would probably put you in the position of not ever running a tank dry. I don't do that anyway.

Is it safe? Of course. Just ask Airbus about electrically dependent systems, err never mind scratch that.... :mad:
 
Why?

Surely a backup electric pump (for a main electric pump) and associated separate battery is going to weigh more than a mechanical pump. I personally would not entertain a solution of two electric pumps on a low wing aeroplane. My premise is KISS (keep it simple stupid) and would use the tried and tested method of backing up the mechanical pump with an electric.
 
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Many engineers before us...

....tried all sorts of systems dating back to WW 1. When I see Cessna and Piper, to name two, use a mechanical primary pump and an electric backup/priming pump, then common sense tells me to emulate them.

Regards,
 
AX-O said:
All,
saw a post today and it indicated the builder removed his mechanical fuel pump and just used his electric one. Is that safe? Would you need to add an additional electric fuel pump in case the first one fails? Does it need a secondary battery in order to have a better system in case of electrical failure?
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have you read some of the threads by frankh? he's got a lot of hours on his plane, he uses two electrical pumps iirc.

add- http://www.vansairforce.com/community/showthread.php?t=30402
 
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Two airplanes all electric

It comes down to this as others have said.

The simplest way is a mechanical pump. electric backup and at least one mag....If you go with dual EI's or dual electric fuel pumps then it would be wise to add either a second battery or second alternator...But even that is debatable..How often to batteries fail?..I would'nt rely on a single battery but the failure rate is close to zero so why not?

If you don't consider the weigth significant..eg because your going to fly IFR then a redundant electrical system is not a big deal.

Now why go for no mechanical pump?..Well at a high altitude airport on a hot day running Mogas, vapour lock is a real possibility...Thats is if you boost pump quits on takeoff you may find your mech pump simply doesn't work becasue it could vapour lock.

The hydraulic location of a mechanical pump is poor..I wanted to do it right hydraulically speaking.

I have about 900 hours over two airplanes running an all electric system without issues so far.

Frank
 
Can it be safe? Probably. It REALLY isn't ideal though, if you have the option at all, you should go mechanical/electric. Our mechanical pump from 1969 with 2000 hours on it is still going strong on the Cardinal, then again so is the electric from '68... that gets tested for 3 seconds every flight.

I don't think I'd plan on using a electric for continuous use if I had the option. I don't think they were ever designed for it, but I don't have any data to back that up.
 
If you look at the certified...

osxuser said:
Can it be safe? Probably. It REALLY isn't ideal though, if you have the option at all, you should go mechanical/electric. Our mechanical pump from 1969 with 2000 hours on it is still going strong on the Cardinal, then again so is the electric from '68... that gets tested for 3 seconds every flight.

I don't think I'd plan on using a electric for continuous use if I had the option. I don't think they were ever designed for it, but I don't have any data to back that up.
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....requirements for Part 23, there is an option for both pumps to be on continuously.... Automotive electric fuel pumps run continuously.

Item (d) says that a failed pump cannot fail the supply.
It also looks like a dual electric pump system with no gravity back-up and no mechanical pump does not meet certification requirements.

23.991 - Fuel Pumps

(a) Main pumps. For main pumps, the following apply:

(1) For reciprocating engine installations having fuel pumps to supply fuel to the engine, at least one pump for each engine must be directly driven by the engine and must meet ?23.955. This pump is a main pump.

(2) - [Turbine stuff only not copied...]

(b) Emergency pumps. There must be an emergency pump immediately available to supply fuel to the engine if any main pump (other than a fuel injection pump approved as part of an engine) fails. The power supply for each emergency pump must be independent of the power supply for each corresponding main pump.

(c) Warning means. If both the main pump and emergency pump operate continuously, there must be a means to indicate to the appropriate flight crewmembers a malfunction of either pump.

(d) Operation of any fuel pump may not affect engine operation so as to create a hazard, regardless of the engine power or thrust setting or the functional status of any other fuel pump.
 
Geico266 said:
I have dual electric pumps in my -3. There is a back up battery and an isolated electrical system in the event I need to seperate the batteries. It is pretty rare that the Facet pump fails, but having two is only smart in my book.
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I have had ONE (1) Facet Electric pump fail on takeoff of my airplane. The engine kept running using the mechanial pump. I had a wingman escort me to my home base. Was not the smartest thing as I should have landed at the airport under me. The wingman could have flown me home where I could pick up my new spare Facet Electric pump then flown back and installed it. There was over 2,100 hobbs hours on the Facet pump when it failed. It was used IAW (In Accordance With) Lycoming recommendations during those hours. On for Takeoff, Climb, fuel tank changes, landing, hot ground operations and my own requirement was on any time lower than 1,000 AGL.
 
My Facet Fuel Pump Failed

Just last month I had to replace my Facet electric fuel pump. It was installed with the engine 350 hours ago. I'm glad the mechanical one was there!
 
Really?

osxuser said:
Can it be safe? Probably. It REALLY isn't ideal though, if you have the option at all, you should go mechanical/electric. Our mechanical pump from 1969 with 2000 hours on it is still going strong on the Cardinal, then again so is the electric from '68... that gets tested for 3 seconds every flight.

I don't think I'd plan on using a electric for continuous use if I had the option. I don't think they were ever designed for it, but I don't have any data to back that up.
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I realise this is all personal opinion of course but your stating this as REALLY isn't a good idea sounds like a statement of fact.
The Cardinal is a completely different situation hydraulically speaking,..I.e its a high wing airplane. Very difficult to get vapour lock on a high wing airplane.

The FACT is (with a low wing airplane) the standard set up is a poor design hydraulically speaking. Sure it works with 100LL but its still marginal..I.e your sucking a high vapour pressure liquid with a hot pump that is a ways uphill.

On the Cardinal you don't have this situation as the fuel is always above the inlet to the mechanical pump. Certainly if I had a high wing airplane then I'd stick with the standard setup.

Many automotive conversions (probably all of them I bet) have an all electric pump setup, so they can be made to be perfectly reliable.

The electric pumps are indeed rated for continuous use as they are in your car.

Frank
 
OR

KentuckyMike said:
Just last month I had to replace my Facet electric fuel pump. It was installed with the engine 350 hours ago. I'm glad the mechanical one was there!
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The second Facet..:)...No difference providing the electrical system is designed correctly.

Frank
 
frankh said:
I realise this is all personal opinion of course but your stating this as REALLY isn't a good idea sounds like a statement of fact.
The Cardinal is a completely different situation hydraulically speaking,..I.e its a high wing airplane. Very difficult to get vapour lock on a high wing airplane.

The FACT is (with a low wing airplane) the standard set up is a poor design hydraulically speaking. Sure it works with 100LL but its still marginal..I.e your sucking a high vapour pressure liquid with a hot pump that is a ways uphill.

On the Cardinal you don't have this situation as the fuel is always above the inlet to the mechanical pump. Certainly if I had a high wing airplane then I'd stick with the standard setup.

Many automotive conversions (probably all of them I bet) have an all electric pump setup, so they can be made to be perfectly reliable.

The electric pumps are indeed rated for continuous use as they are in your car.

Frank
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Frank, so does that mean if the tanks could be pressurized to the equivalent of the fluid head from the high-wing tanks, then boost pumps wouldn't be needed at all? A Cardinal's wings are what, maybe 4 ft. from the belly? So less than 2 PSI would do the trick?
 
Not really

(and I preface this by the fact I have not actually done the calc) the Pressure in the tanks cannot be always assumed to be there..i.e the airpseed is pretty low on take off and of course the fuel flow is at a max. The fuel vents only give you part of the total pressure due to the fact they are cut at 45 degrees..and in a pretty turbulent flow area.

Even a Cardinal could suffer a vapour lock incident if a feed line became partially plugged..but having the extra head of fuel helps.

Of course a Cardinal on Take off has a nose high attitude which does reduce the head available....But if you think about it on a low wing airplane the lack of head situation also gets worse..I.e the engine gets further above the fuel tanks than it does while sitting on the ground.

Frank
 
JHines said:
Frank, so does that mean if the tanks could be pressurized to the equivalent of the fluid head from the high-wing tanks, then boost pumps wouldn't be needed at all? A Cardinal's wings are what, maybe 4 ft. from the belly? So less than 2 PSI would do the trick?
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If I may butt in, the RV tanks can at least be have a positive head of pressure by using a pitot type device at the vent openings at the forward fuselage instead of the cut off screened fittings as per the plan. I've had them installed since day one and the only down side is the need for covers to keep mud bees out one the ground.

The faster you go, the more pressure in the tank to move fuel out. :)
 
Sure thwy can

But there are downsides here too

Namely your not moving very fast whaen your stationary on the runway

and I would'nt fancy a pitot tube vent for IMC work..Unless it was heated of course.

Frank
 
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