Is an engine without a mechanical fuel pump safe?

AX-O · #1 06-23-2009, 07:37 PM

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?

Geico266 · #2 06-23-2009, 07:58 PM

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.

AX-O · #3 06-23-2009, 08:42 PM

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.

DakotaHawk · #4 06-23-2009, 08:48 PM

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.

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.

SteinAir · #5 06-23-2009, 08:57 PM

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

Geico266 · #6 06-23-2009, 09:11 PM

Quote:

Originally Posted by AX-O

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.

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.

airguy · #7 06-23-2009, 09:18 PM

"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.

Ted Farmin · #8 06-23-2009, 09:54 PM

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

asav8tor · #9 06-24-2009, 02:07 AM

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....

islandmonkey · #10 06-24-2009, 02:53 AM

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.