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Fuel Pump (Electric) Switch

S

sportkid

Active Member
Our local A&P (Rotax expert) came by just as I made final checks to the wiring in order to fire up (just a figure of speech) the avionics.
When we turned on the master, the fuel pump clicked away.
He said to turn it off because of the absence of fuel. (Of course).
No switch for the pump. He was confused by this. We pulled the pump fuse and then turned on the avionics. All of the planes he worked on (too numerous to list) - plus the plane I flew (Remos) to get my ticket used a switch to control the fuel pump because you don't need it for
normal flight.
Why no switch?
 
It does have a mechanical fuel pump...the 912ULS. Vans is electric and designed to run all the time. I have flown my 12 with the fuse out from beginning to end and it worked fine. I am assuming its just a safety issue. A bigger problem is getting adequate fuel pressure. Sometimes it turns out to be tank junk that has accumulated in the gascolator screen but more often it takes more than one hole in the gas cap. I finally drilled three (as did others) and the problem was solved immediately.
 
I don't remember if...

... it was on this list, or a comment that Mitch Lock made (Van's East Coast Rep & RV-12 demonstrator) but the answer I remember is that the electric fuel pump has a very low failure rate. It was more likely that the (if installed) pilot operated electric fuel pump switch would fail instead of the pump itself. Because of this high reliability factor, it was decided to wire the pump as "always on" (and provide the fuse in the circuit). It does not adversely affect any operational aspects of the engine.
 
Peterk said:
It does have a mechanical fuel pump...the 912ULS. Vans is electric and designed to run all the time. I have flown my 12 with the fuse out from beginning to end and it worked fine. I am assuming its just a safety issue. A bigger problem is getting adequate fuel pressure. Sometimes it turns out to be tank junk that has accumulated in the gascolator screen but more often it takes more than one hole in the gas cap. I finally drilled three (as did others) and the problem was solved immediately.
Click to expand...

Peter,
As I have said before, I am highly skeptical that adding more vent holes has any effect on fuel flow or fuel pressure.
Numerous other builders have posted that they added extra holes and then realized it was pointless (made no difference).

The current vent hole size is more than adequate for the maximum fuel flow to ever be expected (about 7 gallons per hour). Now if you were pouring from a fuel jug, the current vent hole would be way under sized because you would then want a fuel flow of 5 or 6 gallons in just a couple of minutes (3 gallons per minute) That is a difference factor of about 30 when comparing the two.
 
sportkid said:
Our local A&P (Rotax expert) came by just as I made final checks to the wiring in order to fire up (just a figure of speech) the avionics.
When we turned on the master, the fuel pump clicked away.
He said to turn it off because of the absence of fuel. (Of course).
No switch for the pump. He was confused by this. We pulled the pump fuse and then turned on the avionics. All of the planes he worked on (too numerous to list) - plus the plane I flew (Remos) to get my ticket used a switch to control the fuel pump because you don't need it for
normal flight.
Why no switch?
Click to expand...

The main theme of the Light Sport Aircraft is Simple. Simple to fly, simple to maintain, etc.
Not having a switch that the sport pilot needs to remember to operate is an enhancement in safety (if the engine driven pump fails during flight the pilot has to do nothing except continue to fly). Part of a RV-12 pilots pre-start check should be to note that the electric pump noise is present and that fuel pressure begins to rise when they turn on the master switch.

The electric pump is continuous duty rated (just like the electric pumps used in cars now a days). It can be operated full time just like the engine driven fuel pump (as long as there is fuel present. BTW the RV-12 documentation warns builders not to operate the pump for more than a second or so without fuel present).
 
After posting with Scott, talking with Rotax mechanics, and EAA tech counselors, I decided to add a switch to control the electric back up pump. The factory fix is to "pull the fuse" if you want to shut the pump off. I don't like this idea as spade fuse terminals are not intended to have fuses installed and removed all the time. It may wear out. Further, I just don't like not being able to know the pump is on or off. No other plane I have flown had the back up pump on all the time.


RV-12_night.JPG


The switch is located in the "Auto Pilot Disconnect" hole. I don't intend on installing an AP right now, and I didn't want to drill anymore holes in the panel. In this picture the switch was turned around to have the LED lite on top. It is right next to the throttle so you don't even have to look for it when you need to turn it on. My SOP is anything flight at or below 1,000 AGL the back up pump is on. I only run the pump under 1,000' AGL. One of the down sides is I needed to find a different "K" factor for the Dydnon fuel consumption.

It comes down to a personal preference thing, I prefer a switch. Now if I can just find one with a green LED to match the panel!

JMHO.
 
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Fuel pump switch

I am not really concerned about the pump being on or off when considering its reliability but there is one other consideration.

In the RV6A that I just sold I always noticed that if the fuel pump (boost pump) was on that the Matronics Fuel Flow gauge showed a greater flow rate than was correct. When you turned off the boost pump the flow rate indication decreased. Matt at Matronics has as lot of information about this, and it was discussed at some length a while back on his web site (Matronics.com)
 
For those of you that installed a switch...

...did you do this after the certification / airworthiness inspection, or was your RV-12 licenced as Experimental / Amateur Built??
 
JohnF said:
I am not really concerned about the pump being on or off when considering its reliability but there is one other consideration.

In the RV6A that I just sold I always noticed that if the fuel pump (boost pump) was on that the Matronics Fuel Flow gauge showed a greater flow rate than was correct. When you turned off the boost pump the flow rate indication decreased. Matt at Matronics has as lot of information about this, and it was discussed at some length a while back on his web site (Matronics.com)
Click to expand...

As long as the fuel flow calibration is done in the "normal" condition it doesn't matter.
For the RV-12 with the pump on all the time, the fuel flow calibration process compensates for it.
 
It would seem to me that with the electric fuel pump pressurizing the line going to the engine driven fuel pump that there would be less chance for vapor lock to occur. This is similar to the way modern automobile fuel systems operate, with the electric fuel pumps located iin the tank. As an aside, one John Deere combine we owned used a similar facet fuel pump that operated whenever the combine was running and operated for over 4000 hours without being replaced. Very reliable. I'm going to leave my electric fuel pump on.
 
Just a caution. The fuel pump fuse also supplies power to the 2 fans that cool the avionics. Would not be wise to use the fuse as a switch to turn off the fuel pump during the course of normal flying. OK for short periods of time, but, probably not for long periods during warm weather (I can only hope!!)
 
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