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RV-12 Fuel Boost Pump Usage Question...

TomVal

TomVal

Well Known Member
In reference to Van’s RV-12 Operating Checklist for the Rotax engine I have a question about fuel boost pump usage procedures. In the “Pre-Start and Starting Engine” checklist there is no mention of turning the boost pump on prior to start. Although the first item listed in the “Starting Engine” checklist is “Fuel Pressure – Normal” which would imply the fuel boost pump must be turned “ON” prior to start. If so, there is no mention of turning the boost pump off after start. Does that also imply that the boost pump should run continuously??? As a matter of fact, there is no mention of fuel boost pump usage anywhere in the Operating Checklist!

On the Jabiru, the Operating Checklist does direct you to turn the boost pump “ON” for 10 seconds prior to start then “OFF”. After that, there is no further mention of required boost pump usage.

I find the lack of boost pump usage for both powerplants interesting. The single Bing carburetor on the Jabiru sits approximately at the level of the bottom of the fuel tank (lots of head pressure), whereas the dual carbs on the Rotax sits high on the engine.

These Rotax and Jabiru engines appear to be of a different breed. I would appreciate some RV-12 operator input on their boost pump usage procedures.

Thanks,
 
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The RV fuel pump is hard wired and comes on with the master if the fuse is installed. Some of us have installed a switch. Mine starts equally well with or without the electric pump running.
 
RFSchaller said:
The RV fuel pump is hard wired and comes on with the master if the fuse is installed. Some of us have installed a switch. Mine starts equally well with or without the electric pump running.
Click to expand...

Thanks for the input. So the Rotax fuel system requires continuous boost pump operation, is that correct? Wrt the Jabiru, that puts the question back in my court. The manufacturer only requires the boost pump on for start. My guess it must be due to the low mounted location of the carburetor. I don't like the idea of having the boost pump off for takeoff and landing. I need to research this further.
 
Position of the carburetor has nothing to do with an engine that has a fuel pump. Both the Rotax and the Jabiru have mechanical fuel pumps.

Van's has elected to leave the "boost" pump on continuously because it will not harm the electric pump and it removes a couple of steps of pilot interaction.
The Rotax itself does not require continuous operation of the electric pump.
 
Thanks Mel for the clarification. Wrt to the Jabiru, if the boost pump is required for start, why does the Jabiru engine operating manual not require boost pump operation for takeoff and landing? I need to research this.
 
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I cannot recall just where I got the information, but it is my understanding that the electric pump was an addition, and its presence and constant running was primarily to overcome vapor lock tendencies.
 
My Rotax starts just fine with only the engine driven pump. I'm guessing it's because the carburetor bowls are full and keep things going for the few seconds it takes to get the engine driven pump up to speed. I'll make another guess and say the Jabiru prime step is probably just a precaution to ensure fuel lines are full if someone has a fuel tank located lot enough to allow siphon effect.
 
I fly various aircraft using the Rotax and all if these have a switched boostpump.
The pump is switched on during take off and landing and the word is that this reduces the chances on vapour locks.

I made my pump switched in the 12 and when airborne i use the unused 2 amps of the pump for a second Dynon screen, which works fine for the last 75 flying hours.
I can run the pump and additinal SV simultaneously but in that case the generator is loaded just over 80% of max which is the advised continuous limit.
That is with all options installed incl Adsb an all active.
 
Hotscam said:
I made my pump switched in the 12 and when airborne i use the unused 2 amps of the pump for a second Dynon screen, which works fine for the last 75 flying hours.
Click to expand...

Hey Jack, where did you tap the circuit for the switch? And, where did you mount the newly added fuel pump switch? Your experience could save me a lot of research time.

Thanks..........Tom
 
I'm not Jack, but I mounted mine next to the fuel SOV, as shown by several others in their Mods write-ups. I tapped into the red boost pump power wire that runs through the tunnel there. Mine is a rocker switch with a red LED for ON, and it's bright enough for bright sunshine.

Of of my first mods after Cert, and highly recommended!

Bob Bogash
N737G
 
Red Wire in the Tunnel Harness

Thanks Bob. I'll do a search and see what I can find. I sure hate to dig back in to that tunnel harness, but I sure would like to be able to control the pump. I'm an old Mooney jock and "fuel pump on" was always a part of the GUMP check that was included in every T.O & L. procedure..........Tom
 
Tom,

If you don't have Nav lights you can use that switch for the fuel pump. I made the wiring swap in the wire bundle between the seats. Now the Nav light switch is my fuel pump switch.

Rich
 
I disconnected the pump wire from the fuse panel connector and put a relay in between.
The relay is operated by a switch on both sticks together with trim push buttons. Works great
 
Jabiru USA Response to Fuel Boost Pump Usage...

Concerning fuel boost pump usage on the Jabiru 2200 here is the response that I received from Jabiru USA:

Normally we use the boost pump to fill the carb bowl if the aircraft has not flown for several weeks. Then we use it for takeoff. Otherwise we do not operate the pump.
 
Switch solves incorrect fuel flow indications

Go here for picture of my switch, installed primarily to prevent false high fuel flow indications at higher altitudes.
 
todehnal said:
Thanks Bob. I'll do a search and see what I can find. I sure hate to dig back in to that tunnel harness, but I sure would like to be able to control the pump. I'm an old Mooney jock and "fuel pump on" was always a part of the GUMP check that was included in every T.O & L. procedure..........Tom
Click to expand...

You can access the red wire under the dash also. It is the thickest red wire in the loom going into the tunnel.
 
Thick Red Wire

Thanks guys. I found the wire above the avionics deck, and I have enough lead to tie in the switch wires. I would like the switch as close to the throttle as possible, so I ordered a switch like one of the ignition switches, and will mount it on the lower left corner of the glove box panel, in line with the skyview switches in the center panel. This sounds like a worthwhile mod to me............Tom
 
Wiring the New Switch

Okay, I got my new switch and it is exactly like the ignition switches, and I have the heavy red fuel pump wire that I will cut and add leads and terminal ends to go to the new switch. Anyone know how to wire the thing?? I have two wire and the switch has 5 lugs. Nothing is labeled. Anything else necessary to make the light in the switch work?

Thanks, Tom
 
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http://www.ottoexcellence.com/UserFiles/file/K1%20Schematics.png
Go to above link. The schematic in the second row, third from left is similar to your switch. But I do not think that your switch has an internal resistor to protect the LED from excessive current. Put a 620 or 680 ohm resistor in series with the LED.
Use terminals 1 and C to control the fuel pump.
Terminal 1: +12 volts
Terminal C: fuel pump
Put resistor between terminals C and 4.
Terminal 3: Ground
Joe Gores
 
Fuel Pump Switch Wiring

Mich48041 said:
http://www.ottoexcellence.com/UserFiles/file/K1%20Schematics.png
Go to above link. The schematic in the second row, third from left is similar to your switch. But I do not think that your switch has an internal resistor to protect the LED from excessive current. Put a 620 or 680 ohm resistor in series with the LED.
Use terminals 1 and C to control the fuel pump.
Terminal 1: +12 volts
Terminal C: fuel pump
Put resistor between terminals C and 4.
Terminal 3: Ground
Joe Gores
Click to expand...

Wow!! There are some sharp guys on this forum!!
Joe, I dug around on the above site for almost 2 hrs, trying to verify your wiring recommendations. As far as I can tell, you are absolutely correct!! I don't think that I would ever had figured it out on my own. It was a challenge, even after I knew the answers. Two items gave me trouble. First, I couldn't identify which terminal was which (they are not marked) second, I couldn't verify the LED with a multimeter, but according to their documentation, it looks like 12v on terminal 4, and ground on terminal 3 should light the light. I will add the resistor between C and 4, as you recommended. Thanks so much! You have probably saved me much grief... Next, we'll look for smoke. :)

Tom
 
Updated Information

One last thing Joe:
I just called Otto and asked about the internal resister. They told me that there was a series resister in the switch. In fact, he said that the description (12 volt green LED) doesn't mean that it is a 12 volt LED. It just means that there is a series resister to reduce the voltage from 12 volts, for the LED. Let me know if that makes sense, and I may leave the resister out.

Tom
 
Tom,
Since the manufacturer says there is indeed an internal series resistor, then an external resistor is not needed. Connect terminals C and 4 with a very small wire. Since you have an ohmmeter, you can figure out which two terminals to use so that the pump comes on when the top of the switch is pressed. The only way to smoke the switch is if ground is connected to the wrong terminal. It is not desired to connect 12 volts and ground together when the switch is turned on, even if it is fuse protected.
Since the Otto website schematic does not give complete part numbers, I was not sure if the Otto K1ABBPCADA switch contained an internal resistor or not. You helped me by clarifying that.
Joe Gores
 
Switch Location

I have nav light, so that wouldn't work. I'm Skyview, so I am kind of on my own for a location. Being an old Mooney Jock, I would like it as close to the throttle as possible. My best plan, so far, is to put it bottom left in the glove box panel, as close as possible, and in line with the center panel switch row. That's about as close as I can get it to the throttle, and it will be on the end of the row, for easy identification, with more "heads up" time.

12%20skyview_lg.jpg




Tom
 
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