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RV-12 Boost Fuel Pump

Geico266

Well Known Member
I jumped the panel with 12v to see where I was at with the trim servo as reported by other. Much to my surprise the fuel pump was on..... and there is no switch. :eek: Pull the fuse?....................the electronic cooling fans stop. :eek:

Either the rest of you guys have not hooked the boost pump up yet. There is no way to shut it off! Sure would be nice to be able to shut the boost pump off to work on the avionics. :cool:

Why am I the first to figure this out? This really scares me! :eek:

Can someone else double check me on this? Pull the fuse to the fuel pump and the fans stop? Why?
 
Can someone else double check me on this? Pull the fuse to the fuel pump and the fans stop? Why?

Because they share the same circuit (and same fuse). Very few (simple any way) aircraft have separately protected circuits for every single item in the electrical system

Do not run the pump with no fuel...just pull the fuse. You will not over heat the avionics as long as the system is not powered up for an extended period of time.

I am curious how you previously determined that your trim wasn't working without having the fuel pump turn on?
 
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I am curious how you previously determined that your trim wasn't working without having the fuel pump turn on?

I pulled the fuse for the fuel pump. The trim wasn't working until I put a charger on the battery (13v+) and turned up the trim potentiometer. It works great now. ;)

Hey Scott,

Why is it a good idea to have a fuel boost pump and no way to turn it off? Every RV I have ever been in has a boost pump switch for start up, take off, and flying below 1,000' AGL (pattern). Why not in the RV-12? Why would the -12 be different? Was this an oversite. :eek:
 
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Why is it a good idea to have a fuel boost pump and no way to turn it off? Every RV I have ever been in has a boost pump switch for start up, take off, and flying below 1,000' AGL (pattern). Why not in the RV-12? Why would the -12 be different? Was this an oversite. :eek:

Yea, Like everything else on the RV-12, It's one oversight after another ;).

No, Seriously...if you look at anything that has been previously published about the primary design criteria for the RV-12, one of the top things was a simple uncomplicated airplane to fly. The RV-12 has about as simple of a fuel system as you can have.
Verify there is fuel in the tank, and that the fuel valve is turned on, and you are ready to fly. No tank switching to monitor or aux. pump to remember to turn on and off.
 
Scott, I understand the mission is simplicity, but even the simplist ultra lite I flew with a 912 had a switch for the fuel boost pump.
 
I may be completely wrong - but my understanding of aircraft systems is that the boost pump is being turned off just to save it from premature wear. There is no other effect of having pump switched on or off in the aircraft during flight in normal conditions. Apparently Van decided that this days longevity of fuel pumps is so good that it doesn't make any sense to turn it off to save it few hours of work. Remember - it's an LSA - you won't make 9h cross country hops in it - you _WILL_ land much more often so the fuel pump would be run quite high percentage of total time anyway.
 
Another good reason to switch it off is to make sure that the mechanical or primary pump is still working.....
 
Brian has it right, the need is to switch it off to be sure the engine driven pump is working so that we can assure that we were not flying around on just the boost pump, which like computer hard drives, it is not if it will fail but when.

I am all for simple, it would have been simple if it had its own fuse so we could have pulled it and checked the other pump.

Best regards,
Vern
 
Another good reason to switch it off is to make sure that the mechanical or primary pump is still working.....

Excellent reasoning.

Another reason is over pressurization control.

I guess you can always just pull the fuse. :rolleyes:
 
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Scott, I understand the mission is simplicity, but even the simplist ultra lite I flew with a 912 had a switch for the fuel boost pump.

I guess it wasn't the simplest of ultralights then (and if it had a 912 i it I doubt it was an ultralight)

Brian has it right, the need is to switch it off to be sure the engine driven pump is working so that we can assure that we were not flying around on just the boost pump, which like computer hard drives, it is not if it will fail but when.

I am all for simple, it would have been simple if it had its own fuse so we could have pulled it and checked the other pump.

Best regards,
Vern

Vern,
What prevents you from pulling the fuse with the way the system is now designed?

As for checking the pump(s)...I believe an RV-12 pilot will still know if there is a problem.
When you power on the master if the fuel pressure does not increase from zero, your aux. pump has failed. If when you start the engine you don't see an increase in fuel pressure, your engine pump may have failed.
 
Hi Scott,

It is my understanding that the fuse to the Boost Pump also controls other things and it is not a good practice to pull out a fuse as part of the preflight for each flight as this will in time damage the fuse socket.

I think you may have a misunderstanding of "Engine Driven Fuel Pumps" and Boost Pumps". "Engine Driven Fuel Pumps" by design must produce lower fuel pressure so that we can see a fuel pressure increase when the "Boost Pump" is activated, this is so that the "Boost Pump" is capable of driving fuel through the engine driven pump even if it has failed. In many aircraft you can not hear the "Boost Pumps" when they are turned on so you need to see the increase in fuel pressure to know that they are working.

Have a good day.

Best regards,
Vern
 
Hi Scott,

It is my understanding that the fuse to the Boost Pump also controls other things and it is not a good practice to pull out a fuse as part of the preflight for each flight as this will in time damage the fuse socket.

I think you may have a misunderstanding of "Engine Driven Fuel Pumps" and Boost Pumps". "Engine Driven Fuel Pumps" by design must produce lower fuel pressure so that we can see a fuel pressure increase when the "Boost Pump" is activated, this is so that the "Boost Pump" is capable of driving fuel through the engine driven pump even if it has failed. In many aircraft you can not hear the "Boost Pumps" when they are turned on so you need to see the increase in fuel pressure to know that they are working.

Have a good day.

Best regards,
Vern

Actually I think you have a misunderstanding....
First of all the RV-12 does not have a boost pump it has an auxiliary fuel pump (as do many aircraft). I believe the boost pump nomenclature is typically used for an aircraft that would not even be able to be started without boosting the pressure so that the fuel delivery system (typically fuel injection) will operate. An RV-12 would start just fine without the aux. pump powered on so its only real purpose is as a back up for the engine driven pump (the reason I refer to it as an Auxiliary pump).
I do not believe that any aircraft manufacturers specifically design using an engine driven pump that produces a lower output pressure than the installed electric pump. Simply by the process of them both working together there is typically a slight increase in pressure because the electric pump helps the engine driven pump. If you have data that implies differently, I would be very interested in seeing it.
 
Whole new problem - electrical

My fuel pump was not working, but I was working on the trim motor at the time ( another problem for me ), when another builder ask about turning off the pump. That is when I realized mine was not working at all. After 3 1/2 hours work today, I found out the red wire, at the 37 pin tunnel harness connector, was in the wrong hole. That is not a wire we had to mess with. That was pre-wired before we received the harness. How could it have passed inspection ! Hope no one else has such a nightmare.

John Bender
 
Actually I think you have a misunderstanding....
First of all the RV-12 does not have a boost pump it has an auxiliary fuel pump (as do many aircraft). I believe the boost pump nomenclature is typically used for an aircraft that would not even be able to be started without boosting the pressure so that the fuel delivery system (typically fuel injection) will operate. An RV-12 would start just fine without the aux. pump powered on so its only real purpose is as a back up for the engine driven pump (the reason I refer to it as an Auxiliary pump).
I do not believe that any aircraft manufacturers specifically design using an engine driven pump that produces a lower output pressure than the installed electric pump. Simply by the process of them both working together there is typically a slight increase in pressure because the electric pump helps the engine driven pump. If you have data that implies differently, I would be very interested in seeing it.

I do not care what you call the electric pump, the fundamentals are still the same.

The concept of the "Extra Pump" producing more pressure than the normal pump was started in commercial aircraft back about 1940 and continues on all commercial aircraft today.

No, if you have two pumps in series producing the same pressure you will not get any additional pressure.

If we need two pumps to have safe operation (and in the case of the RV-12 which produces no head pressure from the fuel tank because of its location in relationship to the engine we do need two independent pumps just in case one fails) then we should be able to check each pump for proper function before each flight. It is just good practice like checking both ignition systems before flight.

Best regards,
Vern
 
Trim

My fuel pump was not working, but I was working on the trim motor at the time ( another problem for me ), when another builder ask about turning off the pump. That is when I realized mine was not working at all. After 3 1/2 hours work today, I found out the red wire, at the 37 pin tunnel harness connector, was in the wrong hole. That is not a wire we had to mess with. That was pre-wired before we received the harness. How could it have passed inspection ! Hope no one else has such a nightmare.

John Bender

John,

Did you get your trim problem solved? I used the "Brad" solution and it works great now. Before that all I got was a hum (low voltage). Now it zips right along.
 
Hey Pete

My motor has a problem. It will run, under higher power, but almost stops each time the main gear that goes around the threaded rod, comes around. The gear is not running true, and it binds each time it goes around. Had to take the top off and watch it operate to see what was happening. The threaded rod should move forward and reverse so to speak. It also tries hard to move side to side each time the plasitc gear goes around. If you trim it back to slow it down some, it will NEARLY stop, or stop, each time the gear makes one revolution around the threaded rod, when it gets to the tight spot. Long explanation, but it has a problem due to ONE little plastic gear.

John Bender
 
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I may be wrong here but one of the peculiarities of Mogas is vapour locks.

I think the idea of the electric pump running all the time is to provide a positive pressure to the fuel system to reduce the risk of vapour locks.
 
I may be wrong here but one of the peculiarities of Mogas is vapour locks.

I think the idea of the electric pump running all the time is to provide a positive pressure to the fuel system to reduce the risk of vapour locks.

You are correct.
It is just another one of the many reasons for designing the system in the configuration that it is
 
Is the electric auxiliary fuel pump on the RV-12 rated for continuous operation? I suppose it is, but it is my IMPRESSION that the boost pumps used in most RV's is not so rated. They may be different animals .Comments?
 
Is the electric auxiliary fuel pump on the RV-12 rated for continuous operation? I suppose it is, but it is my IMPRESSION that the boost pumps used in most RV's is not so rated. They may be different animals .Comments?

The facet brand pumps recommended for carbureted fuel system installations in RV's are rated for continuous duty (as is the one in the RV-12).

I believe the Airflow Performance pump recommended for fuel injected installations is also rated for continuous duty but I would need to check to be sure.
 
Separate fuse?

When I read that there is no switch to shut off the electric fuel pump, I thought about installing one after my RV-12 is certificated. Then Scott posted the procedure for checking that the fuel pumps are operating. Turn on the master and watch for fuel pressure. That tells you that the electric pump is working. Start the engine and watch the fuel pressure. An increase in pressure tells you that the mechanical pump is also working. It makes sense to me. But what if I only want to sit in the cockpit and play with the radios? I will have to practice operating the Dynon to become proficient with it. There needs to be an easy way to disable the fuel pump. I can pull the fuse as long as it does not disable whatever I want on. Those cooling fans will be needed if the electronics are on for any length of time. Perhaps the fuel pump should have its own fuse. One could disable only the fuel pump if desired. And a fault in another circuit would not affect the electric pump. Like Scott said, not having a switch makes the plane simpler to operate. I will not have to remember to turn the fuel pump switch on, or put the wheels down, or turn on the carburetor heat, or enrich the mixture before landing. What does that leave for a landing checklist, flaps?
Joe
 
Cooling fans

I don't know why fokes are so concerned about not having the coolong fans on while testing other equipment. Light aircraft have been operating for years without any cooling. The Dynon 180 installation manual page 4-13 says the surrounding air during operation may be no warmer than 50C. For the length of time you are without cooling the instruments won't know the difference.
Tony
 
I don't know why fokes are so concerned about not having the coolong fans on while testing other equipment. Light aircraft have been operating for years without any cooling. The Dynon 180 installation manual page 4-13 says the surrounding air during operation may be no warmer than 50C. For the length of time you are without cooling the instruments won't know the difference.
Tony

Tony,
You may not be aware but the area aft of the instrument panel on an RV-12 is a closed space...no free movement of air (this was done to provide a strong but light fwd fuselage structure BTW) That is the reason it has cooling fans. This probably why builders are concerned about powering up with the fans disabled.
It is not any problem powering up equipment for a short period of time (15 minutes would be no problem at all). If someone wants to spend an extended period of time doing something like learning the operation of the Dynon D180, you can use the power switches on the front of the radio, xponder, and GPS to power them off. You could then operate the D180 for a very extended amount of time without any danger of over heating (it dissipates a very low amount of power and consequently produces very little heat)
 
I ended up not liking the "pump on" all the time. I added the following switch to trun the pump on and off.

RV-12_night.JPG
 
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Fuel pump switch - revisited

Up until now, I have been of the opinion that not having a seperate switch for the electric fuel pump was OK with me. But, there always seems like there is something I want to do on the Dynons or GPS whenever I go to the airplane. So, I am continually pulling and reinstalling the fuel pump fuse. I have made my GPS non-removeable so if I want to put in a flight plan before flight, I will be pulling the fuse again. Also, this is not conducive to safety, as a pilot can forget to put the fuse back in, just like he can forget to switch on the pump if there was a switch. The engine will start and run, but the avionics would likely overheat on a hot day.
Anyway, enough probably has been said on this topic, there is a lot of great information in this thread, and folks can make up their own mind.
I am planning to install a switch, but I can not quite yet as I need to fly off my flight test cards first.
Here is where I think I will put the switch:
861037271_ZM5kc-M.jpg

The switch would be guarded in the ON position. The label works for both the valve and the switch - down is ON. And, the position of the switch makes it easy to check when you check the fuel valve on the takeoff and landing checklists.
Since the wire bundle is directly below it will be an easy matter to cut and put a male and female spade on the red fuel pump wire. The switch can then be installed with spade connectors to make removal of the inspection panel easy.

Tony
 
Tony, If you want to put a switch, put it on the panel where all the other airplanes have it. One day is one day where another pilot is going to fly your plane, when that motor misses a beat and in a panic he is looking for the boost pump switch will he remember to look on the floor??? No he will scan the panel for the switch...just my 2cents!

I like Geico's position, i am going to steal his idea! :D

Kind Regards,
Rudi
 
I like Geico's position, i am going to steal his idea! :D

Kind Regards,
Rudi


Steal away! :D

It is nice to have the switch right next to the throttle. I can flip the switch on during take off, or in the pattern, without even looking at it. The red LED tells you it's on. I'm not sure where I would put it if I add the auto pilot, but it would be on the panel somewhere.
 
Interesting dialogue gents. I have a question for Scott.I discovered yesterday the pump is on same circuit as fans. Also conluded that it's probably not a good idea to run pump w/o fuel,(although I don't recall reading any advisory for that) or all the time for that matter when there is fuel. Scott, I would appreciate your comments on installing a switch in the fuel pump line later for control of pump operation purposes. ie; Is running the pump 100% of the time during operation a requirement for this RV12 system?
Thanks for the feedback.
Dick Seiders
 
Switch location...

Tony, If you want to put a switch, put it on the panel where all the other airplanes have it. One day is one day where another pilot is going to fly your plane, when that motor misses a beat and in a panic he is looking for the boost pump switch will he remember to look on the floor??? No he will scan the panel for the switch...just my 2cents!

I like Geico's position, i am going to steal his idea! :D

Kind Regards,
Rudi

Rudi, you make a good case. Now, where-oh-where on this panel to put it. I can't use Larry's idea because I have the autopilot. Maybe above the fuse panel. It would be in plain site and there is plenty of room behind the panel, plus the red fuel pump wire is easy to tap into. You can see it in the lower wire bundle in this picture.
[url=http://tonytessitore.smugmug.com/RV-12-Project-N577RV/Avionics/10000342_ViPnR#739889731_eks9Y-A-LB][/URL]

Tony
 
Tony, I just had a thought. You could mount a fuel pump toggle switch under the panel. It would be out of the way, not "in" the panel, but still in visual if you keep it close to the edge.

Just a thought.
 
Rudi, you make a good case. Now, where-oh-where on this panel to put it. I can't use Larry's idea because I have the autopilot. Maybe above the fuse panel. It would be in plain site and there is plenty of room behind the panel, plus the red fuel pump wire is easy to tap into. You can see it in the lower wire bundle in this picture.
[url=http://tonytessitore.smugmug.com/RV-12-Project-N577RV/Avionics/10000342_ViPnR#739889731_eks9Y-A-LB][/URL]

Tony

Hi Tony,

Hmm I am not aware of Autopilot issues with the proposed position, becuase we are not there yet. What is the problem with Geiko's position and AP?

If there are problems then Maybe you want to mimic the switch panel on the left panel and put it with the same spacing on the right panel. If you want to be fancy mimic the switch and grey border too it will look like it was designed like that. Below is what I think stolen from your pics if there is a AP position problem. But you can Mimic the same on top of the Fuses, just keep symmetry and type in mind and it will look good.

switch.jpg


OR

switch2.jpg
 
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I like to use the same switch as used in the switch panel. Does anyone have a link were the same switches can be bought?
 
Hi Tony,
Hmm I am not aware of Autopilot issues with the proposed position, becuase we are not there yet. What is the problem with Geiko's position and AP?

Rudi, Larry's switch occupies the hole that is used for the autopilot disconnect button. The red button can be seen in the pictures.
Tony
 
I noticed the circuit board for the switch panel has an empty switch position between the IGN B and Avionics switches. The board's silk screening in this location says "Fuel Pump". I think it goes to a seperate pin on the fuselage connector. I wonder if this was the original design and was abandoned, or if the designer was just leaving his options open.

It looks like it would be possible with a little reverse engineering to put the switch there. Seems like a logical order for the switches, but in my opinion its kinda close to other switches that you wouldn't want to turn off unintentionally in an emergency.
 
Fuel Pump Switch

To put an end to all of this discussion the main reason that there is a fuel pump in the RV-12 is to prevent vapor lock. Although some may not have an issue with the pump turned off, they may not be operating at a fuel type or density altitude that will cause an issue. Others may follow your lead and have an engine failure. There are multiple reports (that I know of) of pilots turning off the pump after reaching pattern altitude and experiencing partial power loss. The pump has been placed close to the tank to provide pressure all the way to the engine thus reducing the chance of vapor lock as the fuel passes through valves and gascolators (which may be points of cavitation/pressure drops where vapor can form) or heated fuel lines on its way to the engine.

A Rotax is not a Lycoming therefore do not add a switch to operate the aircraft in a similar manner. The pump needs to run all the time.

Next about checking the engine-driven pump. When you turn on the aircraft the fuel pressure will rise to a steady-state level. When the engine comes on the fuel pressure will rise to a higher level letting you know that the engine-driven pump is working.

What amazes me is why anyone would not want the electric pump running all the time? The pump is designed to be running all the time. These pumps will last for years and years and are only $40. Are people really risking their lives to save $40 over a period of 15 years (approx how long mine lasted on my RV-7) ... that is $2.67 per year. Not even a gallon of gas or a gallon of milk these days.

My main mission at Van's is to ensure that each and every one of you is safe. Let's do the right thing here and please just leave it running.

Rian Johnson
Van's Aircraft
Vice President Chief Engineer
 
I’ve posted this info on several previous posts. My 12 is a day VFR machine, so it was a simple change to use the nav light switch and wiring as a means to control the electric fuel pump.
 
I’ve posted this info on several previous posts. My 12 is a day VFR machine, so it was a simple change to use the nav light switch and wiring as a means to control the electric fuel pump.

Rich, airplanes crash on a regular bases during the day under VFR conditions because of engine failure, so I am not clear what relevance that has on choosing to operating with a fuel pump off, when it was purposely chosen as a critical design point to be operating 100% of the time.

With all of the discussion in the RV-12 forum about vapor lock and the influence of different vapor rating fuels, hopefully RV-12 owners will take it to heart that the fuel system as designed was done that way for very specific purposes, and will operate it as intended.
 
Scott,

I have posted my reasoning many times. It appears there are two schools of thought that will never reconcile. I understand the argument for continuous running, but I have chosen a different path. I’ve said all I intend to on the reasoning.

Rich
 
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