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What I don’t understand…

Piper J3

Well Known Member
Shutting off electric fuel pump. This is being discussed in several threads again, so let’s just have the debate right here and now.

Rotax Installation Manual, and yesterday, the Vice President - Chief Engineer at Van’s Aircraft, are adamant about running electric fuel pump 100% for purpose of preventing vapor lock.

Can someone explain why they have elected to shut off electric fuel pump? Please explain in technical terms and not feels-good-to-do language. Please be specific and technical.

It is very hard for me to understand this behavior. It parallels what is happening now in this country with COVID vaccine reluctance. When something is proven effective, has scientific basis, explained well to the end user, with imminent danger if not followed – people are sticking their fingers in light sockets.

I experienced vapor lock several weeks ago and never want to have it happen again. I started a specific thread to discuss mitigation and now I read how happy people are to flaunt manufacturer’s advice on a safety related issue.

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Please read once again Van’s Aircraft – Rian Johnson, Vice President Chief Engineer:

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
 
Electric fuel pump.

To begin with, people have different knowledge and experiences.
We see the same things but interpretes it differently.
On a certified aircraft there is a requirement to have two fuel pumps with
separate power supply. Often there is a mecanical and an electric pump.
Recomendations from manufacturers are allmost allways made to protect them from liability. So some people read them in that context.
Both Vans and Rotax recomends an electric fulpump in addition to the mecanical. Vans recomends it to be usead at all times, Rotax does not say
when to use it. Building your own aircraft, you are the manufacturer and
the construction may be different from other aircraft.
You may be a piano teacher or an engineer by profession and have 10 hours or 2000 hours of experience. This results in different pilot decitions.
If the engine lose power and this is because of fuel starvation this can lead to
an accident if not corrected. If having no experience of switching fuel tanks
hearing the engine stumble, this may seem frightening for some people.
For others its buisiness as usual.:D
 
I know a lot of stuff about a lot of things. I don't know everything about everything. I don't feel compelled to make my RV-12 more like some other airplane "just because" that's what was done on a different airplane, with a different engine and a different fuel system. I figure I'll give the designers of the airplane credit for having some clue what they were doing.

The guy who built my plane put a push-pull breaker in line with the fuel pump wiring, right next to the fuel valve. I removed it and spliced the wires. never did see a point to it, and it was just one more potential point of failure. The thing runs just fine with the electric pump on, thanks.
 
When Rian resurrected the 10 yr old boost pump switch thread yesterday, I recalled that I had several posts therein. I have to say that I have vacillated both for and against a separate fuel pump switch ever since then. At that time I decided not to install the switch for the reasons that Van’s states, but it still rankles me that I have to pull the pump fuse to do a database update or other maintenance on avionics. Data base updates come once a month if you care to do them, that relates to a lot of fuse pulls.

To me, a Master Switch controls power to all the system switches. But in the -12 it also is activates the fuel pump system. Overall, I am Ok with that and have been just been pulling that darn fuse when I need to do something on the avionics like put in some user frequencies on the GTR 200 or change a setting on the SV. More than once I have started the airplane to go flying with the fuse removed.

All that being said, I don’t see much harm at all in having a switch as long as it is a “proper” switch. Kind of like the British building a “proper” bridge in the Alec Guinness movie.

If employed, the switch should be in plain sight and have a visual indication that it is ON, like all the other switches on the -12 panel. There should be a line item for it “ON” in the pre-start checklist.

So, I don’t have a switch for the fuel pump, except the one sitting on my workbench and never installed. I have been flying factory original for 11 years and loving it but hating having to pull that fuse for minor avionics ground work!
 
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I don't mean to be non-technical here. I think the issue stems from initial private pilot training. In most (all?) of the low-wing planes that people are taught in, the electric fuel pump is only used in critical phases of flight. Perhaps that carries over into other planes people end up flying ?

That said, the RV-12iS (more specifically the Rotax 912iS engine) has two electric pumps (no mechanical) and they both have switches. Are there any iS pilots who don't use both, always?
 
All that being said, I don’t see much harm at all in having a switch as long as it is a “proper” switch. Kind of like the British building a “proper” bridge in the Alec Guinness movie.

If employed, the switch should be in plain sight and have a visual indication that it is ON, like all the other switches on the -12 panel. There should be a line item for it “ON” in the pre-start checklist.

I agree ... and although I do have a panel mounted fuel pump switch, I NEVER fly with it switched off. It is a line item in the pre-start checklist. I only turn it off while performing SkyView updates, sifting through SkyView menus or while programming frequencies in the GTR200.

For those of us running auto fuel subject to winter RVP changes, I feel it is especially important during the spring and fall to make sure the fuel pump is running during the entire flight as clearly stated by Van's Aircraft. Flying with high RVP fuel during warm days is NOT a good mix ... so keep that fuel circulating (and spike it with 100LL if it is a really warm day).
 
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It is very hard for me to understand this behavior. When something is proven effective, has scientific basis, explained well to the end user, with imminent danger if not followed – people are sticking their fingers in light sockets.

....says the guy who wants to put privy vents in his cowl and is insulating the fuel lines.:)

......you knew that was coming.............
 
It’s kind of like Convective activity. People ducking underneath dodging rainshafts or using data link to dodge buildups always state they have done it hundreds of times without a issues. Those that do end up having a issue often are not around to write about it.
 
That said, the RV-12iS (more specifically the Rotax 912iS engine) has two electric pumps (no mechanical) and they both have switches. Are there any iS pilots who don't use both, always?

The 12iS pumps are switched for two reasons. First, you have to be able to check each one individually before flight. Second, only one pump is used for starting to reduce the electrical load.

Vans warns to never operate in flight with only one pump on for the obvious reason that if the sole operating pump fails the engine will quit.

I'm sure there are pilots who think shutting one down during cruise will save pump life. However, since the one operating pump has to carry the entire load, it is working harder than it would with both pumps running - most likely shortening pump life.
 
It is very hard for me to understand this behavior. It parallels what is happening now in this country with COVID vaccine reluctance. When something is proven effective, has scientific basis, explained well to the end user, with imminent danger if not followed – people are sticking their fingers in light sockets.

RV12 info and debate is interesting. Is this opinion paragraph about medicine VAF appropriate?

I fly an -A model, some pilots choose tailwheel planes. Riskier? Some choose to decline flights in experimental planes like RVs and some refuse experimental medicine. Maybe we all touch light sockets when we get out of bed? Freedom!

Carl
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The only time I turn it off (by pulling the fuse) is when I'm playing with the Dynon on the ground.

Would anyone even think about turning it off if it weren't so freaking noisy?
 
Whenever I decide to power up Master and don't want electric fuel pump running - I pull fuse and tape it to pilot's control stick.

Almost 100% certainty that it will be noticed and placed back in fuse block.
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Hadn’t heard of that one in 79’ but the recent one referenced where the crew TRIED to disconnect automation using a CB in an attempt to diagnose a problem caused a fatal accident. Man, be careful out there!
 
I'm one of those builders who installed a fuel pump switch. I understand why Vans chose not to make it part of the kit, but I find it very handy to be able to easily turn the pump on or off for upgrades or to drain the last of the fuel out of the tank.
However, I never fly with the pump off because Vans designed it that way and it's obviously safer to have it on throughout the flight. Can't see any good reason to do otherwise. Just another checklist item on start-up and shutdown. I'm also curious as to why some choose to go against the recommendations and turn it off in flight.
 
Bob,

I can tell you why I put it on a switch. I didn’t like the idea of the electric pump failing during a flight and not knowing it until next ground operation ((best case) or the mechanical failed on the same flight(worst case). Since I have been flying a Cherokee for 36 years the switch also aligns my routine with my Cherokee habits. I had some concern about wearing out the electric pump, and my experience over 9 years has shown I only need it on very hot Phoenix summer days when I get hi fuel flow and lo fuel pressure alarms due to fuel flashing in the lines. I run the pump on takeoff and landing. I like to do the run up with it off to confirm the mechanical pump capacity. I understand the reasoning behind running it continuously, but I have made a different choice. Spare me the criticism, because I've heard it all over the years on this forum, and I have confidence in my decision from 1100 hours of incident free operation.

Rich
 
Make sure all you are turning off with that ADDED switch is just the electric fuel pump.

The fans for your Transponder if you have the later RV-12's with a Skyview system are on that same circuit. You don't want to overheat that electronic stuff right behind the firewall, it needs airflow.
 
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Mine is on all the time. Yes annoying, yet reassuring. As I previously mentioned I upgraded to the newer Facet pump per Vans' notification a few years ago and carry the old one in my fly away bag.
 
Mine is on all the time. Yes annoying, yet reassuring. As I previously mentioned I upgraded to the newer Facet pump per Vans' notification a few years ago and carry the old one in my fly away bag.

Does the Facet pump come with the same jackhammer "audio alert" function?
 
I believe the old version (slightly less pressure capability) and the newest recommendation are both Facet pumps. And yes, equally annoying.
 
Well, the builder of my RV12 put in a fuel pump switch and the various checklists I got with the airplane included turning on and off the pump during various times as cussed and discussed in the multiple threads here on VAF. I have only been flying it for a couple of months now and the "vapor lock" issue wasn't really on my radar until recently.

In future I will plan to turn the pump ON after start up and check for pressure increase. I will plan to turn the pump OFF after back on the ground. All of this vapor lock talk has got me sort of skittish right now, especially since I live in Texas and summer temps are often higher than 100F around here.

I do find myself needing to turn on the Master switch in the hanger for various reasons from time to time and will use the switch to turn off the pump during those times.

Jim, do you have the upgraded 40135 Facet fuel pump in your airplane as described in Van's Notification Letter 18-07-12?
https://www.vansaircraft.com/service-information-and-revisions/n-2018-07-12/


After reading that "notification" I came away wondering why "vapor lock" wasn't even mentioned by name. Yes, they mention "heat soaked engine", "High OAT", "low fuel pressure" but it seemed to me they didn't want to just come out and call a spade a spade.. With that said, I'm also surprised they didn't issue a Service Bulletin.. It's only a $54 part and a couple of hours of labor.. Since the whole reason for the electric fuel pump is to prevent/reduce vapor lock, switching to a higher pressure pump is still to prevent/reduce vapor lock.. Although it wasn't mentioned, clearly having the electric fuel pump is a redundant fuel pump in case the mechanical pump stops working during flight.

I will be adding the upgraded pump during my Condition Inspection in a couple of weeks. I will also continue using a mixture of 100LL and MOGAS in my plane. During my Light Sport Repairman Inspection Class, the instructor said that he has worked on many Rotax engines that use approximately 50% 100LL and 50% Mogas, and his experience has been that lead build up is non-existent or minimal when both fuels are used. On his own personal aircraft he does the same and he changes the oil every 25 hrs and the filter every 50 hrs. During the first 25 hours I flew the plane, it included a lot of cross country flying NC>TX TX>OH OH>TX and a good deal of 100LL.. I didn't find any lead laying on the bottom of my oil reservoir. I started each cross country flight with a full tank of MOGAS.

Hopefully between always using the upgraded electric pump, 100LL/MOGAS I won't have to deal with vapor lock. It's also nice to know that at the departure end of the runway I use the most, I have a considerable amount of pasture land with no trees to make an off field landing if needed.
 
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Randy,

The Nav Light circuit is completely different from the AV fans. When I used the Nav Light switch to control the electric fuel pump I left the fans on the original power feed and used the original fuel pump lead to power a switched cigarette lighter receptacle on the ELT antenna bracket. I use it to power a backup ADS-B receiver.

Out of curiosity: for you guys running with the original configuration of a continuously running electric pump, don’t you worry about an undetected mechanical pump failure?

Rich
 
Randy,

The Nav Light circuit is completely different from the AV fans. When I used the Nav Light switch to control the electric fuel pump I left the fans on the original power feed and used the original fuel pump lead to power a switched cigarette lighter receptacle on the ELT antenna bracket. I use it to power a backup ADS-B receiver.

Out of curiosity: for you guys running with the original configuration of a continuously running electric pump, don’t you worry about an undetected mechanical pump failure?

Rich

For those with a non-switched electric pump, they can confirm operation of both by confirming electric pump pressure before starting engine. After starting engine, pressure should increase if mechanical pump is working. I do agree that it would be easy to fail to notice a mechanical pump failure (minor pressure drop) while flying, but should be easy to detect during the next engine start up.
 
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While it’s not a -12, I fly two Rotax LSA on a regular basis in Las Vegas, and haven’t had any problems with vapor lock on hot days. Is the -12 really that susceptible to vapor lock? And if the engine can’t run on the mechanical pump alone, and is relying on that $40 electric pump to keep the fan up front turning, why don’t they have a backup in the event that the main electric pump quits? Maybe even go a step further, and may I ask if the mechanical pump alone can’t keep the engine running, what is the purpose of even having the mechanical pump anyway? Just have two electric pumps? Sorry for my lack of knowledge on this, I’m genuinely curious about this. Thanks.
 
I turn on Master switch and both ignitions before I climb into the airplane. I get serenaded by the oscillating pump. Electric fuel pump gets to run a few minutes before engine start to recirculate fuel FWF back to the tank. On successive hot-engine starts, I run the electric pump several minutes to stabilize fuel line temperatures. I have installed a fuel line temperature sensor to confirm this. As part of engine start check list, I confirm fuel valve “on” visually, touch big red knob on floor to feel heartbeat of electric pump, and confirm 1 gallon flow rate and 3 psig pressure. Engine start adds the mechanical pump to increase pressure to 5 psig.

I turn ignition switches on early because there is no harm in doing so and to eliminate the embarrassment of cranking the engine with no spark.

I agree with jrock836 that either pump may fail in flight and not be detected until next engine start. Engine start check list will confirm both pumps working.
 
Jim, do you have the upgraded 40135 Facet fuel pump in your airplane as described in Van's Notification Letter 18-07-12?
https://www.vansaircraft.com/service-information-and-revisions/n-2018-07-12/


After reading that "notification" I came away wondering why "vapor lock" wasn't even mentioned by name. Yes, they mention "heat soaked engine", "High OAT", "low fuel pressure" but it seemed to me they didn't want to just come out and call a spade a spade.. With that said, I'm also surprised they didn't issue a Service Bulletin.. It's only a $40 part and a couple of hours of labor.. Since the whole reason for the electric fuel pump is to prevent/reduce vapor lock, switching to a higher pressure pump is still to prevent/reduce vapor lock.. Although it wasn't mentioned, clearly having the electric fuel pump is a redundant fuel pump in case the mechanical pump stops working during flight.

The answer to your question is that the alternate pump was for the most part introduced for a different reason.
That is why the S.B. takes the time to describe a number of different causes for lower than nominal fuel pressure, and why it was specified as optional and not a mandatory replacement.
People that haven't been around the RV-12 and the Rotax 912 wouldn't know that a lot of the specs and details for operation of the engine have evolved over the years.
Van's began working with the engine in about 2005.
A lot of things have changed in that regard since the RV-12 was first offered to the public as a kit almost 15 years ago. The 912 has gone through a revision of a couple of different engine driven fuel pumps (at least one of which outputted a different nominal system pressure), the normal operating range for fuel pressure was revised, etc.
 
The answer to your question is that the alternate pump was for the most part introduced for a different reason.
That is why the S.B. takes the time to describe a number of different causes for lower than nominal fuel pressure, and why it was specified as optional and not a mandatory replacement.
People that haven't been around the RV-12 and the Rotax 912 wouldn't know that a lot of the specs and details for operation of the engine have evolved over the years.
Van's began working with the engine in about 2005.
A lot of things have changed in that regard since the RV-12 was first offered to the public as a kit almost 15 years ago. The 912 has gone through a revision of a couple of different engine driven fuel pumps (at least one of which outputted a different nominal system pressure), the normal operating range for fuel pressure was revised, etc.

Thanks for the reply.. So is it your stance that upgrading to the 40135 fuel pump will have no effect on whether or not an aircraft is is going to experience vapor lock in the future? I'll save the $54 if there's no benefit related to lessening the chance of vapor lock..
 
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Randy,

The Nav Light circuit is completely different from the AV fans. When I used the Nav Light switch to control the electric fuel pump I left the fans on the original power feed and used the original fuel pump lead to power a switched cigarette lighter receptacle on the ELT antenna bracket. I use it to power a backup ADS-B receiver.

Out of curiosity: for you guys running with the original configuration of a continuously running electric pump, don’t you worry about an undetected mechanical pump failure?

Rich

Rich -

It has been my personal experience that when an engine driven pump fails, the engine usually fails. That includes a Rolls Royce engine on a 757 with 6 wing tank pumps.
 
Randy,

The Nav Light circuit is completely different from the AV fans. When I used the Nav Light switch to control the electric fuel pump I left the fans on the original power feed and used the original fuel pump lead to power a switched cigarette lighter receptacle on the ELT antenna bracket. I use it to power a backup ADS-B receiver.

Out of curiosity: for you guys running with the original configuration of a continuously running electric pump, don’t you worry about an undetected mechanical pump failure?

Rich

I suppose we could worry about an undetected mechanical fuel pump failure, but then, I'm sure that by now, there'd have been more comments about it in Rotax 912 forums, and planes that run the Rotax 912 series engines having more unexpected emergency landings, from failed mechanical fuel pump systems, since almost all other 912's run on strictly the mechanical fuel pump for fuel delivery.

However, the statistics for such occurrences doesn't seem to be showing up on the internet? Knowing how aviators communicate... IMHO, based on my limited observations, it appears to be a non issue, at this point in time, with Mogas, or avgas or E10 fuels.
 
While it’s not a -12, I fly two Rotax LSA on a regular basis in Las Vegas, and haven’t had any problems with vapor lock on hot days. Is the -12 really that susceptible to vapor lock? And if the engine can’t run on the mechanical pump alone, and is relying on that $40 electric pump to keep the fan up front turning, why don’t they have a backup in the event that the main electric pump quits? Maybe even go a step further, and may I ask if the mechanical pump alone can’t keep the engine running, what is the purpose of even having the mechanical pump anyway? Just have two electric pumps? Sorry for my lack of knowledge on this, I’m genuinely curious about this. Thanks.

The RV-12 has NOT shown itself to be any more susceptible to vapor lock than other aircraft using the 912ULS engine.

The RV-12 engine will continue to run if the back-up pump failed (and vise versa) as long as the airplane isn't being operating with an inappropriate fuel (winter blend auto fuel on a hot summer day with a high fuel temp.).
That is not any more true than if the back-up pump on the airplanes you now fly failed (I am assuming they have one because it is a design requirement specified by Rotax).

It is far to much info to write an a single post here in the forum, but if you desire a deeper understanding, I suggest a bit of research on winter blend fuels / fuel vapor pressure, and Rotax 912ULS / vapor lock.

In a not shell, considering the size of the RV-12 fleet (now 700+ airplanes flying) and size of the LSA fleet as a whole which predominately uses this engine, it is not a common problem, but it does happen on occasion. Following proper steps regarding fuel grade to match the current flight conditions and the recommendations in an aircrafts POH, particularly related to operating in extreme temp. conditions, mitigates any vapor lock tendency.

BTW, you didn't mention what fuel is used in the airplanes you fly. There is a huge difference between winter blend auto fuel and 100LL, regarding the likelyhood of vaporlock.
 
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Thanks for the reply.. So is it your stance that upgrading to the 40135 fuel pump will have no effect on whether or not an aircraft is is going to experience vapor lock in the future? I'll save the $54 if there's no benefit related to lessening the chance of vapor lock..

If you research the science of vapor lock, you will find that the higher the system pressure is, the less likely it is to occur.
So in any given system, if the nominal pressure is a bit higher in the acceptable range with one pump than it is with another, it would probably give a slight improvement regarding the likelihood of a vapor lock occurrence.
Changing pumps should not be consider a reason to develop a casual attitude to considerations of what fuel is in the tank relative to the current operating conditions.
 
If you research the science of vapor lock, you will find that the higher the system pressure is, the less likely it is to occur.
So in any given system, if the nominal pressure is a bit higher in the acceptable range with one pump than it is with another, it would probably give a slight improvement regarding the likelihood of a vapor lock occurrence.
Changing pumps should not be consider a reason to develop a casual attitude to considerations of what fuel is in the tank relative to the current operating conditions.

Thank you.. That was my understanding, but wanted to make sure I wasn't misinterpreting something..

New fuel lines going in (over 5 years old). Cleaning all the filters. New engine driven pump (over 5 years old). New electric pump. Running 100LL / Mogas mixture. Plane is stored in hanger, but will cover the canopy when left outside to minimize fuel in tank being heated. Will leave the cowling oil fill door open when stopping for a quick refuel on cross country trips. Hopefully all of this will minimize any issues this summer.
 
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John,

I hope you’re wrong in the case of my 12 and Cherokee! Why have a backup pump if it can’t keep the fan turning? I am familiar with power plant fluid systems that employ booster pumps necessary for net positive suction head at the main pump, and my interpretation of what Scott has said is that Vans intended constant electric pump operation to fulfill that role. What I personally have found in operation of my airplane is that the electric pump is only needed in that role during hot summer days. If I had found otherwise I would have immediately put it back in the original configuration and found some other creative role for that Nav light switch.

Rich
 
John,

I hope you’re wrong in the case of my 12 and Cherokee! Why have a backup pump if it can’t keep the fan turning? I am familiar with power plant fluid systems that employ booster pumps necessary for net positive suction head at the main pump, and my interpretation of what Scott has said is that Vans intended constant electric pump operation to fulfill that role. What I personally have found in operation of my airplane is that the electric pump is only needed in that role during hot summer days. If I had found otherwise I would have immediately put it back in the original configuration and found some other creative role for that Nav light switch.

Rich

Well I can only relay my one and only experience with an engine driven fuel pump failing. Not a 912 of course. And the fuel pressures required in turbofan engines are prob substantially more than those required in prop engines i would think.

Guess it maybe depends on the manner of the failure inside the pump itself?

Great article from Rainbow on the 912 fuel pump...love the cutaways:

https://electricmotorglider.com/wp-content/uploads/2018/09/912-Fuel-Pump-Web.pdf
 
John,

The comment on failure mechanism got me thinking. The aircraft fuel systems I have seen (granted a limited sample) put the boost pump in series with the mechanical pump. When I built my first home built it was a WAR FW-190 with a gas tank just forward of the instrument panel.The O200 had no mechanical fuel pump since it came off a Cessna 150. I designed the fuel system with a continuous run electric pump and a backup in parallel with check valves to prevent back flow. It turned out that it was better off to just use gravity feed. Running the electric pumps tended to make the engine run rough. I wonder if there is a credible mechanical pump failure mode that wold block booster pump flow on the 912.

Rich
 
The engine driven fuel pump on a Rotax 912 ULS is very similar to the pump traditionally used on Lycomings for decades. The internal check valves operate the exact same way and there’s virtually no way for it to fail and restrict fuel flow, Which is why they have been used for so long on Lycomings.
 
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fuel pump switch?

It's interesting how old threads fade away and then rise from the dead several years later. I'll restate what I posted long ago. When I took the Rotax inspection course the experienced instructor advised me to put a switch on the electric pump and turn off the pump during cruise flight. So I did. This advice seems to be accepted general practice for Rotax 912 engines, but not appropriate for the RV-12 because of the fuel tank location. Eventually I had momentary low fuel pressure warnings a couple times and sought education from VAF forums. These convinced me that the fuel pump should indeed be left on all the time on this fuel system configuration, and I have done so ever since. However, it is nice to be able to turn the switch off and not have to listen to the chattering pump when I am tinkering with the avionics.
 
I'll restate what I posted long ago. When I took the Rotax inspection course the experienced instructor advised me to put a switch on the electric pump and turn off the pump during cruise flight. So I did. This advice seems to be accepted general practice for Rotax 912 engines, but not appropriate for the RV-12 because of the fuel tank location.

I think it would be inappropriate for any aircraft where the fuel storage is below the level of the engine drive fuel pump and / or aux pump (as in probably any low wing airplane powered with the 912ULS).

Do you recall the reasoning given by the person that recommended turning off the pump in flight?
 
I think it would be inappropriate for any aircraft where the fuel storage is below the level of the engine drive fuel pump and / or aux pump (as in probably any low wing airplane powered with the 912ULS).

Do you recall the reasoning given by the person that recommended turning off the pump in flight?

I'm also curious what is different about the RV-12 fuel system design versus other low wing LSA aircraft. A quick search for online Operating handbooks for some other low wing LSA aircraft with 912ULS engines yielded the following information.

1. Sling LSA- 912ULS- Electric fuel pump on for takeoff, and off during climb (Not lower than 300 feet AGL)

2. Sport Cruiser - LSA- 912ULS- Electric fuel pump OFF after engine start.....not used for takeoff or inflight except for where called for by checklist

3. Rans S19 - 912 ULS - Fuel pump on Before Takeoff. Cruise flight - Fuel Pump Off and monitor fuel pressure.

Disclaimer- We have an RV-12 running in the stock configuration with the fuel pump ON all the time and have no intention of changing it. Vans is very firm on the fact that the pump needs to be run 100% of the time, so we do. We also run UL94 fuel exclusively.

But it does bring up a few questions...

1. Why does the RV-12 NEED to do this when other manufacturers do not with their airframes? It it mainly the location of the fuel Tank in the -12 versus other designs?
2. Do any other manufacturers recommend the use of the electric fuel pump 100% of the time on the 912ULS?
 
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I'm also curious what is different about the RV-12 fuel system design versus other low wing LSA aircraft.

1. Why does the RV-12 NEED to do this when other manufacturers do not with their airframes? It it mainly the location of the fuel Tank in the -12 versus other designs?

I'm not going to claim that I understand and know all of the various issues, but this is what comes to mind for me..

RV12's fuel tank is lower than the carbs and engine driven pump, so yes tank position / head pressure might be one of the factors.

RV12's fuel tank is inside the cockpit and the fuel inside the tank is subject to being heated under the clear canopy and under a hot sun. Starting out with fuel that is already heated before reaching the engine compartment can't be good! Highly recommend covering the canopy to reduce temps in the cockpit when parked, or leaving the canopy cracked open to help ventilate the cockpit. Also not a bad idea to throw a towel over the fuel tank, if it isn't already covered with carpet.

Every model of LSA has different engine cowlings and cooling, or lack thereof. RV12's under cowling temps can continue to rise after being parked and can clearly cause engine heat soak of the fuel lines. That's why it's recommended to leave the oil door open if you're planning to fly again soon after landing. Those fuel lines are at the very top of the engine next to the cowling where temps are the highest.

Leaving the electric pump on at all times slightly increases the fuel pressure. Higher fuel pressure reduces the chances of vapor lock in any fuel system.

Age and type of gasoline used may have an effect on vapor lock. This includes winter mix vs. summer mix. (Not RV12 specific)

Outside air temp can have an effect on vapor lock. (Not RV12 specific)

Taxi habits (engine speed / time) can have an effect on vapor lock, especially if the RV12 cowling design and cooling efficiency is part of the issue.

There are lots of variables involved and I think that is why it has been nearly impossible to say any one factor is the cause of vapor lock in the RV12. I believe it's a combination (perfect storm) of many things and it's why it's important to recognize the issue and do things such as running the fuel pump 100% of the time, keeping the cockpit cool, try to use fresh fuel (I tend to mix 100LL and Mogas), leave the oil door open when making fuel / lunch stops, just in case the perfect storm happens.

Hard to say why other companies do or don't do something.. Maybe some have a smaller number of flying planes and haven't seen the problem often enough (or ever) to cause alarm. Or maybe due to tank location, cowling design it hasn't been an issue..

I'm sure I have probably missed a few other issues that could be specific to the RV12 that others might also cover. The fact remains that RV12s have had some incidents involving vapor lock, or suspected vapor lock, so doing a few things like running the fuel pump 100% of the time makes sense to me..
 
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I'm not going to claim that I understand and know all of the various issues, but this is what comes to mind for me..

RV12's fuel tank is lower than the carbs and engine driven pump, so yes tank position / head pressure might be one of the factors.

RV12's fuel tank is inside the cockpit and the fuel inside the tank is subject to being heated under the clear canopy and under a hot sun. Starting out with fuel that is already heated before reaching the engine compartment can't be good! Highly recommend covering the canopy to reduce temps in the cockpit when parked, or leaving the canopy cracked open to help ventilate the cockpit. Also not a bad idea to throw a towel over the fuel tank, if it isn't already covered with carpet.

Every model of LSA has different engine cowlings and cooling, or lack thereof. RV12's under cowling temps can continue to rise after being parked and can clearly cause engine heat soak of the fuel lines. That's why it's recommended to leave the oil door open if you're planning to fly again soon after landing. Those fuel lines are at the very top of the engine next to the cowling where temps are the highest.

Leaving the electric pump on at all times slightly increases the fuel pressure. Higher fuel pressure reduces the chances of vapor lock in any fuel system.

Age and type of gasoline used may have an effect on vapor lock. This includes winter mix vs. summer mix. (Not RV12 specific)

Outside air temp can have an effect on vapor lock. (Not RV12 specific)

Taxi habits (engine speed / time) can have an effect on vapor lock, especially if the RV12 cowling design and cooling efficiency is part of the issue.

There are lots of variables involved and I think that is why it has been nearly impossible to say any one factor is the cause of vapor lock in the RV12. I believe it's a combination (perfect storm) of many things and it's why it's important to recognize the issue and do things such as running the fuel pump 100% of the time, keeping the cockpit cool, try to use fresh fuel (I tend to mix 100LL and Mogas), leave the oil door open when making fuel / lunch stops, just in case the perfect storm happens.

Hard to say why other companies do or don't do something.. Maybe some have a smaller number of flying planes and haven't seen the problem often enough (or ever) to cause alarm. Or maybe due to tank location, cowling design it hasn't been an issue..

I'm sure I have probably missed a few other issues that could be specific to the RV12 that others might also cover. The fact remains that RV12s have had some incidents involving vapor lock, or suspected vapor lock, so doing a few things like running the fuel pump 100% of the time makes sense to me..

Joe.....

These are all very possible reasons.

The question I posed was in response to Scott's assertion that "I think it would be inappropriate for any aircraft where the fuel storage is below the level of the engine drive fuel pump and / or aux pump (as in probably any low wing airplane powered with the 912ULS)."

That statement is clearly not the case with other examples of low wing 912ULS powered aircraft. The air frames above have hundreds of flying examples between them, so are also a fairly large population sample to go off of that also are flying around.

We have never experienced vapor lock in our RV-12. But, it also doesn't get flown in the extreme heat I am sure many of them do. Our problem is the cold! :) But the Thermostasis takes care of that!!!!

Steve
 
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Joe.....

The question I posed was in response to Scott's assertion that "I think it would be inappropriate for any aircraft where the fuel storage is below the level of the engine drive fuel pump and / or aux pump (as in probably any low wing airplane powered with the 912ULS)."

I gotcha.. Sorry, I honestly didn't even notice you were directing it towards Scott..

When do the new and improved INSULATED RV12 specific fuel lines get released? :)

Have a great day.. Joe - -
 
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I gotcha.. Sorry, I honestly didn't even notice you were directing it towards Scott..

When do the new and improved INSULATED RV12 specific fuel lines get released? :)

Have a great day.. Joe - -

Joe,

No worries at all. I appreciate your response and think it brings up some great points as to why the RV-12 may experience some of these issues.

The mystery that remains is why other designs don't experience these issues. Perhaps like you said it is a perfect storm of variables coming together in the -12. I love the plane. The question I have wondered from time to time is if running the electric fuel pump 100% of the time is a "bandaid" or a design "feature". :)

As far as the new insulated fuel lines...... how did you hear about that? We are working on a dry ice air conditioning unit for the entire cabin. Benefit is that it keeps not only the fuel lines cool, but also the entire cabin nice and cool in summer. The disadvantage...... turns the entire cockpit into a foggy IFR mess. :):):)

Steve
Steve
 
All of those airplanes were designed by different people, and different people have different opinions about how things should be done. Look at the amount of discussion that's gone on here between the leave-it-on and switch-it-off camps. The design engineers are not omniscient... they're people, and people make decisions based on their own priorities, opinions, and experience. Yes, even when they have more education in a specific area than the average cat.
 
An RV-12 engine will not automatically quit if the aux pump is shut off in flight.

This is obvious since there are some owners that installed a switch and purposely choose to fly with the pump shut off.

Anyone with an understanding of fuel vapor pressure and the issues that it can (Note I didn't say "will") cause, would probably also be able to reason that the pump running all the time is a conservative approach to the fuel system operation.

Particularly in the context of LSA by design being an airplane that is supposed to be a low pilot work load / easy to fly airplane. Since all of the other relevant details have been discussed multiple times in the past I see no reason to do so again now.

As a side note, it is hard not to think that an aftermarket parts supplier asking the questions that they have in that last few posts, must be doing so for some reason other than asking for some explanation, when they have been active in many of the fuel system discussion threads in the past.
 
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As a side note, it is hard not to think that an aftermarket parts supplier asking the questions that they have in that last few posts, must be doing so for some reason other than asking for some explanation, when they have been active in may of the fuel system discussion threads in the past.

Scott

Sorry to disappoint! No secret squirrel Skunkworks projects going on here! :) Our Rotax 912ULS fuel and oil hose installations for the RV12 and many other aircraft are fully mature with many hundreds currently flying.

I greatly respect Rian, the Chief Engineer of Vans. From the post that started this thread, he made it very clear that RV-12 owners have experienced partial power loss when turning off the electric fuel pump, and that it must be run 100% of the time.

Since we personally own and operate an RV-12, (a strong reason for my interest), doing everything possible to understand that situation and avoid putting yourself in it is a smart PIC decision. Maybe it is as simple as the tank location in the RV-12 puts more strain on the mechanical pump than in other aircraft.


I think it would be inappropriate for any aircraft where the fuel storage is below the level of the engine drive fuel pump and / or aux pump (as in probably any low wing airplane powered with the 912ULS).

Do you recall the reasoning given by the person that recommended turning off the pump in flight?

Multiple aircraft manufacturers with 912ULS engines in low wing aircraft specify to turn the pump OFF in their POH. Leaving the pump on all the time as VANS does in the RV-12 likely ran contrary to what the instructor was used to with other low wing Rotax 912 installations.
 
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So the designer's intent is to have the fuel pump operating any time the engine is running, and the objection of some owners is that having the pump going while doing ground maintenance is annoying. To me that suggests a solution where in the pump is powered not through the master switch but through the ignition switches (double-pole). Then you could update your databases in peace but still have the pump running any time you were actually intending to fly.
 
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