Mechanical fuel pump failure?

Fellow RV'ers,

I had a bit scare today: as I was taking off, got low fuel pressure warning and low fuel flow. I turned the electric fuel pump on (have a dedicated switch for that, and normally I keep it on during take off), and everything went back to normal. A few minutes later in flight, while on slow climb, I test again by turning electric pump off, and same thing happened (Rotax 912ULS engine is less than 50 hours) so kept it on and got back to home base (KJYO). Just before touch down, to test it again, turned off electric pump off, this time didn't get the warning. Taxied normally back to parking (with pump off) and did a final high RPM check, still everything seemed normal.

Less significant issue (on the same flight):
Just had my Dynon Skyview Touch serviced and put it back in the airplane and ran setup configured it. Everything worked fine and including autopilot. Set a direct flight plan and wanted to navigate using Autopilot, Roll -> Nav mode. However, NAV mode was not selectable, only HDG and TRK was selectable. Any thoughts?

Any thoughts what happened? Mechanical fuel pump failing or any other theory? I am new owner, so still got lots to learn.

Thank you for your valuable comments.
Regards,
Monir

The reason the RV-12 was designed with a full time operating electric pump is to mitigate vapor lock issues.
The classic instance of high vapor lock potential is late spring hotter weather with winter blend fuel still in the tank.
Are you using auto fuel or 100 LL?

Scott,

I have not put in 100LL yet, using auto fuel. Temp today was around 65F. What's normal practice for RV12, do most use electric pump all the time (I know design is defult full time on, but some, like mine has dedicated elec fuel pump switch).

You think its vapor lock or mechnical fuel pump failing (though as I mentioned engine is only less than 50 hrs).

A pump failure is possible but seems unlikely.
The standard RV-12 design has the electrical on full time for the reason already mentioned.
Stating a cause would only be a guess.
A couple things you could do
Leave the pump on at all times as designed.
Drain the fuel and put in some 100LL and see if the issue reoccurs.

As Scott says? Van?s designed the fuel delivery system for the legacy 12 to have the electric fuel pump running 100% of the time. Van?s designed the fuel system per Rotax recommendations. It?s foolish, and very unsafe, to defeat the electric fuel pump for any reason. It?s important to understand why Rotax and Van?s require this feature. Lycoming and Continental engines have carburetor mounted below the engine where temperatures remain cooler. The 912, with carbs mounted up high on the engine, and nearby hot cylinders, is prone to vapor lock. The 12 is tightly-cowled and retains heat after engine shut down. Heat also builds up during ground operations. The electric fuel pump is added to continually distribute ?cool? fuel through the fuel distribution hoses under the cowling with a return line to the fuel tank. This ?cool? fuel is continually brought to the carbs to mitigate the boiling of fuel in the carb float bowls.

I don?t understand why anyone would want to defeat this safety feature. If the reason is to verify that both fuel pumps are operating prior to takeoff then simply check fuel pressure before and after engine start. The electric pump running by itself should produce ~ 2psig, and when the engine starts, the mechanical pump should add pressure for total of ~ 5psig.

Consider this possibility? someone has added a switch to their RV-12 to shut off the electric fuel pump. Pilot has switch in off position during ground ops and fuel in carb bowls is very near boiling point. Fuel in distribution lines above hot engine is also very near temperature for boiling fuel. Engine runs well at low speeds because very small amount of fuel is required. So, pilot does pre-takeoff checklist and remembers to switch electric fuel pump on and firewalls throttle and begins climb out. What could happen? Carb bowls are at hot temperature and only hot fuel is available as fuel is beginning to vaporized in the float bowls at this critical phase of flight. Cool recirculated fuel needs to be available in abundance during hot climb out.

Piper J3 said:

The 912, with carbs mounted up high on the engine, and nearby hot cylinders, is prone to vapor lock.

Click to expand...

Traditionally, vapor lock refers to the formation of a vapor bubble in the inlet line to a pump, due to the combined effects of reduced pressure and heat.

Boiling the contents of a float bowl is possible, but note the absence of reduced pressure in the bowl. And float bowl boiling would not generate a low fuel pressure warning.

Vapor lock with a 912 is two-fold issue. First, the engine driven pump is forward and high, so the fuel feed line runs the full length of the engine installation, from firewall to nose case, sometimes uphill. There is plenty of opportunity to pick up heat.

Second, the 912 is usually run on mogas. This is spring, when mogas is in transition from winter blends to summer blends. A tankful of winter blend (higher vapor pressure) on a warm day exacerbates the line heating issue.

The RV-12's electric pump location below the tank means it always has positive head pressure at a relatively low temperature, so vapor bubble formation at its inlet is very unlikely. Flip side is that the electric pump, if not operating, further reduces inlet line pressure at the engine driven pump, as the engine pump has to draw fuel through the electric pump check valves, the gascolator, and a long length of line.

Listen to Scott...wire the RV-12 pump to run full time.

An additional thought? Suppose someone adds a switch to shutoff the electric fuel pump and operates the airplane as he sees fit. E-LSA rules say no problem. Maybe the POH is updated to include this new switch and its function ? or, maybe not. Likewise, any checklists may not include the new electric fuel pump switch. For the person who installed the shutoff switch ? maybe no problem.

Then, and this is a big THEN, the plane gets sold and the new owner is maybe unfamiliar with RV-12 or understanding/operation of carbureted Rotax 912. Maybe new owner operates plane like a Cherokee and thinks electric fuel pump is strictly a ?boost? pump. Maybe new owner makes habit of taking off without benefit of electric fuel pump. And, maybe one day, things don?t go well?

Jim, Scott, Dan,

I am never turning that fuel pump off again. Makes sense and thanks for the detailed explanations.

Monir

Mechanical fuel pump failure

I have a question about the cool fuel. First let me say that I have not modified my RV-12 so the electric fuel pump runs all the time. I understand the purpose is to help prevent vapor lock. But my question is doesn't the mechanical fuel pump circulate cool fuel the entire time the engine is running? Is the purpose of the electric pump just to increase the volume of fuel being circulated? Is the volume of cool fuel circulated by the mechanical just not enough?

Thanks.

The electric fuel pump increases the PRESSURE in the line, possibly in addition to the amount of fuel fed and recirculated.

The higher pressurized fuel lines acts to offset the vapor pressure of the Mogas fuel.

Basic chemistry of solid, liquid, and gas or vapor phases of elements and compounds... Think of a balloon and pressure inside the balloon vs outside the balloon at 1 std atm. If you increase the outside pressure, the balloon shrinks in size. Same with vapor lock. Increase the outside pressure with the electric fuel pump, so that it exceeds the ability of the Mogas fuel in question to act like an obstructive "balloon of vapor" not liquid, in your fuel line.

The 912 ULS engine has a fuel return path incorporated into its fuel delivery system.
The return path is via a restrictor orifice that is inside the fuel distribution manifold block on the top of the engine (Clamped to the aluminum balance tube that runs laterally between the two intake manifolds)
The location of this block (and the associated fuel return line) is at the highest point in the RV-12 fuel system which makes it ideal for capturing vapor bubbles and sending them back to the tank.
The flow through the orifice does artificially increase the overall fuel flow a bit as well which also helps keep the fuel cooler but primary reason Rotax says the return is used is to bleed off vapor bubbles.

There is still ~ 18" of fuel line that routes from the block to each carburetor so there is still opportunity for fuel to get additionally heated and not have anywhere for formed vapor bubbles to go.

The electric pump has a different purpose than the fuel return path.
It is a not to increase the volume, it is to assure that as much of the fuel system as possible is always at an elevated pressure.

Vaporization of fuel (in simple terms) occurs when the pressure of the fuel goes below a specific value which is influenced by the vapor pressure rating and temperature of the fuel.

Any time a pump is sucking fuel from a tank, it is lowering the pressure on the suctions side of the system. If the fuel vapor pressure rating, fuel temp., and other factors are just right, vapor will form in the fuel. That vapor will then continue down stream to the pump that is doing the sucking. Once enough vapor enters the pump, the ability of the pump to produce a flow and develop pressure on the output side will begin to degrade. The amount of degradation will depend on how much vapor is being ingested.

So, with the electric pump pump on the RV-12 being just about directly under the fuel tank, it can benefit from gravity to feed the inlet side which reduces the likely hood of there being any pressure drop in that short length of the system as a result of it sucking fuel.
The output of the electric pump is pushing fuel (by using increased pressure) fwd through the system towards the engine drive pump so that for the most part, the engine drive pump is never sucking fuel from the tank. It is just slightly raising the pressure because its pressure output is slightly higher than the rating for the electric pump.

This is the primary reason that modern cars and trucks have the fuel pump located inside of the fuel tank.

The fuel system for the RV-12 Rotax 912 includes a pressure return line to fuel tank. The fuel returns at about 1 gallon/minute regardless if one or both pumps are running. This flow rate is established by use of a restricting orifice in the fuel rail on top of the engine. The orifice limits the flow and establishes the weak (~2 psig) fuel pressure when just the electric pump is running.

So, why run the electric fuel pump by itself? The main problem is actually when the engine isn’t running. Sounds ridiculous, but it isn’t. Suppose you are hopping rides with friends and shut off the engine each time you load passengers. Or, perhaps the airplane is used in a flight school and flown multiple times in short order. The hot engine heat-soaks all the under-cowled components including carbs, mechanical fuel pump, and all the fuel lines. With no fuel flow the propensity for vapor lock is enormous.

If you think you’re going to restart a hot engine – leave the master switch on to allow the electric pump to recirculate fuel forward of the firewall. If airplane has sat for a period of time with hot engine then run electric fuel pump for a few minutes prior to engine start to bring cool fuel into play.

mechanical fuel pump failure

Thanks for the explanations. I understand now and appreciate your help.

Jim,

Your last comment about leaving the master switch and electric pump on is very valuable tips. As I would do that sometime, turn it off take a stroll around some around the airplane and start back up. Will keep the master and elec. pump on if short stops/starts.

And again, a tons of things I learned on this particular thread.
Thank you all.
Monir

I put a switch on my fuel pump too. Here in PHX it gets pretty hot in summer. In summer I run with my electric fuel pump on for taxi, takeoff, climb, descent and cruise until my fuel cools down at altitude. Otherwise I get vapor lock.