RV-14 fuel system

I have some concerns about the stock RV14 fuel system and I am looking for some suggestions. The following pic is from the assembly plans



As can be seen in the picture the fuel flows through the valve , through the filter and pump on the way to the firewall. A drop dead simple fuel system.
This is the good news.

The problem I have is that there is no way to sump that filter, or to know if it is getting full of debris.
We have two independent tanks and it seems a shame to not be able to switch tanks if the filter were to plug.
A filter for each tank would be preferable. Yes there are a lot of single point failures on an airplane but why design one in for the cost of an extra filter?

To clean the filter means that you have to "break into" the fuel system. By that I mean that during servicing the fuel line will have to be removed from the back of the filter. This one action opens the door for leaks or foreign mater to get into the system. I do not think that fuel lines were meant to be opened and closed on an regular basis: over time what does this do to the flare? A simple gascolator is designed to come apart for cleaning without removing fuel lines.

The location of the filter is going to be a bit of a bear to get at under the panel. When service items are not easy to service the natural tendency is to "do it later"
I know that no one here is guilty of that but fuel system problems are the number one cause of engine failures.

In Canada we are required to use a gascolator but I really like the idea of a filter before the fuel pump and the stainless steel tunnel precludes putting a gascolator in that area. A filter, or gascolator in each wing root is an option but that adds a lot of complexity and again makes the system difficult to service, which is not desirable.



Has any one thought of an alternate system?

I really can't offer anything at this point. Don't know enough about how to plump a fuel system on an airplane.

But I talked to two RV8 owners today at Triple Tree's RAF flyin; they each told me that their planes did have a gascolator. I am imagining the other RV's do have gascolators or something as good. It seems to me that an easily accessible gascolator would be more than a good idea and I am surprised that the RV14's don't have one.

I'll stay tuned for either an explanation why they don't or how one can or should be incorporated.

Of the thousands of RV's that are flying, a large percentage of the ones with carburetors have a gascolator, and a large percentage of the ones that have fuel injection, have an inline filter configuration like shown in the above RV-14 drawing.

I've also wondered about the effects of repeatedly opening the fuel lines to replace a filter each year. It might be better if you had a short, flexible fuel hose between the selector and the filter instead of the rigid tube per the plans. One concern is how to ensure there are no leaks after reassembly. Since the filter is before the pump, turning the pump on doesn't pressurize the lines in the area that was opened up. Maybe doing the filter change early during the annual would be a good idea, then turn on the pump after reassembly to draw fuel through the new filter, and check carefully a few hours later before putting the cover back on the center console to make sure there's nothing seeping out.

There are pros and cons any way you design the fuel system. If you install a gascolator it needs to be in an accessible location with the drain sticking out the bottom of the aircraft somewhere. If you put it on the firewall it is another place for the fuel to heat soak which can exacerbate vapor lock issues (usually not a problem in flight but rather on a quick turn fuel stop on a hot day where everything firewall forward is getting really hot while you're shut down). On my -8A I put an Andair gascolator in the left wing root. Overall I was happy with it in that location, but it isn't as easy to open up to service at annual as it would be on the firewall.

The question of whether a gascolator is even needed has been debated in the past on the forums. Most certified aircraft have them, but I was told once by a Grumman owner that the Yankee and Tiger models don't have them. One nice thing about the RV is that the tank sumps are in the very lowest corner of the tank, the wing has enough dihedral to make water move toward that corner, and there are holes very near the aft lower corner of each rib that allow the water to move mostly unimpeded. By contrast, some aircraft like high wing Cessnas have almost no dihedral and to make matters worse some of them use a bladder instead of a wet wing...the bladder tends to have some wrinkles and the result is areas that may keep water from moving to the wing sump while sitting on the ground. So in that design a gascolator somewhere downstream from the tanks is probably a good idea. My personal experience after flying my -8A for a decade is that I never found even a drop of water in my gascolator, and only once found a couple very small drops of water when sumping the tanks. That was after the airplane sat out for a week at Oshkosh and it rained a lot that year, so there was constantly water pooled around the gas caps and I suspect a few drops worked their way in past the o-ring.

I also pulled the gascolator apart each annual and the filter screen was always pretty clean...just a few flecks of dirt here and there.

So, I think overall Vans stock setup for the -14 is pretty good. If you're not seeing any significant water or debris when you sump the tanks, the filter isn't going to clog. If you do see significant water or dirt when sumping tanks, I would not take off until I rocked the wings back and forth and sumped several more times until I was getting clean, uncontaminated fuel out of the sump drain. Large volumes of water or debris can clog up even a gascolator or dual filter design (especially since it's likely that the contaminants were introduced when refueling in which case you've probably got the same issue in both tanks).

As already mentioned, this design aspect of the RV-14 fuel system is nothing new to RV's.
There are RV's that I have been maintaining for 15 + years with this type of a filter installation with no ill effects.
If it ever did become an issue, it would just require making a new short interconnect line which would be a pretty simple process.

BTW, you don't replace these filters. It has a cleanable / reusable screen.

rvbuilder2002 said:

BTW, you don't replace these filters. It has a cleanable / reusable screen.

Click to expand...

Scott, does the filter cartridge disassemble for cleaning or do you just flush it backwards?

I was also wondering if the filter element is just a simple screen or if it's pleated which would provide much more surface area to capture debris before it could become completely clogged up...

It's a bit worse. According to the Precision Airmotive fuel injector installation requirements...

"The fuel supplied to the servo shall be filtered to 32 micron nominal. This filtration may occur prior to the engine driven pump, but all other pumps and valves shall be upstream of the filter."

The RV-14 installation violates this requirement by not having a filter after the boost pump. Add in the Canadian gascolator requirement and it means that a redesign of the fuel system may be at hand.

In my HR-2, I use the 70 micron gascolator prior to the boost pump, then an in-line 40 micron filter after. I have to remove the in-line filter for cleaning at the annual.

Let's narrow the concern to address the potential failure modes.

rvbuilder2002 said:

Of the thousands of RV's that are flying, a large percentage of the ones with carburetors have a gascolator, and a large percentage of the ones that have fuel injection, have an inline filter configuration like shown in the above RV-14 drawing.

Click to expand...

I typically don't like the "thousands are flying" justification answer, but in this case it is appropriate.

I have read about the fuel systems failure mode for many years here - and would conclude most of them were due to poor/accidental assembly errors, or modifications.

Two issues: debris (filtration) and suction head. Unfortunately, the better you filter upstream (before the pumps), then more likely the system is to have higher suction head. If redundancy is the issue, then put the same filter between each tank and the selector valve.

To validate Scott's assertion (thousands . . .) how often has the servo final filter been removed during the annual and found with debris? None that I can find on VAF, but that does not mean it can not happen.

Each new change has new failure modes and consequences, a nice 32 micron final filter after the mechanical pump would do the trick for filtration, but the addition component, and heat, could affect idle fuel temps on hot days.

The filter before the pump is the standard approach. If you get a chunk of crud from the tank you don't want it to go into the pump.

On the filter getting clogged, this risk is significantly reduced if you take standard steps to flush the system before use. Here is a simple process to do your tank sender calibrations, fuel flow checks and flush:
- Level the plane.
- Connect a simple clear plastic inline filter to your firewall side fuel fitting. The function of the filter is to make sure the fuel you are using remains clean, and that you can check for gross contamination as you go.
- Select off on the fuel valve.
- Fill the first tank with fuel, two gallons at a time (or as required by whatever system you are using). When done you have completed the fuel sender calibration.
- After the tank is full (and you know how much fuel you put in it), pump the fuel from the tank using your installed fuel pump and filter, as well as the temporary plastic filter into a series of five gallon gas cans.
- As you pump time how long it takes to fill up a can. This tells you your maximum fuel flow.
- After the tank is empty, sump whatever fuel is still in it. This is your "Unusable Fuel". As you know what you put in the tank you now know your "Usable Fuel".
- Repeat for the other tank.
- When done, remove the installed filter (the one that goes with the plane) and inspect. If you find any crud in it repeat the process.

The objective here is to start off life with clean tanks.

You may want to remove and inspect the filter after the first 10 hours or so to make sure all is well.

I remove and inspect the inline filter in my plane every year. I have never found it to have any crud. To the failure mode discussion, the surface area of the filter is so large that it would take a lot of contamination in the tank to fully clog it. If you had this much debris in the tank I would be amazed of you did not see crud when sumping long before the filter gets clogged.

Carl

There are three or four filters in a typial FI system:
- finger screen in tank,
- gascolator or in-line filter to protect the boost pump from fuel tank debris,
- *in-line filter to protect the FI servo from debris caused by a failing boost pump, and
- final finger screen internal to FI servo to protect the injector nozzles.

*This is the one specified by Precision Airmotive and the one most likely to be omitted, as shown in the RV-14 documentation.

By omitting the (large area) filter after the boost pump, we are taking the chance that the pump won't fail and shed debris that will plug the final (small area) finger screen in the servo.

So I guess the question is: what are the failure modes of the electric boost pumps?

Fuel system

Here's what I wrote back in 04 07 2013 on the fuel system for my RV-9A. Unfortunately still not flying.

I went through this too. When I discussed it with Precision Airmotive, they replied as follows:
"We recommend a nominal 32 micron filter because the finger screen in the inlet of the servo is 70 micron and is not meant to be a primary filter. I agree that most certified aircraft should have a similar filter installed, but I have no way of knowing what they use as they are separated from us by the engine manufacturer which gives engine installation specs. Also from our experience, most do not use a sufficiently fine filter".

It's also worth noting that the finger screen in the inlet of the servo does have a bypass, and so if it gets clogged with 70 micron particles, it bypasses and lets any contamination downstream through to the spider and nozzles.

After a LOT of investigation of certified aircraft fuel systems, and available components I ended up with the following system.
Andair GAS375 gascolators (70 micron) in each wingroot
Andair fuel tank selector valve
Weldon 18000A high pressure fuel pump mounted in the usual place ahead of the selector valve in the tunnel
Falls Filtration 02W05847 main filter mounted approximately where the Van's gascolator for a carb engine is mounted right side low forward of the fireeall. This particular filter is normally used on the Beech King Air.

The specs on the filter are:
Fluid: Avgas or Jet Fuel
Flow Rate: 600 pph, using Jet fuel this works out to 85 GPH
Pressure rating:
-Operating 75 psi
-Proof 150 psi
-Burst 225 psi
Filtration:
-17 micron nominal
-33 micron absolute
Pressure drop: 0.5 psi max at rated flow
Bypass valve cracking pressure: 1.8-2.2 psi

I think most FBO fuel systems have 32 micron or better filtration, but as protection against those that don't or for fueling from a drum I prefer to have an adequate main fuel filter with good contaminant filtration and dirt holding capacity. YMMV
I'm still working on mounting all this stuff and sorry do not have pix.

BillL said:

I typically don't like the "thousands are flying" justification answer, but in this case it is appropriate.

Click to expand...

Bill, I think it is appropriate in many instances were a builder is contemplating a modification "to make it better".

There are lots of accident reports because someone thought they had a better way for a particular detail that had proven it self over many years and 10's of thousands of flight hrs in thousands of RV's with no reported problems.

I agree it is not necessarily appropriate for all situations but I believe it is for a large majority of them.

For anyone contemplating fuel system modifications, you might check out a series of artciles in Kitplanes that started in the Dec 2105 issue and continued in January and February. Written by a sort of well known airplane designer, it can give you a lot of useful information on which to base design decisions. Or understand the decisions that have already been made my most kit designers.

Paul