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In line fuel filter question

MSFT-1

Well Known Member
I spent the weekend with my A&P working on the annual inspection of my RV-10. As I mentioned in a previous post, he wanted to inspect the fuel filter (the RV-10 does not have a firewall mounted gascolator).

After disassembling the center console and taking a look down in the tunnel it was clear that it was going to be a real bear to get to the fuel filter and remove it.

So the questions:

1) Why does the RV-10 have an inline fuel filter instead of a gascolator? According to my A&P, the gascolator is a proven design and is ridiculously easy to inspect and maintain. Is there some advantage to the inline fuel filter in this application? Could I remove it and install a gascolater instead?

2) Other's have mentioned putting an access panel on the side of the tunnel. Any thoughts on the size of the access panel?

3) Is there a recommended periodicity for checking/replacing the inline fuel filter?

thanks,
 
fuel filter

why not have two fuel filters one under the right seat and one under the left seat why? easy to get to with the large panel under the seat and a filter for each tank less heat in that area !!
 
That is a great question

My A&P friend has some thirty years of small plane experience (and has owned several two seat RVs himself).

When he looked at the location of the fuel filter he just shook his head and said roughly "this is one of the dumbest things I have seen".

He said the same thing when I showed him the procedure for removing the front seats.

I kind of like your idea actually. Anyone see any downside to doing that other than the cost of two filters?

bruce
 
No gascolator on FI engine.

Before you?re A&P gets all cranked up about the location of the fuel filter on your 10, and the virtues of gascolators, let me attach a letter I wrote to another gentleman about the subject. Maybe if you show this to you?re A&P he?ll realize there?s more than one reason why we to do things a different way. Just because a process has been around for 70 years it may or may not apply to new technology. If you have, or you?re A&P has additional questions on this subject, feel free to give me a call.

"Per our phone conversation I will reiterate what we discussed concerning gascolator installations on fuel injected aircraft.

To fully understand the situation, you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.

Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.

One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.

Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.

1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated."

Hope this helps. We hear a lot about this topic.

Don
 
Thanks for thi s information

Don:

I appreciate your thoughts and guidance. A couple more questions for you:

1) You mention one of the reasons for a fuel filter is because of the low tanks compared to the engine. This is true on all the RVs. On my RV-8 (sold a couple years ago) it had a gascolator (which as far as I know was the "normal" design). Why is the RV-10 the only one that went away from the gascolator?

2) You mentioned that aerobatics are another reason to use an inline filter. The RV-10 (and the RV-9) is not aerobatic so this does not seem to apply. Am I missing something?

3) Another poster suggested using two inline filters (one for each tank) and put them under each front seat (very accessible). Is there any problem doing that (other than the cost)? Not sure I want to, just wondering.

thanks.
 
2) Other's have mentioned putting an access panel on the side of the tunnel. Any thoughts on the size of the access panel?


Bruce,

In the other thread, I provided this link to Airward's Access Panel Kit. If you don't want to purchase this kit, Dave has enough information online that it's pretty easy to reverse engineer it.

I purchased the kit. By the time you buy the materials, it wasn't worth the time for me to reverse engineer or come up with an original design. It depends on how much you value your time as to which option may be best. It took less than an hour to install.

But to answer your question directly, it's 11 x 7.

bob
 
Vans design for my injected 7A

has no gascolator. I wanted one on the cool side of the firewall and very low in the system, so I replaced the AFP filter with an Andair gascolator in basically the same location. Andair's gascolater is OK with the fuel pressure and has a very nice fine filter screen, much better than the standard gascolators out there. The fit was tight, but I can now check for water there and service the filter without getting fuel all over the inside of the plane. In my opinion, the AFP filter is a poor design in that respect (though they have way more experience with fuel systems than I). To make this easy, I added a rectangular cover plate to the bottom of the fuselage. I still have to take the cover off inside the to remove the filter, but it's relatively easy on the 7...
 
Careful

Bruce,
I plan on putting one of these in each wing root:
http://www.summitracing.com/parts/SUM-230100/

John

Those filters have a very small surface area and could easily plug with contaminated fuel. Remember a partial plugging will drop the fuel pressure dramatically and could easily lead to vapour locking...which can ruin your whole day.

I run a wingroot electric fuel pump system (no mechanical pump) and I use those filters on the DISCHARGE side of my pumps. The only reason they are there is if a chunk of something broke off a pump..To be honest if I were doing it again I wouldn't bother installing them.
On the inlet side of the pumps you really want a much larger surface area...Exactly what the AFP system provides.

Personally however I want a filter on each tank...remember if a single filter system plugs the fuel flow will be interrupted. I concede that this is a very small likelyhood however.

Filter location is never easy as it is dicated by the fuel system design. In my case the filters are in the wingroots but the filter are should be large so should only be a once per year affair.

Frank
 
There is only one RV-10 flying in the UK and he has modified the fuel system to use the Andair Gascolator rather than the in-line filter. Nothing to do with water collection - he has put it in the tunnel. His logic is based on 3 premises:

- the filter is finer
- the filter is removeable and cleanable without disconnecting pipes
- water collects at the fuel drains

I intend to mod the same way.
 
Heat, heat, heat

Don:

I appreciate your thoughts and guidance. A couple more questions for you:

1) You mention one of the reasons for a fuel filter is because of the low tanks compared to the engine. This is true on all the RVs. On my RV-8 (sold a couple years ago) it had a gascolator (which as far as I know was the "normal" design). Why is the RV-10 the only one that went away from the gascolator?

2) You mentioned that aerobatics are another reason to use an inline filter. The RV-10 (and the RV-9) is not aerobatic so this does not seem to apply. Am I missing something?

3) Another poster suggested using two inline filters (one for each tank) and put them under each front seat (very accessible). Is there any problem doing that (other than the cost)? Not sure I want to, just wondering.

thanks.

Basically what I?m saying here is that with a fuel injected engine using Airflow Performance or Precision fuel injection we do not recommend installing a gascolator on the firewall for the reasons stated in #3 above. The RV-10 is not the only RV we do not recommend using a gascolator on. All RV?s that are fuel injected come under that statement and Lancair?s, Glasair?s etc. Maybe your RV-8 was carbureted. If so then, revert to the discussion about why there are gascolators with carburetors. Aerobatics or not, heating the fuel is the problem with this type with FI so the less we expose the fuel to heat the better the fuel injection system will operate.

You can use a filter at each tank if you want. Some customers have.

The filter we use which was designed around our fuel injection system (which has other filters in the system) and is designed as a suction filter. Therefore to satisfy the total flow of our boost pump, a 125-micron filter was used. This has proven to be of adequate filtration to protect the boost pump and not restrict the fuel flow to the engine driven pump when the boost pump is off. In some specialized applications we have changed this filter to a 40-micron filter. But of course in these applications further maintenance is required to insure the filter is not subject to debris restricting the fuel flow.


Don
 
Drifting a bit----

Don, in regards to the fuel heating, what is your take on using a shroud and blast tube on the mechanical fuel pump??

Thanks in advance.
 
Heat and more heat

Add some suction and a higher vapour pressure fuel (namely mogas) and thats really the whole story of potential vapour lock.

Anything you can do to get away from the ugly perfect storm above the better.

Using 100LL, a fuel pump cooling shroud, avoiding a gascolator north of the firewall or avoiding a mechanical pump altogether reduces the risk of vapour lock.

Vitally important to understand the physics of the situation before modifying any standard fuel system however.

Frank
 
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We added two fuel filters (one each side) between the tanks and the fuselage. Very easy to install and very easy to change.
 
gascalator

placing a filter or gascolator on the suction side of the electric boost pump is dangerous because any contamination/obstruction will cause vapor lock.
simply said--the pump can push quite well, but is very limited in suction due to simple physics and vapor pressure of fuel.

i totally disagree with don concerning gascolators on the firewall. he just stated that the boost pump is capable of 30 psi which will guarantee that the boost pump can come to your rescue if the engine pump starves. i maintain that the gascolator is much safer due to preflight sampling, which no one can do on a regular basis to the fuel filter in the tunnel. How can anyone be absolutely certain that the filter is not clogged to some degree when it is located within the tunnel?

As far as heating of the gascolator, i can't see it being a problem when the volume of fuel is so small that the exchange rate with fresh fuel does not allow for significant heating. All this assumes fire sleeve on fuel lines and reasonable care in locating the gascalator where no additional heat is directed at it from the oil cooler, etc.

again, i think all builders should seriously consider the ramifications of installing any kind of filter on the suction side of the boost pump and also should consider the simplicity and long proven reliability of a gascolator.
I believe most filter installations are maintenance traps.

warren
 
I disagree

yes ideally one would NOT have any kind of potential restriction on the inlet to a pump. With a low pressure Facet pump you can in fact get away with doing this.

However the high pressure rollervane pumps have to have absolutely clean fuel or they will wear out fast. Remember the standard fuel system is really dependant on that boost pump...More to the point I would not try to take off without it running on a standard set up.

Secondly the filter area on the filter that AFP rovides is enourmous. It would take a lot of debris to actually plug it and the clean pressure drop is negligible. In reality our fuel supplies (whether mogas or 100LL) are also very clean.

You can also run a filter prior to each pump for further protection, but the bottom line is running a high pressure pump without a filter is a bad idea.

Frank
 
i maintain that the gascolator is much safer due to preflight sampling, which no one can do on a regular basis to the fuel filter in the tunnel. How can anyone be absolutely certain that the filter is not clogged to some degree when it is located within the tunnel?

Well my RV has a gascolator and I used to sump it every flight, now I do it once every couple of weeks. 300ish hours (including flights in mexico and hand fueling at burning man) - I still haven't found anything in there other than fuel.

A poll of the local RV pilots and they have never found anything there either.

In the wing sumps I'll occasionally find a bit of water or debris, but the gascolator seems to be too high to do much good.
 
Did you ever get a response to your 3rd question regarding frequency of replacement? Must be a function of fuel quality; anyone have experience cutting one apart and sampling content?

I spent the weekend with my A&P working on the annual inspection of my RV-10. As I mentioned in a previous post, he wanted to inspect the fuel filter (the RV-10 does not have a firewall mounted gascolator).

After disassembling the center console and taking a look down in the tunnel it was clear that it was going to be a real bear to get to the fuel filter and remove it.

So the questions:

1) Why does the RV-10 have an inline fuel filter instead of a gascolator? According to my A&P, the gascolator is a proven design and is ridiculously easy to inspect and maintain. Is there some advantage to the inline fuel filter in this application? Could I remove it and install a gascolater instead?

2) Other's have mentioned putting an access panel on the side of the tunnel. Any thoughts on the size of the access panel?

3) Is there a recommended periodicity for checking/replacing the inline fuel filter?

thanks,
 
fuel filter

the planes that have two filters one for each tank are you using
a shut off valve before the filter so when you service the filter
the tank does not drain ?
 
Don't over complicate things

the planes that have two filters one for each tank are you using
a shut off valve before the filter so when you service the filter
the tank does not drain ?

Glenn,
Adding shut off valves just adds weight and another potential leak into the system. During filter cleaning you can do one of two things.

#1 Check the filters with nearly empty fuel tanks. I doubt the tanks would siphon fuel out when the level is below 1/3rd full. I'm not familiar with the RV10 setup, but on the single and two place RVs, fuel level would need to be above 1/3rd full for fuel to flow out of the tanks.

#2 Have on hand one or two 3/8" [AN6] line plugs to seal the line while you have the filters off. These can be purchased at any hydraulic supply store. Aircraft spruce also carries them. See

http://www.aircraftspruce.com/catalog/hapages/an806-2d.php

Most Ford automotive fuel filters come with plastic or rubber caps installed on the fuel filter. They are installed to keep dirt out of the filter before use. They will also work as a low cost alternative. I'll bet that your locate auto mechanic has a bunch of these in his tool box. I sure do.
Charlie Kuss
C&E Automotive owner
 
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Airflow fuel filter periodicity replacement

Hi,

Followup to this thread...
What is the periodicity for checking/replacing the Airflow inline fuel filter?
Could not find any answer to that...

THanks
Rickard
 
Hi,

Followup to this thread...
What is the periodicity for checking/replacing the Airflow inline fuel filter?
Could not find any answer to that...

THanks
Rickard
Vast majority of people including myself do it at each annual, needless to say, after the first year/time, I have hardly ever found anything in the fuel filter.
 
Last edited:
For posterity

Bruce,
I plan on putting one of these in each wing root:
http://www.summitracing.com/parts/SUM-230100/

John

Very bad idea, IMO. My secondhand kit came with these under each seat and it wasn't long before vapor lock made its appearance on 93 mogas. $500 to Aircraft Specialty for quality teflon hoses and 40u Holley filters and several days deep in the bowels of the airframe and the issue is resolved.

Don't cut corners here.
 
Well here's another idea... I removed and checked the filter from the tunnel for the first two condition inspections and found it pristine but a pain to access as others have noted. In addition, I had a slight fuel leak at the inlet fitting which I was ultimately able to stop with a DEL seal. Since the second condition inspection, I have not disturbed the fuel filter in the tunnel. Instead, I disconnect the fuel line at the throttle body and measure fuel flow with the boost pump on. The last check showed 54 gpm, roughly twice the max the engine requires at full power. As long as this number remains stable I do not plan to inspect the fule filter again.
 
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