In line fuel filter question

MSFT-1 · #1 08-30-2009, 10:24 PM

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,

glenn72pc · #2 08-31-2009, 06:53 AM

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 !!

MSFT-1 · #3 08-31-2009, 07:18 AM

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

Don at Airflow · #4 08-31-2009, 07:45 AM

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

MSFT-1 · #5 08-31-2009, 07:58 AM

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.

rleffler · #6 08-31-2009, 08:42 AM

Quote:

Originally Posted by MSFT-1

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

frazitl · #7 08-31-2009, 08:45 AM

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...

johngoodman · #8 08-31-2009, 08:55 AM

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

John

glenn72pc · #9 08-31-2009, 11:32 AM

does any one know what the micron size in the filter vans is using on the rv10 fuel filter?

frankh · #10 08-31-2009, 12:06 PM

Quote:

Originally Posted by johngoodman

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