Fuel System Questions

[JimParker256]

I've got one of those filters (mine is the "Mr. Funnel" brand purchased via Amazon) for refueling my Rotax-powered RANS S-6ES with auto gas, which is the "preferred" Rotax fuel). My local airport does not dispense auto gas, so I have to haul it myself, using 5-gallon cans that will fit in the trunk of my car. That means the odds introducing some "junk" are a bit higher than with a properly-managed avgas fuel farm, and the Mr Funnel takes care of that nicely.

[svyolo]

Rolls Royce a few years back had a couple of "rollbacks" of their engines powering a 777 (British Airways) and if I remember right a A330. The 330 was at altitude and regained thrust, the 777 crashed on short final with a few minor injuries. It was caused by excess water in the fuel that froze in the filter/bypass. The engines met certification requirements for fuel filtering. They got a couple of bad tanks of gas the exceeded certification requirements.

[rodsmith]

Alaska bush pilots use the same type of funnel, especially while fueling from stashed barrels. I believe some will pass fuel but not water.

[svyolo]

I attended a seminar on fuel system design at OSH a few years ago. The presenter was a highly experienced aero engineer with multiple production airplane fuel system designs to his credit, including both military and civilian designs. I took a lot of notes, thinking it would come in handy later on. The bottom line: His "ideal" fuel system for a gravity-fed airplane looked identical to Bob Barrows' design. Front and rear fuel pickups (with screens), fuel lines laid out so that they flow downhill all the way to the low point (fuel drain) that is preferably a gascolator. If that was not possible due to tailwheel stance, then install at least a u-shaped bend to serve as a "sump" and install a fuel drain at the low point. (A la the Citabria design.)

And he was adamant about the use of a gascolator instead of an inline filter. He was very opposed to the use of automotive-type inline filter, unless that filter had a "bypass" capability in case it became clogged. (And those filters, while supposedly available, are also supposed to be extremely expensive.) He cited more than a dozen NTSB reports about aircraft engines failing due to fuel starvation caused by those inline filters becoming clogged. Notably, one of those had dual filters in parallel, which the builder thought would mitigate the risk of a filter clogging. Both filters became clogged at the time of the engine failure due to fuel starvation. In every one of those cases, the pilot was unaware the filter had become clogged. There was plenty of fuel on board the aircraft, but no way to get it to the engine!

He especially hated the all-metal cylinder type, because short of disassembling them, they cannot be visually inspected. He grudgingly agreed with a question from the audience that the transparent barrel type filters were marginally better, but only if they were installed where the pilot could actually see them easily during pre-flight – preferably with some back-lighting to show the filter media better. But even those can be difficult to determine if/when they are becoming clogged. His last statement on them was "If you do use them, please, please replace them at LEAST every annual, if not more often."

A bad tank of gas can take down any airplane. If you have fuel filters, the filters can clog, stopping fuel flow, or overheating your fuel pump or popping the CB. If you don't, it can clog injectors, flow dividers, carb floats, etc. Sometimes it is just bad luck. More often, it is poor maintenance.

One of the local DAR's here is a Boeing fuel system guy. He doesn't like filters or strainers, because they can clog. Without them, what is the next failure point/mode?

When I used to cruise on a sailboat, I had a "Baja filter". It was a fuel strainer/big funnel. It strained fuel while fueling, and you could see what was strained out afterwords.

[Nev]

There’s a very interesting discussion on fuel filters and Gascolators here on the Vans forum, particularly in relation to fuel injected engines vs carbureted. Bear in mind that the RV’s are low wing aircraft and any water in the system is likely to collect in the rear of the fuel tanks, as opposed to the Bearhawk where the lowest point by design is usually the gascolator.

Apologies in advance if this has been linked to before.

[Nev]

That’s interesting Jim. My EFII System came with a spare filter, to be installed at the first annual with instructions to buy another and replace at each annual thereafter.

[JimParker256]

I attended a seminar on fuel system design at OSH a few years ago. The presenter was a highly experienced aero engineer with multiple production airplane fuel system designs to his credit, including both military and civilian designs. I took a lot of notes, thinking it would come in handy later on. The bottom line: His "ideal" fuel system for a gravity-fed airplane looked identical to Bob Barrows' design. Front and rear fuel pickups (with screens), fuel lines laid out so that they flow downhill all the way to the low point (fuel drain) that is preferably a gascolator. If that was not possible due to tailwheel stance, then install at least a u-shaped bend to serve as a "sump" and install a fuel drain at the low point. (A la the Citabria design.)

And he was adamant about the use of a gascolator instead of an inline filter. He was very opposed to the use of automotive-type inline filter, unless that filter had a "bypass" capability in case it became clogged. (And those filters, while supposedly available, are also supposed to be extremely expensive.) He cited more than a dozen NTSB reports about aircraft engines failing due to fuel starvation caused by those inline filters becoming clogged. Notably, one of those had dual filters in parallel, which the builder thought would mitigate the risk of a filter clogging. Both filters became clogged at the time of the engine failure due to fuel starvation. In every one of those cases, the pilot was unaware the filter had become clogged. There was plenty of fuel on board the aircraft, but no way to get it to the engine!

He especially hated the all-metal cylinder type, because short of disassembling them, they cannot be visually inspected. He grudgingly agreed with a question from the audience that the transparent barrel type filters were marginally better, but only if they were installed where the pilot could actually see them easily during pre-flight – preferably with some back-lighting to show the filter media better. But even those can be difficult to determine if/when they are becoming clogged. His last statement on them was "If you do use them, please, please replace them at LEAST every annual, if not more often."

[Mark Goldberg]

I have only used fuel filters outboard between the aux tanks and the transfer pumps. Because the Facet pumps recommend it and I have heard those transfer pumps do not like debris. Filters to be changed out at annual. Out there the worst that could happen is not being able to transfer fuel inboard to the mains. Fuel filters have clogged and planes have had engines quit when filters have been used before the engine. But as others have said it would seem that any debris would show up soon after flying. Unless you got some bad avgas sometime later. Mark

[zkelley2]

I started installing my SDS fuel injection parts on the engine a couple parts a day, at the end of the day. After a lifetime of doing a huge variety of building/fixing stuff, I would say the SDS stuff is as high quality as anything I have ever put my hands on. I am pretty sure it is not as well designed as something that went on the space shuttle. I am pretty sure it IS as well made as something that went on the space shuttle. Stunning quality. I am taken aback.

I feel like an Apple fan boy (I am not one) with a the newest iPhone. When I got the 2-3 cubic foot box delivered, I looked at it and realized how much it cost. It is worth it.

This is how I felt about the sds product I put in as well. Theres not a single certified part on my engine as well engineered.

[Nev]

Ok that’s all good to know. Thanks. I hadn’t thought of changing the screen size in the gascolator.

[whee]

It’s hard to recommend against a manufacturer but I also recognize the perspective of the guys you talked to.

Two thoughts: 1. the “filter” EFII calls for isn’t so much of a filter as a course screen. While it is possible to plug it it won’t do so in the same way nor as quickly as a filter. 2. I think it was Battson that commented the latest time we had this discussion and said he had some swarf for a few hours right after the build but after that the screens have been clean. I really don’t think plugging the screens is going to be an issue.

My perspective; if the filter is actually screen and after the first few hours there is no longer much of a risk of plugging the screen, because any debris is fine enough it will pass through the screen, then there really isn’t much need to keep the screen in place after those first few hours.

My solution; The gascolator has a screen in the top of it. I removed that screen and replaced it with a screen that met EFIIs spec for the pre-filter. It think it was 90 micron or something. So far it has caught some junk.

[Nev]

Has anyone installed an auxiliary fuel pump without an inline filter ? The EFII pump I’ve installed comes with a filter recommended by the manufacturer. However I’ve now had two very experienced aircraft engineers query this with me. One strongly suggested having a drain on the fuel filter (not sure how to do this, and it’s also right next to the gascolator drain). They also suggested removing the filter entirely and allowing for earlier replacement of the the electrical pump as required. Presumably without a filter in the system, the pump could potentially be damaged, but the engine should continue to run and I can see the logic here. What is best practice? What are others doing ?

I’m struggling to reconcile between the manufacturers recommendation (for a system that is not certified), and the caution raised by two very experienced engineers.

[svyolo]

I started installing my SDS fuel injection parts on the engine a couple parts a day, at the end of the day. After a lifetime of doing a huge variety of building/fixing stuff, I would say the SDS stuff is as high quality as anything I have ever put my hands on. I am pretty sure it is not as well designed as something that went on the space shuttle. I am pretty sure it IS as well made as something that went on the space shuttle. Stunning quality. I am taken aback.

I feel like an Apple fan boy (I am not one) with a the newest iPhone. When I got the 2-3 cubic foot box delivered, I looked at it and realized how much it cost. It is worth it.

[whee]

Just to clear things up in regard to flow testing:

14 CFR 23.955

23.955 Fuel flow.

(a)General. The ability of the fuel system to provide fuel at the rates specified in this section and at a pressure sufficient for proper engine operation must be shown in the attitude that is most critical with respect to fuel feed and quantity of unusable fuel. These conditions may be simulated in a suitable mockup. In addition -

(1) The quantity of fuel in the tank may not exceed the amount established as the unusable fuel supply for that tank under § 23.959(a) plus that quantity necessary to show compliance with this section.

(2) If there is a fuel flowmeter, it must be blocked during the flow test and the fuel must flow through the meter or its bypass.

(3) If there is a flowmeter without a bypass, it must not have any probable failure mode that would restrict fuel flow below the level required for this fuel demonstration.

(4) The fuel flow must include that flow necessary for vapor return flow, jet pump drive flow, and for all other purposes for which fuel is used.

(b)Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

(c)Pump systems. The fuel flow rate for each pump system (main and reserve supply) for each reciprocating engine must be 125 percent of the fuel flow required by the engine at the maximum takeoff power approved under this part.

(1) This flow rate is required for each main pump and each emergency pump, and must be available when the pump is operating as it would during takeoff;

(2) For each hand-operated pump, this rate must occur at not more than 60 complete cycles (120 single strokes) per minute.

(3) The fuel pressure, with main and emergency pumps operating simultaneously, must not exceed the fuel inlet pressure limits of the engine unless it can be shown that no adverse effect occurs.

(d)Auxiliary fuel systems and fuel transfer systems. Paragraphs (b), (c), and (f) of this section apply to each auxiliary and transfer system, except that -

(1) The required fuel flow rate must be established upon the basis of maximum continuous power and engine rotational speed, instead of takeoff power and fuel consumption; and

(2) If there is a placard providing operating instructions, a lesser flow rate may be used for transferring fuel from any auxiliary tank into a larger main tank. This lesser flow rate must be adequate to maintain engine maximum continuous power but the flow rate must not overfill the main tank at lower engine powers.

(e)Multiple fuel tanks. For reciprocating engines that are supplied with fuel from more than one tank, if engine power loss becomes apparent due to fuel depletion from the tank selected, it must be possible after switching to any full tank, in level flight, to obtain 75 percent maximum continuous power on that engine in not more than -

(1) 10 seconds for naturally aspirated single-engine airplanes;

(2) 20 seconds for turbocharged single-engine airplanes, provided that 75 percent maximum continuous naturally aspirated power is regained within 10 seconds; or

(3) 20 seconds for multiengine airplanes.

So in a pumped systems the pumps must be ON during the testing.

[Battson]

Yes that is the AC I was thinking of! Thanks for putting two and two together.