Fuel System Questions

[svyolo]

Jared;
I am doing an EFI, so I need return lines. The returns will be plumbed to the front half of the inboard side of the tanks, near the top. I will probably put everything in the location of the fuel valve and gascolator on the stock fuel system, under the front seats/floorboards. The gascolator is omitted with EFI. It sure would be easier to put it all overhead, I just don't think I want that many fuel fittings over my head. Sure would make troubleshooting a fuel leak easy though. LOL.

[Mark Goldberg]

John, with the plans you will see how Bob designed the fuel system. As Jared said - this is one of the areas where homebuilders can really have problems when trying to "improve" a system that is designed carefully and works. I feel a gascolator is needed to allow somewhere to drain water that might accumulate in the low point of the fuel system. There is a good reason why a gascolator has been used this way in planes for a long time. Mark

[svyolo]

Mark;
I like the idea of a gascolator at the low point as well, but both companies I talked to about their EFI either didn't recommend one, or said it was unnecessary. They both said "it serves no purpose". I don't think it would hurt anything to have one, as that part of the fuel system is pressurized only by gravity. EFI deals with small amounts of water better than a carb. I think almost all diesel powered vehicles (boats, heavy equipment, my Chevy Colorado) have a fuel/water separator that do the same thing. But most EFI gasoline vehicles that I know of don't.

I can't say I have decided to have one or not. There is plenty of room for one I believe in the "plans" location, plus space for the duplex fuel valve and 2 feed and return lines.

[svyolo]

Mark;
I have been thinking about Bob's fuel system, and I think the recommendation to run in Both (especially takeoff and landing) is good. But I also think the gascolator might also be functioning as a bit of a small header tank when the tanks are low on fuel, and the plane is maneuvering. So I am starting to think the gascolator might be a good idea on and engine with EFI as well.

I will make sure to get a duplex valve with a "both" position, and using that position will be part of my TO and landing checklist. But because of the uncertainty of how much fuel is returned to each tank, I will use L or R position the rest of the flight.

The EFI's use 25gph electric pumps, and the engine is consuming much less (5-13). But I think that will exacerbate any fuel starvation problem with low fuel, in maneuvering flight.

I think recommending using Bob's fuel system is a good one. Again.

[Mark Goldberg]

Good decision. It is well thought out and proven by many 1,000 hours of flight by many Bearhawks. We like the SPRL fuel valve which I stock. L/R and BOTH positions. MG

[alaskabearhawk]

I'm planning the plumbing for my fuel system and re-read this thread. I am using an IO-540 and an EFII boost pump. After reading the entire post I saw no mention of considering a float installation. When sitting on the water the entire fuselage is raised up by the tail, so finding a low point for both a wheel and float installation may be a bit problematic.

[AZBearhawk272]

The worst case condition is tail low 3 point.
A level condition fuel still needs to drain downhill to reach the engine. In most cases the low point drain will still be the same.
I know mine works tail low or in level condition. I followed Bob’s recommendations.

[whee]

Just to close the loop here.

We completed our fuel flow testing today. Our system is installed per the sketch posted earlier except we installed a gascolator and no filter. Since our engine is fuel injected we needed to check the flow with the electric pump on. The critical angle used was 19* nose high. For our plane that required the distance from the ground to the axle to be 40” with the tailwheel removed (spring installed on the plane). Front fuel pickups in the tanks were unported during the testing.

The left tank flows 40gal/hr and has 1.5gal unusable.

Right tank flows 38gal/hr and has 1gal unusable.

I think the difference between the tanks is due to a minor testing error.

[Battson]

Just to close the loop here.

We completed our fuel flow testing today. Our system is installed per the sketch posted earlier except we installed a gascolator and no filter. Since our engine is fuel injected we needed to check the flow with the electric pump on. The critical angle used was 19* nose high. For our plane that required the distance from the ground to the axle to be 40” with the tailwheel removed (spring installed on the plane). Front fuel pickups in the tanks were unported during the testing.

The left tank flows 40gal/hr and has 1.5gal unusable.

Right tank flows 38gal/hr and has 1gal unusable.

I think the difference between the tanks is due to a minor testing error.

I interpreted AC41.13 to mean testing for pumped systems required 1.25x max flow with pumps off? Whereas an pump-less system needs 1.5x max flow. Have I misunderstood that?

[svyolo]

I am not sure either. Also I don't know if a high wing gravity feeding to a boost pump is what the FAA was referring to as a "pumped" system. The boost pump is still gravity fed.
I kind of assumed that a pumped system was referring to a low wing plane that needed a pump to feed fuel.

But I was not sure, nor am I now.

[Mark Goldberg]

A gravity flow system means no pumps. Not an engine driven pump nor a boost pump that is required if you have an engine driven pump. The boost pump is there to force fuel through a broken engine driven pump that has stopped working. Any obstruction will decrease the flow. I understand there is one fuel flow transducer made for gravity systems that has minimal flow restriction. Mark

[whee]

I interpreted AC41.13 to mean testing for pumped systems required 1.25x max flow with pumps off? Whereas an pump-less system needs 1.5x max flow. Have I misunderstood that?

Wish I had a solid answer. I couldn’t determine how to interpret the FAA guidance (AC90-89B). The AC says: “Make sure the aircraft is tied-down and chocked. With minimum fuel in the tanks, disconnect the fuel line to the carburetor. The fuel flow with a gravity flow system should be 150 percent of the fuel consumption of the engine at full throttle. With a fuel system that is pressurized, the fuel flow should be at least 125 percent. When the fuel stops flowing, the remaining fuel is the “unusable fuel” quantity.”

Since I have a fuel injected engine my fuel system is “pressurized.” But only part of my fuel system is pressurized and I didn’t want my pumps sucking fuel from the tanks; I wanted gravity to keep the pumps “flooded” with fuel. I checked with my DAR who had nothing to offer. EAA said I needed to do the test with the boost pump on. This seemed reasonable since the boost pump is supposed to be ran during takeoff and landing.

To make sure the boost pump is flooded we checked the flow at the pump inlet. We only did this at 3-point attitude and gravity does indeed supply more fuel than the electric pump delivers downstream.

[Battson]

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

[svyolo]

The FAA has many rules written decades ago that have never, and probably will never, be changed. I would not argue what it means one way or another. To my mind, since the boost pump (or my EFI pump) is gravity fed, I think I need to test it to gravity (150%) standards. If I had an in tank pump, or one fed at the tank outlet, I think it would be the "pressured" standard.

But like I said, I am not sure what was meant.

[whee]

The FAA has many rules written decades ago that have never, and probably will never, be changed. I would not argue what it means one way or another. To my mind, since the boost pump (or my EFI pump) is gravity fed, I think I need to test it to gravity (150%) standards. If I had an in tank pump, or one fed at the tank outlet, I think it would be the "pressured" standard.

But like I said, I am not sure what was meant.

I think we are thinking the same thing. Need 150% to the fuel pump inlet.

My pump is under the floor so I wanted a standard to test the remaining portion which is pressurized. I think "pressurized systems" are typically found in low wing planes which use in wing pumps. which have to be turn on during fuel flow testing.

Perhaps the best (most conservative) method for testing a system like mine (high wing, fuel injected, with belly mounted auxiliary pump) would be: 1. Make sure that 150% of needed fuel is supplied to the auxiliary pump inlet. 2. 125% of needed fuel is supplied to the engine driven fuel pump with the aux pump off. 3. 125% of needed fuel is supplied to the engine driven fuel pump with the aux pump on.

[svyolo]

Yup. My thinking exactly. And no DAR could question it. Perhaps some would question it if you assumed 125% simply because you had a boost pump in your system, somewhere.

This is my logic, but I won't argue it. It is a bit of a guess, and gut feeling.

[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.