Fuel Flow Test - Pitch Angle

[robcaldwell]

Just following up after completing the fuel flow test, I came in with the below results.

Aircraft with 8.5 tires positioned on 2 concrete blocks with the tail on the floor gave me 20 degree pitch angle. Mine was a boosted fuel flow test with a Air Flow Performance boost pump after the gascolator. I disconnected the fuel line at the inlet of the fuel servo. I achieved more than double the required 125% BSFC calculation.

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[rodsmith]

Those are great numbers! Guess it is easier to pass this test with a pump system versus gravity alone. Is your fuel flow sender ahead of the fuel servo or between it and the fuel spider? I have some 90s in my system and have the Andair boost pump. Well after purchasing it I found out it creates a significant pressure drop when bypassing. Besides the test with the boost pump on I will check flow to the mechanical pump with the boost pump off to make sure it is adequate.

[robcaldwell]

My fuel flow transducer is located after the fuel servo. It is located on top of the case just before the divider. This placement is at the direction of Don Rivera at Airflow Performance as well as Tom Sweringen at TS Flightlines.

I did a gravity test as well just to see what it would do. I saw an average of about 8GPH in all three conditions. Not great, but the airplane would probably still fly, although anemically.

[schu]

Rob, it's very doubtful your airplane would fly on just the gravity feed. I doubt the servo and flow divider would get anything into the cylinder with 2-3 psi of fuel pressure.

[Battson]

I thought the requirement was to test with the pump off, if you have a pumped system? 150% with gravity flow and 125% with a pump (pump off).

For a 260hp fuel injected engine with no return lines, 125% of peak fuel flow is about 113 to 115 L/hr - AKA - 30.0 GPH

If your system cannot get 125% of max flow with the pump off, based on practical experience from testing and then flying our fuel system - then the fuel pressure will nose-dive when you take-off without the boost pump running or if one pump fails during take-off (measured upstream of the pump).

Our fuel system is Bob's design with a filter, transducer, and boost pump included under the floor. No other changes.

Even with compliant fuel flow, it is still possible get ahead of the fuel system and cause a low pressure condition. It's not common, but it can happen. This situation requires the electric boost pump to stabilize the pressure until you reduce throttle and let the system catch up.

Low fuel pressure can cause issues, although it is rare... normally missing / rough running and lower power output. The bendix injection system will run at cruise power with just a handful of psi.

[zkelley2]

That was my understanding as well. Or at least 125% flow to the fuel pump inlet in the worst condition as described by the AC. I cannot imagine a setup that would not flow a crap ton of fuel to the servo with a pump on. Anyone running a bearhawk with a pump is likely running a hybrid gravity/pressurized setup. Gravity to the inlet of the pump.

I guess we could be wrong, I mean if It's flowing 55gph with the pump on and fuel line to a can, why would it not do that hooked up with the engine running? With a fuel pump driven airplane it is assumed that a fuel pump will work for the entire flight and a failure of both pumps will kill the engine.

Capable of flowing 55gph, actually flowing about 24gph at full power, there's going to be some pressure there I would think. I'm guessing there's some math that would verify it. I guess pressure wise that's going to be a really important ground test, boost pump off, full power, watch the fuel PSI. Possibly go do that in a flight test at a really safe altitude as well where you pull up into a really high pitch at full power with the boost pump off. Or figure out a way to get that pitch on the ground and full power.

Lot of guessing on my part. Just some thoughts.

[auburntsts]

My interpretation of AC 90-89B is just the opposite— if you have a pressurized fuel system for a fuel injected engine the 125% stipulation is with a pump on. And there is no expectation to meet the 125% gravity only as the assumption is the system will always be pressurized either primarily from the mechanical fuel pump or a booster pump. Look at this way, AC 90-89B isn’t worded high wing vs. low wing, it’s gravity vs pressurized. If it was almost no low wing would ever pass.

[robcaldwell]

I also performed a non-boosted gravity flow test as well. No boost pump, and obviously no engine driven pump. With 5 gallons per side, and the plane up on blocks, I saw about 8 gallons per hour in all three selector settings. My system routes through the selector, back to the gascolator, through a pre-filter, through the boost pump, out the firewall, through the engine driven pump, to the servo. That's a lot of bends and components. I ran that configuration past Don Rivera at Air Flow Performance and he said he would have set it up the same way.

I agree with Zach and will test on the ground with no boost pump.

[AKKen07]

So the Flight Testing Handbook reference seems vague to me but here is an excerpt from AC 23-16A pertaining to certified aircraft. It talks about gravity fed systems in much the same way as the FAA Flight Testing Handbook for EABs does. Then it goes into a little more detail about planes with pumps:

Paragraph 23.955(c) ADC Cross-reference: 5.11(c) How can I demonstrate the fuel flow rate compliance for a pump fed fuel system? These tests are required to demonstrate that the airplane fuel system can deliver an adequate fuel supply to the engine. Conduct these fuel-flow rate tests by placing the airplane in the same attitude as outlined in the procedure for gravity-feed fuel system tests. Operate the fuel pump either driven by the engine with the pump mounted on the regular engine-fuel pump drive pad or driven by a separate external power source. If conducted with the pump mounted on the engine, use a separate source of fuel, external to the airplane, to operate the engine at takeoff r.p.m. If an external power source is used to drive the fuel pump, conduct the test with the pump operating at the same speed as the pump operates when the engine is running at takeoff r.p.m. Mount the pump at the same height as it would be if mounted on the fuel pump pad on the engine...

I think it stands to reason that having better psi without the pump running is a very good idea - but perhaps not required per the FAA. I believe Rob's results are valid for that purpose.

[AKKen07]

Fun story: I know of at least one model of certified aircraft that would undoubtedly have an engine quit without the boost pump on in either takeoff or go-around. I taught in the Diamond Eclipse planes several years ago and I used that phenomenon to reinforce the use of the landing checklists. Once the engine quit on my students (I managed the situation appropriately to keep it safe of course) they never forgot again.

[zkelley2]

I think all the TCM injection systems get upset with full power and the boost pump not on the correct position? From the couple time's I've flown a 206.... but it's been awhile.

[whee]

There isn’t really a standard for all aircraft with TCM fuel injection. I looked up a bunch of procedures and they were all over when it comes to boost pump operations. Some have a high and low speed positions, some are just on and off, some you run the pump on takeoff and landing, others only used the pump for startup and emergencies. I settled on using the boost pump during startup and emergencies.

[Battson]

In our setup, with the mechanical diaphragm pump running, at cruise power we see between 23 and 25 psi measured after the mechanical pump. With the electric boost pump running, we see about 28 psi.

At full power we see about 19 psi with just the mech pump, and about 23 with the boost pump. In some cases you can catch the engine with low pressure and I have seen it drop to about 13 psi with just mech pump.

Our system acheived the 125% fuel flow test without the pumps running, but only with a small margin. We did not achieve 150% flow. Flows were measured at the inlet of the mech pump.

With hot fuel system after engine shutdown, fuel pressure can rise to 31 psi (engine off). Turning on the electric boost pump at this stage can create pressures up to 50 psi between the two pumps. Care is necessary, but it hasn't blown our system.

[schu]

In our setup, with the mechanical diaphragm pump running, at cruise power we see between 23 and 25 psi measured before the mechanical pump. With the electric boost pump running, we see about 28 psi

That seems really odd to me. You have fuel pressure before the mechanical pump with the boot pump off? How is that possible? If anything the mechanical pump should be lowering the pressure at the inlet and there is nothing behind it to make 23-25psi. What am I missing?

[Battson]

Sorry - typo - AFTER the pump, I have fixed it.

[schu]

Okay, that makes way more sense.

[schu]

Rob,

Regardless of what the FAR says or if you interpret the fuel flow to be measured before or after the pump, what you really care about is that the inlet of the pump is always flooded to make sure it's not sucking, then verify that the pumps are able to make the pressure you need. As Zach points out, this is a hybrid gravity/pressurized setup, so, you want to verify both parts independently.

If it were me, I would take a fuel flow test at the inlet of the mechanical pump. I'd look for 31GPH. If you don't have that, then you could in theory have a situation where that pump is sucking. Next, I would do a fuel pressure test at the inlet of the servo with the boost pump on. If you see 25+ PSI, then the pump is working right and you can assume it's inlet is flooded as you saw it flooded all the way to the mech pump. After that I would start the engine and confirm that you get 25+ PSI with the boost pump off.

That process confirms:
Both pumps are flooded to more than 125% fuel flow.
Both pumps are making pressure needed to run the engine.

schu

[robcaldwell]

All good points. I am no longer up on blocks and don't intend to return to that attitude (too nerve racking), but I can perform some additional testing, especially gravity flow at the mechanical pump inlet.

[whee]

Rob,

Regardless of what the FAR says or if you interpret the fuel flow to be measured before or after the pump, what you really care about is that the inlet of the pump is always flooded to make sure it's not sucking, then verify that the pumps are able to make the pressure you need. As Zach points out, this is a hybrid gravity/pressurized setup, so, you want to verify both parts independently.

If it were me, I would take a fuel flow test at the inlet of the mechanical pump. I'd look for 31GPH. If you don't have that, then you could in theory have a situation where that pump is sucking. Next, I would do a fuel pressure test at the inlet of the servo with the boost pump on. If you see 25+ PSI, then the pump is working right and you can assume it's inlet is flooded as you saw it flooded all the way to the mech pump. After that I would start the engine and confirm that you get 25+ PSI with the boost pump off.

That process confirms:
Both pumps are flooded to more than 125% fuel flow.
Both pumps are making pressure needed to run the engine.

schu

This is what I did.

[schu]

Additional thoughts:

I like the fuel system with injection/pumps better than carb/gravity. I know everybody loves gravity because it's so reliable, but is it? Even my 170 has placard about single tank operation during cruise due to unexplained engine stoppages, and both tanks are vented together with an in-air scoop that slightly pressurizes the tank. There are a lot of bearhawks that have hundreds of hours with gravity based systems, but I also know at least one BH member that won't operate with 'BOTH' tanks selected. I suspect the difference is the variety of fuel system routing, especially when people move the fuel valve as they don't like the factory location.

In the case of fuel injection, assuming your boost pump is mounted under the floor, I think it would be really hard to have fuel problems as the fuel line routing is pretty vertical directly to the boost pump, and once it hits the pump, it can force the fuel into the engine. I would bet that most people would have more than 50GPH gravity feed to the pump at that location.

All that said, I think it's important to have 30GPH to the mechanical pump, so that single pump operation is reliable, but should something happen, turning that boost pump on should fix it.

Of course the con to fuel injection is no mogas, or perhaps mogas in one tank and 100ll in the other, then using mogas in cruise only, but I think that's worth it for a fuel system that balances fuel delivery to each cylinder.

Otherwise will disagree, and I have no idea what I'm talking about, so consider that too....

[Bcone1381]

I think the fuel flow test is to be performed with electric boost pump on. If my thinking is wrong, help me understand.

Cut/Pasted text from AC90-89b p.33 below are in Italics. They helped me to form my opinion.

The FAA's Fuel Flow Test objective is to have enough Fuel Flow for proper engine operation until fuel exhaustion.

Any aircraft that has fuel supplied to the engine under pressure has a "fuel system that is pressurized. It is clear to me that the fuel flow test on these aircraft must be performed with the pump ON. How else is an RV-6 going to pass the test?

A Bearhawk with a properly installed Fuel Boost Pump has a very very low bar to pass and can pass the fuel flow test easily with little effort.
_____________
Fuel Flow.
A fuel flow and unusable fuel check is a field test to ensure the aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude, and is accomplished by:

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.

Changing Fuel Flow or Pressure.
If the aircraft’s fuel flow rate is less than planned, there is a volume or pressure problem. An increase in the fuel flow volume may necessitate installation of larger fuel line fittings on the fuel tanks, fuel selector, and carburetor in addition to larger internal diameter fuel lines. To increase fuel pressure, install an electrically driven or engine-driven mechanical fuel pump prior to the first flight.
____________

[Battson]

Think about it this way, will you be flying with the boost pump on at all times? Or, do you want to write an operational procedure that the pump MUST be on any time you are developing more than X-horsepower?

If you answered "no" to both of those, then you want the pump off for flow testing.

Otherwise your test will reflect a need to have the pump on to meet the minimum performance criteria. If you need the pump running to get 125% flow, then the engine could be sucking fuel at full power which can lead to air-locking the pump and premature fuel starvation while there is still fuel in the tanks.

[svyolo]

I thought the requirement was to test with the pump off, if you have a pumped system? 150% with gravity flow and 125% with a pump (pump off).

For a 260hp fuel injected engine with no return lines, 125% of peak fuel flow is about 113 to 115 L/hr - AKA - 30.0 GPH

If your system cannot get 125% of max flow with the pump off, based on practical experience from testing and then flying our fuel system - then the fuel pressure will nose-dive when you take-off without the boost pump running or if one pump fails during take-off (measured upstream of the pump).

Our fuel system is Bob's design with a filter, transducer, and boost pump included under the floor. No other changes.

Even with compliant fuel flow, it is still possible get ahead of the fuel system and cause a low pressure condition. It's not common, but it can happen. This situation requires the electric boost pump to stabilize the pressure until you reduce throttle and let the system catch up.

Low fuel pressure can cause issues, although it is rare... normally missing / rough running and lower power output. The bendix injection system will run at cruise power with just a handful of psi.

Zkelley is of similar thinking as me. The BH system is gravity fed to the pump inlet. Then it is pressure fed. In my perfect world, the pump inlet would never "suck". But then again, in low wing aircraft, the pump inlet always "sucks".

What standard do we test too? If a low wing aircraft is allowed to test to 125%, obviously pump on, why can't we? The high wing gives us an about 1 psi head pressure over a low wing. But I still like the idea of no sucking.

I still kind of think Whee got it right with 1/2" fuel lines.

[whee]

Zkelley is of similar thinking as me. The BH system is gravity fed to the pump inlet. Then it is pressure fed. In my perfect world, the pump inlet would never "suck". But then again, in low wing aircraft, the pump inlet always "sucks".

What standard do we test too? If a low wing aircraft is allowed to test to 125%, obviously pump on, why can't we? The high wing gives us an about 1 psi head pressure over a low wing. But I still like the idea of no sucking.

I still kind of think Whee got it right with 1/2" fuel lines.

The issue with a “sucking” pump only comes into play on tanks with dual ports. If the fuel level in one tank gets below one of the ports the pump will suck air and not fuel. I don’t know about all low wing airplanes but many have single port tanks which eliminates the unporting issue. This is the same reason high wing airplanes with fuel injection have header tanks.

I agree with Battson’s perspective on fuel testing but the challenge is testing a fuel system that requires an engine driven pump but not being able to run that pump. However, I do think the FAA guidance expects that the pump will be running during the test.

I chose to test my system by ensuring the electric boost pump was always flooded with fuel. So I tested the gravity fed fuel to the inlet of the boost pump. Then I tested to see if gravity supplied sufficient fuel to the inlet of the engine pump. Then I performed additional testing once initial flight testing was complete.

[svyolo]

If I remember right suction fed low wing fuel systems, by AC 43, are required to have a single fuel pickup, and no "both" setting on the fuel system. Suction on Bob's system might cause problems. I think for o-360 engines Bob's fuel system is fine. For O-540's it is marginal, but it seems to have a good track record, so maybe good enough.
I am still using a small header tank (2 liters) which is OK for how SDS runs their fuel system. I think EFII's requires either return to the mains, or a much larger header tank. SDS's fuel system inputs less heat into the fuel that is returned. I am not sure about Conti's.

[Bcone1381]

This is a good discussion.

My answer to both of Battson's questions in post #25.1 is......NO, I do not want the boost pump on during all phases of flight, and I do not want it ON at all times above a certain power setting. The fuel system should tolerate the boost pump to be turned off after take off, then be turned on before landing, and be on when moving the fuel selector valve.

Battson says "The engine could be sucking fuel." If either pump will ever suction feed fuel then a header tank (a tank with a single fuel port) seems to be a prudent installation. If a fuel flow test is done with the boost pump off and it flows 125% at the intake of the second fuel pump I can see we have proven it will not suction feed.

[schu]

This is a good discussion.

My answer to both of Battson's questions in post #25.1 is......NO, I do not want the boost pump on during all phases of flight, and I do not want it ON at all times above a certain power setting. The fuel system should tolerate the boost pump to be turned off after take off, then be turned on before landing, and be on when moving the fuel selector valve.

Battson says "The engine could be sucking fuel." If either pump will ever suction feed fuel then a header tank (a tank with a single fuel port) seems to be a prudent installation. If a fuel flow test is done with the boost pump off and it flows 125% at the intake of the second fuel pump I can see we have proven it will not suction feed.

There is a time and place for a header tank, however where you install it and how you vent it matters a lot. You will effectively have a gravity fed fuel system to the header tank, but since the tank will likely be at or below the floor, the fuel up to the mechanical pump may not flow as well causing your mechanical pump to suck slightly. This shouldn't be a problem because you can't unport the header tank, however if you ran the airplane out of gas while on one wing tank, it might be more difficult to restart.

I break down the bearhawk to 3 fuel systems:

Gravity/carb. Build it exactly like Bob says, but I'd also add a cross vent.

Lycoming returnless injection: Build it like Bob says, but make sure you have the fuel flow to the mechanical pump to ensure the gravity part works up to the pump where you pressurize the fuel.

EFI/contiental return injection: This is the complex one. You really want to try and make sure that any parts of the system that are gravity fed are able to flow more than 80GPH as dual electric pumps with an idling engine can cycle this much fuel. Most EFI vendors recommend a header tank. Personally, I avoided this because it's complex, but if I was forced to go this route, I'd probably run 1/2 fuel lines to the dual pump mounted under the pilot seat. This ensures that the lines to the pump hold a bit of fuel and are vertical. I suspect that would keep the pump flooded even at 80GPH flow rates. The con is that this setup wouldn't work with a 'both' setting. You could build a header tank and return the fuel to that, but I'd be very careful to make sure that it's positioned such that the pump is always flooded even under very high fuel flows, is vented to the main tanks, and large enough that the fuel doesn't get hot.

But like I said, I don't have a clue, so it's up to the builder to think this through....

[svyolo]

There was a pretty good discussion on running both fuel pumps during takeoff and landing on VAF. One of the theories was that running both (the pumps are constant displacement) would increase the likelihood of vapor lock. They would create so much suction that they would cause the more volatile distillates out of suspension. It sounded plausible to me.

[zkelley2]

I think the fuel flow test is to be performed with electric boost pump on. If my thinking is wrong, help me understand.

Cut/Pasted text from AC90-89b p.33 below are in Italics. They helped me to form my opinion.

The FAA's Fuel Flow Test objective is to have enough Fuel Flow for proper engine operation until fuel exhaustion.

Any aircraft that has fuel supplied to the engine under pressure has a "fuel system that is pressurized. It is clear to me that the fuel flow test on these aircraft must be performed with the pump ON. How else is an RV-6 going to pass the test?

A Bearhawk with a properly installed Fuel Boost Pump has a very very low bar to pass and can pass the fuel flow test easily with little effort.
_____________
Fuel Flow.
A fuel flow and unusable fuel check is a field test to ensure the aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude, and is accomplished by:

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.

Changing Fuel Flow or Pressure.
If the aircraft’s fuel flow rate is less than planned, there is a volume or pressure problem. An increase in the fuel flow volume may necessitate installation of larger fuel line fittings on the fuel tanks, fuel selector, and carburetor in addition to larger internal diameter fuel lines. To increase fuel pressure, install an electrically driven or engine-driven mechanical fuel pump prior to the first flight.
____________

Technically an RV6 is gravity fed to the electric pump(s) which sit just below the fuel tank level in the fuselage correct? The electric pumps have to be on for startup even with a carb, but then in cruise, yes, the engine driven pump is sucking the whole time. But as whee pointed out, there's only 1 port on the tank and no both.

[svyolo]

I haven't calculated it, but my gut feeling is that the head pressure is so low in most low wing airplanes like an RV that the pump is almost always sucking. 1 psi isn't much, but if you or a friend have a boat, try creating a leak 3 feet below the waterline. That 1 psi creates a gusher. I have done that, both accidentally and on purpose. I doubt 1/6 of that head pressure would be anything like it.

[zkelley2]

The height above the liquid is quite low, but there's roughly 0 resistance on the way to the pump was well. I'd be surprised if it couldn't flow 30gph a couple inches inboard and a few inches below the tank on gravity.