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Fuel Flow Tests and Reports

sglynn

Well Known Member
What is really needed for fuel flow tests and reports for airworthy certificate and safe flight?

I see the EAA packet for final registration and airworthiness has a form for fuel flow results but it doesn't really say it is required. And is it required that we do a flow test in a climbing, nose up attitude? I've done several tests including on bench, and two on the RV-7A and am seeing 30+ GPH using the Andair Aux boost pump. I've ensured the vent lines are open and working.

I've heard Van's did fuel flow tests and that I should request a copy of theirs. I asked, but they didn't know of such a report due to builder options.

So does everybody jack up their RVs into a climb configuration and test with the boost pump? Is that what's needed to ensure it will work?
 
So does everybody jack up their RVs into a climb configuration and test with the boost pump? Is that what's needed to ensure it will work?

Yes that is what I did before my DAR A/W inspection on my RV-10. Prior to my DAR inspection, my DAR had a list of things he would require to see were completed, and fuel flow was on it. YMMV depending on who you use. I think it is a good idea to know anyway.
 
Steve, I did three tests.

1.Pressure leak test between the fuse wall and servo. Air pressure (30 psi) for 24 hrs. Had zero drop, but if anything leaked (a 10 friend did) it dropped pretty fast as there is not a lot of volume.
2. Flow at pressure while running to determine unusable fuel. Bypassed the servo, disconnected the supply to the spider and hooked a clear section prior to a ball valve. Did each side and recorded how much fuel was still in the tank. The "not used" method.
Flow was measured by the red cube, clear section was observed to detect air bubbles in case of a suction leak. The ball valve set the system flow to max take-off flow (or near it), and recorded the pressure to ensure it was within Lycoming limits.

3. #2 was repeated with nose down and nose up. Nose up left very little fuel in the tank.

An initial 1/2 gallon of fuel was run through first and the tank drained, that purged the line and the fuel looked bad. After the purge all fuel was able to be reused.


EDIT: We tested at the over delivery required by the FAA test as well to be sure the boost pump delivered through the mechanical pump.
 
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I did a fuel flow test with the aircraft level. It easily exceeded the requirements. I then taxied over to an incline we have on the airport that approximates the climb attitude and ran it for 3 minutes plus. It continued to work fine. I felt better for it and showed my flo results and incline position to my DAR and he was satisfied for my plane. If u want to do more, maybe a Canadian will chime in - I believe they have a very specific testing requirement.
 
I did a fuel flow test with the aircraft level. It easily exceeded the requirements. I then taxied over to an incline we have on the airport that approximates the climb attitude and ran it for 3 minutes plus. It continued to work fine. I felt better for it and showed my flo results and incline position to my DAR and he was satisfied for my plane. If u want to do more, maybe a Canadian will chime in - I believe they have a very specific testing requirement.

No need to go to Canada...:)

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-89B.pdf

The FAA gives the fuel flow test requirements in AC 90-89 on pages 33 and 34. This same requirement is in the FAA AC 91-116 for a second pilot in Phase I testing. It also says how you can document it.
 
I have to give Gil credit, he always seems to have the technical or source credit with a copy or link to the many questions that are posed by us VAF?rs. Good job Gil - keep it up.
 
Nose Up Fuel Test

Thanks all. I have one more fuel test to do. Nose Up. I'll have to find a hill on the airport and pull the tail down until the skid hits and put a ramp or blocks under the nose gear. Not going to jack the mains.

One thing the documents seem to overlook is whether the vent lines are working well enough. One test I did was a simple drain test. I let fuel drain out of the cocks and on one of them I noticed the flow slowing down to trickle. Caps are on. Something was partially blocking the vent line. I blew it out and all works fine.
 
Thanks all. I have one more fuel test to do. Nose Up. I'll have to find a hill on the airport and pull the tail down until the skid hits and put a ramp or blocks under the nose gear. Not going to jack the mains.

One thing the documents seem to overlook is whether the vent lines are working well enough. One test I did was a simple drain test. I let fuel drain out of the cocks and on one of them I noticed the flow slowing down to trickle. Caps are on. Something was partially blocking the vent line. I blew it out and all works fine.


Nah, it's not overlooked.

If your fuel flow meets the requirements at climb angles then the vents are allowing in enough replacement air into the tank. If the vents were clogged then you would not get the proper fuel flow.

Implied is that the fuel flow test should be done in a fully operational mode testing from each tank with the fuel caps on. :)
 
No need to go to Canada...:)

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-89B.pdf

The FAA gives the fuel flow test requirements in AC 90-89 on pages 33 and 34. This same requirement is in the FAA AC 91-116 for a second pilot in Phase I testing. It also says how you can document it.

Awesome! Thanks Gil. My EAA chapter just did an OpsLims presentation and started me thinking on docs and DAR prep. An AC that covers a bunch of this and things I've not yet thought of yet is money! Now to order the EAA packet....
 
AC 90-89B's Fuel Flow test procedure calls out a few measurements that I don't know how to find:

1. "Place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle."

How do I determine my particular airplane's highest anticipated climb angle prior to flight testing? Does Vans publish this number somewhere for each model they sell? I see some people here just testing at 20 degrees and calling it a day. Is there some literature behind that 20 number?

Is there any value in also doing a fuel flow test at 0 degrees?

2. "[...] 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."

Where can I find the engine's expected fuel consumption at full throttle? For example, for a Lycoming engine, I assume it's hidden away somewhere in the Operator's Manual. Any section or page number references that could help me find it? I'm aware of the 0.55 x horsepower x safety margin estimation, but it would be nice to be able to just cite Lycoming's actual documentation for a given engine type.
 
Thanks all. I have one more fuel test to do. Nose Up. I'll have to find a hill on the airport and pull the tail down until the skid hits and put a ramp or blocks under the nose gear. Not going to jack the mains.

One thing the documents seem to overlook is whether the vent lines are working well enough. One test I did was a simple drain test. I let fuel drain out of the cocks and on one of them I noticed the flow slowing down to trickle. Caps are on. Something was partially blocking the vent line. I blew it out and all works fine.

In an A model, a little weight on the tail to hold it down will give you 15 degrees nose up...certainly plenty for test requirements.
 
AC 90-89B's Fuel Flow test procedure calls out a few measurements that I don't know how to find:

1. "Place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle."

How do I determine my particular airplane's highest anticipated climb angle prior to flight testing? Does Vans publish this number somewhere for each model they sell? I see some people here just testing at 20 degrees and calling it a day. Is there some literature behind that 20 number?

Is there any value in also doing a fuel flow test at 0 degrees?

2. "[...] 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."

Where can I find the engine's expected fuel consumption at full throttle? For example, for a Lycoming engine, I assume it's hidden away somewhere in the Operator's Manual. Any section or page number references that could help me find it? I'm aware of the 0.55 x horsepower x safety margin estimation, but it would be nice to be able to just cite Lycoming's actual documentation for a given engine type.

This is an insanely aggressive climb at 95 knots indicated, solo weight, light on fuel. You won't be climbing this aggressive on your first flight but the plane is capable of it. Normal climb angle is more like 9-10 degrees.

I put the tail as low as I could get it when I did the test. Fuel pickups are in an ideal location for a tail low attitude. Test results didn't change one bit from level to tail low either.

As for max fuel flow the only thing in the operators manual you can go on is the fuel flow chart which is set at best power mixture. On my M1B that calculates to 14.5 gal/hr. That number in reality is way too low. Most people see 16-18 as a max fuel flow. So worst case scenario (on my IO-360-M1B) is 18x1.25=22.5 gph. Just make sure and do the math for your specific engine.

Hope this helps.
 

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Keep in mind that wing angle of attack, and actual pitch angle can be very different values.

Some of the RV models with a high power to weight ratio, when lightly loaded, are capable of full power climbs at pitch angles of 45 degrees or more.
This would not be a normal flight operation, but if the goal of testing is to assure engine operation in all possible flight conditions, the test should be done at way more than 15 degrees (approx. level flight stall angle of attack).

There is some basis for the argument that if an RV is built with an entirely by the plans fuel system, nothing other than a simple fuel flow test should be required. Unfortunately there has been a small number of instances over the years where because of a builder mistake, proper fuel flow was not available.
That alone makes doing a though test a good idea.
 
AC 90-89B's Fuel Flow test procedure calls out a few measurements that I don't know how to find:

1. "Place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle."

How do I determine my particular airplane's highest anticipated climb angle prior to flight testing? Does Vans publish this number somewhere for each model they sell? I see some people here just testing at 20 degrees and calling it a day. Is there some literature behind that 20 number?

Is there any value in also doing a fuel flow test at 0 degrees?

2. "[...] 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."

Where can I find the engine's expected fuel consumption at full throttle? For example, for a Lycoming engine, I assume it's hidden away somewhere in the Operator's Manual. Any section or page number references that could help me find it? I'm aware of the 0.55 x horsepower x safety margin estimation, but it would be nice to be able to just cite Lycoming's actual documentation for a given engine type.
This is a very good test to do, and can help find issues that otherwise would occur during first flight - not what you want.

My andair backup electrical fuel pump put out over 50GPH at all angles, which is much higher than the 1.5x needed for an io-360. I'd guess other electric fuel pumps have similar performance. Kind of hard to test the mechanical fuel pump obviously, but you can do that later at altitude.

I flowed many gallons through both tanks to make sure that there was no difference due to clogging intakes or filters. Even after that, I found some debris over the first few tanks in the sump. Nothing big, but still.

Before testing the fuel pumps, I had flushed the tanks several times with fuel, both on and off the wings. All this outside of course, and with good grounding to the jerrycans, through a Mr. Funnel filter to check for debris.

If you have not run fuel through your pumps yet, check with the manufacturer to ensure you don't damage them at first start - running some pumps dry for too long can damage them.
 
This was timely for me. I had conducted fuel flow tests from each tank and the Andair pump provided almost 200% of max engine requirement. Tests were done in level attitude and my unusable fuel was quite low, based on exact weight of fuel in tank and fuel removed. However I haven't tested it in other attitudes yet; I've contacted my DAR and I'm sure he will advise as necessary.
 
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