By Vic Syracuse, EAA Lifetime 180848

This piece originally ran in Vic’s Checkpoints column in the May 2023 issue of EAA Sport Aviation magazine.

The piston engines with which we all power our aircraft are reliable pieces of equipment if we take care of properly feeding them. With the right dosage of air, fuel, and spark, they will start, and with the proper ancillary systems, such as cooling and lubrication, they should keep running.

A high percentage (33 percent) of engine stoppages, or unplanned landings, during the early phases of amateur-built aircraft test flying are due to improperly constructed or modified fuel systems. As a designated airworthiness representative, I always ask if there have been any modifications to the aircraft or its fuel system other than what the kit manufacturer recommended.

One of the required tests for using the Additional Pilot Program as described in Advisory Circular (AC) 90-116 is that there must be a documented fuel flow test of the aircraft. Typically, we want to ensure that there is 150 percent of the maximum required fuel flow for the installed engine in the worst possible attitude. That usually means the aircraft should be placed in a climb attitude on the ground. For taildraggers this is quite easy. For nose-gear aircraft, you have to elevate the nose. Be sure to ground the aircraft to the fuel container. The fuel line should be disconnected at the carburetor or the fuel servo. Then, using a stopwatch, check the amount of fuel flowing into the container.

Most of the time, if constructed by the kit manufacturer’s recommendations, the flow should be adequate. A good rule of thumb is that that a 3/8-inch fuel line works for engines up to 300 hp, and a 5/16-inch one works with some of the smaller engines such as Rotax 912s.

Modifications to the fuel system can be simple things such as adding fuel filters in the wing roots. You might not even think much about it, as adding a fuel filter seems like a good safety investment. I would agree with you, but you need to pay attention to the 90-degree fittings that possibly get added to accommodate the filters. Each 90-degree fitting is like adding about 10 feet of line into the system. It’s best to use two 45-degree fittings if room permits.

An interesting note in AC 90-116 says if modifications are made to the fuel system after the initial tests, the tests must be performed again. Seems like a good idea.

Many of you may know I recently completed a Hummingbird helicopter. Fuel systems were very much on my mind during the construction process. The Hummingbird (300L) is basically a refined Sikorsky helicopter (HO5S-1) that was certified, with about 94 of them brought to production before the program was terminated.

Originally it was designed as a wounded troop transport, with the ability to load two stretchers inside the cabin, with one on top of the other. You can see the last remaining HO5S-1, owned and piloted by Alex Anduze, land on the aircraft carrier in the movie Devotion. (For more on Alex’s rare helicopter, see “A True Test of Determination,” November 2015. — Ed.)

The fuel system is quite simple. There is a bladder tank that connects to a gascolator, which continues to an electric fuel pump. From there it routes to the engine-driven fuel pump, the outlet of which is connected to the fuel servo. The Lycoming HIO-540 is an injected engine. Typically we don’t use a gascolator for an injected engine. That was the first thing I didn’t like. I believe the gascolator was left over from the initial installation of the Franklin engine. There was no provision for a fuel filter, which was the second thing I didn’t like.

I’ve built 11 other aircraft, and inspected and performed maintenance on hundreds of others, so I felt experienced enough to make some changes. However, that statistic regarding fuel system modifications kept whispering in my ear, so I didn’t make any changes. The last thing I want is to be is a statistic. However, that gascolator kept nagging at me. I also kept telling myself that this was basically a certified helicopter in a kit, so I shouldn’t mess with it.

My normal practice for the other aircraft I’ve built has been to thoroughly inspect the aircraft at 10 hours, which means taking a hard look at the fuel system for any construction debris. The day after my Hummingbird was certificated, I traveled to Vertical Aviation in Sanford, Florida, to fly with Alex Anduze prior to making the first flight. He has probably forgotten more about helicopters than I will ever learn. He’s a super guy. I met and flew with him about a year ago for a demo flight. I wanted his opinion on whether I would be safe enough to make the first flight of my own. That worked out and is a topic for another column. However, Alex made a comment that amplified that nagging voice in my head. He said to check the gascolator.

So, I did just that after less than three hours of engine run time, which probably equated to about 55 gallons or one full tank. I was shocked at what I found. The gascolator screen was about 80 percent blocked at the outlet. It appeared to be lint and black rubber particles, which I assumed to be from the inside of the bladder. On other aircraft, I am used to finding some metal shavings or fiberglass strands in the screen after 10 hours or so, but this was a dramatic difference. While the gascolator screen was doing its job, I did not think the small surface area was adequate for the HIO-540. I decided to make a change.

I thought about completely removing the gascolator, and I may still do that. In the meantime, I decided to add a fuel filter I had experience with and knew was sufficient. The 74-micron fuel filter manufactured by Airflow Performance has always worked well in my RV-10 and hundreds of others — and there was room to add it in the fuel line ahead of the gascolator. I did that, and I’m confident I won’t have to worry about the gascolator potentially restricting the fuel flow. I used only straight fittings, so I didn’t add any extra restrictions in the fuel line. However, I did retest and ensured there was adequate fuel flow through the new setup. I also like to have a quality fuel filter ahead of the electric fuel pump, so that helped drive the location for me.

The helicopter climbs in a somewhat flat attitude, so I did not have to jack anything up for the fuel flow test. I have a fuel flow gauge and fuel pressure gauge and have not noticed any difference, so I am comfortable with the change so far after two hours of flight testing. I will continue to monitor it and will definitely take apart both the gascolator and the screen at 10 hours to check them.

I also added a high-quality, remote-operated fuel cutoff valve right where the fuel exits the tank. I wanted a way to completely turn off the fuel if there was an engine compartment fire. It also makes maintenance of the fuel system possible without the need to drain the fuel tank.

My goal is to keep my fun factor alive during flight testing by not having any unplanned engine stoppages.

Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA mechanic, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more than 10,000 hours in 74 different types. Vic founded Base Leg Aviation, has authored books on maintenance and prebuy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.