I’d like to get feedback about the differences between a gravity fuel system and one assisted by a pump or two. Here is my particular situation:
Background and Tests
I am building a Sonex (VFR only) with a 120 hp 3300 Jabiru engine. The fuel tank is under the glare shield. Most people run a strictly gravity system and remove the mechanical fuel pump. For my tests, I’ve elevated the engine 20 degrees and put 2 gallons of fuel in the tank. This leaves only 6” between the level of gas in the tank and the level of the intake manifold. The fuel exits the hose at the intake manifold level in each test. I have a finger strainer in the tank and a gascolator (with a fine mesh filter) at the low spot on the hot side of the firewall. 3/8” hose has been used throughout. All unnecessary 45 and 90degree fittings have been removed.
Flow tests through this system were 20 GPH. WOT fuel flow for the 3300 has been reported to be 10-12 GPH. So there’s plenty enough reserve to meet the 150% rule stated in the FAR. But this is without installing a fuel flow robber…er, I mean fuel flow sensor. I get only 8.5 GPH when this is added. I will repeat the tests when a larger, less restrictive fuel flow sensor arrives. I expect this will improve flow rates, but doubt they will increase to15-18GPH, the minimum I’d need for 150%.
Reports from Others
There are several people flying with 3300’s in their Sonexes using a gravity fuel system like I’ve described, INCLUDING the flow sensor. Most say they have never experienced a problem. One person reported a performance drop when he added a fuel flow sensor. He has since removed the sensor.
Other people have added the mechanical pump to the system. Some have added the mechanical pump by itself, but most have added an electric backup pump (usually a Facet) to the system as well as a fuel pressure sender. The mechanical pump is reported to put out ~5 psi and the Facets only 2-4 psi.
If the mechanical pump system is added to my flow sensor equipped system, it seems I would want an electric backup in case of mechanical pump failure. Most people with a system like this have the electric pump plumbed in series with the mechanical pump, turning on the electrical pump only for takeoffs and landings.
There are also people that have chosen to add either a mechanical or electrical pump to an already adequate gravity system. They had tested more than150% of their WOT fuel flow rate by gravity alone and chose to add a backup pump to increase the systems dependability. For example, there is one Kitfox owner I know of that installed an electric Facet backup pump. He turns it on only for suspected fuel starvation emergencies.
Assuming that fuel flow tests through the larger fuel flow sensor gives me 10 GPH (~80-100% ‘takeoff’ flow rate) here are my choices:
- Remove the flow sensor. Gravity only.
- Remove the flow sensor and add either the mechanical or electric pump, assuming the 150% rate can still be obtained through a failed or turned off pump.
- Keep the flow sensor and add both pumps plus a pressure sender. The mechanical pump runs continuously, electric pump for backup.
Option 1, gravity with no fuel flow sensor, would be the simplest. But I wouldn’t have the advantage of the data provided by a flow sensor.
Option 2, one pump only, would be the next up the complexity scale. Adding either pump would not be a major undertaking. And each pump weighs about a pound. Not trivial, but not like an extra battery either. Choosing one pump over the other is not obvious to me.
Option 3, the whole 9 yards, is the most complicated, but still easily doable. It adds 2.5-3.5 pounds and ~$100 to the gravity only system.
Which of these systems is the most dependable? A gravity only system doesn’t have a pump to fail, leak, or squirt gas. But a system with pumps might provide the extra pressure needed to overcome a clogged filter, a plugged vent, or a tank with only a few ounces remaining. And I would have fuel flow data.
I look forward to hearing what your insight is.