Recirculating Fuel Systems

A recirculating fuel systems (sometimes known as vapor return system) is a very effective "band aid" that can be used to control vapor if all other remedies have failed (click Vapor to learn about other remedies).  The use of a vapor return system introduces other complexities in the fuel system that increases cost and invites fuel system mismanagement.    The letter below was written to an aircraft kit manufacturer discussing some of the issues relating to this subject.

April 23, 1997

Dick Van Grunsven
Van’s Aircraft
P.O. Box 160
North Plains, OR 91733

Re: Vapor return lines used with Ellison Throttle Body Injectors on RV aircraft.

Dear Dick,

Several of our RV customers have contacted us regarding vapor return lines in their fuel systems ....... From talking to them it is my understanding that they are plumbing the return line into the gascolator or some other point in the fuel system downstream of the tank. This routing exacerbates the problem of vapor formation because heated fuel returning from the engine compartment is mixed with cooler fuel coming from the tank, with the result that the temperature of the fuel penetrating the firewall on its way to the engine is at a higher temperature than it would be if the vapor return line were not installed.

Our position on vapor return lines has always been that in aircraft with more than one fuel tank, they should be used only as a last resort; when all other efforts to control fuel vapor have been tried and found to be inadequate. The reason for this is that to be effective the return fuel must be routed back to the fuel tank where it will have an opportunity to cool. When the aircraft has more than one tank, the pilot’s fuel management becomes complicated by the need to worry about where the return fuel is going. He either needs to be able to switch the return fuel to keep it returning to the tank feeding the engine, or he must remember to always start his flight on the tank which receives the return fuel and then carefully monitor fuel use out of each tank to assure that returning fuel does not fill up the return tank and be pumped overboard through the tank vents.

I would welcome any comments you may have on this topic, maybe we could find someone to build a rotary valve that would switch both the fuel source and the return fuel with the movement of a single handle.

Best regards,

Ellison Fluid Systems, Inc.

Andair makes a fuel valve that can switch the routing of the return fuel at the same time the fuel tank feeding the engine is selected.

A typical vapor return schematic diagram is shown below:

Note that the fuel return must go to the top of the tank above the surface of the fuel.  If the return is positioned below the surface of the fuel, then the return line must include a one way check valve preventing unfiltered fuel from entering the TBI.    If the aircraft has more than one tank, this system can create a fuel management problem which is discussed in the above letter.  A fuel tank selector valve which also switches the return line will solve this situation.

An excellent article about fuel system design by Lyle Powell, may be accessed by clicking on Fuel Systems for Homebuilt Aircraft.