e. 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:
(1) Place the aircraft’s nose at an angle 5 degrees above the highest anticipated climb angle. The easiest and safest way to do this with a conventional gear aircraft is to dig a hole and place the aircraft’s tail in it. For a nose gear aircraft, build a ramp to raise the nose gear to the proper angle.
(2) Make sure the aircraft is tied-down and chocked. 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.
(3) The formula for fuel flow rate for a gravity-feed fuel system is 0.55 times
engine horsepower (HP) times 1.50. This gives a fuel flow rate in pounds of fuel per hour. Divide the pounds-per-hour number by 60 to calculate pounds per minute, and divide again by 6 to calculate gallons per minute. To get gallons per hour for Avgas divide pounds per hour by 6; or multiple gallons per minute by 60. For a pressurized system, substitute 1.25 for 1.50 to calculate the fuel flow rate.
(4) The fuel consumption rate of most modern engines is about 0.55 pounds per hour per brake HP. The chart below illustrates the results of this calculation for a 100 HP engine: