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The differences between the elbows, fittings, pipe sizes, hoses and routing methods used during installation
and the fuel pressure for your aircraft can, for any fuel flow gauge, cause the flow transducer to output a
different number of electrical pulses per gallon (K Factor) than when it was tested on the bench. To correct
for any errors in the K Factor, keep track of the Actual Fuel Used (fuel required to bring the tanks back to
full) and compare this with the FP-5L Fuel Used (Full Tank Level minus Fuel Remaining as displayed on the
FP-5L). If your error is less than 3 gallons for a single tank, you should average the error over 3 tanks of
fuel. Use the following formula to correct the K Factor:
Note: The K Factor can be automatically corrected by using the "Auto Calibrate" Mode (see the Display
Modes and Operation Features section of this manual).
Note: When refueling an aircraft fuel tank, it is not easy to fill the tanks to exactly the same level each time.
Some variables are: 1. How level the wings are during refueling. 2. The affects of any crossover tubes.
3. The ability of the line attendant to fill the tank to the same mark. Also, the shape of the tank and the
dihedral of the wings can aggravate the situation.
Note: K Factor settings below 580 will cause the display resolution to increase above .1 Gal/Hr. when used
with a Display Update Time setting of "UP2." K Factors below 180 will cause the display resolution to
increase above .1 Gal/Hr. when used with a Display Update Time setting of "UP1." The FT-90 Flow Trans-
ducer has a K Factor around 198.
4. Filter:
There are two filters used by the FP-5L to remove fluctuation or jump in the fuel
flow readings. Selecting either filter will not affect the accuracy of the FP-5L. Use
filter “F 1” for most fuel-injected engines. This filter has the fastest response time
(eight times faster than other flow gauges). Use filter “F 2” for most carbureted
engines. This filter has a slightly slower response time than F1 (still faster than other flow gauges) but will take
out any fluctuation or jump you may have seen using the F1 filter.
5. Fuel Flow at 75% Power (rich mixture) Setting:
This setting calibrates the % Horsepower Display Mode to your engine. This
procedure should be done after your have verified the calibration of the FP-5L.
Changing the K Factor on the FP-5L will change the displayed Fuel Flow and the
displayed % Horsepower.
Fly your aircraft at cruise altitude (or, if necessary, at lower altitude where you can achieve 75% power) with
a rich mixture. Refer to operating charts from your aircraft or engine manufacturer to determine your power
setting. Record the fuel flow displayed on the FP-5L at 75% power (rich mixture). For this setting, program the
recorded fuel flow into the FP-5L. When displaying the Fuel Flow at 75% Power an "r" will be displayed in the
lower left corner of the display.
(FP-5L Fuel Used) x (Current K Factor)
New K Factor = -----------------------------------------------------
Actual Fuel Used
F 1
Blinking Digit
r 17.0
Gal/Hr
Blinking Digit
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