Thanks to Rares Vernica, the cause of the fuel-flow issue found on the C172SP Integrated Fuel Gauges (INTFG) for X-Plane has been spotted. It is very likely caused by the “rounding” function in Logitech’s X-Plane Plugin for the command.
The value of the “sim/cockpit2/engine/indicators/fuel_flow_kg_sec” dataref from X-Plane for the C172SP is in the range approximately between 0.003550 to 0.009475 (or 0.003xxx to 0.009xxx) when engine is running.
As shown in the image above, the 3rd decimal number on the Flight Instrument Panel (FIP) is rounded to either 0 or 5 regardless of the actual output from the dataref in X-Plane.
Since the value of “fuel_flow_kg_sec” of the C172SP is so small and starts from the 3rd decimal number, rounding the 3rd decimal number to 5 or 0 simply fail to render the fuel flow value correctly for the aircraft.
Even worse, this “rounding bug” not only affects the C172SP, but also all other small aircraft in X-Plane, including the Beechcraft Baron B58, Cirrus Vision SF50, Stinson L5 Sentinel, etc — making all fuel flow gauges developed for FIPs unusable.
It should be true to all other third-party small aircraft, such as the Carenado CT210M, as well.
For larger aircraft, such as the Boeing 737, 747 and MD-82, the rounding of the 3rd decimal place will not cause any issue because the fuel flow values on these aircraft are generally greater than 0.01.
I am currently communicating the issue with Logitech support. I hope the company cares more about customers than Saitek in the past, so that a feasible solution or an update of the X-Plane Plugin could come soon to fix the issue.