The value that set by setAirplaneThrottle =/= the value that can be get by airplaneThrottle when the plane's config throttleToThrustLogFactor isn't 1.
There're two planes in Vanilla Arma 3 that the value isn't 1, which are V-44 Blackfish and Y-32 Xi'an.
vehicle player setAirplaneThrottle 0.5;
airplaneThrottle vehicle player; // 0.793701 if the value is 3
airplaneThrottle vehicle player^(getNumber (configFile >> "CfgVehicles" >> typeOf vehicle player >> "throttleToThrustLogFactor")); // 0.5