you will get false as a BOOL when trying to use the IsPedInAnyVehicle(ped, false) native in a aircraft. Try using the IsPedInAnyHeli() native and make a new variable that checks if the ped is in a heli, for example:
local inHeli = IsPedInAnyHeli(ped)
if inHeli then
SetVehicleEngineOn(vehicle, true, true, false)
MotorOn = true
else
MotorOn = false
end
keep in mind that this code is just an example