A combined cycle power plant utilizing an air injection apparatus for lowering the temperature and raising the mass of the exhaust gas provided to the heat recovery steam generator from the gas turbine portion of the plant. Also a method for the detection of bit errors in the processing of digitalized transmitted data is disclosed whereby the data are transmitted by an optical transmission system. An injection pulse width output to injectors 15 is corrected to reduce an air-fuel ratio fluctuation due to the supply of this desorbed fuel to the engine 10.