Abstract: An improved gas turbine engine diagnostic system is disclosed which is particularly adapted for direct measurement of compressor performance, including efficiency and mass flow, for aircraft engines, without having to remove the engine form the aircraft. The system includes an instrumented compressor air discharge tube which has probes for total pressure, static pressure and temperature. The instrumented air discharge tube may be substituted for an existing air discharge tube for the engine to be tested, without having to remove the engine. Transducers attached to these probes provide signals which are connected to a data recorder or computer. Data can be taken as the engine is run, either on ground test or flight test. The total mass airflow output from the compressor stage for various engine performance settings can then be calculated using the static pressure, total pressure and air temperature measurements.