A process for testing a combustor or a compressor of a gas turbine engine, where a large volume of compressed air is stored in a large reservoir of at least 10,000 m3 such as an underground storage cavern, compressed air from the storage reservoir is passed through an air turbine to drive a compressor to produce high pressure and temperature compressed air, and where the compressed air can be discharged into a combustor and burned with a fuel for testing of the combustor under simulated conditions of a real gas turbine engine.