Shakedown testing of an aircraft design involves test flights of the prototypes, a process that actually starts months or years before first flight with simulator flights and hardware testing. This process often incorporates an iron bird test rig in which all the flight control systems are brought together in an engineering lab, while test-articles of the physical structure will be subjected to stress and fatigue loads beyond anything the aircraft is likely to encounter in service. The aircraft systems are gradually commissioned on board the prototypes; first on external power, then, once engines are fitted, on internal power, progressing to taxi trials and eventually first flight. Flight-testing proceeds conservatively, demonstrating that each test condition can be safely achieved before proceeding to the next. Prototype aircraft are generally heavily instrumented in order to support these flight-test objectives by capturing large amounts of data for both live analysis (which on larger aircraft such as airliners may happen at dedicated flight-test engineer stations on board) and for analysis post-flight. The ultimate aim of testing is to demonstrate the aircraft can operate safely throughout its flight envelope and that all regulatory requirements of the relevant National Aviation Authorities have been met, allowing the design to receive its Certificate of Airworthiness.