How quickly should N1 rise after activating the starter switch during an immediate airstart attempt?

The rise of N1 after activating the starter switch is an important parameter during an immediate airstart attempt. N1 should begin to rise within 5 seconds after the starter switch is engaged. This timeframe is crucial because a prompt N1 rise indicates that fuel has been successfully introduced into the engine and that the ignition process is functioning correctly.

If N1 does not rise within this timeframe, it may indicate a failure in the ignition or fuel system, necessitating a different course of action. Additionally, understanding this critical timing helps ensure safety and efficiency in the airstart process, as pilots must be prepared to respond appropriately if the engine does not start or if there are indications of a malfunction.