After how many minutes post-start must N1 not exceed 75%?

The correct response indicates that after 2 minutes post-start, N1 should not exceed 75%. This limitation is in place to ensure the engine operates within safe parameters during the critical initial phase after engine start. Immediately after starting, the engine requires time to stabilize and reach optimal operating conditions. During this stabilization period, exceeding 75% N1 can pose risks, such as overheating or stressing the components of the engine that may not yet be adequately lubricated or cooled.

Having a defined time frame, such as 2 minutes, allows pilots to ensure that the engine has safely transitioned to a stable operating condition before increasing power settings beyond that threshold. This practice is crucial for maintaining engine longevity and preventing potential mechanical issues that could arise from premature high thrust operations.

While options like 1 minute, 3 minutes, and 5 minutes suggest different considerations for engine start limitations, they do not align with the established protocol of ensuring that the engine is well within safe operating ranges specifically after the first 2 minutes of operation.