The starter should not be engaged while N2 is above what percent?

• A. 60%
• B. 55%
• C. 50%
• D. 65%

Answer: (B)

Engage the starter only when the N2 spool is at a low speed. The starter is designed to accelerate a stationary or near-stationary compressor, not to be connected when the high-pressure/secondary spool is already turning quickly. If N2 is above the limit, engaging the starter can load or back-drive the starter mechanism and the starter clutch, leading to damage or a failed start.

The published limit is 55% N2. That means you should not energize the starter once N2 has risen above about half of the engine’s spool speed. When N2 is above that threshold, you wait until it drops to a safe level before attempting to start. Values higher than 55% would allow starting while the spool is already turning too fast, which is not allowed; a value like 50% would be below the limit, but the documented requirement is specifically 55%.