During a starter assisted engine start, you should not try to engage the air turbine starter if the N2 RPM exceeds what percentage?

• A. 30%
• B. 45%
• C. 50%
• D. 60%

Answer: (B)

The starter is used to provide initial rotation of the engine core until ignition can take over. The critical speed you don’t want to push the starter past is about 45% N2. At that point the engine’s own air and fuel will carry the rotation, and continuing to run the air turbine starter risks overspeeding the starter/gear train and stressing the engine. So if N2 is already above roughly 45%, you should not engage the air turbine starter. If N2 is below that threshold, engaging the starter helps bring the core up to speed for ignition. Some engines may have a similar range, but 45% is the typical guideline used in training.