Starter disengagement should occur at what percent N2?

• A. 30% N2
• B. 50% N2
• C. 70% N2
• D. 90% N2

Answer: (B)

Starting a turbine engine requires bringing the high-pressure spool up to speed so ignition can be established reliably. Disengaging the starter around 50% N2 hits a balance: the core is spinning fast enough that light-off and stable operation are likely once fuel is introduced, but the starter isn’t left to drive the engine past what it can safely handle. If you cut out too early, ignition may fail to light; if you wait too long, the starter can be overstressed or the engine could overspeed once ignition starts. Because of this, about half of N2 is the standard reference for starter disengagement in many training and maintenance procedures, with small variations depending on the engine type.