At what N1 percentage does the ENG START switch disengage?

• A. 40%
• B. 60%
• C. 50%
• D. 30%

Answer: (A)

When starting a turbine engine, the ENG START switch powers the starter to spool the engine up. The switch disengages once the low-pressure spool (N1) reaches about 40% speed. At that point the engine has enough airflow and ignition to sustain operation on its own, so the starter is cut out to prevent overheating or damage from prolonged start. If it disengaged too early, the engine might not reach stable, self-sustaining speed; if it stayed engaged too long, the starter could overwork. So about 40% N1 is the designed disengagement point.