Before attempting an inflight restart, N1 must reach what percent?

• A. 5%
• B. 40%
• C. 0%
• D. 90%

Answer: (C)

Starting from a non-spinning core is how an inflight restart is designed to work. N1 represents the low‑pressure compressor speed, and for a restart you want the engine to be brought from rest by the starter so fuel can light off cleanly and the engine can accelerate through the light‑off window to idle. If N1 is already at some nonzero value, the starter and fuel control may not be able to re‑establish proper light‑off timing, risking flameout, surge, or starter damage. From zero percent N1, the starter can smoothly re‑spool the core and ignition can be timed correctly as it accelerates to idle. The other options would imply the engine is already spinning, which is not suitable for initiating a controlled inflight restart.