Consider a hypothetical incident involving a wide-body freighter. The crew performed a walk-around and noted a faint gray line on the main landing gear wheel flange. It was dismissed as a paint scratch. During a max-weight takeoff, just before V1, the #2 engine ingested a bird causing a surge. The crew rejected takeoff (RTO).
The most reliable method for real-time detection is AE. During a ground test (engine run-up with brakes applied), piezoelectric sensors listen for the distinct "ping" of a crack jumping forward. If AE sensors detect a spike in RMS voltage at 80-100% N1 (fan speed), an active takeoff crack is present. active takeoff crack