As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING AN ANNUAL INSPECTION, A 4.5 INCH CRACK WAS FOUND IN THE SKIN AT THE AFT EDGE OF THE BOTTOM CHANNEL STIFFENER, REAR SPAR LOCATION. STOP DRILLED THE CRACK AND INSTALLED A DOUBLER. AIRFRAME HAS 6261 HRS AND WAS USED FOR FLIGHT TRAINING. REPETITIVE STRESS LOADING AND FLEXING IN THAT AREA OF STRESS CONCENTRATION MOST LIKELY CAUSED THE SKIN TO CRACK FROM FATIGUE.