What was found
PILOTS SUDDENLY FELT A VIBRATION AND HEARD A HOWLING NOISE. THEY RETURNED TO THE LANDING SITE AND SHUT DOWN. AT INSPECTION, MAINTENANCE FOUND 1 OF 3 NAS628-15 BOLT BROKEN IN HALF ON THE NR 2 HIGH SPEED SHAFT ON THE FORWARD END, WHERE IT ATTACHES TO THE FREE TURBINE COUPLING. BOTH HALVES OF THE BROKEN BOLT ARE BEING RETURNED TO THIS REPAIR STATION AND THE MANUFACTURER, (ERICKSON) FOR INVESTIGATION. THIS REPORT IS BEING MADE AT THIS TIME TO COMPLY WITH REPORTING REQUIREMENTS. NOTE THAT PART TT, TC AND TIME SINCE WERE ALL SET TO 0.0 BECAUSE THE REPORT DOES NOT ALLOW "UNKNOWN".