What was found
DURING FIREWALL CLEANING/INSPECTION DURING ANNUAL/100HR INSPECTION, A CRACK, APPROXIMATELY 2.3 IN LENGTH WAS DISCOVERED IN THE UPPER RADIUS OF THE LEFT HAND SIDE OF THE LOWER FIREWALL DOUBLER DIRECTLY THROUGH THE TOP RIVET THAT HOLDS THE PARKING BRAKE BRACKET (PN 0713070-20) TO THE FIREWALL ON THE CABIN SIDE. THERE WERE SOME SMALL DIMPLES IN THE FIREWALL ITSELF ABOVE THIS AREA. IT WOULD APPEAR THAT THE ENGAGING OF THE PARKING BRAKE (PERHAPS TOO OVERZEALOUSLY) FLEXES THIS AREA AND CAUSE THE CRACK OVER TIME. WE RECENTLY (WITHIN THE LAST 3 MONTHS) HAD ANOTHER CAP AIRCRAFT WITH THE EXACT SAME CRACK, AT SIMILAR TIME IN SERVICE.