A B737-500 WAS DISSIPATED IN NON COMPLIANCE WITH UNRPTED APU DOOR DAMAGE. DENT AND CRACK CAUSED BY GND EQUIP.

Narrative

FLT WAS AN ORIGINATOR AND I WAS CAPT. FLT WAS ALSO THE FIRST FLT OF THE DAY FOR ACFT. NORMAL PREFLT INSPECTIONS AND PREPARATIONS WERE CONDUCTED BY THE FLT CREW; AND NO DISCREPANCIES WERE NOTED. THE ACFT HAD A 6-DAY SVC CHK ACCOMPLISHED THE PREVIOUS EVENING; WHICH SERVED AS THE CURRENT AIRWORTHINESS RELEASE FOR THIS ACFT ON THIS FLT. THE FLT DEPARTED GATE ON TIME AND ARRIVED UNEVENTFULLY IN ZZZ. SINCE THE FO HAD FEWER THAN 100 HRS SINCE IOE AND ZZZ IS A 'SPECIAL ARPT;' I MADE THE APCH AND LNDG. WHILE AT THE GATE IN ZZZ ONE OF THE GND PERSONNEL RPTED TO THE FO THAT A SMALL CRACK WAS EVIDENT IN THE SKIN OF THE ACFT UNDERNEATH THE APU. THE FO INSPECTED THE ACFT AND FOUND A FRACTURE IN THE SKIN OF THE APU ACCESS DOOR THAT WAS APPROX 5 INCHES LONG. I WAS AWAY FROM THE ACFT AT THAT TIME. UPON MY ARR BACK AT THE ACFT; THE FO RPTED AND SHOWED THIS CONDITION TO ME. I NOTIFIED BOTH DISPATCH AND MAINT CTL OF THE CONDITION. I WROTE THE CONDITION UP IN THE ACFT LOGBOOK; AND; AT THE DIRECTION OF DISPATCH/MAINT CTL; CONTRACT MAINT WAS CALLED. AFTER CONTRACT MAINT ARRIVED; TOOK PICTURES AND MEASUREMENTS OF THE FRACTURE; AND CONFERRED WITH MAINT; THE RESULT WAS AN ACFT SWAP. MY CREW THEN PREFLTED A NEW ACFT AND CONTINUED UNEVENTFULLY WITH THE REMAINDER OF THE DAY'S SCHEDULE. THE FO STATED THAT HE DID NOT NOTICE THE SKIN FRACTURE DURING THE PREFLT INSPECTION. FROM THE TIME THE FLT CREW ARRIVED AT THE ACFT UNTIL THE ACFT WAS PUSHED BACK; THERE WAS NOTHING NOTICEABLE THAT JARRED THE ACFT (SUCH AS A VEHICULAR COLLISION OR MAINT STAND MAKING PHYSICAL CONTACT WITH THE ACFT). FROM THE TIME THE ACFT PUSHED IN THE ORIGINATING CITY UNTIL IT BLOCKED IN AT ZZZ NOTHING COLLIDED WITH THE ACFT; AND THE ACFT ITSELF DID NOT COLLIDE WITH ANYTHING. FROM MY POINT OF VIEW; FLT WAS NORMAL IN EVERY RESPECT FROM THE TIME THE CREW RPTED FOR DUTY IN OUR ORIGINATING CITY UNTIL THE FRACTURED APU ACCESS DOOR WAS DISCOVERED IN ZZZ. AFTER VIEWING THE FRACTURE IN ZZZ ITS LOCATION ON THE ACFT; AND THE MANNER IN WHICH IT WAS RUPTURED/DEFORMED (THE METAL WAS DEFORMED AND RUPTURED IN AN UPWARD; NEAR VERT DIRECTION WITH THE LINE OF THE RUPTURE NEARLY PARALLEL TO THE GND). I BELIEVE THE FRACTURE WAS CAUSED BY DIRECT PHYSICAL CONTACT WITH SOME SORT OF VEHICLE IMMEDIATELY BENEATH OR IN THAT PROX TO THE APU ACCESS DOOR THAT EXERTED EXCESSIVE FORCE UPWARD INTO THE APU CAVITY. TO ADDRESS THE QUESTION OF WHEN IT OCCURRED; I CAN ONLY SPECULATE. IT DID NOT OCCUR WHILE THE FLT CREW WAS OPERATING THE ACFT; AND I THINK IT UNLIKELY THAT IT OCCURRED AT THE GATE IN ZZZ SINCE NO ONE IN ZZZ WOULD HAVE REASON TO BRING MECHANICAL EQUIP DIRECTLY UNDER THE APU ACCESS DOOR AT THAT ELEVATION; AND TIME WAS SHORT BTWN WHEN THE ACFT BLOCKED IN AND WHEN THE RUPTURE WAS DISCOVERED. I BELIEVE; THEN; THAT IS MOST LIKELY OCCURRED SOMETIME BEFORE THE FLT CREW ARRIVED AT THE ACFT IN OUR ORIGINATING CITY. IT COULD HAVE BEEN MINS; HRS; OR EVEN DAYS BEFORE; BECAUSE AFTER SEEING THE FRACTURE; I CAN SAY WITH CERTAINTY THAT IT WOULD HAVE BEEN VERY EASY TO MISS; PARTICULARLY UNDER LOW LIGHTING CONDITIONS. I COMMEND AND THANK THE GND PERSONNEL IN ZZZ WHO DISCOVERED THE FRACTURE.