Narrative:

Flight was an originator and I was captain. Flight was also the first flight of the day for aircraft. Normal preflight inspections and preparations were conducted by the flight crew, and no discrepancies were noted. The aircraft had a 6-DAY service check accomplished the previous evening, which served as the current airworthiness release for this aircraft on this flight. The flight departed gate on time and arrived uneventfully in ZZZ. Since the first officer had fewer than 100 hours since IOE and ZZZ is a 'special airport,' I made the approach and landing. While at the gate in ZZZ one of the ground personnel reported to the first officer that a small crack was evident in the skin of the aircraft underneath the APU. The first officer inspected the aircraft and found a fracture in the skin of the APU access door that was approximately 5 inches long. I was away from the aircraft at that time. Upon my arrival back at the aircraft, the first officer reported and showed this condition to me. I notified both dispatch and maintenance control of the condition. I wrote the condition up in the aircraft logbook, and, at the direction of dispatch/maintenance control, contract maintenance was called. After contract maintenance arrived, took pictures and measurements of the fracture, and conferred with maintenance, the result was an aircraft swap. My crew then preflted a new aircraft and continued uneventfully with the remainder of the day's schedule. The first officer stated that he did not notice the skin fracture during the preflight inspection. From the time the flight crew arrived at the aircraft until the aircraft was pushed back, there was nothing noticeable that jarred the aircraft (such as a vehicular collision or maintenance stand making physical contact with the aircraft). From the time the aircraft pushed in the originating city until it blocked in at ZZZ nothing collided with the aircraft, and the aircraft itself did not collide with anything. From my point of view, flight was normal in every respect from the time the crew reported 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 aircraft, and the manner in which it was ruptured/deformed (the metal was deformed and ruptured in an upward, near vertical direction with the line of the rupture nearly parallel to the ground). I believe the fracture was caused by direct physical contact with some sort of vehicle immediately beneath or in that proximity 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 flight crew was operating the aircraft, and I think it unlikely that it occurred at the gate in ZZZ since no one in ZZZ would have reason to bring mechanical equipment directly under the APU access door at that elevation, and time was short between when the aircraft blocked in and when the rupture was discovered. I believe, then, that is most likely occurred sometime before the flight crew arrived at the aircraft in our originating city. It could have been mins, hours, 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 ground personnel in ZZZ who discovered the fracture.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.