AN EMB120 L HYD BAY ACCESS DOOR OPENS INFLT; DAMAGING THE L WING AND INBOARD FLAP. FO RPTR ATTRIBUTES THIS INCIDENT TO ACFT AGE AND HRS.

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CRUISE FLT FL200. AFTER APPROX 1 HR FLT TIME A LOUD THUMP AND SHUDDER OF THE ACFT. BELIEVING A GEAR DOOR HAD OPENED INFLT THE ACFT WAS SLOWED TO APPROX 190-200 IAS; UNEVENTFUL LNDG AT GTR WAS ACCOMPLISHED. I BELIEVE; BECAUSE OF THE AGE OF THE ACFT AND THE HIGH HRS ON THE AIRFRAME AND COMPONENTS; THAT THE L HYD BAY DOOR BECAME UNSECURED AND OPENED INFLT. THE HYD BAY DOOR WAS STILL ATTACHED; HOWEVER IT WAS DESTROYED. THERE WAS DAMAGE TO THE L INBOARD WING AND L INBOARD FLAP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO STATED THAT THE ACFT HAD BEEN SVCED AND INSPECTED FOR A MALFUNCTIONING NOSE GEAR. THE GEAR HAD TO BE 'SWUNG' AND TO DO THIS THE NOSE GEAR ACCESS DOOR HAS TO BE OPENED. IT WAS POSSIBLE THAT THE HYD ACCESS DOOR WAS OPENED AT THIS TIME AS WELL. THE ACFT WAS ACTUALLY PREFLTED TWICE BY THE FO AFTER THIS PROC AND THE BAY ACCESS DOOR CHKED TWICE. IT WAS CLOSED AT THAT TIME. HE STATED THAT IF IT HAD NOT BEEN PROPERLY CLOSED; THE DOOR WOULD HAVE OPENED ON THE INITIAL CLB; A CLASSIC SIT. SINCE THIS EVENT OCCURRED 1 HR INTO THE FLT HE FEELS THAT IT WAS EITHER AIRFRAME EXPANSION; VIBRATION OR 'AGE' THAT CREATED THE INCIDENT. HE HAS NO KNOWLEDGE OF SIMILAR INCIDENTS OCCURRING. THE HYD BAY ACCESS DOOR IS NOT INCLUDED IN THE CIRCUITRY FOR ANY DOOR WARNING LIGHTS.