A B727-200 IN CRUISE AT FL240 INCURRED THE LOSS OF THE L MAIN LNDG GEAR DOOR DUE TO INCORRECT INSTALLATION AFTER TIRE AND WHEEL REPLACEMENT.

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ON SEP/XA/00 AT APPROX XA30Z AT FL240; APPROX 80 DME FROM SAC VOR ON THE 070 DEG RADIAL; THE ACFT SHUDDERED AS IF PASSING THROUGH AN INVERSION LEVEL OR A POSSIBLE BIRD STRIKE. NO APPARENT STRUCTURAL PROBS; FLT CTL MALFUNCTIONS; NO WARNING LIGHT INDICATIONS IN THE COCKPIT; NOR WAS THERE ANY SIGN OR INDICATION OF DAMAGE TO THE ACFT. THE POSSIBILITY OF A BIRD STRIKE AT FL240; THOUGH REMOTE; WAS ASSUMED. UPON LNDG; WALKAROUND REVEALED NO INDICATIONS OF A BIRD STRIKE. HOWEVER; UPON PREFLT; THE FE FOUND THE L GEAR DOOR MISSING AND STRUCTURAL DAMAGE TO THE UNDERSIDE OF THE L WING. MAINT WAS NOTIFIED AND THE MISSING GEAR DOOR WAS RECORDED IN THE LOGBOOK. THE CAUSE OF THE GEAR DOOR SEPARATING FROM THE ACFT IS NOT EXACTLY KNOWN AT THIS TIME. THE FE DID INSPECT THE GEAR DOOR ON HIS WALKAROUND AND IT APPEARED TO BE PROPERLY ATTACHED. A TIRE CHANGE WAS ACCOMPLISHED PREVIOUSLY IN THE DAY BEFORE THE FLT; AND IT IS SUSPECTED THAT THE ATTACHING BOLTS WERE IN PLACE BUT RETAINING NUTS TO HOLD THE BOLTS IN PLACE WERE NOT PROPERLY ATTACHED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A B727-200 AND THE L MAIN GEAR TIRE AND WHEEL HAD BEEN REPLACED THE DAY BEFORE THE FLT. THE RPTR SAID THE INBOARD FLAP JACKSCREW FAIRING WAS GOUGED AND DENTED BY THE GEAR DOOR DEP.