ACFT EQUIP PROB -- THE EMER EXIT WINDOW (OVERWING) DEPARTS FROM A LIGHT TWIN.

Narrative

I DEPARTED TORRANCE ARPT (TOA) IN A CESSNA 402 FOR SHORT REFRESHER FLT. FBO INSTRUCTOR IN CAPACITY OF OBSERVER WAS IN COPLT SEAT. MY SON WAS SEATED IN CABIN ON L SIDE (REAR FACING SAT) -- THE ONLY (AGE 11) PAX. AFTER NORMAL DEP FROM RWY 29R; I TURNED SW AT THE REDONDO BEACH SHORELINE AND BEGAN LEVELING AT 3000 FT MSL. AT ABOUT 2 MI OFF- SHORE; THE ACFT SHUDDERED AND WIND NOISE INCREASED. THE CABIN EMER EXIT WINDOW (R SIDE OF CABIN) HAD DEPARTED THE AIRPLANE. THE CABIN PAX WAS STILL BUCKLED INTO THE L HAND FORWARD CABIN SEAT. I REDUCED PWR TO 20 INCHES MANIFOLD PRESSURE WHILE THE OBSERVER/PLT LOOKED BACK FOR STRUCTURAL DAMAGE. NO TAIL DAMAGE WAS OBSERVED; AND THE CABIN PAX RPTED THE WINDOW POPPED OFF AND PASSED UNDER THE TAIL. WE RETURNED TO TOA FOR A NORMAL LNDG ON RWY 29R. ACFT FLT CHARACTERISTICS REMAINED NORMAL. TWR ASKED ACFT ON PARALLEL IFR APCH TO BREAK OFF AND APPROVED MY REQUEST FOR RWY 29R. POSTFLT INSPECTION REVEALED THE EMER RELEASE D-RING STILL CLIPPED IN PLACE BELOW THE MISSING WINDOW; AND NO APPARENT REASON FOR LOSS. I AM CERTAIN THE PAX (SON) WAS SEATED AND BUCKLED IN TO THE L CABIN SEAT AT ALL TIMES FROM ENG START TO COMPLETION OF FLT. FBO MECHS RPT NO KNOWN REASON FOR WINDOW LOSS. THERE WERE NO INJURIES OR DAMAGE OTHER THAN THE MISSING EXIT WINDOW. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE PLT SAID THAT AFTER THE INITIAL INSPECTION BY THE FBO'S MAINT DEPT THE FAA INSPECTED THE ACFT AND CONCLUDED THAT THE PAX; THE PLT'S 11 YR OLD SON; MUST HAVE DONE SOMETHING TO CAUSE THE EMER EXIT WINDOW TO DEPART. ANOTHER WINDOW WAS ORDERED FROM CESSNA FOR THIS 402. THE SECOND WINDOW WAS INSTALLED AND INSPECTED BY THE FBO'S MECHS. THE CESSNA 402 WAS TEST FLOWN AND THE WINDOW DEPARTED. ANOTHER WAS ORDERED FROM CESSNA. THE 3RD WINDOW WAS INSTALLED BY THE MECHS AND INSPECTED BY BOTH THE FBO AND THE FAA. THE CESSNA 402 WAS AGAIN TEST FLOWN AND AGAIN THE WINDOW DEPARTED. ANOTHER WINDOW WAS INSTALLED TEST FLOWN SUCCESSFULLY AND THE ACFT WAS THEN SOLD TO AN OVERSEAS COMPANY. CESSNA TOLD THE FBO AND THE PLT THAT THEY HAVE NEVER HAD THIS PROB BEFORE. THE RPTR SAID THAT THE FBO; LATER; TOLD HIM THAT THE 10000 HR ACFT WAS PROBABLY DIMENSIONALLY CHANGED AND THAT MAY BE A CAUSE FOR THE SEPARATIONS. THIS IS NOT A PRESSURIZED MODEL.