A MIL TRAINER ACFT HAS A RAPID DECOMPRESSION DUE TO LOSS OF A LARGE SECTION OF THE CANOPY.

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INFLT EMER ON T33; A MIL TRAINER ACFT. WE ENCOUNTERED AN ACFT CANOPY FAILURE AND RAPID CABIN DECOMPRESSION DURING FLT. THE FLT CONDITIONS WERE STRAIGHT AND LEVEL FLT AT APPROX FL250 AND 280 KIAS. A PORTION OF THE CANOPY; ABOUT 20%; OF THE L; AFT SECTION DEPARTED THE ACFT -- ESTIMATED AS APPROX 5 FT BY 2 FT. THE AIR CREW DOES NOT KNOW IF THE PART WHICH LEFT THE ACFT REMAINED IN ONE PIECE OR WAS SHATTERED DURING THE BLOWOUT. THE AIR CREW DSNDED TO LOWER ALT; DECLARED AN EMER; AND RETURNED FOR AN UNEVENTFUL LNDG. NOTE ON ACFT MAINT: THIS ACFT IS OPERATED AND MAINTAINED IN ACCORDANCE WITH FARS 91 AND 43; RESPECTIVELY. ACFT IS LICENSED AS 'EXPERIMENTAL' CATEGORY AND COMPLIES WITH FAA REGS FOR IFR IN UNITED STATES AIRSPACE. MAINT PROCS ARE DEFINED IN FAA APPROVED D950-10434-1; 'INSPECTION PROGRAM FOR CANADAIR CT-133 MK3 (T33) ACFT.' THE ACFT WAS INSPECTED FOR CONTINUED AIRWORTHINESS (ANNUAL INSPECTION) ON JUN/MON/04; AND A PREFLT INSPECTION WAS COMPLETED IN THE MORNING; PRIOR TO TKOF. I CONSIDER ALL MAINT AND PROCS FOR THIS ACFT TO BE SUPERB; AND WERE OF NO FACTOR IN THIS EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CABIN ALT AT THE TIME OF THE INCIDENT WAS APPROX 11000 FT. THE CREW WAS WEARING OXYGEN MASKS AND WAS NOT AFFECTED BY THE INCREASED CABIN ALT. THEY ROUTINELY ATTEND A RECURRENT ALT CHAMBER TRAINING AND WERE AWARE OF WHAT TO EXPECT. SPARES AVAILABLE FOR REPAIR WERE DETERMINED TO BE SVCABLE BUT QUITE OLD. THE NEWEST ONE AVAILABLE WAS INSTALLED AND AN ORDER HAS BEEN MADE TO ACQUIRE A NEWLY MANUFACTURED ONE FOR THE FINAL REPAIR. THERE HAS NEVER BEEN A RECOVERY OF THE LOST CANOPY RPTED EVEN THOUGH PRECISE LOCATION OF THE INCIDENT IS KNOWN.