ACR WDB LNDG OVERSHOOT AND RWY EXCURSION.

Narrative

A LONG AND FAST LNDG ON A WET RWY RESULTED IN STOPPING BEYOND THE RWY END AND INTO THE PAVED OVERRUN. BACKGND AND DESCRIPTION OF EVENTS. THE CAPT PERMITTED THE INTL RELIEF OFFICERS (IRO) TO FLY THE LAST LEG OF 3 DAYS OF INTL FLYING BY THE SAME CREW. I WAS THE DESIGNATED FO AND OCCUPIED AN OBSERVERS SEAT BEHIND AND BTWN THE CAPTS AND FO SEATS. DURING APCH TO KELT; THE IRO FAILED TO INITIATE A TIMELY DSCNT XING THE FAF AND INTERVENTION BY THE CAPT WAS NOT ADEQUATE TO CORRECT A HIGH AND FAST APCH. WHEN THE ACFT HAD NOT YET TOUCHED DOWN AFTER PASSING APPROX 40 PERCENT OF THE RWY; THE CAPT PULLED THE SPDBRAKE AND A FIRM TOUCHDOWN RESULTED ON RWY 23 ABOUT HALF WAY BTWN THE ROMEO AND BRAVO INTXNS. MAX EFFORT BRAKING AND REVERSE THRUST STOPPED THE ACFT IN THE PAVED OVERRUN. NO ACFT DAMAGE OCCURRED. ANALYSIS AND OPINION: AT OUR AIRLINE THE IRO RECEIVES THE SAME INITIAL TRAINING AS THE CAPT/FO BUT DOES NOT MAINTAIN LNDG CURRENCY IN LINE OPS. HE MERELY FILLS IN DURING ENRTE OPS. I QUESTION THE VALUE OF THIS 'INFLT RELIEF' WHEN NORMAL PROCEDURAL FAMILIARITY IS NOT MAINTAINED. THE DECISION TO PERMIT THE IRO TO FLY WAS A MISTAKE; IN HINDSIGHT. CREW ROOM SCUTTLEBUT HAD MADE THE PRACTICE OF IRO FLYING KNOWN TO ME SO THE DECISION TO GIVE THE IRO THE LAST LEG DID NOT AROUSE ANY OBJECTIONS ON MY PART. THE IRO'S PERFORMANCE IN EARLIER PORTIONS OF THE FLT WAS A LITTLE SLUGGISH ALTHOUGH HIS FLYING WAS NORMAL; THIS MINOR SLUGGISHNESS; HOWEVER; GAVE EARLY WARNING THAT SPECIAL CARE WOULD BE REQUIRED DURING APCH. THE PRESENCE OF PASSING RAIN SHOWERS AND WET RWYS RAISED ANOTHER FLAG. THE ASSIGNMENT OF RWY 23 WITH ITS BACKCOURSE APCH (NO GS); DOWN SLOPE; AND MODEST LENGTH PRESENTED ADDITIONAL APCH CONSIDERATIONS. ULTIMATELY; THE FINAL FACTOR IN THIS UNCOMFORTABLE SITUATION WAS THE FAILURE TO EXECUTE A GAR WHEN THE APCH REMAINED WELL OUTSIDE THE BOUNDS OF A NORMAL APCH PROFILE. I DO NOT FULLY UNDERSTAND WHY I DID NOT YELL 'GAR!'