FLC OF A B727 LOST DIRECTIONAL CTL DURING LNDG DUE TO WX ENCOUNTER (MICROBURST) RESULTING IN A HARD LNDG AND GOING OFF THE R SIDE OF THE RWY INTO A GRASSY AREA. THE PAX WERE DEPLANED THROUGH THE REAR AIRSTAIR DOOR.

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I WAS FLYING THE ACFT. WE ENTERED THE MSY TERMINAL AREA FROM THE N; DEVIATING FOR WX BUILD-UPS. WE WERE VECTORED FOR THE ILS RWY 28 APCH. THE APCH SPDS WERE DETERMINED AT VREF; 128 AND VREF +10; 138. WITH THE ILS TUNED AND IDENTED; AND THE FLT DIRECTOR ON; I INTERCEPTED THE LOC RWY 28 AT APPROX 12 MI FROM THE FIELD AT 3000 FT AND STARTED DOWN THE GS WITH A CAPTURE ON THE FLT DIRECTOR. TWR ADVISED THAT SHOWERS WERE APCHING THE FIELD AND THAT A BEECH ACFT HAD RPTED FLUCTUATIONS IN AIRSPDS (BUT NO AMOUNT OF FLUCTUATIONS WERE GIVEN). I ADVISED WE WOULD ADD 5 KTS TO MY LNDG SPD FOR PRECAUTIONS. AT APPROX 1500 FT AGL THE RWY ENVIRONMENT WAS IN SIGHT AND I COULD SEE RAINSHOWERS AT THE OTHER END OF RWY 28. THE APCH WAS CONTINUED WITH NO ABNORMALITIES. THE WIND SEEMED TO BE MINIMAL (AS THE ATIS RPT ADVISED) SINCE ALMOST NO CORRECTION WAS NEEDED TO REMAIN ALIGNED WITH THE RWY CTRLINE. AT 100 FT AGL SOME WIND GUSTS WERE ENCOUNTERED; BUT I WAS ABLE TO REMAIN CTRLINE CTLED WITHOUT USING LARGE AMOUNTS OF FLT CTLS. AT THIS TIME; WE ENCOUNTERED LIGHT RAIN AS WAS EXPECTED. AT 40 FT I STARTED TO BREAK THE DSCNT WITH A SLOW FLARE. AT 30 FT HVY RAIN WAS ENCOUNTERED AND WIND GUSTS INCREASED DRAMATICALLY; BUT THE ACFT WAS STILL CONTROLLABLE AND THE RWY ALIGNMENT AND ENVIRONMENT WAS IN SIGHT. AT THIS TIME; THE CAPT TOOK CTL OF THE ACFT AND I FOLLOWED ALONG WITH HIM ON THE CTLS. BTWN 20 FT AND 10 FT A SUDDEN BURST OF WIND AND BLINDING RAIN HIT THE ACFT AND ALL VISIBILITY WAS LOST. THE ACFT IMMEDIATELY HIT THE RWY WITH A HARD IMPACT. WITH NO VISIBILITY THE CAPT TRIED TO MAINTAIN MINIMAL BANK ANGLE AND DIRECTIONAL CTLS. MY FIRST CONTACT WITH THE OUTSIDE ENVIRONMENT WAS OUT MY R WINDOW AND I COULD SEE THE GRASS; THUS WE WERE PARTIALLY OR ALL OFF THE R SIDE OF RWY 28. FULL USE OF THE FLT CTLS WERE USED BY THE CAPT (I FOLLOWED ALONG ON THE CTLS); TO BRING THE ACFT TO THE L; BUT WE JUST CONTINUED ON A R SIDED PATH ONTO THE GRASS. THE CAPT USED FULL BRAKING AND REVERSE THRUST TO STOP THE ACFT. ONCE STOPPED THE ENGS WERE SHUT DOWN; I CALLED THE TWR TO ADVISE THEM WE WERE OFF THE RWY. THEN I LOOKED OUT THE R SIDE AND COULD SEE THAT THE WINGTIP WAS ABOUT 1 1/2 FT OFF THE GND; AND ADVISED THE CAPT AS SUCH. SINCE THE RAIN WAS STILL COMING DOWN HARD AND THE WINDS WERE GUSTING; THE CAPT HAD THE PAX REMAIN SEATED; AND SENT THE ENGINEER TO THE BACK OF THE ACFT TO ASSESS THE SIT AND VERIFY IF WE COULD EXIT OUT THE REAR STAIRS SAFELY. THE CAPT COORDINATED WITH THE TWR FOR RESCUE VEHICLES AND TRANSPORTATION OF THE PAX. THE ENGINEER ADVISED THE CAPT THAT THE AFT AIRSTAIRS WAS A SAFE EXIT. THE PAX WERE EXITED OUT THE REAR STAIRS WITH THE REST OF THE CREW AND TAKEN TO THE TERMINAL.