ATX SMA ENCOUNTERS EXTREME TURB AND MICROBURST ON APCH TO LAND.

Narrative

AUTOPLT UNABLE TO HOLD ALT (CLB INDICATED); WHILE AIRSPD BEGAN TO INCREASE INTO THE YELLOW ARC. ADVANCING WARM FRONT PRODUCING SUPERCOOLED RAIN BELOW 3000 FT OVCST WITH A 50 KT HEADWIND AND A FULL 1 INCH PRESSURE DROP WITHIN THE LAST 50 MI RPTED BY ATC FOR ALTIMETER SETTING. ENCOUNTERED A CLOUD WHICH PRODUCED VIOLENT TURB ABOUT ALL 3 AXES. NOTHING HAD SHOWN ON STORMSCOPE PRIOR NOR HAD PREVIOUS CLOUD ENCOUNTERS PRODUCED THIS EFFECT. 180 DEGS TO E INITIATED WITH VECTORS BACK TO CHARLESTON FOR THE VOR DME #3 APCH. STRATUS AT 600 FT AND HVY RAIN REDUCED VISIBILITY FURTHER AS MICROBURST ENVELOPED ACFT. GEAR DOWN; THROTTLES IDLE AND AIRSPD IS STILL IN THE YELLOW ARC. AS WE APCHED 800 FT AND UNABLE TO HOLD MINIMUM APCH ALT; PWR WAS RESTORED TO FULL DESPITE HIGH AIRSPD IN CONTINUING VIOLENT TURB. 100 FPM CLB WAS ATTAINED UP TO 1000 FT AS ATC VECTORED ACFT DIRECT TO RWY 3 AS COCKPIT NAVIGATIONAL INSTS WERE UNREADABLE IN THE TURB. DESPITE ACCEPTING CLRNC FOR VOR APCH; THE ACFT WAS UNABLE TO HOLD THE ASSIGNED ALT OF 1600 FT IN THE DOWNBLAST. THE ACFT LANDED WITH NO FURTHER PROBS.