CAPT OF AN EXECUTIVE JET MOMENTARILY LOST CTL OF ACFT JUST PRIOR TO TOUCHDOWN DUE TO ACFT ICING. THE R WING DROPPED AND THE ACFT INITIALLY TOUCHED DOWN ON THE R LNDG GEAR. RPTR REGAINED ACFT CTL AND TAXIED TO PARKING. POSTFLT INSPECTION DISCLOSED AN UNEVEN DISTRIBUTION OF RIME ICE ON THE WINGS. HOWEVER; THERE WAS MORE ICE ON THE L THAN ON THE R WING.

Narrative

ON THE EVENING OF DEC/TUE/97; WE WERE CLRED TO 3000 FT MSL TO BEGIN A GPS RWY 12 APCH INTO MARSHALLTOWN; IA. WE ENTERED THE OVCST AT BTWN 3000 FT AND 3100 FT MSL AND STARTED PICKING UP RIME ICE IMMEDIATELY. ENG ANTI-ICE WAS ON PRIOR TO ENTERING THE OVCST AND THE WING ANTI-ICE WAS SELECTED WHEN THE ICE WAS FIRST DETECTED. SINCE THE WINGANTI-ICE HAD BEEN USED EXTENSIVELY DURING THAT FLT AND THE PREVIOUS DAY; THE TKS FLUID WAS AVAILABLE TO THE WING PANELS IMMEDIATELY. WHEN WE ENTERED THE TOP OF THE OVCST AT ABOUT 3100 FT AND STARTED TO PICK UP ICE; WE CLBED TO 3200 FT MSL TO GET BACK ON TOP OF THE OVCST AND OUT OF ICING CONDITIONS. WE WERE CLRED TO OSAZE (IAF) TO BEGIN THE GPS APCH AND UPON REACHING OSAZE WE DSNDED TO 2600 FT MSL AS PER THE APCH PROC. AT THIS POINT WE WERE 15 MI FROM TOUCHDOWN AND THE APCH PROCEEDED NORMALLY AND WE BROKE OUT OF THE OVCST AT 700 FT MSL. THE RPTED WX AT THE TIME WAS OVCST CEILINGS AT 700 FT AND 6 MI VISIBILITY WITH LIGHT WINDS FROM 170 DEGS AT 4 KTS. THE COMPUTED VREF AT OUR LNDG WT WAS 120 KTS AND WE CROSSED THE RWY THRESHOLD AT THAT SPD. WHILE IN THE FLARE AND WHAT FO AND I WOULD GUESS TO BE PERHAPS 1-2 FT OFF THE RWY; THE R WING SUDDENLY AND WITHOUT WARNING DROPPED. SINCE WE WERE SO CLOSE TO THE RWY AT THE TIME THE WING DROPPED; A REASONABLY LIGHT TOUCHDOWN WAS MADE. THE STALL WARNING AND STICK SHAKER WAS EVIDENT PRIOR TO THE WING DROPPING AND OUR TOUCHDOWN SPD WAS APPROX 5-8 KTS BELOW VREF WHICH IS NORMAL FOR THIS AIRPLANE. AFTER LNDG WE INSPECTED THE LEADING EDGE OF THE AIRPLANE AND FOUND APPROX 3/8 INCH OF RIME ICE COVERING ABOUT 60% OF THE WING. THE R WING SEEMED TO HAVE SLIGHTLY LESS ICE ON IT THAN THE L WING BUT STILL IT WAS THE R WING THAT DROPPED. IN THE ACFT FLT MANUAL; THE MINIMUM SPD FOR TKS (ANTI-ICE FLUID) DISTRIBUTION ON THE WING AT OUR LNDG WT OF 20000 LBS IS JUST OVER 200 KTS. SINCE WE WERE IN ICING CONDITIONS FOR ABOUT 15 MI UNDER 200 KTS OF SPD I SUSPECT THE TKS DISTRIBUTION WAS NOT SUFFICIENT UNDER THE CONDITIONS WE ENCOUNTERED THAT EVENING. ALL WING AND TAIL ICE WAS REMOVED PRIOR TO OUR NEXT TKOF WHICH WENT WITHOUT INCIDENT.