WAKE ENCOUNTER -- ACR CREW OF A BE-1900D HITS THE WAKE OF A DC-9; AND AFTER PARTIALLY LOSING CTL; CONTINUES TO A SUCCESSFUL LNDG.

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A HALF HR PRIOR TO ARR; I RECEIVED THE ATIS INFO FOR THE DETROIT METRO ARPT. WIND VELOCITY WAS VERY LIGHT AND THEREFORE THE CAPT AND I BRIEFED FOR THE POSSIBILITY OF WAKE TURB. AS WE APCHED THE ARPT AT 4000 FT MSL; DC-9 JET TFC ABOVE AND AHEAD BEGAN TO DSND TO LAND ON RWY 21R. WE WERE INSTRUCTED TO FOLLOW TFC FOR A VISUAL APCH TO THE SAME RWY. AS WE FOLLOWED THE TFC WE REMARKED ON THE EXHAUST TRAIL EASILY VISIBLE TRAILING FROM THE DC-9 AHEAD. THE TRAIL WAS IMMEDIATELY AHEAD AND ABOVE OUR FLT PATH; BUT IT WAS 'DSNDING' INTO OUR FLT PATH. AS WE FLEW INTO THE EXHAUST WE EXPERIENCED A SLIGHT ROLL TO THE L; THEN A RAPID ROLL TO THE R. BECAUSE WE ANTICIPATED THE TURB WE BOTH FOUGHT THE UNCOMMANDED ROLL IMMEDIATELY AND REGAINED CTL. DURING THE ENCOUNTER WE ROLLED R ABOUT 30 DEGS AND GAINED 150 FT. I ADVISED ATC OF OUR ENCOUNTER AND THEN REMAINED 2 DOTS ABOVE GLIDE PATH FOR THE REMAINDER OF THE APCH. THE LNDG WAS PERFORMED WITHOUT INCIDENT. HINDSIGHT IS ALWAYS 20/20. IN THIS CASE THE CREW ANTICIPATED CORRECTLY AND PREPARED FOR THE WAKE TURB ENCOUNTER. HOWEVER; DUE TO THE RARITY OF ENCOUNTERS WITH THIS TYPE OF TURB; WE WERE 'REACTIVE' TO THE SIT INSTEAD OF 'PROACTIVE.' PERHAPS THE MOST VALUABLE LESSON LEARNED FROM THIS ENCOUNTER WILL BE THAT YOUR INTUITION SHOULD BE LISTENED TO AND TREATED AS ANOTHER CREW MEMBER. ALSO; IT SHOULD BE KNOWN THAT FLT INTO OR THROUGH JET ACFT EXHAUST IS NOT RECOMMENDED. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTR SAID THAT THE FLC EXPECTED THAT THEY MAY ENCOUNTER WAKE ON SUCH A CALM WIND DAY. THEY SAW THE DC-9'S ENG SMOKE TRAIL; BUT THEY MADE NO MANEUVERS TO AVOID IT. THEIR BE-1900D ROLLED L 10 DEGS THEN VIOLENTLY TO THE R ABOUT 30 DEGS AND A SLIGHT PITCH UP. THERE WAS NO PRIOR WARNING; OTHER THAN THE SMOKE TRAIL; OF THE WAKE POSSIBILITY. THE FO SAID THAT HE WILL; IN THE FUTURE; STEER AROUND OBVIOUS WAKE TRAILS.