AN ACR B737-200 HIT THE WAKE OF A B727-200 WHILE ON A 6 MI FINAL APCH TO RWY 27R AT ORD. THE FLC USED ALMOST ALL OF THEIR ROLL AUTH AND RUDDER TO STOP A SUSTAINED ROLL TO THE L AT 25 DEGS OF BANK. THEY RECOVERED AND CONTINUED THE APCH AND LNDG.

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ATC GAVE DSCNT TO 4000 FT MSL AND TURN TO THE RWY 27R LOC FOR A VISUAL APCH 3 MI BEHIND B727. WHILE ON GLIDEPATH AT ABOUT 3000 FT MSL WHILE I WAS TUNING RADIOS; PF (STUDENT CAPT ON IOE) ENCOUNTERED A STEADY ROLL TO THE L. HE APPLIED FULL R RUDDER AND AILERON. THE ROLL STOPPED; BUT DID NOT LEVEL ACFT. PWR WAS APPLIED WITH LEVEL PITCH TO ACFT GAR PWR. THE ACFT BEGAN TO RESPOND AFTER IT POWERED OUT OF THE WAKE VORTICES. I ESTIMATE THE BANK ANGLE REACHED 25 DEG L BANK. WE CONTINUED THE APCH TO A SAFE NORMAL LNDG. PLT AT CTLS SAID UNUSUAL ATTITUDE TRAINING WAS AN IMPORTANT FACTOR TO HIS PROPER RESPONSE. NEITHER ONE OF US HAD EVER ENCOUNTERED THIS TYPE OF ROLL BEFORE. MY C130 FORMATION FLYING HAD NEVER PRODUCED A SIT WHERE I ENCOUNTERED SUCH VORTICES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS IN A B737- 200 AND THEY HAD THE B727 IN SIGHT. THE INITIAL ENCOUNTER WAS A GENTLE ROLL TO THE L THAT THE FLYING CAPT WAS ABLE TO COUNTER; BUT THE SECOND ROLL WAS MUCH STRONGER AND ALTHOUGH THE AILERON AND RUDDER DISPLACEMENT WAS FAIRLY EXTREME THE BEST THAT THEY WERE ABLE TO DO WAS STOP THE ROLL PROGRESSION AT ABOUT 25 DEGS OF L BANK. THE IOE CAPT THEN HELD THE CTL DISPLACEMENT WHILE ADVANCING PWR TO HOLD ALT AND PREPARE FOR A GAR. THIS LATTER INPUT ALLOWED THE ACFT TO GET ABOVE THE WAKE AND RECOVER AND CONTINUE THE APCH.