SAAB 340B ACFT ON FINAL APCH TO RWY 31; JFK; ENCOUNTERED SEVERE WAKE TURB FROM PRECEDING B767 AT 100 FT. A 50 DEG ROLL TO THE L OCCURRED INSTANTLY; THE FLC REGAINED ACFT CTL AND EXECUTED A GAR. GAR ALT ASSIGNED WAS 2000 FT; BUT DUE TO INCREASED FLC WORKLOAD AND HIGH PWR SETTING FOR CLB; THEY WENT THROUGH THE ASSIGNED ALT WHICH REQUIRED CTLR INTERVENTION.

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AT 100 FT AGL WE ENCOUNTERED WAKE TURB FROM THE PRECEDING B767. (DURING LNDG) OUR ACFT ROLLED APPROX 40-50 DEGS TO THE L. THE CAPT (PF) INITIATED A GAR. WE THOUGHT WE HEARD TURN R 090 DEGS MAINTAIN 3000 FT. WE WERE VERY BUSY DURING THE GAR AND CALLED TO VERIFY OUR ALT. THE CTLR TOLD US TO MAINTAIN 2000 FT. AT THIS TIME WE WERE AT 2400 FT AND IMMEDIATELY RETURNED TO 2000 FT. WE THEN LANDED WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FO ATTRIBUTES WAKE ENCOUNTER AT LOW ALT TO THE CLOSE IN TRAIL SPACING OF THEIR ACFT BEHIND A B767. RPTR SAYS; HAD THE ENCOUNTER OCCURRED AT A HIGHER ALT; THEY PROBABLY WOULD HAVE BEEN ABLE TO CONTINUE THE APCH. SINCE IT HAPPENED AT JUST 100 FT; BOTH FLC MEMBERS WERE VERY ALARMED AT THE SEVERE BANK; 50 DEGS; THAT RESULTED. THEN; DURING THE GAR THE WORKLOAD INCREASED DRAMATICALLY; CONTRIBUTING TO CLBING THROUGH THEIR ATC ASSIGNED ALT. RPTR SAYS THAT THEY SHOULD HAVE SET THEIR ALT ALERT TO THE ASSIGNED ALT WHEN GIVEN ON THE INITIATION OF THE GAR.