ACR MDT DECLARED AN EMER FOR AIRFRAME VIBRATION AFTER TKOF FROM MKE.

Narrative

TAXI AND TKOF WERE NORMAL. ALL CHKLISTS COMPLETED PROFESSIONALLY AND LEGALLY. SHORTLY AFTER GEAR RETRACTION; ACFT BEGAN TO SHAKE AND SHUDDER VIOLENTLY. INITIAL REACTION WAS THAT ACFT WAS ENTERING A STALLED CONDITION. THIS WAS IMMEDIATELY RULED OUT DUE TO THE FACT THAT WE HAD POSITIVE ACFT CTL; ALBEIT IN A VIOLENT SHAKING/SHUDDERING CONDITION. AS WE HAD NO IDEA WHY THIS WAS HAPPENING; OR WHAT WAS CAUSING IT TO OCCUR; WE ELECTED TO RETURN TO MILWAUKEE FOR IMMEDIATE LNDG; AS WELL AS TO DECLARE AN EMER AND HAVE EQUIP STANDING BY. THE VIOLENT MOTION OF THE ACFT SUBSIDED AFTER 3-5 MINS WHICH WAS FOLLOWED BY AN UNEVENTFUL LNDG. COMPANY MAINT PERSONNEL COULD FIND NO PROBLEM WITH THE ACFT; WHICH WAS SIGNED OFF AS 'COULD NOT DUPLICATE' AND SUBSEQUENTLY RETURNED TO REVENUE SVC. THIS WAS A MOST VIOLENT SHUDDERING OF STILL UNKNOWN CAUSES AND FOR THE SHORT PERIOD OF TIME THE ACFT WAS AIRBORNE; A GREAT DANGER EXISTED. THERE ARE NO SIMULATOR DRILLS FOR THIS TYPE OF EMER; NOR ARE THERE ANY CHKLIST PROCS PUBLISHED (NO EXCEEDENCES; ENG OR AIRFRAME LIMITATIONS APPEARED) FOR THIS SITUATION. THE ONLY THING THAT SAVED THE FLT WAS THE PROFESSIONALISM; COOLNESS; AND PAST TRAINING BY ALL CREW MEMBERS ABOARD. I AM EXTREMELY LUCKY TO HAVE BEEN SCHEDULED WITH THE CREW I WAS WITH. MY HAT IS OFF TO EACH AND EVERY ONE OF THEM.