AN ACR FLC EXPERIENCES A VIOLENT ROLL TO THE L AND THEN TO THE R AS THEY STOW THE SPD BRAKES.

Narrative

WE WERE ON THE ARR INTO ZZZ AND CENTER SLOWED US TO 280 KTS. TO STAY ON PROFILE AND MEET OUR XING RESTR THE SPD BRAKE WAS DEPLOYED BY THE FO AND PF. PASSING APPROX 17000 FT AND BACK ON PROFILE HE EASED OFF THE SPD BRAKE AND STOWED IT. AT THAT EXACT MOMENT THE ACFT ROLLED VIOLENTLY TO THE L. THE FO HAD HIS HANDS ON THE CTLS; EVEN THOUGH THE AUTOPLT WAS ENGAGED; WITH VNAV AND LNAV SELECTED. HE QUICKLY APPLIED R AILERON AND STOPPED THE ROLL BTWN 30-45 DEGS BANK. THE ACFT ENTERED A DUTCH ROLL; ROLLING AND YAWING FOR A COUPLE SECONDS; THEN ROLLED L AGAIN. THE FO CORRECTED THE SECOND ROLL MUCH QUICKER THAN THE FIRST AND COMPLETELY DISENGAGED THE AUTOPLT AND I TURNED OFF THE YAW DAMPER. AT THE TIME OF THE SECOND ROLL; THE ROLL CHANNEL OF THE AUTOPLT WAS PROBABLY OFF; DUE TO THE MANUAL INPUT MADE TO CORRECT FOR THE FIRST ROLL. AFTER MUCH DISCUSSION WE WERE UNABLE TO DETERMINE THE ORIGIN OF THE UNCOMMANDED ROLL; HOWEVER THE SPD BRAKE SEEMED TO BE INVOLVED SINCE THE ACT OF STOWING IT COINCIDED WITH THE EVENT. THE RUDDER OR AILERONS COULD HAVE BEEN INVOLVED AS WELL; BUT SINCE THE ROLL HAPPENED SO FAST AND CONSIDERABLE YAWING WAS ALSO INVOLVED THERE WAS NO WAY TO DETERMINE THE ORIGIN. WE WERE IN CLR CALM AIR AND NO OTHER TURB WAS EXPERIENCED WITH THE EXCEPTION OF THE 2 DISTINCT UNCOMMANDED ROLLS. I RECOMMEND ALL PCU'S BE BENCH TESTED; FLT CTLS CHKED; AND THE FLT DATA RECORDER ANALYZED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT HE AND THE FO RPTED THE EVENT TO THE COMPANY AND ENTERED IT INTO THE MAINT LOG. THE FLT DATA RECORDER ONLY SHOWED 1 UNCOMMANDED ROLL/YAW AND IT WAS ONLY TO 11 DEGS. THE FLC WAS SURE OF WHAT THEY SAW. THE CAPT SAID THAT HE WAS LOOKING AT A CHART WHEN THE FIRST ROLL STARTED; BUT HE WAS FULLY ALERT WHEN THE SECOND OCCURRED AND HE DISAGREES WITH THE RECORDER. THE CAPT FURTHER SAID THAT HE IS EXPERIENCED IN AEROBATICS AND WAKE TURB AND HE HAS NOT FELT ANYTHING LIKE THIS BEFORE. MAINT CONCLUDED THAT THE YAW DAMPER HAD MALFUNCTIONED.