MD80 FLT CREW INDUCES RUDDER DEFLECTION AT FL280 ATTEMPTING TO RESET DFGS BY PULLING CIRCUIT BREAKERS.

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UPON DEP; AUTOTHROTTLE WOULD NOT ENGAGE. MANUALLY SET PWR AND CONTINUED TKOF. WX ESSENTIALLY CAVU. AIRBORNE; AUTOPLT WOULD ALSO NOT ENGAGE. I WAS PF. FLEW RNAV DEP MANUALLY SO AS TO TROUBLESHOOT OUTSIDE OF THE TERMINAL AREA. AFTER CTR HDOF AND ABOVE 10000 FT; ATTEMPTED TO SWAP DFGS TO #2 AND HAD SAME RESULT -- NO AUTOTHROTTLE OR AUTOPLT. CONTINUED CLB. WX ENRTE AND AT DEST WAS VFR; SO WE ADVISED ATC THAT WE WERE GOING TO STAY OUT OF RVSM AIRSPACE AND WE BEGAN TROUBLESHOOTING AFTER LEVELOFF AT FL280 (ASSIGNED). I XFERRED CTL TO FO AND REFED QRH. THE ONLY PROC WE HAVE EVER HAD OR USED FOR RESETTING THE DFGS(S) IS TO CYCLE THE 3 CIRCUIT BREAKERS C17; C18; AND C19 TO RESET THE DFGS SWITCHING (IT HAS WORKED IN PAST PER PREVIOUS REFS). WHEN I PULLED C17 WE HAD WHAT FELT LIKE A FULL (RESTR TRAVEL) SCALE RUDDER DEFLECTION -- EXTREME YAW; AND FO HAD ALMOST FULL OPPOSITE ROLL TO COUNTERACT THE ACFT ROLL FROM THE RUDDER INPUT. I IMMEDIATELY RESET C17 AND THE YAW INSTANTLY CEASED. I CONTACTED MAINT. THE FIRST SUGGESTION WAS TO DO EXACTLY WHAT I HAD DONE. WHEN I TOLD HIM ABOUT THE YAW; HE SAID THAT IT WAS A KNOWN PROB AND THAT THE AUTOPLT WAS GOING INTO THE TEST MODE AND SEVERE RUDDER OSCILLATIONS COULD OCCUR. NEEDLESS TO SAY; WE ABANDONED THAT AVENUE OF TROUBLESHOOTING. I HAD MAINT PUT ON ALERT AND WE HAND FLEW THE FLT TO ITS COMPLETION. ONCE ON GND WITH PAX DISEMBARKED; WE KILLED ALL PWR TO THE ACFT AND RE-POWERED ALL SYS. BOTH DFGS'S RESET AND WE REGAINED USE OF AUTOTHROTTLE; AUTOPLT. THE PROB IS THE POTENTIAL FOR A SUDDEN AND UNEXPECTED RUDDER HARD-OVER IF EITHER C17 IS PULLED OR (AND I DON'T KNOW; BUT CAN ASSUME) POPS (THE RESULTING LOSS OF PWR WOULD CREATE THE SAME SCENARIO). MAINT TOLD ME THAT THIS HAS HAPPENED BEFORE AND YET IN OVER 20 YRS OF FLYING THIS ACFT; I HAD NEVER HEARD OF IT. I SUSPECT FURTHER RESEARCH NEEDS TO BE DONE AND EITHER A CAUTION OR WARNING NEEDS TO ACCOMPANY THE PROC NOT TO PULL THE DFGS SYS SWITCH CIRCUIT BREAKERS IF THIS IS A POSSIBLE RESULT. LOSS OF CTL OR A STRUCTURAL PROB COULD RESULT DEPENDING ON AIRSPD/ALT/FLT CONDITIONS. I SPOKE WITH CHIEF PLT AT THE END OF THE WORK DAY AND ADVISED HIM OF THE ANOMALY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: IT WAS DETERMINED AFTER THE EVENT THAT THIS CIRCUIT BREAKER (C17) AFFECTS YAW DAMPER OPERATION IN ADDITION TO DFGS SWITCHING AND CAUSES THE RUDDER TO DEFLECT TO THE YAW DAMPER LIMIT OF 2 DEGREES. THE REPORTER HAS MANY YEARS OF EXPERIENCE ON THE MD80; AND THIS PROCEDURE WAS CARRIED OVER FROM HIS EARLY TRAINING ON THE ACFT. NEEDLESS TO SAY; IT WILL NOT BE USED AS AN AIRBORNE PROCEDURE IN THE FUTURE. IN THE EVENT THAT THIS CIRCUIT BREAKER POPPED ON ITS OWN; NORMAL QRH PROCEDURES WOULD HAVE LED TO TURNING OF THE YAW DAMPER ALLOWING THE RUDDER TO CENTER.