STRUCK ACFT PARKED AT GATE DURING POWERBACK MANEUVER.

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ALL CHKLIST AND PROCS WERE ROUTINE. ENGS START AND CLRED FOR PWRBACK FROM GATE X AT MEMPHIS; TN. ALL HAND SIGNALS WERE PROMPTLY FOLLOWED. AT THE COME AHEAD SIGNAL I CAME OUT OF REVERSE AND WAITED FOR THE TAXI SIGNAL FROM THE GND SIGNALMAN. IT DID NOT COME--A HOLD SIGNAL FOLLOWED. FROM THE ACTION ON THE RAMP I REALIZED THAT SOMETHING HAD OCCURRED BEHIND MY ACFT. UNABLE TO GET ANY INFO FROM COMPANY RADIO. AN AGENT YELLED UP THAT MY ACFT HAD STRUCK ANOTHER (LGT) PARKED AT GATE Y. AT THIS TIME A F/A CAME TO THE COCKPIT AND ASKED IF EVERYTHING WAS OK. SHE STATED THAT AN UNUSUAL NOISE WAS HEARD FROM THE REAR OF THE ACFT. F/A AND PAX INFORMED OF PROB. ENGS WERE SHUT DOWN AND WE WERE TOWED BACK TO ORIGINAL GATE. LOGBOOK ENTRIES COMPLETE AND CHKLIST COMPLETED. A VERBAL RPT WAS MADE TO FLT DISPATCH. ACFT CHANGE COMPLETED AND FLT CONTINUED TO ORD. CONTRIBUTING FACTORS: IT IS UNUSUAL TO HAVE AN LGT ON THIS GATE. THE LGT EXTENDS FURTHER OUT INTO TAXI AREA THAN AN MLG (NORMALLY AT THIS GATE). ALSO; MY MLG MODEL IS LONGER THAN OTHER MLG COMING OFF OF GATE X. WE ARE NOT ALLOWED TO USE BRAKES TO STOP REARWARD MOTION. ACFT COULD HAVE BEEN STOPPED VIA BRAKES IF SIGNALMAN THOUGHT WE WERE NOT STOPPING FAST ENOUGH. ACFT DID CONTINUE BACK A FEW FT COMING TO FORWARD THRUST. I DID NOT REALIZE THAT WE HAD CONTACTED ANYTHING AND I BELIEVE OUR SIGNALMAN DID NOT REALIZE THIS UNTIL POINTED OUT. DAMAGE: MLG LEFT HORIZ STAB TRIM TABS (DENT/CUT) AND LGT UPPER RUDDER TRIM TAB; RIGHT SIDE (VISUAL INSPECTION ONLY). CORRECTIVE ACTION: 1) PUSH BACK AT ALL TIME USING INTERCOM. 2) NOT ALLOWING PWRBACK BY MLG AND LGT WHEN THEY ARE ON OPP GATE SINCE THEY; BY THEIR COMBINED LENGTH; ALLOW LESS MANEUVERING AND STOPPING ROOM. DURING COMPANY MEETING WITH CHIEF PLT I WAS INFORMED THAT THE COMPANY CTL TWR (RAMP) SAW THIS INCIDENT ABOUT TO HAPPEN. ALSO; THE WINGWALKER SAW THE CONTACT COMING. HE STARTED RUNNING TOWARDS THE COCKPIT TO WARN ME/FORWARD SIGNALMAN (ONE I WAS WATCHING). (I CANNOT SEE THE WINGWALKER DURING PWRBACK.) WINGWALKER DID NOT APPEAR IN MY LINE OF SIGHT IN TIME. I RECOMMENDED TO THE CHIEF PLT THAT AN EMER STOP HAND SIGNAL FOR THE WINGWALKER (TO FORWARD SIGNALMAN) BE USED SO CAPT COULD STOP REARWARD MOVEMENT BY BRAKES. IN MY CASE ONLY PROB TO RESULT WOULD BE AN UPSET (AND POSSIBLE INJURY TO THE F/A). THEY WERE STANDING; GIVING SAFETY ANNOUNCEMENTS. BRAKING WHEN GOING REARWARD WOULD PROBABLY RESULT IN NOSE STRUT EXTENTION. A STANDARD SIGNAL TO TIGHTEN OR OPEN THE TURN RADIUS WOULD ALSO GIVE SIGNALMAN MORE CTL TO DIRECT CAPT. AT PRESENT; SIGNAL IS ONLY TO TURN LEFT OR RIGHT; NOT AMOUNT OF TURN DESIRED.