B727 LOSES FLUID IN ITS 'B' HYD SYS AT FL280. AFTER 'B' ELECTRIC PUMPS WERE TURNED OFF THE ACFT STARTED A CLB WHICH WAS CTLED ONLY AFTER THE AUTOPLT WAS DISCONNECTED AND RETRIMMED TO REGAIN THE TEMPORARY LOSS OF ACFT CTL. ACFT HAND FLOWN TO DEST ARPT AT DEN WHERE AN EMER; PARTIAL FLAP; LNDG WAS PERFORMED.

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IN CRUISE FLT ON AUTOPLT AT FL280; 'B' SYS HYD LIGHT CAME ON. NOTICED FLUID DECREASING; TURNED OFF 'B' ELECTRIC PUMPS. JUST AFTER ATTN WAS DRAWN TO HYD PANEL; THE NOSE PITCHED UP AND THE ACFT BEGAN TO CLB DESPITE THE FACT THAT THE AUTOPLT WAS ON ALT HOLD AND IN THE 'A' CHANNEL. WE DISCONNECTED THE AUTOPLT AND THE TRIM WAS VERY NOSE HIGH. WE RETRIMMED WHILE STILL KEEPING THE CTL COLUMN TO STOP THE NOSE FROM PITCHING UP FURTHER. WE GAINED APPROX 400 FT IN ALT BEFORE WE STARTED BACK DOWN TO FL280. WE DECLARED AN EMER GOING INTO DEN ARPT AND EXECUTED AN UNEVENTFUL PARTIAL FLAP LNDG. IT WAS NOT CLR WHAT THE CONNECTION WAS BTWN THE 'B' SYS HYD AND THE 'A' CHANNEL AUTOPLT BUT AFTER THE HYD SYS WAS REPAIRED; THE AUTOPLT WORKED NORMALLY. (A CHANNEL AUTOPLT USES 'A' SYS HYD.) THE DISTR OF THE HYD FAILURE WITH ATTENDANT PROCS CONTRIBUTED TO THE ALTDEV. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID THAT HE NOTICED THE 'B' HYD LIGHTS ON THE PLT'S PANEL FLICKER JUST PRIOR TO THE SO CALLING OUT THE FAULT AND ADVISING THE CAPT THAT HE WAS TURNING OFF THE 'B' SYS HYD ELECTRIC PUMPS. THE CAPT TURNED THE ACFT OVER TO THE FO AND WAS MONITORING THE SO'S ACTIONS WHEN THE ACFT SIMPLY STARTED TO CLB. AFTER TURNING OFF THE AUTOPLT AND RETURNING TO FL280 THE ACFT WAS HAND FLOWN TO DEN ARPT WITH THE PARTIAL FLAP LNDG PERFORMED AFTER AN EMER WAS DECLARED. THE GND CREW COULD NOT FIND ANY OTHER PROB WITH THE AUTOPLT OTHER THAN THE FACT THAT THEY COULD NOT GET THE AUTOPLT TO ENGAGE ON THE GND. THE AUTOPLT WAS DEFERRED IN THE LOGBOOK AND PLACED ON THE MEL. THE HYD LEAK WAS FOUND TO BE IN THE TRAILING EDGE FLAP ACTUATOR AND WAS REPAIRED. THE ACFT LEFT FOR ITS NEXT TRIP WITH THE ACFT BEING HAND FLOWN. NO FEEDBACK WAS RECEIVED FROM MAINT CTL AS TO THE CAUSE OF THE AUTOPLT MALFUNCTION AND ITS TIE IN WITH THE 'B' HYD SYS SHUTDOWN.