B757-200 CREW HAD ALT DEV ATTEMPTING AN AUTO COUPLED APCH AT VHHH.

Narrative

ARR AT HONG KONG INT'L ARPT; AFTERNOON LOCAL TIME IN VMC WX CONDITIONS. ON APCH TO RWY 25L WE WERE GIVEN THE CLRNC 'TURN L HEADING 270 MAINTAIN 4500 FT REPORT ESTABLISHED'. I ENTERED 4500 FT IN THE MCP ALTITUDE WINDOW AND THE FO WHO WAS THE PF TURNED THE HEADING BUG TO 270 PUSHED FL CHANGE TO DSND AND ARMED THE APCH MODE. THE LOCALIZER FOR 25L IS UNUSABLE PAST 7 DEGS LEFT OF COURSE AND UP UNTIL THIS TIME I HAD BEEN UNABLE TO IDENTIFY IT. AT SOME POINT WHILE THE AIRPLANE WAS DSNDING TO 4500 FT AND I WAS LOOKING AT MY CHART TO IDENTIFY THE LOCALIZER; THE GLIDE SLOPE CAME ALIVE AND THE AUTOPLT CAPTURED IT AND THE LOCALIZER. WHEN I HAD FINISHED IDENTIFYING THE LOCALIZER AND LOOKED AT THE ADI GLIDE SLOPE AND LOCALIZER WERE CAPTURED AND THE ACFT WAS DSNDING. SHORTLY THERE AFTER WE RECEIVED AN AUTOPLT CAUTION WITH AN AMBER LINE THROUGH THE G/S FMA INDICATING A LOSS OF SIGNAL. THE AUTOPLT CONTINUED THE DSND AND WE WAITED SEVERAL SECONDS TO SEE IF IT WAS A TEMPORARY LOSS OF SIGNAL. WE DISCUSSED WHY IT HAD HAPPENED AND THE FO STATED IT COULD BE A FALSE GLIDE SLOPE AND MAYBE WE SHOULD SELECT ALTITUDE HOLD. HE THEN ASKED FOR ALTITUDE HOLD AND I SELECTED IT BUT IT WOULD NOT ENGAGE SINCE THE LOCALIZER AND G/S WERE CAPTURED. THE ALTITUDE ALERT CAUTION THEN WENT OFF SINCE WE HAD DSND BELOW 4200 FT. THE FO THEN COMMENTED THAT WE NEEDED TO BE AT 4500 FT UNTIL LOTUS INTXN. WE HAD EXTENDED OUR RTE OFF THE FINAL FIX SO LOTUS WAS NOT ON OUR MAP DISPLAY. I LOOKED AT THE APCH CHART TO GET THE DME FOR LOTUS AND WAS SWITCHING MY MAP DISPLAY TO ILS MODE FOR THE DME WHEN THE APCH CTLR CALLED AND SAID WE WERE TOO LOW; CLB TO 4300 FT. THE FO DISCONNECTED THE AUTOPLT AND BEGAN TO CLB. WE THEN REINTERCEPTED THE G/S AND CONTINUED THE APCH TO LNDG. ON A PHONE CALL TO THE ATC SUPVR I WAS TOLD THE G/S AT HONG KONG WAS UNUSABLE BEYOND 15 MILES. I COULD NOT FIND THIS INFO ON THE APCH CHART OR ANY WHERE ELSE IN THE MANY PAGES OF FOR HONG KONG. CONTRIBUTING FACTORS WERE; OUR FAILURE TO KEEP TRACK OF OUR POS WITH REGARDS TO THE PROPER GLIDE SLOPE INTERCEPT. A CLRNC THAT DID NOT SPECIFY TO MAINTAIN 4500 FT UNTIL LOTUS INTXN. OUR ANTICIPATION OF STARTING DOWN THE GLIDE SLOPE SHORTLY AFTER THE FINAL VECTOR AND CLRNC. NOT REMEMBERING THAT ALTITUDE HOLD WAS NOT AVAILABLE WHEN THE LOCALIZER AND GLIDE SLOPE WERE CAPTURED. SOME DEGREE OF COMPLACENCE CAUSED BY DAY VMC CONDITIONS. NOT CHKING THE FMA TO SEE THAT ALTITUDE HOLD HAD ENGAGED. NOT DISCONNECTING THE AUTOPLT IN A TIMELY MANNER AND STOPPING THE DSCNT.