FO; PF; ON A B747-400 TELLS OF DIFFICULTY IN ACQUIRING THE ILS OF A PARALLEL RWY FOR A 'SIDE-STEP MANEUVER.' PRIMARY RWY HAD JUST BEEN CLOSED BY RAIN SHOWERS AT A FOREIGN ARPT. CTLR ASKED FLT TO MOVE TO THE PARALLEL RWY. THEY COULD NOT GET THE CDU TO ACCEPT THE NEW ILS FREQ FOR APCH UNTIL BOTH THE AUTOPLT AND BOTH FLT DIRECTORS WERE CYCLED OFF AND BACK ON. THIS RESULTED IN A DESTABILIZED APCH WITH A 200 FT CEILING. RPTR CALLS FOR AN ADDITIONAL OPTION ON THE ARR DEP PAGE SO AS TO AVOID THIS 'OFF AND ON' OP. AN ANONYMOUS RPT.

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RECENTLY I WAS INVOLVED IN A HARROWING EXPERIENCE WHILE FLYING OVERSEAS IN A B747-400. ONE OF THE 'FEATURES' OF THIS PLANE IS A FAIL SAFE OR 'FAIL OPERATIONAL' DESIGN USED FOR FLYING INST APCHS. UNFORTUNATELY; THIS DESIGN CRIPPLED THE COCKPIT CREW FOR ABOUT 1 MIN WHILE WE WORKED FEVERISHLY TO RECTIFY THE PROB. I WAS FLYING A CAT I ILS APCH TO THE ACTIVE RWY. THE AUTOPLT WAS ON AND THE LOC WAS 'CAPTURED.' A RAINSTORM OVER THE FIELD CLOSED THE ACTIVE RWY AND THE CTLR ISSUED US CLRNC TO LAND ON A PARALLEL RWY. (THE RAIN WAS HVY; BUT NOT RED ON WX RADAR.) SINCE WE HAD ALREADY CAPTURED THE LOC TO THE PREVIOUS RWY; HDG SELECT MODE WAS DISABLED. SO IN ORDER TO COMPLY WITH THE CTLR'S NEW HDG TO INTERCEPT THE NEW LOC; I HAD TO TURN OFF THE AUTOPLT AND FLY TO THAT HDG MANUALLY. (FLT DIRECTORS ARE ALSO INOP AT THIS POINT.) MEANWHILE; THE CAPT IS TRYING TO RESELECT THE NEW RWY SO WE CAN PICK UP THE NEW LOC. THE FAIL OPERATIONAL DESIGN WOULD NOT ALLOW HIM TO SELECT A NEW RWY EITHER ON THE 'NAVRAD' PAGE OR THE DEP/ARR PAGE. WHILE WE 'FLAILED ALONG' ON THIS MISERABLE APCH; I THOUGHT OF GOING MISSED APCH; BUT GUESS WHAT? I HAVE NO WAY TO FLY IT BECAUSE THE LOC IS STILL TUNED TO THE WRONG RWY! MY MAP DISPLAY IS NO GOOD AND ALL I HAVE IS A $150 MILLION J-3 CUB WITH 440 PEOPLE ON BOARD! FINALLY; WE REMEMBERED FROM TRAINING THAT THE ONLY WAY TO FREE UP THE CDU IS BY PHYSICALLY TURNING OFF BOTH FLT DIRECTORS AND THE AUTOPLT. THEN AND ONLY THEN CAN YOU SELECT A NEW RWY. I CAPTURED THE LOC AND WAS HIGH ON THE GS; BROKE OUT AND LANDED. PLEASE ADVISE BOEING THAT THEY MUST ADD A NEW SELECTION ON THE DEP/ARR PAGE THAT ALLOWS SWITCHING RWYS WITHOUT TURNING OFF THE AUTOPLT AND FLT DIRECTORS.