B737-400 FLT CREW SELECTED FLAPS 40 DEG POS AND THEN RETRACTED THEM AFTER NOTING THEY HAD EXCEEDED THE AIRSPD LIMITATION.

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ATC KEPT US HIGH ON APCH TO LAX AND THEN CLRED US FOR A VISUAL APCH. WE HAD BRIEFED A FLAPS 30 DEG LNDG AND SET THE BUGS AND CDU APCH REF PAGE ACCORDINGLY. THE FO CALLED FOR FLAPS 40 DEGS AND I SELECTED FLAPS 40 DEGS. I THEN SAW OUR AIRSPD WAS 170 KTS AND IMMEDIATELY RETRACTED THE FLAPS BACK TO 30 DEGS. THE FLAP GAUGE SHOWED NO MOVEMENT BEYOND FLAPS 30 DEGS. THE LNDG WAS NORMAL. HOW DID WE GOT TO THE POINT WHERE I ACTUALLY MOVED THE FLAP HANDLE TO A SETTING WHILE OUR AIRSPD WAS ABOVE THE PLACARD SPD FOR THAT SETTING? AS WE DEBRIEFED; WE REALIZED THAT THE FO THOUGHT OUR SPD WAS OK FOR FLAPS 40 DEGS BECAUSE HE DID NOT SEE THE HOLLOW YELLOW BAR ON THE SPD TAPE WHICH REPRESENTS THE PLACARD SPD FOR THE NEXT NORMAL FLAP POS. HERE IS WHY HE DID NOT SEE IT: WE HAD SELECTED FLAPS 30 DEGS ON THE CDU APCH REF PAGE AND; THEREFORE; THE NEXT FLAP PLACARD SYMBOL IS BLANKED WHEN CURRENT FLAP POS EQUALS THE SELECTED LNDG FLAP CONFIGN. HERE IS THE KEY THOUGH: AS A CREW WE SHOULD NOT HAVE OPTED TO USE FLAPS 40 DEGS WHEN THE APCH WAS BRIEFED AS A FLAPS 30 DEG LNDG. THE IDEA OF USING FLAPS 40 DEGS TO HELP GET DOWN AND THEN TRANSITIONING BACK TO FLAPS 30 DEGS FOR LNDG IS A BAD IDEA. BESIDES BEING PROCEDURALLY INCORRECT AND UN-BRIEFED; IT ALSO ROBS THE CREW OF THE INDICATION OF THE NEXT FLAP PLACARD SPD HOLLOW YELLOW BAR ON THE SPD TAPE. AT A SAFE DISTANCE FROM THE ARPT AN OPTION MAY BE TO RE-BRIEF AND RESELECT LNDG FLAP CONFIGN IF A CHANGE IS DESIRED. IN CLOSE; IF A STABILIZED APCH CANNOT BE COMPLETED USING PLANNED LNDG FLAP CONFIGN; A GAR SHOULD BE INITIATED.