B777 CREW DSNDED BELOW THE GS INSIDE THE OM.

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ARR TO LAX FROM PARIS (CDG). AFTER 10 HR FLT; FO BRIEFED ARR AND APCH TO LAX. VISUAL CONDITIONS CLR; BUT HAZY. BRIEFING ADEQUATE; BUT HURRIED. SOCAL APCH CLRED US 'AFTER FUELR; CLRED ILS RWY 25L; MAINTAIN 250 KTS.' FIRST PART OF APCH ROUTINE USING ALL RWY 25L STEPDOWN ALTS. FO (PF) HELD 250 KTS TO APPROX 13-14 DME (TOO LONG). SOCAL SAID '190 KTS TO LIMA; CALL LAX TWR ON 120.95.' FO BEGAN SLOWING AND ATTEMPTED TO MAINTAIN DSCNT PROFILE. HE DISCONNECTED AUTOPLT AND AUTOTHROTTLES; WITH FLAPS 1 DEG; GEAR WAS LOWERED AT 210 KTS. JUST INSIDE LIMA (FAF); WE DSNDED BELOW GS -- ENOUGH TO ACTIVATE GPWS GS WARNING. I TOLD HIM TO LEVEL OFF. HE DID; SLOWED; CAPTURED GS WITH FLT DIRECTOR; COMPLETED APCH AND LNDG. STABILIZED AT 500 FT AGL (SOP REQUIREMENT); BUT NOT AT 1000 FT AGL (MY REQUIREMENT). PROBS: 1) COMS -- BRIEFING TOO SHORT AND HURRIED. BREAKDOWN AS WE TRIED TO SLOW AND MAINTAIN THE DSCNT PROFILE. 2) SOP -- MCP DISCIPLINE. PF MAKES MCP CHANGES WITH AUTOPLT AND AUTOTHROTTLES CONNECTED. WHEN HE DISCONNECTED; HE CONTINUED TO MAKE THOSE CHANGES AS I TALKED TO TWR AND APCH (I DID NOT CORRECT HIM!). I MADE SOME MCP CHANGES AS WELL. TOO MANY HANDS TURNING KNOBS -- NOT SOP FOR OUR COMPANY. 3) COCKPIT MGMNT -- I ALLOWED A VERY EXPERIENCED FO TO LEAD US INTO A POTENTIALLY DANGEROUS SCENARIO. WE DID NOT GET DANGEROUSLY LOW; BUT WE WERE TOO LOW. CLR BREAKDOWN ON MY PART. LACK OF SOP FOR FO.