A B777 PIC ALLOWS HIS AIRSPD TO DETERIORATE 20 KTS UNDER TARGET SPD ON A ROLE REVERSAL OF PLT FLYING INSIDE THE LOM DURING A VISUAL APCH TO RWY 27R AT ORD; IL.

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THE FO AND I TALKED ABOUT THE FACT THAT I WOULD MAKE THE LNDG AT ORD. BUT I LET HIM START THE APCH WITH THE IDEA IN MIND THAT I WOULD TAKE THE ACFT AT THE MARKER. APCH CTL TURNED US JUST INSIDE THE MARKER AND WE WERE SLIGHTLY HIGH. THE FO REACHED THE MARKER ON GS WITH THE GEAR DOWN ON SPD WITH THE CHKLIST COMPLETE. I INFORMED THE FO THAT I HAD THE ACFT AND HE INFORMED ME THAT HE HAD BEEN HAND FLYING. AT ABOUT 1200 FT; THE FO STATED THAT WE HAD GOTTEN SLOW AND WE WERE 20 KTS BELOW TARGET (REF +14 KTS). I RESPONDED BY ADVANCING THROTTLES AND SELECTED THE AUTOTHROTTLE ON. THE SPD RETURNED TO REF +14 KTS. BELOW 900 FT NORMAL APCH THE REST OF THE APCH AND LNDG WERE NORMAL AND ON PROFILE ON SPD. FROM A CRM STANDPOINT; I CERTAINLY SHOULD HAVE TAKEN THE ACFT FOR LNDG NO LATER THAN DOWNWIND SO AS TO AVOID THIS KIND OF PROB. IN THE FUTURE; I WILL NOT CHANGE CTL OF THE ACFT ON FINAL APCH UNLESS ABSOLUTELY NECESSARY. SUPPLEMENTAL INFO FROM ACN 529971: CAPT WAS UNDER NEW CAPT REQUIREMENTS TO DO FIRST 10 TKOFS AND LNDGS. CAPT ANNOUNCED THAT HE HAD THE ACFT. SHORTLY AFTER THAT; WE WERE FULLY CONFIGURED ON SPD; ON PROFILE. I RESPONDED THAT I HAD THE RADIOS AND HE SAID TO CONTACT THE TWR. AFTER CHANGING FREQS; I NOTICED THAT THE AIRSPD WAS DECREASING THROUGH OUR TARGET AND SAID WATCH YOUR AIRSPD. I THEN SAID AIRSPD AGAIN; 20 KTS SLOW. CAPT PUSHED UP THE PWR AND ENGAGED THE AUTOTHROTTLES. WE REMAINED ON GS AND WERE BACK ON SPD BY APPROX 800 FT. DESPITE THE MOMENTARY LAPSE OF AIRSPD; WE MET THE STABILIZED APCH CRITERIA.