B777 PIC HAS PROBS MANAGING THE FMS TO MAINTAIN A DEFINED AIRSPD AND RATE OF DSCNT TO INSURE IN-TRAIL SEPARATION FROM PRECEDING ACFT IN FOREIGN AIRSPACE. POTENTIAL CONFLICT AS SPD DEV AND ALT CTL ALLOWED TO VARY.

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I WAS THE PIC AND FLYING A FLT FROM CHICAGO OHARE TO FRANKFURT GERMANY. WE LEFT CHICAGO ABOUT AA00 LCL ON DEC/XX/96; AND HAD AN ALL-NIGHT UNEVENTFUL OCEANIC XING. AT ABOUT BA30 LCL WE WERE LEVEL AT FL240; 6 NM FROM REUDESHIEM NDB AND IN CONTACT WITH FRANKFURT CTL. I RECEIVED CLRNC TO THE REUDESHIEM NDB TO HOLD; EXPECT FURTHER CLRNC; BB59 LCL; TO MAINTAIN AIRSPD; HE WOULD CALL THE SPD REDUCTION; AND TO DSND TO FL120 AT EXACTLY 1500 FPM. I SELECTED VERT SPD AND 1500 FPM DOWN. THE FO WAS BUSY SETTING UP THE HOLD AND DOING THE APCH/DSCNT CHKLIST. THE 3RD PLT WAS GETTING ATIS. I NOTICED OUR VERT SPD WAS ONLY 1000 FPM DOWN AND THAT OUR AIRSPD HAD INCREASED FROM 320 KTS TO ABOUT 328 KTS NEARING VMO (MAX OPERATING SPD). I DEPLOYED THE SPD BRAKES TO ACHIEVE THE REQUESTED RATE OF DSCNT. NOW THE AUTOTHROTTLES ADVANCED AND I RETRACTED THE SPD BRAKES; NOTICING THAT AIRSPD HAD DROPPED TO 305 KTS. I DECIDED THAT VERT SPD CTL WAS ERRATIC AND SELECTED FLT CHANGE MODE OF DSCNT. SINCE THE AIRSPD WAS LOW; THE NOSE OF THE ACFT DROPPED TO ACHIEVE THE SELECTED SPD. RATE OF DSCNT INCREASED TO ABOUT 2100 FPM. I ATTEMPTED TO SMOOTHLY INCREASE THRUST AND EVENTUALLY ACHIEVED 1500 FPM DOWN WITH 320 KTS. ABOUT THIS TIME FRANKFURT CTL INSTRUCTED US TO MAINTAIN FL180. WE WERE AT FL183 AT THIS TIME. HE ASKED WHAT OUR RATE OF DSCNT WAS; TO WHICH THE FO RESPONDED 1500 FPM DOWN. AS IT WAS AT THAT INSTANT. HE THEN INFORMED US THAT WE WERE ONLY 4 MI BEHIND AND 800 FT ABOVE THE PRECEDING ACFT. HE ASKED THE OTHER ACFT IF THIS HAD CAUSED THEM ANY PROB AND THE RESPONSE WAS NO. NO EVASIVE ACTION WAS TAKEN AND WE CONTINUED TO AN UNEVENTFUL LNDG. I WAS SOLELY RESPONSIBLE FOR AND ACCEPT RESPONSIBILITY FOR THIS INCIDENT. I FEEL THAT SEVERAL FACTORS CONTRIBUTED TO THIS MISTAKE. DUE TO THE LONG; BACK OF THE CLOCK; NIGHT FLT WE WERE ALL FATIGUED. IN ADDITION THIS WAS MY 1ST FLT AFTER VACATION AND I WAS NOT AS SHARP AS I WOULD LIKE TO BE. THE TCASII ON THIS ACFT WAS INOP; SO POS OF OTHER ACFT WERE UNKNOWN. THE INCIDENT OCCURRED AT A BUSY POINT IN THE FLT WHEN ALL 3 OF US WERE OCCUPIED WITH DIFFERENT DUTIES. PERHAPS A CLRNC TO DSND AT 1500 FPM OR LESS WOULD HAVE BEEN A BETTER CHOICE THAN AT EXACTLY 1500 FPM.