A B757 PLT COMMENTS ON THE FMS CAPTURING A SPD WHILE LEVELING AT 10000 FT AND ALLOWING THE ACFT TO EXCEED 250 KTS BECAUSE THE CAPTURED SPD IS ABOVE 250 KTS.

Narrative

THE B757'S FMS HAS A UNIQUE QUIRK THAT OFTEN CAUSES AIRSPD TO EXCEED 250 KTS BELOW 10000 FT. ON THIS PARTICULAR FLT; I WAS HAND FLYING THE BRAVES RNAV DEP OUT OF ATL. INITIAL CLRNC WAS TO 10000 FT WITH PLANNED CRUISE AT FL400. COMPANY POLICY HAS US USE FULL CLB PWR RATHER THAN DERATED TO SAFE FUEL; AND THE RATE OF CLB WAS BTWN 3000-4000 FPM. THERE IS ALSO A SPD RESTR TO STAY AT 250 KTS UNTIL A CERTAIN FIX. APCHING 10000 FT WITH AUTOTHROTTLES ENGAGED; I LOWERED THE NOSE TO REDUCE THE CLB RATE. THE AUTOTHROTTLES WERE SLOW TO RESPOND; SO I PULLED THEM BACK MANUALLY. AT 10000 FT; THE FMS SWITCHED FROM VNAV TO LEVEL CHANGE; AND THE AIRSPD WINDOW OPENED AT THE CURRENT SPD; WHICH HAD CLBED UP TO 260 KTS OR 265 KTS. THE AUTOTHROTTLES NOW ATTEMPTED TO MAINTAIN THIS SPD IN LEVEL FLT. AS THE PF; AND HAND FLYING THE RNAV DEP; IT IS EASY TO BECOME SATURATED. FLT PATH MUST REMAIN PRECISELY ON TRACK THROUGHOUT THE TWISTING DEP; AND THE HIGH CLB RATE MUST BE REDUCED IN THE LAST 200 FT FOR PAX COMFORT AS WELL AS ACCURATE LEVELOFF AT ALT WITHOUT OVERSHOOT. AIRSPD WILL SOMETIMES COME UP ABOVE 250 KTS DURING THIS MANEUVER; AND IS EXASPERATED BY THE FMS DROPPING OUT OF VNAV INTO LEVEL CHANGE. AN FAA INSPECTOR OBSERVING THE FLT COMMENTED ON THE SPD AND MENTIONED HE HAD NOTICED THE PROB ON OTHER BOEING PRODUCTS; INCLUDING THE B737 NG SERIES. I WILL MENTION THAT WITH THE AUTOPLT ENGAGED; THE PROB IS RARELY SEEN. AS SOMETHING PRACTICAL; I HAVE LEARNED THROUGH THIS; NOW I PLAN TO ENGAGE THE AUTOPLT IF HIGHER ALT HAS NOT BEEN GIVEN 1000 FT PRIOR TO 10000 FT OR OTHER LOW ALT LEVELOFFS UNTIL SOME SORT OF FIX HAS BEEN DEVELOPED.