Narrative:

While en route from tampa, fl, to washington dc, in an medium large transport, the flight had been uneventful until the initial descent from the cruising altitude of FL370. Center issued a crossing restriction of FL270 at 80 NM south of richmond VOR (ric). I programmed the FMC for the descent and allowed it to initiate the idle-power descent for the intercept in VNAV mode. Speed during descent was mach .75 and approximately 260 KIAS with the KIAS slowly increasing toward the crossover figure of 293 KTS as programmed for economy profile. During the descent, ATC stated that 'when able,' we should maintain 300 KTS. At this time, we were passing approximately FL320 with mach .75, increasing through about 285 KTS. I called up the 'cruise' page on the CDU and entered '300' into the cruise airspeed line, expecting the computer to continue the descent and 'trap' the 300 KT cruise speed at level-off. As soon as I 'executed' this instruction, the throttles advanced and the aircraft began to level off at about FL300 maintaining 300 KIAS. I spent about 8 seconds saying 'what's going on' and trying to get a 'level change' idle descent going before realizing it was too late -- I disconnected the autoplt and manually reestablished a 3000 FPM descent but crossed the ric 80 NM arc at about FL285 (1500 ft high) and leveled off at FL270 at approximately 77 NM south of ric (3 NM late). I still do not know why the FMC 'captured' the book and leveled off early. A logical explanation would be that I entered '300' into the 'cruise altitude' line instead of the 'cruise airspeed' line, but I know I didn't do that. The FMC 'nails' these crossing restrictions very well most of the time, but I would like to see a 5 NM or so 'pad' that would give us a little margin for error in that 1/10 percent occurrence of winds not being quite as forecast, late descent initiation, etc. I am going to begin programming the FMC to be down 5 NM early effective immediately.

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Original NASA ASRS Text

Title: ALT BUST. XING RESTRICTION NOT MET.

Narrative: WHILE ENRTE FROM TAMPA, FL, TO WASHINGTON DC, IN AN MLG, THE FLT HAD BEEN UNEVENTFUL UNTIL THE INITIAL DSCNT FROM THE CRUISING ALT OF FL370. CTR ISSUED A XING RESTRICTION OF FL270 AT 80 NM S OF RICHMOND VOR (RIC). I PROGRAMMED THE FMC FOR THE DSCNT AND ALLOWED IT TO INITIATE THE IDLE-PWR DSCNT FOR THE INTERCEPT IN VNAV MODE. SPD DURING DSCNT WAS MACH .75 AND APPROX 260 KIAS WITH THE KIAS SLOWLY INCREASING TOWARD THE CROSSOVER FIGURE OF 293 KTS AS PROGRAMMED FOR ECONOMY PROFILE. DURING THE DSCNT, ATC STATED THAT 'WHEN ABLE,' WE SHOULD MAINTAIN 300 KTS. AT THIS TIME, WE WERE PASSING APPROX FL320 WITH MACH .75, INCREASING THROUGH ABOUT 285 KTS. I CALLED UP THE 'CRUISE' PAGE ON THE CDU AND ENTERED '300' INTO THE CRUISE AIRSPD LINE, EXPECTING THE COMPUTER TO CONTINUE THE DSCNT AND 'TRAP' THE 300 KT CRUISE SPD AT LEVEL-OFF. AS SOON AS I 'EXECUTED' THIS INSTRUCTION, THE THROTTLES ADVANCED AND THE ACFT BEGAN TO LEVEL OFF AT ABOUT FL300 MAINTAINING 300 KIAS. I SPENT ABOUT 8 SECONDS SAYING 'WHAT'S GOING ON' AND TRYING TO GET A 'LEVEL CHANGE' IDLE DSCNT GOING BEFORE REALIZING IT WAS TOO LATE -- I DISCONNECTED THE AUTOPLT AND MANUALLY REESTABLISHED A 3000 FPM DSCNT BUT CROSSED THE RIC 80 NM ARC AT ABOUT FL285 (1500 FT HIGH) AND LEVELED OFF AT FL270 AT APPROX 77 NM S OF RIC (3 NM LATE). I STILL DO NOT KNOW WHY THE FMC 'CAPTURED' THE BOOK AND LEVELED OFF EARLY. A LOGICAL EXPLANATION WOULD BE THAT I ENTERED '300' INTO THE 'CRUISE ALT' LINE INSTEAD OF THE 'CRUISE AIRSPD' LINE, BUT I KNOW I DIDN'T DO THAT. THE FMC 'NAILS' THESE XING RESTRICTIONS VERY WELL MOST OF THE TIME, BUT I WOULD LIKE TO SEE A 5 NM OR SO 'PAD' THAT WOULD GIVE US A LITTLE MARGIN FOR ERROR IN THAT 1/10 PERCENT OCCURRENCE OF WINDS NOT BEING QUITE AS FORECAST, LATE DSCNT INITIATION, ETC. I AM GOING TO BEGIN PROGRAMMING THE FMC TO BE DOWN 5 NM EARLY EFFECTIVE IMMEDIATELY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.