WDB FAILS TO MAKE XING RESTR.

Narrative

THIS FLT WAS IN THE FINAL 1/2 HR OF AN 8:30 HR FLT. THE TRIP HAD BEEN ROUTINE; SEASONED CREW. THIS PLT WITH ALMOST 4 YRS ON THE ACFT; ALL ON THE N ATLANTIC. WE HAD BEGUN THE DSCNT EARLY TO FIND A BETTER RIDE AND WERE NOW AT FL280. WHEN ATC CLRED US 'DIRECT MANTA' I KNEW THE NEXT CLRNC WOULD LIKELY BE TO CROSS 35 NM N OF MANTA AT FL200. WHEN WE GOT THIS CLRNC SHORTLY THEREAFTER I LOADED THE COMPUTER AND TOLD THE CAPT IT'S IN THE BOX WITH A 10 NM BUFFER. (SO NOW WE HAVE TOLD THE ACFT TO CROSS THIS NEW FIX AT 45 NM N OF MANTA.) I FIGURED EVERYTHING WAS CORRECTLY SET. A SHORT WHILE LATER AS I WAS GETTING OUT OF MY SEAT TO USE THE LAVATORY I HEARD ATC ASK IF WE WERE GOING TO START DOWN SOON. THE CAPT LOOKED AT HIS 'PROGRESS PAGE' AND SAID 'YES;' IN 10 NM. THE CTLR SAID WE WERE ONLY 8 NM FROM OUR RESTR. WE DSNDED QUICKLY AND SMOOTHLY BUT I ESTIMATE WE CROSSED 10-15 HUNDRED FT HIGH. THE CTLR SAID 'NO PROB.' AFTER LNDG THIS 3 PLT CREW DISCUSSED OUR EXPERIENCE AT LENGTH. I DON'T THINK ANY OF US WILL EVER KNOW WHETHER THIS WAS AN FMC GLITCH; OR AN INCORRECT ENTRY ON MY PART INTO THE FMC. COMPLACENCY? ONLY 1 PLT WATCHING THE STORE? LESSON LEARNED!