A CL65 PIC DSNDS OVER 500 FT BELOW HIS ASSIGNED ALT WHEN HE HAD RESET HIS LCL QNH TO A HIGHER SETTING THAN ISSUED PASSING JAYBO INTXN; MD.

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THIS HAS TO BE ONE OF THE MOST SHOCKING NASA FORMS I'VE FILLED OUT. I CANNOT BELIEVE IT HAPPENED. SO HERE IT IS. THE FO WAS OFF GETTING ATIS; TALKING TO COMPANY; AND ALL THAT GOOD STUFF. I WAS GIVEN A LATE DSCNT CLRNC FROM ABOUT FL240 TO 15000 FT AT JAYBO INTXN. THE RADIOS WERE A LITTLE BUSY AS THEY ALWAYS ARE IN THE NE CORRIDOR. WHEN I REALIZED HOW MUCH TIME I HAD TO MAKE THE XING RESTR I NOTICED THE VNAV TELLING ME I MUST DSND AT 2900 FPM. SO; I STARTED DOWN. I WAS GIVEN 30.54 ON THE ALTIMETER SETTING -- SO I THOUGHT. THE FO CAME BACK OFF THE RADIOS AT ABOUT 16000 FT. HE SAID TO ME; 'THE AFT IS NOT CAPTURING THE ALT.' I SAID 'YES IT IS.' MY SIDE WAS CAPTURING WHAT IT THOUGHT WAS 15000 FT. HIS SIDE HAD DSNDED 1000 FT BELOW. THE ALTIMETER WAS ACTUALLY 29.54. IT TOOK US A COUPLE OF MOMENTS TO DETERMINE THE CAUSE OF CONFUSION. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND CLBED TO THE TRUE 15000 FT. ATC NEVER SAID A WORD. IN FACT; THEY TOLD US WE WERE CLRED DOWN TO 11000 FT ALMOST RIGHT AWAY. I NEVER HAVE HAD THIS HAPPEN BEFORE. I WAS SHOCKED AND A LITTLE STARTLED TOO. THIS SHOULD NOT HAPPEN. WE DEBRIEFED AND DECIDED IT WAS LACK OF CREW COM AND POSSIBLY LAST MIN TASK SATURATION. THIS WILL HOPEFULLY NEVER EVER HAPPEN TO ME AGAIN -- YIKES! SUPPLEMENTAL INFO FROM ACN 504280: WE DEBRIEFED AND DETERMINED THE DSCNT WAS PROBABLY A LITTLE TOO RUSHED AND MAYBE WE WERE BOTH A LITTLE TASK SATURATED.