CL65 ALTDEV FROM MISSET ALTIMETER TIMES TWO NEAR IAD.

Narrative

I WAS PERFORMING PNF DUTIES; THE FO WAS FLYING USING THE AUTOPLT. I WAS OFF THE ATC FREQ MAKING IN-RANGE CALL TO COMPANY AND ARR CABIN ANNOUNCEMENT. WHILE I WAS DOING SO; THE FO RECEIVED CLRNC TO CROSS ROBRT INTXN AT 10000 FT. HE HAD BEEN ISSUED AN ALTIMETER SETTING OF 30.38 BUT HAD INSTEAD SELECTED 31.38. WHEN I FINISHED WITH PREVIOUS MENTIONED DUTIES I REJOINED THE FO ON THE ATC FREQ. HE ADVISED ME OF ALL CLRNCS AND CHANGES WHILE I WAS OFF ATC FREQ (XING RESTR AND NEW ALTIMETER SETTING). I SET THE CAPT'S ALTIMETER AND VERBALLY CONFIRMED THIS WITH THE FO; AS PER OUR POS PROCS. IT IS NOT REQUIRED IN OUR OPS PROCS THAT I INSPECT THE VALUE THE FO SETS IN HIS ALTIMETER. HOWEVER; AS A MATTER OF ROUTING I DO SO. THIS TIME I DID NOT AND HE SET THE WRONG VALUE. OUR COMPANY REQUIRES A 3000 FPM DSCNT RATE WHEN COMPLYING WITH A XING RESTR. AS WE APCHED 10000 FT; I NOTED A LACK OF ANY INDICATION THAT THE AUTOPLT INTENDED TO LEVEL AT THE REQUIRED ALT. I ALSO NOTED A LARGE DIFFERENCE IN THE ALT READING BTWN THE 2 MAIN ALTIMETERS. I TOLD THE FO TO STOP THE DSCNT AND LEVEL THE ACFT. BECAUSE OF THE HIGH RATE OF DSCNT AND THE MOMENTARY CONFUSION OF CONFLICTING INFO COUPLED WITH A MOMENTARY DELAY IN COMPLYING WITH MY ORDER; THE ACFT REACHED 9300 FT INDICATED ON THE CORRECT ALTIMETER. AT THIS TIME DULLES APCH ASKED US ABOUT OUR CURRENT ALT. I RESPONDED BY REQUESTING THE CORRECT ALTIMETER SETTING AND ADVISING THE CTLR THAT WE WERE QUICKLY RETURNING TO 10000 FT. (THERE WAS DOUBT IN MY MIND AT THIS POINT AS TO THE CORRECT SETTING.) AFTER THE FLT; THE FO AND I DISCUSSED THIS INCIDENT AND DETERMINED THAT HE HAD HEARD THE CORRECT SETTING; READ IT BACK CORRECTLY; BUT HAD NONETHELESS DIALED IT IN INCORRECTLY. WE WERE OPERATING IN THE EARLY MORNING HRS COMPLETING A CONTINUOUS DUTY OVERNIGHT TRIP. LEAVING US BOTH WITH INADEQUATE REST. I THINK THAT IS THE REASON FOR THE FO'S MENTAL LAPSE; AS I HAVE FLOWN WITH HIM BEFORE AND KNOW HIM TO BE BOTH A GOOD AND COMPETENT PLT. I KNOW MY OWN LACK OF DILIGENCE WAS ALSO DIRECTLY RELATED TO MY LACK OF ADEQUATE REST.