SPD DEV NEAR BOWIE; TX.

Narrative

WE WERE CRUISING AT FL330; TRACKING DIRECTLY TO UKW AS ASSIGNED BY ATC; WITH MYSELF AS THE PF. AT APPROX 110 NM W OF UKW; WE RECEIVED CLRNC TO CROSS 40 NM W OF UKW AT FL240; TRANSITION TO 300 KIAS; AND TO BEGIN OUR DSCNT NOW. AT THIS TIME MY FO WAS MAKING THE DSCNT PA; SO I READ BACK THE CLRNC; THEN BEGAN THE DSCNT TO FL240. AFTER ESTABLISHING THE DSCNT; I THEN DIALED IN THE DESIRED 300 KT AIRSPD; BUT ELECTED NOT TO SELECT IT; SINCE WE WERE INDICATING 40 KTS LESS THAN THE 300 KIAS CHANGEOVER SPD. MY INTENTION WAS TO MAKE THE SPD SELECTION AT THE MACH/IAS CHANGEOVER ALT. WHEN THE FO RETURNED; I ADVISED HIM THAT WE WERE DSNDING TO CROSS 40 NM W OF UKW AT FL240; AND TRANSITION TO 300 KIAS. AS WE PASSED THROUGH FL280; I ALLOWED MYSELF TO BE DRAWN INTO CONVERSATION WITH OUR JUMP SEAT RIDER. THE DISTR LED ME TO MISS THE MACH/IAS CHANGEOVER; AND THE ACFT SUBSEQUENTLY LEVELED OFF AT THE SELECTED MACH .764. AT THIS POINT WE WERE APPROX 55 NM W OF UKW. AS WE LEVELED OFF; I REALIZED THAT WE WERE STILL IN MACH MODE. I SELECTED IAS ON THE AUTOPLT; AND AT THE SAME TIME ATC INQUIRED ABOUT OUR AIRSPD. THE FO RESPONDED THAT OUR AIRSPD WAS 325 KIAS. ATC THEN REITERATED THE PREVIOUS AIRSPD INSTRUCTIONS; AND WE IMMEDIATELY COMPLIED.