FLC OF A WDB OVERSHOT DSCNT ALT DUE TO THE PF MISUNDERSTANDING OF AN AIRSPD RESTR FOR ALT.

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I WAS INSTRUCTOR PLT FOR FIRST TIME CAPT DURING INITIAL IOE. FLT WAS ON DSCNT; CLRED FROM CRUISE ALT TO FL290. THE PF WAS THE IOE CAPT. HE ENTERED FL290 IN THE FLT CTL PANEL AND I CONFIRMED THE ENTRY AS DSCNT WAS INITIATED. I TOOK OUT OUR PERFORMANCE COMPUTER TO ENTER OUR LNDG INFO. AS I WAS MAKING THE ENTRIES; ZDV CALLED AND INSTRUCTED US TO MAINTAIN 280 KTS. THE PF DIDN'T CATCH THE XMISSION FULLY AND I SAID TO HIM '280' (TWO EIGHTY) AS HE POINTED AT THE FLT CTL PANEL. I RETURNED TO THE PERFORMANCE COMPUTER AND AFTER ABOUT 2 MINS LOOKED AT OUR ALTIMETER AND REALIZED (WITH HORROR) THAT WE WERE LEVEL AT FL280! AGGHH! I IMMEDIATELY RPTED THE MIX-UP TO DEN; AND AFTER A LONG LONG PAUSE; THE CTLR SAID; 'THAT'S OK; CONTINUE DSCNT.' CAUSE: 1) MY FAILURE AS INSTRUCTOR PLT TO EFFECTIVELY MONITOR THE 'NEW GUY.' 2) THE 'NEW GUY'S' FAILURE TO USE COMPANY ALT VERIFICATION PROCS. HE THOUGHT I MEANT FOR HIM TO ENTER FL280. HE THEN ENTERED THAT ALT AND DID NOT ASK FOR VERIFICATION OF THE NEW ALT FROM THE PNF (ME) -- A VIOLATION OF SOP. HAD HE ASKED FOR VERIFICATION; THE ERROR WOULD HAVE BEEN CAUGHT IMMEDIATELY.