PILOT LEAD OF FLIGHT OF TWO MILITARY FIGHTERS REPORTS MISCOMMUNICATION AND ASSUMPTIONS THAT LEAD TO DESCENT FROM ASSIGNED ALTITUDE IN ZLA AIRSPACE.

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FLT OF 2; WAS RETURNING AFTER COMPLETING TRAINING. THE LEAD PLT WAS COMMUNICATING ON THE AUX RADIO WITH THE STUDENT WINGMAN AND THE LEAD WX SYSTEMS OFFICER WAS FAMILIARIZING HIMSELF WITH THE 'ATFLIR' ON BOARD THE ACFT. THE LEAD WX SYSTEMS OFFICER RPTED TO THE PLT 'BINGO DOWN TO FOUR O.' INDICATING THAT HE WAS MOVING THE LOW FUEL INDICATOR FROM 5000 LBS DOWN TO 4000 LBS. THE PLT THOUGHT THE WX SYSTEMS OFFICER SAID 'CLRED DOWN 240;' AND ASSUMED THAT HE HAD MISSED CTR'S CALL WHILE TALKING ON THE AUX RADIO AND THAT CTR HAD DIRECTED THE ACFT TO DSND FROM FL270 TO FL240. THE WX SYSTEMS OFFICER IN THE LEAD ACFT CONTINUED TO OPERATE THE 'ATFLIR' AND DID NOT NOTICE THE ACFT WAS IN A DSCNT; THE STUDENT IN THE WING ACFT ASSUMED HE HAD ALSO MISSED CTR'S CALL. THE NEXT CALL RECEIVED WAS FROM ZLA DIRECTING FLT TO LEVELOFF AT FL250. AT THAT POINT THE ACFT HAD BEEN IN A DSCNT FOR APPROX 60 SECONDS AND WAS PASSING THROUGH FL251. ZLA ASKED IF WE HAD HEARD A CLRNC TO DSND AND LEAD WX SYSTEMS OFFICER RESPONDED 'NEGATIVE.' AT THIS POINT THE CREW IDENTED THE MISCOM AND RPTED TO CTR THAT THE PLT HAD THOUGHT THE FLT HAD BEEN CLRED FOR THE DSCNT. TOO MANY ASSUMPTIONS CONTRARY TO BASIC FLT DISCIPLINE AND THE 'SWISS CHEESE' MODEL LED TO THIS ERROR. NUMEROUS THOUSAND FT CLBS ON THE WAY TO THE AREA CAUSED THE PLT TO ASSUME THE ACFT WOULD BE STEPPED DOWN IN A SIMILAR MANNER ON THE RETURN. THE WX SYSTEMS OFFICER ASSUMING HE DID NOT NEED TO MONITOR ACFT FLT PARAMETERS CAUSED HIM TO MISS THE FACT THE ACFT WAS DSNDING. THE LEAD AND STUDENT PLT ASSUMING THEY HAD MISSED A CALL ON THE PRIMARY RADIO CAUSED NEITHER TO QUESTION THE DSCNT. FINALLY; THE LEAD PLT'S FAILURE TO 'ROGER UP' THE WX SYSTEMS OFFICER STATEMENT CAUSED THE WX SYSTEMS OFFICER TO ASSUME THE PLT HEARD HIM ACCURATELY.