A B737-300 SUBJECT TO A SPD RESTR DELAYS ITS DSCNT FROM FL290 TO FL280. A DSNDING; OVERTAKING B727 EXPERIENCES A POTENTIAL CONFLICT WITH THAT ACFT WHEN CLRED TO FL290.

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WHILE IN CRUISE FLT AT FL290 ATC ISSUED INSTRUCTIONS TO SLOW TO 280 KIAS (APPROX 20 KIAS). I CHANGED AUTOFLT MODES FROM 'VERT NAV' TO 'MCP SPD' TO COMPLY. SHORTLY AFTER ATC CLRED US TO FL280. I SELECTED AND EXECUTED FL280 IN THE FMC; HOWEVER THE AUTOPLT DID NOT BEGIN A DSCNT BECAUSE I DID NOT RESELECT VNAV AND IT WAS THEREFORE NOT IN THE CORRECT MODE TO RESPOND TO FMC COMMANDS. THE CTLR THEN CLRED A B727 BEHIND US TO 'ACCELERATE MAX FORWARD SPD; DSND AND MAINTAIN FL290.' A CONFLICT DEVELOPED BECAUSE OF OUR DELAY VACATING FL290. ON TCASII THE B727 WAS AT OUR CLOSEST POINT 2 NM LATERALLY AND 700 FT VERTLY AND IN SIGHT VISUALLY. ATC QUERIED US ABOUT OUR ALT AND I EXPEDITED OUR DSCNT. SEPARATION WAS LOST FOR A MATTER OF SECONDS. CONTRIBUTING FACTORS MAY HAVE BEEN: FATIGUE. THIS WAS THE LAST OF 5 LEGS AND APCHING THE END OF AN 8 HR DUTY DAY AND 7 HR HARD TIME FLYING DAY. COMPLACENCY -- THIS WAS OUR THIRD SLC-DEN LEG OF THE DAY. DISTRACTIONS -- THE CAPT WAS EATING LUNCH AND SETTING UP FOR THE ARR. MISTAKING AUTOTHROTTLE MOVEMENT AND ACFT PITCH CHANGE (TO AIRSPD CHANGE; NOT ALT CHANGE) FOR PROPER RESPONSE RATHER THAN PROPER PRIMARY INST VERIFICATION (AUTOMATION COMPLACENCY). DIFFERENCE IN EQUIP SPD BTWN B737 AND THE B727 AND THE COMBINATION OF A 'SLOW DOWN; DSND CLRNC' AT AN ALT BELOW A 'SPD UP; DSND' CLRNC TO A HIGHER AND MUCH FASTER ACFT. SUPPLEMENTAL INFO FROM ACN 423282: I SET FL280 AND POINTED AT THE MCP WINDOW UNTIL THE FO CONFIRMED IT. I THOUGHT IT MIGHT GET BUMPY; SO I TURNED ON THE FASTEN SEAT BELT SIGN AND TOLD MY FO I WAS OFF THE RADIO TO MAKE THE FINAL ANNOUNCEMENT AND NOTIFY THE FLT ATTENDANTS THAT I'D BE SEATING THEM SHORTLY. AS WE RECEIVED THE INITIAL INSTRUCTION TO SLOW; I LOOKED TO MY L TO GET MY MOUTHPIECE AND CHK MY MAP. AT THIS POINT MY FO DESELECTED VNAV. I NEVER REALIZED WE HAD CHANGED MODES. WHEN WE WERE INSTRUCTED TO DSND; I SAW HER (FO) TYPE IN THE NEW ALT. OUR COMPANY HAS A PROC; WHERE THE PNF SETS THE NEW ALT IN THE MCP AND POINTS AT THE MCP WINDOW UNTIL THE PF ACKNOWLEDGES. I DID THIS; AND THINKING WE WERE IN VNAV ASSUMED THE ACFT WOULD BEGIN ITS DSCNT. INACTIONS BY CAPT: NEVER REALLY CONFIRMED WE WERE IN DSCNT. DID NOT MAINTAIN SILENCE UNTIL DSCNT BEGAN. THIS MAY HAVE DISTRACTED FO. INACTIONS BY FO: NEVER ENTERED PROPER MODE TO BEGIN DSCNT. NEVER CONFIRMED DSCNT BEGAN. RECOMMENDATIONS: WHEN A NEW MODE IS SELECTED ON MCP; IT SHOULD BE ANNOUNCED. PNF AFTER POINTING AT MCP SHOULD POINT AT THE VERT SPD INDICATOR TO CONFIRM DSCNT; IF IT IS TO BEGIN SHORTLY. IF NOT; HE SHOULD POINT AT FMA AND CONFIRM PROPER MODE SELECTED. CAPT SHOULD PROVIDE FOR AN UNDISTRACTING WORK ENVIRONMENT; BY HOLDING OFF ON THE ANNOUNCEMENTS AND TALKING UNTIL DSCNT HAS ACTUALLY BEGUN. SUPPLEMENTAL INFO FROM ACN 423428: ZDV ASKED THE B737 TO SLOW TO 280 KTS AND ASKED US TO PICK OUR SPD UP. VERY SHORTLY AFTER THIS ZDV CLRED THE B737 TO DSND TO FL280 FROM FL290; THEN IMMEDIATELY GAVE US A CLRNC TO FL290 AND TOLD US TO KEEP OUR SPD UP. I BEGAN LOOKING FOR THE B737 AGAIN. I SAW ON THE TCASII THAT THE B737 WAS STILL AT FL290. WE WERE DSNDING THROUGH FL300. I ADJUSTED THE SCALE ON TCASII AND FOUND WE ONLY HAD ABOUT 3 MI HORIZ SEPARATION. I POINTED THIS OUT TO THE CAPT WHO LEVELED US OFF AT FL295. SOON AFTER THAT ZDV CALLED THE B737 AND ASKED IF HE HAD BEGUN HIS DSCNT AND HE REPLIED THAT HE WAS STARTING DOWN NOW. ONCE OUR TCASII INDICATED WE HAD 1000 FT SEPARATION; WE CONTINUED OUR DSCNT TO FL290. CONTRIBUTING FACTORS: I DON'T THINK THAT ATC CONSIDERED THE FACT THAT A B727 TOLD TO DSND AND KEEP THEIR SPD UP IS GOING TO COME DOWN MUCH MORE RAPIDLY THAN A B737 WITH A SPD RESTR. GIVING BOTH OF THESE ACFT THEIR DSCNT CLRNCS SIMULTANEOUSLY WHEN THEY ARE ALREADY AT THE MINIMUM VERT IFR SEPARATION IS GOING TO CAUSE A CONFLICT. ALSO THE B737 BEING A GLASS COCKPIT; IS GOING TO TAKE LONGER TO INITIATE THE DSCNT DUE TO THE TYPING INVOLVED FOR THE FMS AND THE SLOW PITCH RATE OF THE AUTOPLT. THE B727 IS NON- GLASS. YOU JUST POINT THE NOSE DOWN AND YOU ARE DSNDING. IT IS A LOT LESS INVOLVED. TO AVOID THIS IN THE FUTURE I THINK IF ATC NEEDS TO DSND 2 ACFT AT ADJACENT FLT LEVELS WHO DON'T HAVE HORIZ SPACING; THEY SHOULD DELAY GIVING THE DSCNT CLRNC TO THE SECOND ACFT UNTIL THE FIRST ACFT HAS REACHED ITS NEWLY ASSIGNED ALT.