AN MLG ACR MISSED A XING ALT.

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AT FL290; APPROX 10 NM SW OF EKN; CLRNC WAS RECEIVED TO CROSS 30 NM SW OF MGW AT FL240. PNF ACKNOWLEDGED CLRNC AND PF ENTERED RESTRICTION IN FMC. (FLT WAS OPERATING IN VNAV AND LNAV MODES OF AUTOPLT.) AFTER ENTRY IN FMC; PF NOTICED A 'BYPASS' ANNUNCIATION IN FMC ROUTING IN ASSOCIATION WITH THE ENTRY OF '30 SW OF MGW' FIX. THIS WAS DUE TO IMPROPER ENTRY OF THIS FIX BTWN MGW AND 'ROTON' INTXN. AS OPPOSED TO WHERE IT REALLY WAS; BTWN EKN AND 'TYGAR' INTXN. PF WAS ATTEMPTING TO RESOLVE THIS PROBLEM WHEN IT BECAME APPARENT THAT INITIATION OF DSCNT WAS NECESSARY TO MEET RESTRICTION. PF ATTEMPTED TO USE VERT SPD TO INITIATE DSCNT BUT ACFT REMAINED AT FL290. IT WAS THEN NOTICED THAT ALT 'SET' WINDOW WAS STILL AT FL290; DISALLOWING AUTOPLT TO DSND. ALT WAS RESET TO FL240; DSCNT WAS INITIATED CONCURRENT WITH INQUIRY FROM CTR AS TO ABILITY TO MEET RESTRICTION. WE RESPONDED 'UNABLE' AND FLT WAS GIVEN A 50 DEG R TURN; PRESUMABLY TO AVOID A POTENTIAL CONFLICT. AT FL250; FLT WAS CLRED DIRECT MGW TO RESUME ROUTING. THIS SITUATION OCCURRED BECAUSE OF PF'S FAILURE TO FOLLOW HIS OWN SOP IN SUCH A CIRCUMSTANCE WHICH IS TO 1) RESET ALT; 2) MAKE A QUICK DETERMINATION IF ACFT IS NEAR THE REQUIRED DSCNT POINT AND INITIATE A DSCNT USING VERT SPD OR LEVEL CHANGE IF NECESSARY; 3) MAKE APPROPRIATE ENTRIES IN FMC AND USE VNAV FOR DSCNT IF APPROPRIATE. PF DID THESE OUT OF SEQUENCE AND BECAME DISTRACTED BY AN FMC IRREGULARITY; THUS OVERSHOOTING DSCNT POINT.