FLC OF MIL C130 ACCEPTED A REVISION TO THEIR ASSIGNED ALT DURING TKOF ROLL AND THEN CLBED THROUGH THE RESTR.

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MXF TWR PROCS INSTRUCT ACFT TO MAINTAIN 4000 FT ON DEP. HOWEVER; NORMAL MGM APCH PROC ARE TO MAINTAIN 2000 FT. IN THIS CASE THE ACFT (MIL X/C130) WAS GIVEN 4000 FT BY DEP CLRNC DELIVERY THEN GIVEN 2000 FT BY MXF TWR (LCL) WHEN IFR RELEASE WAS GIVEN. ACFT READ BACK 2000 FT BUT CLBED TO 4000 FT. I KNOW OF TWO OTHER TIMES THIS HAS HAPPENED; AND HAVE HEARD OF SEVERAL OTHERS. MGM APCH RARELY ISSUES DEP ALTS ABOVE 2000 FT AND MXF CLRNC DELIVERY SHOULD NOT ISSUE ALTS ABOVE 2000 FT AS WELL. SUPPLEMENTAL INFO FROM ACN 355567: THE ORIGINAL CLRNC ISSUED TO OUR ACFT INCLUDED 'CLRED AS FILED; CLB AND MAINTAIN 4000 FT EXPECT...' THE COPLT AND NAVIGATOR BOTH COPIED THIS ORIGINAL IFR CLRNC. ABOUT 2 MINS LATER WE WERE 'CLRED FOR TKOF.' THE COPLT ACKNOWLEDGED THE TKOF CLRNC WITH OUR CALL SIGN AND REPEATED THE ORIGINAL CLB RESTR ('CLRED FOR TKOF; RWY HDG TO 4000 FT'). A FEW SECONDS LATER THE CTLR AMENDED THE CLBOUT TO 'TURN TO HDG 300 DEGS WITH 2 DME; CLB AND MAINTAIN 2000 FT.' OUR CREW ACKNOWLEDGED WITH OUR CALL SIGN WHILE STILL FOCUSING ON THE TKOF. THE ALTDEV COULD HAVE BEEN AVOIDED IF OUR CREW DID NOT ACKNOWLEDGE INSTRUCTIONS DURING A CRITICAL PHASE OF FLT; AND ATC AVOIDED (IF POSSIBLE) ISSUING CLBOUT INSTRUCTION WHEN ACFT IN CRITICAL PHASE OF FLT.