ACR MLG ALT DEVIATION OVERSHOT DURING CLIMB WHEN PF DEACTIVATED ALT CAPTURE BY CHANGING FGCP FROM IAS TO MACH MODE.

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WE WERE CLBING AS CLRED BY ATC FROM 23000' TO FL290 WITH THE AUTOPLT ENGAGED. THE F/O CALLED; 'OUT OF 28 FOR 29;' AS WE WERE PASSING THROUGH 28000'; I NOTICED I HAD NOT PUSHED THE IAS-MACH BUTTON TO CHANGE FROM IAS HOLD TO MACH HOLD; SO I SELECTED MACH HOLD AT THIS TIME. WHEN I DID THIS I DID NOT NOTICE THAT ALT CAPTURE TURNED OFF. DUE TO THE CHANGE FROM IAS TO MACH SPD; THE VERT CLB SPD INCREASED. THE ACFT WAS PASSING THROUGH APPROX 29200' BEFORE I REALIZED WHAT HAD HAPPENED. THE ACFT REACHED APPROX 29600' BEFORE I COULD START DSNDING BACK TO 29000'. THE ATC CTLR REQUESTED OUR ALT ABOUT THIS TIME. I'M NOT SURE OF THE REPLY WE GAVE SINCE WE WERE CONCERNED WITH GETTING BACK DOWN TO 29000'; BUT IT WAS SOMETHING LIKE 'DSNDING TO 29000'' OR 'LEVELING AT 29000'.' HE THEN INFORMED US OUR TFC WAS OPP DIRECTION; 20 NM AHEAD. AT THIS POINT THE FLT RETURNED TO NORMAL. WHILE RECOGNIZING THE FACT AS CAPT I AM TO CTL THE ACFT AT ALL TIMES; I DO FEEL THERE IS A DESIGN PROB WITH THE FLT GUIDANCE CTL PANEL (FGCP) OF THE MLG X. HAVING FLOWN THE MLG Y FOR OVER 3 YRS; I KNOW THAT WITH THE PROPER ALT SET ON THE FGCP THE ACFT WILL CAPTURE AND HOLD THE SELECTED ALT REGARDLESS OF WHAT OTHER MODES OR SPD CHANGES ARE SELECTED. THIS TYPE OF SOFTWARE DESIGN IS MUCH BETTER THAN THE PRESENT DESIGN OF THE MLG X FGCP.