PA46 PILOT REPORTS ALTITUDE DEVIATION AFTER CONFUSING AUTO PILOT DISCONNECT BUTTON WITH PTT BUTTON. LAYOUT IS OPPOSITE OF ACFT REPORTER NORMALLY FLIES.

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PIPER MERIDIAN; ACFT X; WAS IFR CRUISE WBOUND ON AUTOPLT WITH NO TCAS TFC NEARBY. ATC...'MERIDIAN PLT'S DISCRETION; DSND AND MAINTAIN 9000 FT.' I SET 9000 FT IN THE S-TEC 1500 AUTOPLT PRESELECT; THEN UNINTENTIONALLY PUSHED THE AUTOPLT/TRIM DISCONNECT BUTTON (BRB) INSTEAD OF THE PUSH TO TALK (PPT) ON THE YOKE. I CANCELED THE AUTOPLT DISCONNECT WARNING AND RESET AUTOPLT; THEN CALLED CTR. ACFT X 'PD LEAVING 1-2 TWELVE THOUSAND FOR 9 THOUSAND; ACFT X.' ATC...'ACFT X ROGER.' IT WAS MY INTENT TO BEGIN DSCNT PD IN A FEW MI. WITHIN 1 MIN I NOTICED THE ALT DRIFTING HIGHER AND NEARING 12300 FT; PUSHED AUTOPLT DISCONNECT AND PUSHED THE NOSE DOWN. ATC...'ACR X SAY ALT.' ACFT X 'LEAVING 12 THOUSAND FOR 9 THOUSAND.' ATC...'ACFT X; ROGER.' I AM FULLY RESPONSIBLE FOR THE ALT DRIFT TO NEAR 12300 FT WHILE CLRED TO 12000 PD TO 9000. I WOULD LIKE TO OFFER COMMENTS; NOT AS A DEFENSE; BUT ADDITIONAL INFO. I FLY 3 ACFT; A CIRRUS SR22; A TBM850 AND THE PIPER MERIDIAN. FOR THE PRIOR WKS I FLEW THE TBM FOR OVER 20 HRS. BOTH THE TBM; CIRRUS (AND MOST?) HAVE THE BRB IN THE THUMB POS ON THE FRONT OF THE YOKE AND THE PTT FOR THE INDEX FINGER ON THE BACK SIDE. THE PIPER MERIDIAN IS THE OPPOSITE. WHEN SWITCHING AIRPLANES IN THE PAST I HAVE PUSHED THE BRB FOR THE PTT OR VICE-VERSA; BUT UNTIL NOW IT HAS NOT BITTEN ME. HIGH HR PROFESSIONAL PLTS HAVE RELATED TO ME DOING THE SAME THING WHEN SWITCHING TO/FROM PIPER FROM/TO OTHER ACFT. WHEN I ATTEMPTED TO RESPOND TO ATC; I PUSHED THE BRB TO TALK BUT INSTEAD DISCONNECTED THE AUTOPLT. WHEN THE AUTOPLT IS RESET IN THE S-TEC IT DOES NOT RECAPTURE THE ALT AND WE BEGAN TO CLB; BUT THE G1000 IN THE TBM DOES CAPTURE ALT. BECAUSE I HAD PROPERLY SET THE ALT PRESELECT TO 9000 FT; THIS SET MY ALT AUDIBLE ALERTS TO 9000 FT +/-200 FT WHICH DIDN'T HELP ME WHEN I DRIFTED ABOVE 12000 FT. I CERTAINLY WANT TO SEE THE ACFT MANUFACTURERS HAVE ALL THE FLEXIBILITY POSSIBLE IN DESIGN AND LAYOUT WHICH HELPS ADVANCEMENT; HOWEVER THE POS OF THE AUTOPLT/TRIM DISCONNECT AND PTT BUTTONS PERHAPS SHOULD BE STANDARDIZED. IT'S A BIT LIKE THE POS ON AUTOMOBILE GEAR SHIFTS ALL BEING 'P R N D L.' IF ONE CAR MANUFACTURER WAS TO HAVE R AND P SWITCHED FROM THE REST OF THE CARS I THINK THERE WOULD BE ISSUES. A PLT UNDER HVY WORKLOAD IN IMC WITH A TRIM RUNAWAY COULD PUSH THE PTT.