C90 KING AIR PLT HAS ALTDEV DURING PRACTICE APCH.

Narrative

I WAS FLYING AN INST PROFICIENCY TRAINING FLT ON AN IFR FLT PLAN ORIGINATING AT SAT ON VICTOR AIRWAY TO AUSTIN BERGSTROM FOR AN ILS LOW ACH AND CLBOUT BACK TO SAT FOR AN ILS RWY 12R FULL STOP. PRIOR TO ENG START; I PLUGGED IN MY PERSONAL HEADSET IN THE L SEAT TO USE IN LIEU OF THE ACFT HEADSET. I DID NOT REALIZE THAT MY R SEAT SAFETY OBSERVER PLT COULD NOT HEAR ME XMIT ON VHF WITH THIS NEW ARRANGEMENT. WHILE RETURNING BACK TO SAT I WAS GIVEN VECTORS BY APCH CTL; FOR AN EVENTUAL CLRNC FOR THE ILS RWY 12R. I WAS CLRED OUT OF 8000 FT MSL FOR 6000 FT MSL BY APCH ON A NORTHWESTERLY HEADING. I THEN DIALED IN 6000 FT IN THE ALT REMINDER WINDOW; WHICH DOES NOT PROVIDE ALTDEV ALERT CAPABILITY. WHILE DSNDING TO 6000 FT I WAS RUNNING THE DSCNT CHKLIST. I INADVERTENTLY DSNDING TO 5800 FT MSL; AND MY SAFETY OBSERVER; WHILE THINKING I HAD ACKNOWLEDGED A DSCNT TO 5000 FT MSL; CHANGED THE ALT REMINDER TO 5000 FT AND CHALLENGED ME BY ASKING 'WE'RE CLRED DOWN TO 5000 FT; RIGHT?' I GLANCED AT THE ALT REMINDER; SAW 5000 FT DIALED IN; AND REPLIED '5000 FT; RIGHT!' A FEW SECONDS LATER APCH CTL CALLED US AND SAID 'YOU NEED TO CLB BACK TO 6000 FT AND TURN R 20 DEGS. I HAVE A CITATION JET IN YOUR 11 O'CLOCK POS CLBING TO 5000 FT.' AT THIS TIME OUR ALT WAS 5300 FT MSL. I ACKNOWLEDGED AND PROMPTLY TURNED WHILE CLBING BACK TO 6000 FT. APCH DID NOT COMMENT ANY FURTHER ON OUR ALTDEV AND PASSED US ONTO ANOTHER APCH CTL FREQ. WE COMPLETED THE ILS AND LNDG WITHOUT FURTHER INCIDENT. WHEN WE FINISHED OUR POSTFLT DUTIES; MY OBSERVER AND I DISCUSSED MY ALTDEV AT LENGTH. THIS WAS WHEN I COMPREHENDED THAT MY OBSERVER COULD NOT HEAR MY RADIO XMISSIONS. HE COULD HEAR ATC; BUT HE WAS GETTING ONLY HALF OF THE RADIO SITUATIONAL AWARENESS PICTURE. AS I WAS PRACTICING SINGLE PLT OPS; HE FELT HE WAS TOO QUICK TO CHANGE THE ALT WINDOW BEFORE ASKING ME ABOUT OUR ALT CHANGE. BUT I HAD ALREADY DEVIATED BELOW OUR ASSIGNED ALT BEFORE HE CHANGED THE WINDOW; AND THAT WAS MY FAULT. I WOULD HAVE CONTINUED DSNDING UNTIL MY OWN SITUATIONAL AWARENESS KICKED IN OR THE ATC CTLR SAW THE DEV. I FULLY APPRECIATE THE ALERT VIGILANCE BY THE CTLR. HE DID A GREAT JOB. I HAVE REMOVED MY PERSONAL HEADSET FROM THE PLANE SO THE R SEATER CAN HEAR ALL XMISSIONS. I HAVE IMPROVED MY CHKLIST USE SO I WILL NOT DISTRACT MYSELF FROM MY PRIMARY DUTY OF FLYING THE AIRPLANE. AS FURTHER NOTE; THE CENTURY IV AUTOPLT (A/P) DOES NOT CAPTURE SELECTED ALTS. DURING MY DSCNT INTO SAT; I WAS USING THE CENTURY IV AUTOPLT AND HAVE SINCE LEARNED TO CLOSELY MONITOR CLBS AND DSCNTS TO MANUALLY CAPTURE MY ALT WITHOUT THE AUTOPLT. I DID THIS ON PREVIOUS AUTOPLT SYS I HAVE USED; BUT THIS ACFT HAS NO SAFEGUARDS SUCH AS ALTDEV ALERTS.