A SA227 INCURRED AN EXCURSION FROM AN ASSIGNED ALT DUE TO TROUBLESHOOTING THE PRESSURIZATION SYS.

Narrative

ZMA TOLD ME TO 'CROSS RAJAY AT 11000 FT.' I STARTED MY DSCNT FROM 13000 FT. I WAS WORKING ON A PRESSURIZATION PROB WITH AN AIRPLANE OUR COMPANY HAD JUST PURCHASED. TO GIVE THE MECHS A BETTER IDEA OF A PRESSURIZATION PROB WHICH WAS MEL'ED; I NOTICED THE AIRPLANE WOULD PRESSURIZE IN THE MANUAL MODE. FROM MIAMI ARPT I REQUESTED 17000 FT AND NOTICED THE CABIN WOULD NOT HOLD ABOVE AT 9000 FT. SO I REQUESTED 13000 FT AND THE CABIN HELD AT ABOUT 2000 FT. MY VSI WAS A COMBINATION TCASII WHICH HAD THE BACK-LIGHTING KNOB SHEARED; SO I THOUGHT THE VSI WAS INOP. THE BLEED AIR IN THE AIRPLANE WAS TREMENDOUS. A HIGH RATE OF AIR WOULD COME IN THROUGH THE EYELETS; SO MUCH THAT IT WAS A DISTRACTION. EVEN THROUGH MY HEADSETS I WAS DEAFENED BY THE 'SSH' OF THE INTENSE BLEED AIR FLOW. WITH A NEW ACFT I HAD NEVER FLOWN BEFORE; AND THE ABOVE DISTRACTIONS; I WAS ALSO MANUALLY TUNING THE CABIN PRESSURIZATION AND I DSNDED THROUGH MY ASSIGNED ALT OF 11000 FT; TO 10500 FT. I QUICKLY CLBED TO 11000 FT. I BELIEVE MY PREOCCUPATION WITH THE PRESSURIZATION; NO VSI AND DISTRACTION OF THE INTENSE BLEED AIR WERE FACTORS. THE BLEED AIR SYS WAS SO LOUD I DIDN'T HEAR THE ALT 'DING.'