CGA CL60 FLC; WITH MAINT PERSON; BECOME DISTRACTED WITH MAINT CHK ITEM AND ALLOW ACFT TO DSND BELOW ASSIGNED ALT. CTLR ALERTS FLC TO ALTDEV. ACFT RETURNS TO ASSIGNED ALT UNTIL VISUAL APCH CLRNC IS ISSUED.

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ON DSCNT INTO DAL; AS PART OF THE MAINT REQUIREMENT; WE DEPLOYED THE AUX GENERATOR TO TEST ITS OP; PRIOR TO THE ACFT GOING FOR INSPECTION AT THAT ARPT. THE PROC IS SOMEWHAT INVOLVED; AND; AS PNF; I WAS DEVOTING SOME TIME IN ASSISTING THE MAINT SPECIALIST IN ACCOMPLISHING THE NECESSARY ITEMS; WHILE DOING MY NON FLYING DUTIES. WE HAD BEEN CLRED TO 3000 FT IN ANTICIPATION OF A VISUAL APCH TO RWY 13L AT DAL. BECAUSE OF THE ABNORMAL CONFIGN OF THE ELECTRICAL SYS WITH THE AIR DRIVEN GENERATOR DEPLOYED; THE COPLT WHO WAS PF IN THE L SEAT; WAS HAND FLYING THE ACFT. IN MY DUAL ROLE; I PROBABLY WAS NEGLIGENT IN XCHKING HIM; AND HE MAY HAVE BECOME DISTRACTED BY WHAT WE WERE DOING; AND INADVERTENTLY; WE DSNDED TO APPROX 2650 FT. AT THIS POINT; WE WERE ADVISED BY ATC TO MAINTAIN 3000 FT. WE CLBED IMMEDIATELY TO 3000 FT; AND CONTINUED TO A VISUAL APCH TO RWY 13L. THE NOISE LEVEL IS EXTREMELY HIGH DURING THE AIR DRIVEN GENERATOR DEPLOYMENT AND CAN BE DISTRACTING. WE SHOULD HAVE INITIATED THE TEST PROC A LITTLE SOONER; AND I SHOULD HAVE MORE STRONGLY EMPHASIZED TO THE SIC THE IMPORTANCE OF 'FLYING THE AIRPLANE.' FORTUNATELY; THERE WERE NO CONFLICTS.