AN INSTRUCTOR AND STUDENT PLT DSNDED BELOW THE PROC ALT ON APCH TO BOEING FIELD.

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DURING A TRAINING FLT INVOLVING A PRACTICE INST APCH (ILS RWY 31L) TO BFI. OUR ACFT RECEIVED A FINAL RADAR VECTOR TO MAINTAIN 2000 FT UNTIL ESTABLISHED ON THE LOC COURSE AND CLRNC FOR THE APCH. DURING THIS PROCESS MY INST STUDENT WAS DEMONSTRATING HIS AUTOPLT TO ME; WE WERE CONDUCTING A COUPLED APCH. THIS WAS MY FIRST EXPOSURE TO USE OF AN AUTOPLT FOR A COUPLED APCH. AFTER THE ACFT WAS ESTABLISHED ON THE INBOUND COURSE; I REMINDED MY STUDENT THAT THE NEXT SEGMENT OF THE APCH ALLOWED FOR DSCNT TO 1800 FT. SO WE DID SO. HOWEVER; WE SHOULD HAVE WAITED UNTIL PASSING LACKR INTXN; A STEP-DOWN FIX SEVERAL MI OUTSIDE THE FAF; BEFORE DSNDING TO 1800 FT. A FEW SECONDS AFTER LEVELING AT 1800 FT; THE APCH CTLR REPEATED INSTRUCTIONS TO MAINTAIN 2000 FT UNTIL ESTABLISHED ON COURSE. I REALIZED THEN WHAT HAD HAPPENED. JUST AS I WAS ABOUT TO HAVE US RETURN TO 2000 FT; WE ARRIVED AT LACKR INTXN. AT THE SAME TIME THE CTLR ASKED IF I HAD THE ARPT INSIGHT AT 12 O'CLOCK AND 9 MI. I DID (WX CONDITIONS WERE CAVU); AND WE RECEIVED INSTRUCTIONS TO THEN CONTACT BOEING TWR FOR LNDG CLRNC. CONTRIBUTING FACTORS HERE WERE OUR FIXATION WITH THE AUTOPLT SYS. EVEN DURING THIS FAMILIAR APCH OUR OVEREMPHASIS ON THE AUTOPLT SYS ALLOWED OMISSION OF SCANNING THE OTHER NAV INSTS. ALSO BECAUSE THE WX WAS EXCELLENT AND I HAD THE ARPT IN SIGHT; MY SCANNING DILIGENCE WAS; I BELIEVE; DEGRADED TO A LOWER LEVEL THAN HAD WE BEEN IN IMC. I HAVE EXPERIENCED THIS KIND OF DISTR RECENTLY AS AN INSTRUCTOR WHEREIN AN ACFT OWNER/PLT ENROLLS IN INST TRAINING. IT SEEMS THE AVERAGE ACFT OWNER HAS MORE SOPHISTICATED EQUIP ON BOARD THAN MANY (MOST?) INSTRUCTORS HAVE BEEN EXPOSED TO IN TRAINING ACFT. IN PARTICULAR; IFR CERTIFIED GPS UNITS AND AUTOPLTS ARE BEING ENCOUNTERED. IN MY EXPERIENCE; ACFT OWNER/INST STUDENT DOES NOT KNOW HOW TO PROFICIENTLY OPERATE THE GPS UNIT THEY HAVE AND CERTAINLY NEITHER DOES THE INSTRUCTOR. ALTHOUGH; PRIOR TO FLT; STUDENT AND INSTRUCTOR STUDY THE GPS MANUFACTURER'S VOLUMINOUS OPERATING MANUAL; THE FLT ITSELF IS WHERE BOTH STUDENT AND INSTRUCTOR GRAPPLE WITH THE TRIAL AND ERROR PROCESS OF SETTING UP THE GPS UNIT FOR EXECUTION OF PRACTICE INST APCHS. THE MOST SIGNIFICANT PROB I HAVE NOTED IS THAT I HAVE ALLOWED MYSELF TO BECOME ENGROSSED WITH PUSHING BUTTONS WHILE NOT KEEPING MY EYES SCANNING OUTSIDE FOR TFC. I HAVE CAUGHT MYSELF PREOCCUPIED IN THIS MANNER AND HAVE HAD TO REMIND MYSELF TO RESUME MY DUTY OF SCANNING OUTSIDE AND MAINTAIN SITUATIONAL AWARENESS. I HAVE BECOME AWARE OF HOW DISTRACTING TO THE UNINITIATED INSTRUCTOR A COMPLICATED NEW DEVICE CAN BE INFLT. FLT INSTRUCTORS NEED TO BE DOUBLY ALERT TO THE POWERFULLY DISTRACTING POTENTIAL IN THEIR MIDST WHILE PROVIDING TRAINING TO THEIR WELL-HEELED STUDENTS WHO BRING TO THE TABLE AMPLY-STOCKED COCKPIT PANELS.