BE58 PLT HAD ALTDEV WHILE PRACTICING OPS WITH A GPS NAV SYS.

Narrative

BACKGROUND: THIS FLT ORIGINATED AT BFI IN SEATTLE FOR THE PURPOSE OF COMPLETING A POST; ANNUAL INSPECTION TEST FLT AND CONTINUED FAMILIARIZATION TRAINING FOR THE OWNER/OPERATOR/PIC ON NEWLY INSTALLED; DUAL; GARMIN GNS 430 GPS NAV SYS. THE PIC IS A HIGH TIME (12000+ HRS) PLT WITH EXTENSIVE XCOUNTRY; IFR EXPERIENCE. HIS ACFT; (BEECH P58 BARON) IS PRESSURIZED AND VERY WELL EQUIPPED. THE PIC IS 73 YRS OF AGE AND NOT COMPUTER LITERATE; ATTEMPTING TO MASTER A NEW STATE-OF-THE-ART; GPS BASED AREA NAV SYS WITH BUILT IN COMMERCIAL CHART DATABASE CAPABLE OF FLYING GPS AND GPS OVERLAYS OF EXISTING APCHS. PROB: CHINOOK APCH WAS VECTORING ACFT TO THE FINAL APCH COURSE; (FAC) AT 5000 FT AND TOLD US TO 'INTERCEPT THE FAC AND PROCEED INBOUND; AT THE 11 DME YOU ARE CLRED FOR THE APCH.' ONCE ESTABLISHED ON THE FAC AT 5000 FT; THE PIC CONFUSED VARIOUS DISTANCE READOUTS; BTWN 2 DIFFERENT GPS WAYPOINTS AND THE DME RECEIVER AND INITIATED DSCNT FROM 5000 FT PRIOR TO REACHING THE 11 DME FIX AS DEPICTED ON THE APCH CHART. CHINOOK APPROACH NOTICED THE ALT AND REQUESTED RETURN TO 5000 FT UNTIL REACHING THE 11 DME FIX; THIS ALTDEV WAS AT APPROX THE 14.5 DME FIX ABOUT 10 NM FROM THE FINAL APCH FIX (FAF) AT DONNY. MY BELIEF IS THE PIC WAS THINKING HE WAS WITHIN THE 10 NM AS DEPICTED ON THE PROFILE VIEW OF THE APCH CHART AND WAS SAFE TO DSND TO 4000 FT. HOWEVER; AND AS I POINTED OUT TO THE PIC AFTERWARDS; HE CONFUSED THE APCH CLRNC TOO; MEANING HE WAS EXPECTED TO REMAIN AS HIS LAST ASSIGNED ALT UNTIL SUCH TIME AS HE ARRIVED AT THE POINT; FROM WHICH HE WAS CLRED FOR THE APCH; THE 11 DME FIX IN THIS CASE. PERFORMANCE CONSIDERATIONS: NEW EQUIP KNOWLEDGE; TRAINING AND PROFICIENCY OF USE COUPLED WITH PIC PERCEPTIONS AND DECISIONS; LEADING FIRST TO DISTRACTIONS; THEN TO PERHAPS PIC NOT HEARING OR UNDERSTANDING THE ATC CLRNC WERE FACTORS LEADING TO THIS DEV. ALSO; THE ACTUAL APCH PLATE IS QUITE BUSY; GIVING THIS PIC; WHO HAD NEVER FLOWN INTO YKM; PLENTY OF OPPORTUNITY TO DEVELOP SOME CONFUSION AND MISUNDERSTANDING. CONSIDERABLE ADDITIONAL TRAINING ON THIS NEW EQUIP IS SCHEDULED TO REDUCE THE DISTR FACTOR; TO THE MAX EXTENT POSSIBLE TO CORRECT THIS AND THE OTHER HUMAN FACTORS; WHICH LEAD TO THIS ERROR.