THE PLT OF A BE90 DSNDED BELOW 15000 FT TOO SOON DURING AN APCH TRANSITION PROC. THE ARTCC RADAR CTLR SAW THE ERROR AND INSTRUCTED THE PLT TO CLB TO PUBLISHED ALT.

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ON MAY/XA/97 I WAS ENRTE FROM BOISE; ID; TO JACKSON; WY; IN A 1983 C90-1 KING AIR WITH MYSELF AND MY WIFE ABOARD. I WAS ON AN IFR FLT PLAN; HAVING DEPARTED BOISE AROUND XA20 LCL TIME. I WAS IN VFR CONDITIONS AT FL200 ABOVE A BROKEN CLOUD LAYER. THE CONDITIONS IN JACKSON WERE: GREATER THAN 6 MI VISIBILITY ON THE GND WITH CALM WIND AND THE CLOUD BASES AT 2500 FT. THE ARPT IS 6500 FT SO THE CLOUD BASES WERE AT APPROX 8000 FT MSL. THE TOP OF THE CLOUD LAYER WAS APPROX 11000 FT MSL. I WAS CLRED TO CROSS THE JACKSON VORTAC AT OR ABOVE 16000 FT AND THEN CLRED FOR THE ILS DME APCH INTO JACKSON. I WAS ASKED TO SWITCH TO ADVISORY UNICOM FREQ AND ADVISE ZLC OF MY CANCELLATION OR DOWN TIME ON THE CTR FREQ. THE APCH PLATE SHOWS THAT ONE SHOULD FLY THE TRANSITION FROM THE JACKSON VORTAC OUTBOUND ON THE LOC TO THE QUIRT INTXN AT 15000 FT AND THEN DO A PROC TURN WHILE DSNDING TO 12000 FT TO INTERCEPT THE LOC AT 12000 FT. AFTER XING THE JACKSON VORTAC AT 16000 FT; THE NEXT ALT I DIALED IN WAS 12000 FT. I CONTINUED MY DSCNT ALONG THE LOC TO APPROX 13600 FT WHEN CTR ADVISED AN IMMEDIATE CLB TO 15000 FT. I RESPONDED WITH AN IMMEDIATE CLB TO 15000 FT AND RPTED THAT I WAS STILL IN VFR CONDITIONS (CLOUD TOPS WERE AT APPROX 11000 FT). AT QUIRT INTXN; I RPTED TO CTR LEVEL AT 15000 FT FOR A PROC TURN AND DSNDED TO 12000 FT. I ENTERED THE CLOUDS AT 11000 FT AND FLEW THE ILS TO WELL BELOW THE BASES AT 8500 FT WHEN I CANCELED IFR WITH CTR; CONTACTED UNICOM AND LANDED UNEVENTFULLY. THIS PROB WAS CAUSED BY MY FIXATION ON THE INTERCEPT ALT OF 12000 FT; INSTEAD OF MORE CAREFULLY READING THE APCH CHART FOR THE PROPER TRANSITION ALT. FORTUNATELY; I WOULD NOT HAVE HIT ANY TERRAIN AND DID NOT INTERFERE WITH ANY OTHER ACFT. THIS WAS AN IMPORTANT PERSONAL LESSON IN PLT ERROR WHICH HAS DRIVEN HOME THE NEED FOR CONTINUAL TRAINING AND HYPERAWARENESS IN ALL PHASES OF FLT.