Narrative:

I was checking out a new copilot on this trip and instructing him on setting up the navaids and LORAN. We were using sea in navigation #2 while being vectored to the localizer back course final for runway 31 bfi. I set navigation #1 for the ILS frequency but did not switch the DME display from navigation #2 to navigation #1. During the period that I was setting up navigation #1 and identing it, I was interrupted from my normal flow by instructing the copilot on how to enter an intersection into the trimble (LORAN). I was further distraction when I couldn't figure out why the intersection was not coming up on the argus display. I decided to forget about the distraction and just fly the localizer back course using DME. But, with the wrong DME being read out on the HSI, I descended from 2600 ft to 1600 ft too early. Approach called bfi tower who relayed to me a low altitude alert. I climbed while figuring out and setting in the correct DME display and, at this point, I was past the 2600 ft restr (lacre) and cleared to descend to 1600 ft. Approach saved my life! While there were distrs, they were minor. I just need to be more diligent in checking the NAVAID setup. I need to doublechk it just like I check twice for 3 green. I was also dividing my attention between instruments and checking the buildup under the heated leading edge of the wing.

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Original NASA ASRS Text

Title: A C501 CAPT BECAME CONFUSED WITH HIS INST SETUP AND DSNDED BELOW A MANDATORY XING ALT. ERROR ADMITTED.

Narrative: I WAS CHKING OUT A NEW COPLT ON THIS TRIP AND INSTRUCTING HIM ON SETTING UP THE NAVAIDS AND LORAN. WE WERE USING SEA IN NAV #2 WHILE BEING VECTORED TO THE LOC BACK COURSE FINAL FOR RWY 31 BFI. I SET NAV #1 FOR THE ILS FREQ BUT DID NOT SWITCH THE DME DISPLAY FROM NAV #2 TO NAV #1. DURING THE PERIOD THAT I WAS SETTING UP NAV #1 AND IDENTING IT, I WAS INTERRUPTED FROM MY NORMAL FLOW BY INSTRUCTING THE COPLT ON HOW TO ENTER AN INTXN INTO THE TRIMBLE (LORAN). I WAS FURTHER DISTR WHEN I COULDN'T FIGURE OUT WHY THE INTXN WAS NOT COMING UP ON THE ARGUS DISPLAY. I DECIDED TO FORGET ABOUT THE DISTR AND JUST FLY THE LOC BACK COURSE USING DME. BUT, WITH THE WRONG DME BEING READ OUT ON THE HSI, I DSNDED FROM 2600 FT TO 1600 FT TOO EARLY. APCH CALLED BFI TWR WHO RELAYED TO ME A LOW ALT ALERT. I CLBED WHILE FIGURING OUT AND SETTING IN THE CORRECT DME DISPLAY AND, AT THIS POINT, I WAS PAST THE 2600 FT RESTR (LACRE) AND CLRED TO DSND TO 1600 FT. APCH SAVED MY LIFE! WHILE THERE WERE DISTRS, THEY WERE MINOR. I JUST NEED TO BE MORE DILIGENT IN CHKING THE NAVAID SETUP. I NEED TO DOUBLECHK IT JUST LIKE I CHK TWICE FOR 3 GREEN. I WAS ALSO DIVIDING MY ATTN BTWN INSTS AND CHKING THE BUILDUP UNDER THE HEATED LEADING EDGE OF THE WING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.