Narrative:

The B737 was in an 8000 ft MVA. I cleared it to maintain 8000 ft until established on the localizer; then cleared for the back course runway 28L approach. I was momentarily distraction and when I next observed the aircraft; it was at 6900 ft at the boundary of an 8000/6500 ft MVA. I am not sure of when the aircraft began the descent. I am also unsure if it was legally safe for it to descend below 8000 ft after established on the localizer that far out. The MSAW did not alarm to indicate an unsafe ground proximity alert. I will review the situation and determine if an aircraft can descend below 8000 ft on the localizer between 25-20 mi.

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Original NASA ASRS Text

Title: BOI CONTROLLER DESCRIBED EVENT WHEN AIRCRAFT CLEARED FOR LOC BC 28L; INSTRUCTED TO MAINTAIN 8000 UNTIL ESTABLISHED; DESCENDED TO 6900 FT; QUESTIONING MVA VERSUS CROSSING RESTRICTIONS.

Narrative: THE B737 WAS IN AN 8000 FT MVA. I CLRED IT TO MAINTAIN 8000 FT UNTIL ESTABLISHED ON THE LOC; THEN CLRED FOR THE BACK COURSE RWY 28L APCH. I WAS MOMENTARILY DISTR AND WHEN I NEXT OBSERVED THE ACFT; IT WAS AT 6900 FT AT THE BOUNDARY OF AN 8000/6500 FT MVA. I AM NOT SURE OF WHEN THE ACFT BEGAN THE DSCNT. I AM ALSO UNSURE IF IT WAS LEGALLY SAFE FOR IT TO DSND BELOW 8000 FT AFTER ESTABLISHED ON THE LOC THAT FAR OUT. THE MSAW DID NOT ALARM TO INDICATE AN UNSAFE GND PROX ALERT. I WILL REVIEW THE SITUATION AND DETERMINE IF AN ACFT CAN DSND BELOW 8000 FT ON THE LOC BTWN 25-20 MI.

Data retrieved from NASA's ASRS site as of May 2009 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.