Background: Boise Air Terminal/Gowen Field (KBOI), Nampa Municipal Airport (KMAN) and Caldwell Industrial Airport (KEUL) have experienced tremendous growth in air traffic over the past 4 years. Because of this increase in traffic and required separation, KBOI final approach course for RWY 10 has extended farther west, beyond what has been normal for many years. Procedures to operate on the final approach course for KBOI RWY 10 require IFR aircraft to be at or descending to 6,000' MSL up to 18 NM west of KBOI. KMAN/KEUL traffic often operate just outside of Class C airspace with no restrictions.

Result: There has been an increase in traffic alerts and TCAS resolution advisories in the area west of KBOI. IFR aircraft landing KBOI, who are in communication with BOI ATCT/TRACON, operating outside of Class C airspace are conflicting with VFR pilots operating in and out of KEUL/KMAN who are not required to be in communication with ATC. These occurrences are continuing to increase.

Advisory: VFR traffic operating in and out of KEUL/KMAN are advised to use extreme caution when operating above 4,500' MSL in the area west of KBOI (see depiction on VFR sectional). Avoiding this area when operating above 4,500' MSL will greatly reduce the likelihood of conflicts with traffic landing KBOI. This advisory applies at all times, but will have the most impact when KBOI is on an east flow (RWY 10).