Narrative:

Crew deviated from ATC instructions in interest of safety of flight. Weather was VMC: hazy twilight with some virga above 4000 ft. Afl (above field elevation) in the vicinity but not in the approach sector. Aircraft was on a dog leg to final when switched to tower for visual approach to cos runway 35R. Tower immediately advised of a microburst alert. Crew requested a turn to the right; eastward; away from terrain; to break off the approach and avoid the microburst hazard. Tower instructed crew to maintain present heading; which was a northwest heading toward rapidly rising terrain. Crew replied 'unable.' tower assigned a heading of 350; approximately runway heading and toward the microburst area. Crew responded that it was not going to fly into the microburst. Tower advised of traffic to the northeast at 9000 ft. Crew roger'd the traffic and swept TCAS for targets. With no TCAS targets in the immediate vicinity; PIC exercised safety of flight authority and executed a turn toward the east; open terrain away from the microburst area. Tower issued a climb to 8000 ft. To which the crew complied with.ATC; tower; prioritized its own concerns; traffic separation; above the physical requirements of continued flight: microburst and terrain avoidance.controllers must have sufficient aviation understanding to prioritize threats and the latitude to compromise procedures in the obvious conflict of priorities. For example; if the choice is between losing separation between two aircraft or losing separation between an aircraft and a mountain; sacrifice separation between aircraft. Similarly; if the choice is between an aircraft flying too close to another and that aircraft not flying at all because of a deadly meteorological hazard; sacrifice IFR separation between aircraft.

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Original NASA ASRS Text

Title: Air Carrier flight crew reported deviating from ATC instructions due to safety of flight.

Narrative: Crew deviated from ATC instructions in interest of safety of flight. Weather was VMC: hazy twilight with some virga above 4000 ft. AFL (Above Field Elevation) in the vicinity but not in the approach sector. Aircraft was on a dog leg to final when switched to Tower for visual approach to COS RWY 35R. Tower immediately advised of a microburst alert. Crew requested a turn to the right; eastward; away from terrain; to break off the approach and avoid the microburst hazard. Tower instructed crew to maintain present heading; which was a northwest heading toward rapidly rising terrain. Crew replied 'unable.' Tower assigned a heading of 350; approximately runway heading and toward the microburst area. Crew responded that it was not going to fly into the microburst. Tower advised of traffic to the northeast at 9000 ft. Crew roger'd the traffic and swept TCAS for targets. With no TCAS targets in the immediate vicinity; PIC exercised safety of flight authority and executed a turn toward the east; open terrain away from the microburst area. Tower issued a climb to 8000 ft. to which the crew complied with.ATC; Tower; prioritized its own concerns; traffic separation; above the physical requirements of continued flight: microburst and terrain avoidance.Controllers must have sufficient aviation understanding to prioritize threats and the latitude to compromise procedures in the obvious conflict of priorities. For example; if the choice is between losing separation between two aircraft or losing separation between an aircraft and a mountain; sacrifice separation between aircraft. Similarly; if the choice is between an aircraft flying too close to another and that aircraft not flying at all because of a deadly meteorological hazard; sacrifice IFR separation between aircraft.

Data retrieved from NASA's ASRS site 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.