Date: 201403
Local time of day: 0601-1200
Location: BZN.Airport, MT
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Final Approach
Airspace: Class E BZN
ATC / advisory: Tower BZN
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: ATC Issue All Types
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Procedure; Human Factors; ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: While on ATC assigned heading of 300; for an ILS 12 approach in visual conditions at night; Approach Control [communication] became very weak and was lost. We decided to turn toward final approach course to stay away from terrain; and contacted the Tower; and told them what happened. Tower told us to go to 118.97 and advise Approach Control what we were doing and that we could no longer hear them on the previously assigned frequency of 119.55. Approach Controller told us to be sure to file a report on this because of the hazard of lost comm in high terrain; and said he would be filing a report as well. [I] used Captain's emergency authority to maintain a safe condition of flight. [We] returned to Tower and were cleared to land.


Second narrative: [We] lost radio contact with Approach Control at BZN while being vectored for ILS 12. [We] turned base at our discretion two miles outside FAF to avoid high terrain before regaining radio contact with Tower and Approach. Weather was VMC. No other traffic in pattern.Approaching BZN from the southeast we were given step down altitudes and headings to fly across the mountains by Big Sky Approach Control on frequency 119.55. Communication was good until we descended in low approaching the airport where the last transmissions were a bit weak and scratchy. Abeam the airport; the final transmission heard assigned a northwesterly vector and a descent to 8;000 FT which placed us on a perfect right downwind for the ILS 12. It was quiet on the radio and apparent from our position that no other instructions would be needed until the base turn. When no assignment was heard as expected; we called Approach. Nobody answered. The Captain initiated a base turn to avoid high terrain to the northwest and I made contact with the Tower to advise. Tower directed us back to a different Big Sky Approach frequency of 118.97. This frequency worked well. They assigned a turn to final and cleared us the approach. Before sending us back to Tower frequency the Approach Controller asked us to write a report on the loss of radio contact. He said he would do the same. It was apparent from my short discussion with him that this was a trend and not an isolated event.


Synopsis: Air Carrier flight crew experiences lost communications while on vectors to the ILS 12 at BZN in night VMC. The Captain elects to turn inbound to the airport to avoid terrain and contact Tower. Tower advises to try a different frequency which is successful.