What happened
On September 20, 2014, a Cessna C525 was conducting a flight at FL 400. At approximately 10:56, the crew established contact with the Prague Radar air traffic control center on frequency 132.805 MHz. During this phase of flight, the crew reduced the volume of the second onboard radio to a silent level due to heavy interference; this specific radio was intended for monitoring the emergency frequency 121.5 MHz.
After entering the Austrian Flight Information Region (FIR) at approximately 11:12, the crew did not receive instructions to change communication frequencies. Between 11:10 and 11:10, the crew performed a "radio check" and, upon receiving confirmation, believed they were still communicating with the correct air traffic control sector. At 11:30, the crew observed two military aircraft in close proximity, though no visual interception signals were observed. Because the emergency frequency radio had been muted, any attempts by the military aircraft to establish contact on 121.5 MHz likely went unheard by the crew.
The crew eventually contacted Prague Radar on their current frequency and was instructed to switch to 126.280 MHz, where they received clearance to continue their flight plan. The military aircraft were observed by the crew until approximately 11:38. The aircraft remained without active communication with the relevant controller for approximately 25 minutes before exiting Austrian airspace at 11:50. During this period, no attempts were made by Prague Radar to contact the aircraft on the existing frequency.
The investigation
The PKBWL investigation focused on the breakdown in communication between the aircraft and air traffic control, as well as the crew's monitoring procedures during the transition between FIR boundaries.
Findings
- The investigation identified the lack of a frequency change instruction from Prague Radar when the aircraft crossed the FIR boundary, alongside a lack of appropriate response to the crew's "radio check" once the aircraft was outside the controller's area of responsibility.
- The crew had completely muted the radio station responsible for monitoring the 121.5 MHz emergency frequency.
Safety action
Following the investigation, pilots were briefed on the incident, with specific emphasis placed on maintaining situational awareness regarding airspace boundaries and frequency management, as well as the dangers of muting radios intended for emergency monitoring.