Incident Overview
On 20 September 2014, a Cessna C525 (model C525) was conducting a planned flight at FL 400. At approximately 10:56, the crew was instructed to contact Praha Radar on frequency 132.805 MHz. Communication was established and the flight continued according to the flight plan.
Communication Issues
During this phase, due to strong interference, the crew reduced the volume of the second onboard radio, which was monitoring the emergency frequency 121.5 MHz. At around 11:12, the aircraft crossed the Austrian FIR boundary. The crew did not receive an instruction to change frequency from Praha Radar. Between 11:10 and 11:20, the crew requested a radio check and, upon receiving confirmation, believed they were still in contact with the appropriate air traffic control sector.
Military Aircraft Encounter
At 11:30, the crew noticed two military aircraft in close proximity, which did not transmit any visual signals suggesting an interception procedure. Any attempts to communicate on the emergency frequency (121.5 MHz) might have been missed by the crew due to the complete silencing of that radio. The crew then re-established contact on the previous frequency with the Praha Radar controller and were instructed to switch to 126.280 MHz, where they received clearance to continue according to plan. The military aircraft were observed until approximately 11:38.
At 11:50, the aircraft exited Austrian airspace, where it had been without communication for about 25 minutes. During that period, communication with Praha Radar was possible, but no attempt to call the aircraft on that frequency was recorded.
Causes
The investigation identified two causes: 1. Lack of instruction from the Praha Radar controller to change frequency when crossing the FIR boundary, and lack of proper response to the 'radio check' request when the aircraft was already outside his area of responsibility. 2. Complete silencing by the crew of the radio that should have been monitoring the emergency frequency.