What happened
On June 18, 2004, an instructional flight involving a Piper PA-28A-161, registration OK-AKA, was operating from Karlovy Vary (LKKV) to Prague (LKPR) at flight level 5000. During the flight, the crew experienced a total loss of two-way radio communication with Prague Control (ACC LKPR).
After approximately 10 minutes of flying near the RAK VOR, the pilot failed to respond to repeated calls on the primary frequency. Attempts to establish contact on the emergency frequency (121.5 MHz) were also unsuccessful. Furthermore, the aircraft's SSR transponder signal disappeared after passing the VOR. The instructor-pilot discovered that the aircraft's radio transmitter was spontaneously and intermittently activating, which effectively blocked the frequency and prevented other aircraft from communicating.
In an attempt to manage the situation, the instructor tried to control the radio from the student's left-hand controls, but the signal remained weak and unintelligible. After switching from COM2 to COM1 and attempting to change frequencies without success, the pilot set the transponder to code 7600 to signal the radio failure. The crew eventually established contact via an alternative frequency (119.00 MHz). Following radar vectors, the aircraft landed safely at Prague (LKPR). The incident resulted in the suspension of all departures from Prague, with an average delay of 20 minutes for other traffic.
The investigation
The ÚZPLN investigation examined flight plans, air traffic control audio recordings, the pilot's statements, and the aircraft's maintenance documentation. The investigation confirmed that both the instructor and the student pilot held valid qualifications and medical certificates, and that the aircraft was fully airworthy at the time of the flight. The investigation also reviewed the radio equipment's maintenance history, noting that the last radio navigation equipment check had been performed in January 2004.
Findings
- The loss of two-way communication and the interference with other users of the frequency were caused by a technical failure of the Push-to-Talk (PTT) button.
- The button on the right-hand control was stuck in the active position, causing the transmitter to cycle on and off automatically.
- The malfunction caused the aircraft to block radio correspondence for other participants in the airspace.
- The incident had no impact on the aircraft's structural integrity, and there were no injuries to the crew.