Incident Overview
On June 4, 2019, between 18:14 and 21:00 UTC, air traffic controllers in the Murun sector of Mongolia's area control center reported interference on the 128.0 MHz frequency, affecting communications with aircraft transiting the sector. The interference caused aircraft transmissions to be unreadable or severely degraded, particularly in the vicinity of the EPKUT waypoint on airway B208, as well as near the SERAT and LAGON waypoints on airway Y746.
Chronology of Events
According to the investigation, the first signs of interference on 128.0 MHz were noted on May 24, 2019, when occasional external noise began disrupting communications. On June 4, the situation worsened. At 18:14 UTC, the sector's radar controller attempted to contact aircraft GEC8400 at FL10700 near EPKUT to transfer control to the Gobi sector. Despite five calls on the Hengertei and Murun RCAG 128.0 MHz stations, no response was received. The controller then switched to the Bulgan RCAG 128.0 MHz station at 18:15 UTC and made six to seven additional calls, but could not establish communication.
Between 18:32 and 18:37 UTC, the controller attempted to transfer other aircraft (CCA968, CCA840, CSN348) near EPKUT, but their readbacks were heavily distorted, intermittent, or inaudible. The controller notified the shift supervisor at 18:38 UTC, who then contacted the Communications, Navigation, and Surveillance (CNS) duty radio engineer. The engineer checked the equipment on the Hengertei, Bulgan, and Murun RCAG systems and found no faults. He switched from VSAT to optical lines and transferred receivers and transmitters from primary to standby, but at 18:52 UTC, a test call to aircraft CPA260 near EPKUT still resulted in an unreadable transmission.
A joint team from the Communications Regulatory Commission (CRC) and CNS engineers had been deployed from May 31 to June 6 to search for the interference source in Orkhon, Bulgan, and Khuvsgul provinces. At 19:04 UTC, the sector controller tried again with aircraft CXA826 at FL11900 using Bulgan RCAG 128.0 MHz, but the aircraft's transmission was absent and noisy. At 19:06 UTC, the shift supervisor (Duty Chief of ATC) turned off the Hengertei and Murun RCAG transmitters, leaving only Bulgan operational, but still could not hear the aircraft. He reported the issue again to the CNS engineer, who checked the equipment yet again and found it normal.
At 20:14 UTC, the Duty Chief informed the CNS shift coordinator. Further equipment checks and BIT tests were performed, all showing normal operation. The controller was advised to continue using Bulgan RCAG. At 20:21 UTC, the Duty Chief reported that aircraft could be heard 7-8 minutes after passing EPKUT. The interference gradually diminished after 21:00 UTC, and normal communications resumed.
The interference on 128.0 MHz persisted from May 24 to July 1, 2019, but only on June 4 did it reach a level causing loss of communication with aircraft. A fault log (C-PAE-114) was opened on May 24 at 11:40 UTC and closed on July 1 at 12:30 UTC after the interference subsided.
Radio Communications Equipment
The Murun sector used three RCAG stations: Hengertei, Bulgan, and Murun, all operating on 128.0 MHz. The ground equipment included PAE T6/T6 Mk6 radio stations and GAREX 220 voice communication systems. During the event, all ground equipment passed self-tests and showed no anomalies. Radio checks with aircraft outside the affected area confirmed that the ground transmissions were normal, indicating that the aircraft communication equipment was functioning correctly.
Air Traffic Control Structure
The ATC service provider, the Air Navigation Service Organization (ANSO) under the Civil Aviation Authority of Mongolia, operates 24/7. The Area Control Center (ACC) comprises four radar control sectors (Center, Murun, Gobi, and Eastern) and two procedural sectors (Altai and Bayan). The Murun sector is responsible for upper airspace above 6150 meters over western Mongolia.