A Dassault Falcon 900 operated by Volkswagen and a LOT Embraer 170 flying from Warsaw to Istanbul narrowly averted a midair in Bulgarian airspace on June 30, 2015, passing within 0.9 nm while both were flying at FL370. According to a recently issued final report from Bulgaria’s Air Accident Investigation Unit (AAIU), the near-collision was caused by the loss of the transponder signal from LOT Flight 7293 due to the transponder being in “standby mode” and subsequent “unintentional interruption of air traffic service.”
At 11:56:19 UTC, 10 minutes after entering Romanian airspace at FL370, the LOT Warsaw-to-Istanbul flight experienced a failure and reset of its Honeywell Primus avionics, which put the transponder in “standby mode.” The LOT pilots did not notice the discrete “TCAS OFF” message, nor did their Embraer 170 have updated Honeywell Primus load software that displays a more prominent warning of this condition.
This was exacerbated by Romanian ATC staff, who mistakenly believed that LOT 7293 “overflew the sector an hour before” and at 12:01 UTC “deleted LOT 7293…without a reason, and…did not try to establish radio communication.” The Romanian Air Force queried ATC about an aircraft without a transponder at 12:10 UTC, and a NATO AWACS plane spotted it at 12:18, but ACC Bucharest “did not discover the connection between the unknown aircraft and LOT 7293.”
Radio contact was re-established with LOT 7293 at 12:24, when the pilots informed Romania that they had passed the DINRO waypoint, about 30 nm from Bulgarian airspace. ATC then instructed the crew to contact Bulgaria’s Varna East sector, but incorrectly provided the frequency for Varna West.
According to the report, Bulgarian ATC tried to identify the aircraft without the transponder, including warning the Falcon 900 to look out for traffic, which it saw but incorrectly estimated to be at FL400. LOT 7293 called Varna West at 12:25:29, reporting its altitude and destination, and ATC informed the LOT crew of the transponder problem. Flight 7293 then reappeared on the radar screens at 12:27:58—a minute after passing the Falcon at 12:26:50.