An Austrian Airlines Airbus A320 flying from Tel Aviv to Vienna diverted to Sofia, Bulgaria, on the evening of Sunday 18 October after a bomb threat. Austrian Airlines confirmed that "a threat" against flight OS82 made a stop in Sofia "necessary". The aircraft landed safely at 20:42 Central European Summer Time with 51 passengers and six crew on board12. Bulgarian MiG-29 fighters escorted it to the ground, according to the Bulgarian Ministry of Defence3. Passengers were taken off and the aircraft was searched41.

According to the Bulgarian Ministry of Transport, the crew reported near the border with Romania that they had received a bomb threat signal, and decided to make an unscheduled landing in Sofia4. The aircraft was the A320 registered OE-LZE51. No injuries have been reported.

What the flight data shows

Open ADS-B data from adsb.lol shows OE-LZE taking off from Ben Gurion Airport at 16:04 UTC (19:04 Israel time)5. It turned north over the sea, climbed to 38,000 ft and flew across Cyprus and Turkey towards Bulgaria51. It was still at that level, heading for Vienna, as it crossed northern Bulgaria towards the Danube.

At around 18:14 UTC the aircraft turned west near the Danube, and at 18:15 UTC (21:15 Bulgarian time) the transponder showed 7700, the general emergency code5. At that point it was just north of the river, which forms the border between Bulgaria and Romania there, and had just started to descend from cruise. Just over two minutes later it was heading southwest, towards Sofia5. The Bulgarian Ministry of Defence gives 21:17 local time as the moment the aircraft signalled a bomb threat on board, while in Bulgarian airspace3. AFP reports that the aircraft had entered Romanian airspace shortly before6.

What followed was a steady descent averaging about 1,800 ft per minute. Northeast of Sofia, at around 12,000 ft, the aircraft flew an orbit to lose height before approaching the airport from the east5. It was on the ground at 18:42 UTC (21:42 in Sofia), just over 26 minutes after the first 7700 and two hours and 38 minutes after leaving Tel Aviv51. According to AFP, the airline said the aircraft landed safely about half an hour after the threat6. The transponder stayed on 7700 until the aircraft was on the ground5.

At the moment of the squawk, Sofia was about 94 NM away by our calculation from the ADS-B position. Vienna was still more than 430 NM ahead and Tel Aviv more than 850 NM behind5. Bucharest was closer, at about 68 NM. Neither Austrian Airlines nor the Bulgarian authorities have said why the crew chose Sofia.

Who reported the threat

Accounts of where the threat came from differ. The Bulgarian Ministry of Transport speaks of a signal received by the crew4. The Israeli newspaper Haaretz reports that Austrian Airlines was alerted by an Austrian national who was not on board7. Israel's public broadcaster Kan, as reported by The Jerusalem Post, says Austrian authorities received a phone call from Vienna about a suspicious object on the aircraft8. Passengers told Ynet they were informed on board that "an anonymous report of a bomb on the plane" had been received2.

Ynet reports that initial indications point to a false alarm2. That has not been officially confirmed: neither Austrian Airlines nor the Bulgarian authorities have announced the outcome of the search. The Bulgarian Ministry of Transport says a crisis team has been set up and the responsible institutions have opened an inquiry4.

The MiG-29s and NATO

According to the Ministry of Defence, the Bulgarian Air Force "immediately" launched its duty MiG-29s. This was done under NATO's Air Policing mission, which guards allied airspace, and in coordination with NATO's Combined Air Operations Centre at Torrejón in Spain3. The fighters escorted the A320 until it landed43.

For the passengers it was a long evening. Austrian Airlines said they would be rebooked on another flight as quickly as possible once the search was complete. "The safety of our passengers and crews always has the highest priority," the airline said19. According to Ynet, a delegation of young people due to speak at the United Nations was on board210. When the passengers reached Vienna was not known at the time of publication.

The Israeli context

The Israeli Air Force said, according to Kan, that the incident did not take place in or near Israeli airspace and that it was therefore not involved8. Even so, the threat comes at a tense time for flights to and from Israel. Last week, according to Israeli media, two anonymous bomb threats were made against Delta Air Lines flights from New York to Israel. Both proved false8. At the end of September, the captain of flydubai flight FZ1073 to Tel Aviv was attacked in the cockpit, a case that is still under investigation.

A diversion to Sofia on this kind of route is not new. In 2014 an El Al flight from Tel Aviv to Munich landed in Sofia after a passenger started shouting about bombs. An Israeli air marshal restrained him and the flight then continued to Munich11.

What a crew does in this situation

A threat received in flight leaves a crew little time. Whether it is real cannot be established in the air, so the question becomes how quickly the aircraft can be safely on the ground. Squawking 7700 shows air traffic control at once that there is an emergency. The FAA's pilot manual sets out the radio side: a distress call starts with MAYDAY, an urgent but not immediately life-threatening situation with PAN-PAN, and a crew without contact selects 770012. An emergency gets priority over all other traffic; controllers can stop departures and hold or divert other arrivals for it13. Whether the OS82 crew made a mayday or pan-pan call is not known. We have not found a public recording of the radio traffic.

Choosing a diversion airport is about more than distance: the runway, the weather, the emergency services and the ability to isolate an aircraft safely all count. Many airports have a designated part of the apron for aircraft suspected of being subject to unlawful interference, well away from terminals, other aircraft and critical facilities. It is used, among other things, for a (suspected) bomb threat14. Communicating false information that endangers an aircraft is itself an act of unlawful interference14.

Landing quickly can mean landing above maximum landing weight. SKYbrary lists three options: burn fuel in a hold, jettison fuel or land overweight. An overweight landing is permitted in the interest of safety, for example when landing quickly reduces exposure to a hazard, but it calls for a special inspection afterwards15. Whether OS82 was above its maximum landing weight has not been disclosed. At the time of the diversion the aircraft had already been airborne for more than two hours.

An aircraft escorted or intercepted by military jets follows the international rules in ICAO Annex 2. The FAA summarises them: try to contact the interceptor or air traffic control on the emergency frequency 121.5 MHz, select 7700 unless ATC instructs otherwise, and follow the interceptor's instructions until positively released16. In the OS82 case Bulgaria describes an escort to landing3.

Last week a hijack code also brought out fighters for a Corendon flight from Corfu to Nuremberg, and in Ben Gurion a Delta aircraft from New York was evacuated after an anonymous bomb threat.