For about an hour on Wednesday morning, Israel thought it might have a hijacked airliner heading its way. flydubai flight FZ1073, a Boeing 737 MAX 8 from Dubai to Tel Aviv with around 174 people on board, suddenly switched its transponder to the emergency code 7700, and then, seven minutes later, to 7500: the code a crew uses to signal unlawful interference with an aircraft. In plain language, a hijacking.

The Israeli Air Force scrambled fighter jets. Prime Minister Benjamin Netanyahu and Defence Minister Israel Katz convened an urgent security meeting. The aircraft turned away from Israel, descended and landed at Tabuk in north-western Saudi Arabia.

Then the story changed. Israeli officials said it was not a hijacking at all. According to Israeli security sources quoted by Israeli media and CNN, the alarm was set off by a violent altercation between the two pilots in the cockpit.

How the morning unfolded

The flight took off from Dubai International at 03:05 UTC, just after 7 a.m. local time, and was due at Ben Gurion around 9:30 Israel time. Flightradar24’s tracking shows the squawk changing to 7700 at 05:31 UTC, about two and a half hours into the flight, and to 7500 at 05:38, while the jet was over Jordan and approaching Israeli airspace.

The aircraft then turned back towards Saudi Arabia, left ground-based tracking coverage and descended to around 15,000 feet. Israel’s security establishment went to full alert. Reports that the pilots had stopped communicating normally with Israeli controllers only added to the tension.

The jet landed safely at Tabuk shortly before 10 a.m. Israel time. Shortly afterwards, the prime minister’s office said the incident was not believed to be a hijacking.

What 7500 means, and why Israel reacted so fast

Every airliner carries a transponder that broadcasts a four-digit code to air traffic control. Three codes are reserved for trouble: 7700 for a general emergency, 7600 for a radio failure and 7500 for unlawful interference. A crew that dials 7500 is telling the ground, silently, that someone else may be in control of the aircraft. Controllers are trained to treat it as real until proven otherwise.

For Israel, which has lived with the threat of aircraft being used as weapons for half a century, a 7500 squawk on a flight approaching Tel Aviv is about as serious as a signal gets. Scrambling fighters to intercept and identify the aircraft is the standard response, and it is exactly what happened on Wednesday.

What we still do not know

A great deal. At the time of writing, flydubai had not issued an official statement explaining what happened in the cockpit, and neither the airline nor the Saudi authorities had confirmed the reports of a fight between the pilots. It is also not yet clear who set the 7500 code, or whether it was selected deliberately.

Nor is it clear how and when the passengers, most of them Israelis, will complete their journey. Investigators will want to interview both pilots, examine the cockpit voice recorder and establish exactly why a routine flight of under three and a half hours ended with fighter jets in the air and an emergency landing in another country. We will update this story as official information emerges.

Sources: Flightradar24, Ynet, The Times of Israel, The Jerusalem Post, Reuters (via Cyprus Mail), CNN, Calcalist