Air-ground communications show that the depressurisation which affected an Alaska Airlines Boeing 737 Max 9 occurred just after it was transferred to Seattle en route air traffic centre while climbing out of Portland.

The aircraft had been climbing to 15,000ft when it contacted Seattle controllers for clearance to a higher altitude.

But just 2min later one of the pilots transmitted an urgent request, stating: “We’d like to go down.”

The pilot followed up by declaring an emergency and saying that the aircraft was descending to 10,000ft – a standard procedure during cabin depressurisation.

According to communications archived by LiveATC, the pilot explained the nature of the emergency, mentioning pressurisation and requesting a return to the airport.

“[We] need a turn back to Portland if we can get one,” the pilot said, repeating the declaration of an emergency and adding that the aircraft had 177 passengers and was carrying 18,900lb (8,570kg) of fuel.

Alaska 737 Max 9-c-Alaska Airlines

Source: Alaska Airlines

Preliminary indications point to loss of an inactive exit between the wing and main aft door

About 1min later the crew was transferred back to Portland approach control, informing the controller that there was an emergency and requesting a descent.

“We’d like to get lower if possible,” the pilot stated, after which the aircraft was cleared to 7,000ft. “We are [in an] emergency. We are depressurised.”

The controller asked whether the crew needed time to burn off fuel before landing, to which the pilot responded that this was unnecessary but the crew would take a few minutes to prepare for the approach.

“Can you get down from there?” the controller asked, and the pilot replied: “We can get down.”

The aircraft subsequently conducted an ILS approach to runway 28L and landed without further incident, about 20min after it had taken off.

Images circulating on social media, of the cabin interior during flight and the fuselage exterior after arrival, indicate that a de-activated left-hand mid-cabin emergency exit door separated from the jet, although the circumstances have yet to be determined.

While the Max 9 is capable of seating up to 220 passengers, Alaska Airlines configures its Max 9s with a lower-density three-class interior, comprising 178 seats – including 16 in the first-class and 24 in the premium-class cabins.

This means it is able to operate the type with a reduced number of exits. The carrier retains the main forward and aft exits, as well as the two overwing exits on the type, but not the mid-cabin exits between the wing and the rearmost doors.

Door de-activation normally involves removing the actuation and exit slide systems from the door, effectively turning into a fuselage window panel.