Boeing 737: Everything you need to know about the aircraft after terrifying mid-flight blowout

The Man Who Pays His Way: Portland depressurisation was unrelated to two fatal Boeing 737 Max crashes

The entire fleet of Boeing 737 Max 9 aircraft flying in the US has been grounded after a panel blew out from an Alaska Airlines aircraft as it climbed from Portland, Oregon.

The plane, on a routine flight to Ontario near Los Angeles, suffered immediate depressurisation and declared an emergency. All 177 passengers and crew aboard flight AS1282 were safe when the aircraft landed back at Portland.

But problems involving the fuselage panel have been discovered by both of the main airlines flying the jet – Alaska Airlines and United, which is the biggest operator.

The incident has once again raised questions about an aircraft type that was involved in two fatal crashes.

The Boeing 737, first launched in 1967, is the world’s most successful aircraft – with around 10,000 delivered. But the latest version, the Max, was involved in two terrible tragedies.

On 29 October 2018, a faulty sensor triggered an anti-stall system that caused Lion Air flight 610 to crash shortly after take-off from Jakarta. All 189 passengers and crew died.

Less than six months later, Ethiopian Airlines flight 302 from Addis Ababa to Nairobi was lost, along with 157 lives, in similar circumstances.

The plane was grounded worldwide shortly afterwards. After a rigorous redesign and certification, the aircraft is flying again – including for Europe’s biggest budget airline, Ryanair.

What does the latest event mean for passengers? These are the key questions and answers.

High flyer: Boeing 737 Max 9. The optional exit can be seen between the wing and the tail – a window slightly separated from those left and right (Boeing)

What caused the two fatal crashes?

Both tragedies involved Max 8 aircraft. They were caused by software known as the maneuvering characteristics augmentation system (MCAS). The software was installed as a precaution due to the unusual structure of the latest variant of the 737: it has large engines mounted further forward than is usual.

MCAS was intended “to provide consistent handling qualities”. But neither the main safety regulator, the Federal Aviation Administration (FAA), nor the airlines that bought the plane, were fully informed of the new system and its power to defy pilots.

In both crashes, incorrect data from a faulty sensor caused the software to push the nose of the aircraft down repeatedly while the pilots struggled for control.

Following the Ethiopian Airlines tragedy, the Max was grounded for 20 months while safety enhancements were made. The plane re-entered service in December 2020 and has been flying routinely since then.

What appears to have happened to Alaska Airlines jet?

An “exit plug” – also called a door in some media – containing a window blew out of the port side of the fuselage of the Boeing 737 Max 9.

The FAA describe it as an “in-flight departure of a mid cabin door plug”.

The plug was located between the overwing and aft emergency exits. On Max 9 aircraft it fills the space for what is termed a “deactivated mid-cabin exit”.

The plug fell 16,000 feet into the back yard of a Portland-area schoolteacher, Bob Sauer.

Why would any aircraft have a ‘deactivated exit’?

Because the law does not require one for the maximum number of passengers aboard the aircraft as operated by the airlines.

The Boeing 737 Max 9 can hold up to 220 seats, but Alaska Airlines’ Max 9 jets are fitted with just 178 – with first class and a premium economy product with extra legroom. United Airlines has fitted 179 seats.

In these configurations, the forward, overwing and aft exits are sufficient to allow an emergency evacuation that meets the 90-second requirement for everyone leaving the aircraft.

Were a high-density configuration to be fitted, either by the original buyer or a later owner, then an extra exit would be needed. Rather than invasive engineering to allow this, the panel/plug/door is a design feature to make it ease to retrofit an additional emergency exit.

Why were all Max 9 planes grounded?

The FAA says: “Boeing 737-9 aircraft will remain grounded until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners.

“Operators must also complete corrective action requirements based on findings from the inspections prior to bringing any aircraft back into service.”

What have inspections found?

Alaska Airlines says it is found “loose hardware” in the affected area. The statement read: “As our maintenance technicians began preparing our 737-9 Max fleet for inspections, they accessed the area in question. Initial reports from our technicians indicate some loose hardware was visible on some aircraft.

“The safety of these aircraft is our priority and we will take the time and steps necessary to ensure their airworthiness, in close partnership with the FAA.”

United Airlines said in a statement: “Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug.

“For example, bolts that needed additional tightening.”

What does Boeing have to say?

“As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings. We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards.

“We regret the impact this has had on our customers and their passengers.”

Is there any connection between this latest event and the two tragedies?

The fatal accidents were due to catastrophically designed software on Max 8 aircraft. This latest event is a structural issue on a different variant, the Max 9. The connection is that they involve the latest version of the Boeing 737.

Could the same thing happen to a jet I am travelling on?

Most unlikely. By far the largest operator of Boeing 737 Max aircraft in the UK and the rest of Europe is Ryanair. The airline has more than 100 Max 8 aircraft – in a special edition known as a 737 Max 200 or the 737-8200.

This type does not have the “deactivated exit” as featured on the Max 9 planes. In fact, Ryanair required Boeing to fit an extra, real emergency exit for its Max aircraft.

This is an extra door between wing and aft, allowing Ryanair to boost maximum capacity from 189 to 197 passengers (and to be able to sell some more seats with extra legroom). So it is a real emergency exit, rather than one an airline might press into service later.

Would you step aboard a Boeing 737 Max?

Yes, I have found the aircraft excellent to fly on, and I trust the judgment of the captain and his or her airline.

Simon Calder, also known as The Man Who Pays His Way, has been writing about travel for The Independent since 1994. In his weekly opinion column, he explores a key travel issue – and what it means for you.