WASHINGTON – The fuselage section that ripped away from a Boeing 737 Max jet mid-flight on Jan 5 reflects a design feature in use for many years, suggesting investigators are likely to zero in on issues in the manufacturing process rather than a design flaw.

The plane was built with modular cutouts in the frame that can house additional emergency exits for high-density configurations. Some airlines order planes with the doors installed to maximise the number of seats.

Others, like Flight 1282 operator Alaska Airlines, do not require the extra exits and have the holes permanently plugged up. 

From the inside, a plug is indistinguishable from the sidewall on the aircraft, while on the outside, an outline of the opening can be seen. The Boeing 737 cutouts date back to the mid-2000s, and hundreds have been installed.

“This has all the earmarks of a manufacturing deficiency, a quality escape from Boeing,” said aviation safety expert Jeff Guzzetti, the Federal Aviation Administration’s (FAA) former accident investigation chief.

“We can’t help to not look at this recent event in the context of all the problems that Boeing has had with manufacturing quality deficiencies.” 

The FAA on Jan 6 ordered certain Max 9 aircraft grounded temporarily for inspections before returning to flight. The move, affecting 171 planes worldwide, followed decisions by Alaska Air and United Airlines Holdings, the two biggest Max 9 operators, to ground dozens of jets in their fleets.  

Cutouts like the one in the Alaska Air incident are aimed at increasing production efficiency and making seating arrangements more flexible. 

They allow manufacturers to make one standard fuselage section, instead of tailor-making different designs for various airlines. This reduces complexity and cost, and facilitates changes in future. For example, a low-cost carrier purchasing the plane second-hand would be able to restore the exit and add seats. 

The Alaska Air Max 9 used on flight 1282 was delivered in late October.

The blowout occurred shortly after take-off from Portland, Oregon for a flight to Ontario, California. It reached an altitude of about 4,876m before losing pressure. It turned around and landed safely after being in the air for about 20 minutes.

Fuselages for the model are made by Boeing’s biggest supplier, Spirit AeroSystems Holdings. 

Investigators will have to look into how the doors are plugged and why they exist if they can come open, said Mr Richard Healing, a former member of the National Transportation Safety Board (NTSB). It is investigating the Flight 1282 incident. 

“It’s going to be serious engineering work and maintenance work, and certainly an opportunity for NTSB to dig in on this, and the FAA as well,” said Mr Healing, who is chief executive of consultancy Air Safety Engineering. “My sense is that this is just the beginning of something.”

The grounding marks a major setback for Boeing, which has worked to improve manufacturing quality after defects and costly repairs hobbled operations in recent years. Many of the most recent issues have been supplier-driven.

In a statement, the US plane maker said it agrees with the FAA action and that it is in close touch with the regulator and customers.

In 2023, Boeing slowed deliveries to fix misaligned drilling holes in the rear section of the 737 supplied by Spirit. Most recently, the FAA said it is monitoring targeted inspections of the 737 Max to look for a possible loose bolt in the rudder control system.

Boeing’s Max 9 and the coming Max 10 are manufactured with emergency door cutouts. The Max 9 is certified to seat up to 220 passengers, which would require 10 emergency exits – five on each side. However, most airlines want cabins that are less dense to accommodate higher-paying passengers. 

Alaska Air seats 178 people in its Max 9 jets, including 24 in its premium class with extra legroom, and 16 seats in first class. United offers 179 seats across three classes and also plugs the extra emergency door spaces.

Boeing began installing the cutouts in 2006 with the previous-generation 737-900ER. European rival Airbus uses flexible door designs in some of its A321 narrow-body aircraft. BLOOMBERG