What to know about the Boeing 737 MAX 9 and the MAX series
An Alaska Airlines flight departing Portland International Airport (PDX) on Friday night experienced a sudden cabin decompression as a fitting on its fuselage shot away from the plane, leaving a gaping hole in the airplane as frightened passengers scrambled to put on emergency oxygen masks.
The flight, AS 1282, bound for Ontario, California, safely returned to Portland with 171 passengers and six crew members, the airline said. Only a few minor injuries were reported. However, the force of the decompression was strong enough to pull smartphones from passengers' hands, open the cockpit door and rip the pilots' headsets off, National Transportation Safety Board chair Jennifer Homendy, who is leading the investigation, said in a briefing on Sunday.
On Saturday, the Federal Aviation Administration ordered the grounding of all 737 MAX 9 aircraft outfitted with a door plug aft of the wing. The NTSB found that the door plug had become unsecured on the Alaska Airlines flight, causing it to shoot off the jet and fall to the sky as the cabin rapidly decompressed.
What is the door plug on the Boeing 737 MAX 9?
The 737 MAX 9 features a spot for an extra emergency exit, which is required on aircraft models with certain higher-density seating configurations. Airlines that put fewer seats on the aircraft, such as Alaska and United Airlines, can choose to install a plug in its place instead.
The plug is installed by Spirit AeroSystems, which manufactures the fuselage for Boeing's 737 MAX aircraft in Witchita, Kansas, and ships the completed bodies to Boeing's Renton, Washington, factory for final assembly. The NTSB said that the plugs are effectively secured by just four bolts, along with other hardware.
Airlines have found improperly installed door plugs on other 737 MAX 9 aircraft
United said on Monday that it had found installation defects during its inspections of the door plugs on Boeing 737 MAX 9 jets, including loose bolts.
The finding, which was first reported by industry outlet The Air Current ahead of United's confirmation, significantly raises the stakes of the nascent crisis for Boeing and its supplier that builds the aircraft fuselage and installs the plug, Spirit AeroSystems. The Air Current reported that at least five United aircraft had been found to have defects.
Alaska Airlines, the other major U.S. airline operating the aircraft subtype, said later on Monday that it had found similar loose hardware while preparing its planes for the formal inspections.
The 737 MAX 9 is the only MAX model with a door plug
The grounding applied to about 171 MAX 9 aircraft, the FAA said. There are approximately 215 of the aircraft subtype in service globally, according to aviation data firm Cirium.
That compares to the 737 MAX 8, which has been the far more widespread of the two midsize 737 MAX models. Boeing has delivered 1,039 of the smaller MAX 8 to airline customers, according to Cirium data.
Because the MAX 8 can't be configured in a way that would make it require more or fewer emergency exits, there's no need for the space to outfit either a door or a plug. The same applies to the two MAX variants awaiting certification by the FAA: the smallest, the 737 MAX 7, and the largest, the 737 MAX 10.
It's unclear when the 737 MAX 9 will return to service
The grounding is expected to continue for at least a few days, although that timeline could stretch.
"The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service," the FAA said on Tuesday.
Boeing proposed a set of instructions for airlines, telling them what needs to be done to inspect and potentially repair door plugs on affected aircraft. However, the planemaker must revise those instructions, the FAA said, "because of feedback received in response."
Once Boeing submits the revised instructions, the FAA will review them before allowing airlines to proceed. The FAA may also inspect each aircraft before it returns to service.
"Every Boeing 737-9 Max with a plug door will remain grounded until the FAA finds each can safely return to operation," the agency said Tuesday.
In the meantime, airlines with the subtype are continuing to cancel flights.
The Boeing 737-900 also has a door plug
An older version of the 737 still in service today, the 737-900, also features a door plug.
Those jets, which were manufactured up until the late 2010s, do not appear to be affected by the loose bolts.
But that generation of aircraft — the 737NG, or "New Generation" — did not have the same troubled past as the MAX aircraft. It was designed — along with its production line — when Boeing was in a very different place.
How the Boeing 737 MAX came to be
Boeing introduced its first 737, the 737-100, in 1967. That jet with a longer -200 variant represented the first generation of what would become the workhorse of airline fleets around the globe.
Since then, the 737 has undergone a few major overhauls.
The "Classic" series, which debuted in the 1980s, included the 737-300, -400 and -500. In the late 1990s and early 2000s, Boeing introduced the 737NG, which was built and delivered to customers through 2019. Many of those jets, which featured upgrades to the airframe, wings and flight deck, are still in service today. This includes the 737-600, -700, -800 and -900.
In the mid-2000s, Boeing began to discuss a successor for the NG, debating between replacing the 737 with a brand new airframe or re-engining the 737 with modern, more fuel-efficient engines.
In late 2010, rival Airbus announced plans for the A320neo, or "new engine option" — a reconfigured version of its single-aisle jet that served as the main competition for Boeing and the 737.
Airbus found its new design met with steep demand, and it took orders for 667 neos at the 2011 Paris Air Show.
Then, in July 2011, American Airlines — which had exclusively purchased from Boeing through the 2000s and had reportedly grown frustrated waiting for Boeing to announce the NG's successor — announced an order for 260 Airbus jets, including 130 neos, along with 100 Boeing 737NGs and 100 of a re-engined 737, if Boeing chose to make one.
A month later, Boeing announced the 737 MAX, which would retain most of the NG's design but feature brand-new engines.
The 737 MAX debuts and crashes
The 737 MAX was certified by the FAA in March 2017, and the first jet was delivered to the launch customer, Indonesia-based Malindo Air, in May. Deliveries quickly ramped up, with carriers worldwide — including Southwest Airlines, United and American — clamoring for the jet in the U.S.
A big part of its appeal was that it shared commonality with the 737NG. That meant pilots could transition fairly easily between the NG and the MAX with just some simple training rather than having to become licensed to fly a new type. That kept costs lower for airlines with the 737 and gave them significantly greater flexibility.
About 230 MAX jets were in service worldwide on Oct. 27, 2018, when Lion Air Flight 610 took off from Jakarta, Indonesia. Pilots quickly began experiencing flight control problems, with confusing warning lights and indicators lighting up the cockpit as pilots struggled to keep the aircraft's nose from pitching downward. Twelve minutes later, the flight crashed into the Java Sea off the north coast of Jakarta, killing all 181 passengers and eight crew members aboard the flight.
Investigators quickly began to focus on a flight control system that Boeing had developed for the 737 MAX called the Maneuvering Characteristics Augmentation System, or MCAS.
Boeing and the FAA issued an Airworthiness Directive several days after the crash. They told airlines and pilots that trim stabilizer control issues were possible on the new jet if certain angle-of-attack sensors failed, and they pointed to the pilot's manual, which had the procedure to override the flight control system.
Deliveries of the jet continued, and by March 10, 2019, there were 386 MAX jets in service globally.
That morning, Ethiopian Airlines Flight 302 departed from Addis Ababa Bole International Airport (ADD) in Ethiopia, bound for Nairobi, Kenya. Pilots reported flight control problems within moments as the flight control system fought against their input and tried to point the aircraft toward the ground. Pilots followed the procedure to disable the system and reset the aircraft trim by hand but, crucially, left the engines on full thrust. Aerodynamic forces on the horizontal stabilizer made it impossible to adjust the trim manually. Within six minutes, the flight slammed into the ground, killing all 157 passengers and crew.
Within three days, regulators worldwide ordered the Boeing 737 MAX grounded, pending an investigation into what had happened on these two flights.
What did MCAS have to do with the 737 MAX crashes?
MCAS was designed to compensate for the fact that the engines on the 737 MAX were larger than those on the 737 NG; consequently, it had to be positioned farther forward and higher up on the aircraft wing in order to clear the ground during taxi, takeoff and landing. However, moving the engine caused the aircraft to have different aerodynamics and to behave differently. For instance, the nose could pitch upward in some situations.
Boeing designed the MCAS system to automatically nudge the MAX's nose downward as needed, making it handle just like the older jets. It did this to keep the plane behaving similarly to the NG and make it so that the FAA and airlines would approve a simpler transition for pilots from the NG to the MAX, with both planes sharing a type rating.
Boeing did not include information about MCAS in the pilots' manual, intending the system to work in the background — and only in situations where the plane could be at risk of stalling — and never even be noticed by the pilots.
But there was a flaw. The system was designed to be activated based on input from just one angle-of-attack sensor. If that sensor, which sits outside the aircraft, became damaged or faulty, it meant that MCAS could activate inappropriately.
Investigators found this to have been the case on both crashed jets. The plane behaved counterintuitively, and the sensors gave the pilots bad information. So, the crew lost control of the jets.
What was the 737 MAX grounding?
The aircraft was grounded for 20 months, with the FAA clearing the plane to fly again on Nov. 18, 2020.
Throughout the grounding and various investigations, numerous concerning aspects of the aircraft and its certification were found and addressed.
As scrutiny remained on Boeing, other issues suggested a quality control problem. Foreign object debris was found in stored jets, and internal emails between Boeing employees emerged in which the workers suggested the aircraft's design and production had been subpar.
By December 2020, however, regulators and airlines felt that the various issues had been investigated and addressed to ensure the plane was safe again. As the COVID-19 vaccination began to be introduced in early 2021, leading travel demand to begin its climb from pandemic-era lows, the 737 MAX became a crucial part of airlines' fleets. It operated without any issues until the Alaska Airlines accident this month.
Where do things stand with the Boeing 737 MAX now?
Until this incident, the 737 MAX had proven to be a reliable and nimble workhorse for airlines around the world since the grounding was lifted. The aircraft have flown more than 2.6 million commercial flights with nearly half a billion passenger seats, according to Cirium.
Boeing, however, has suffered reputationally resulting from the MAX crisis. The company has been accused of prioritizing profits over engineering, which critics say has led to sloppy quality control and rushed designs.
Spirit AeroSystems, which builds the fuselage and installs the door plug, has similarly been accused of failing to manage its quality.
Meanwhile, numerous other potential issues have been found on the MAX, such as bolts on the aircraft's horizontal stabilizer needing to be inspected.
To a degree, even the most routine incidents involving a MAX aircraft have tended to draw disproportionate attention from the public when equivalent maintenance issues are routinely flagged for all commercial aircraft.
Still, Boeing has found itself under a spotlight for its safety practices and records since the crashes, and it seems that the current episode will only fuel that scrutiny.
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