NTSB Publishes Updates On Boeing 737-9

The NTSB has published updates on the Boeing 737-9 with details of how the door blew out, alongside a plan for the future.

The NTSB has held another press conference where its Chair, among other associates, highlighted new findings and developments in investigating the door plug incident occurring on a routine Alaska Airlines flight.

These developments have shed further light on what took place during that flight, alongside the features plus clarity over what’s next for the NTSB’s investigation.

Timeline Of Events

The NTSB says that the aircraft departed from Runway 28L of Portland at 1706

At 1712, cabin pressure dropped from 14.09 to 11.64 psi. Data indicates the plane was at 14,830ft.

A further dramatic pressure drop within the cabin was recorded seconds later, declining to 9.08psi. The aircraft continued to climb for the next minute before reaching a maximum altitude of 16,320 feet.

Following this, the aircraft began descending.

By 1717, the aircraft had descended below 10,000 feet.

At 1726, the plane landed back in Portland at 28L.

This is the primary timeline for what occurred per the NTSB chair; however, it excludes details of sensors going off throughout the ordeal.

Inside The Cabin

The flight attendants described the situation as stressful from when the blowout occurred to when the aircraft touched down back in Portland.

It was highlighted through interviews that there was a lack of understanding of what was happening, primarily driven by an obstructed view of the attendants. Additionally, they mentioned the difficulty in communicating throughout the incident.

Inside the cockpit, there was much of the same distress over the situation. The cockpit door is designed to open during a rapid decompression, as it did at the time of the blowout. However, it has been revealed that the pilots aboard were unaware of this. Boeing has said they’ll change the manual to reflect this.

Additionally, the pilots aboard the flight said one of their checklists flew into the cabin alongside a headset coming off. It was reported that communicating with each other and hearing instructions from air traffic controllers were difficult.

More Aircraft To Be Inspected?

A topic of conversation following the incident involving the 737-9 has been the safety of other variants in the MAX series. The NTSB said their current priority is the incident at hand.

However, they have made it very clear that they are not afraid to go broader in the future and focus on other variants included in the series, something they’ve done before. However, this would be based on recommendations and if believed to be necessary.

The NTSB noted they were aware of United Airlines and Alaska Airlines reporting loose bolts on some of their 737-9s only hours earlier. Teams are currently collecting all the relevant information from this, with follow-up inspections to take place afterwards.

While the bolts have been a focal point, especially with new findings, the NTSB must follow the evidence trail before venturing to other aircraft types.

Understanding What Happened

Following their initial investigations, the NTSB said no discrepancies were found on the installed right aft door plug today. However, they did provide further clarity on what happened.

In simple terms, the door plug moved upward, resulting in the 12-stop gaps becoming disengaged. The door blew out, resulting in the guide tracks fracturing. What’s very important to note is that recovery of the four bolts crucial to not having the door plug move upward has not been recovered.

This means that the NTSB cannot yet determine if they were installed, damaged, not fitted correctly, or many other possibilities.

The NTSB says there are hinges on the bottom and guides at the top. The hinges and guides aim to get the door onto the aircraft and allow it to move outward for maintenance.

The High Five Analogy

During the briefing, those presenting used an analogy of high-fiving to explain how the stop fittings and plug door mechanics work. Essentially, the features above hit each other to prevent the door from coming out, as there’s substantial pressure inside and outside the cabin.

The stop fittings don’t also have a locking mechanism. This is where the high five element come into the fray. They press against each other from inside outward, holding the door in place. The upward motion is needed to get the door in place on installation and to move the door outward during inspection and maintenance.

As a result, you require this movement when installing the door, among other moments. However, when you aren’t doing these set tasks, there needs to be no movement, and that’s where the bolts come in. If you never allowed any movement, then you’d never be able to get the door out or off.

So, the highly discussed bolts don’t keep the door in place. The four bolts in the mechanism ensure the door doesn’t translate upwards following these inspections or maintenance. Ideally, these bolts ensure the door doesn’t fly off and disengage from the stop fittings.

The NTSB says, however, that the bolts can break or have several other things take place with them, which need to be worked out.

Question Marks Remain

The NTSB is unclear on how many United Airlines 737-9 aircraft have loose bolts, and this is information they say the FAA will have. In regards to other issues besides bolts on the aircraft are currently unclear. More will come to light as the investigation continues.

Clarification was provided that the stop fittings faced fractures after the plug door disengaged, but that doesn’t explain why the door disengaged.

What’s Next?

The door plug is set to be sent back to Washington, D.C. The plug will be crated and shipped along with other parts and closely inspected in a lab. The NTSB says a time will be set up which is convenient for all employees to look at what occurred.

Timeframes are unknown. However, it was noted that investigations can take anywhere from a year to 18 months. However, investigators can better understand what occurred and present these findings once the items get into a lab.

One of the most significant points of interest will be if the bolts were present or tightened correctly, which could’ve directly led to the door disengaging and blowing off the aircraft.

The NTSB says it doesn’t ground aircraft. However, it’s up to the FAA to determine when the 737-9 can fly again based on their findings and constant communication between all relevant parties.

Daniel Fowkes
09 Jan 2024
· Aircraft 

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