Thread by @sameo416

A few words about the NTSB preliminary report into the Alaska Air door plug departure on 5 Jan 24, Boeing 737-9 Max, N704AL.

NTSB site if you want to read the report or review the photos: ntsb.gov/investigations…

First, calling this 'rapid' decompression, so not technically 'explosive' decompression. The definition has to do with rate of change of pressure.

Missing door. Can see insulation (yellow) and the structural pads - 2 upper left, 1 upper right - of the door. 12 of those in total.

Technical name for the door plug is the MED plug - 'mid-exit door'.

Love forensic investigators: "The separation of the MED plug from the airplane adversely affected the
pressurization performance of the airplane".

A hole in the pressure vessel means it wouldn't hold pressure.

Very experienced aircrew, 12,700 and 8,300 total hours, 6,500 and 1,500 on type.

After decompression, captain's headset almost off, head pushed into the heads-up display. FO loses her headset. Flight deck door blows open. Crew handles the emergency expertly.

You can hear the FO say on the radio 'we need to go down', likely an inadvertent transmission. Declares an emergency. ATC is a little slow. (asks 2x nature of emergency)

You can hear the FO breathing under the oxygen mask with a bit of positive pressure. She has to stop talking.

To inhale. Under positive pressure breathing you can't talk without effort, so you stop talking, do controlled inhale, start talking again. This is like talking into a strong wind in the face.

The FO is really on top of the comms and the crew has obvs good situational awareness

ATC on approach is slow too, "did you declare an emergency?"

Flight crews should not need to repeat themselves multiple times once they've called mayday - ATC needs to sort its crap up. The approach controller goes back to confirm they don't need a hold

You can hear a bit of frustration in the FO, she jumps right on to say we're an emergency aircraft. Even states 'fly present heading' even though ATC hadn't given a vector, then asks ATC for a vector.

ATC "Are you going to need assistance on the ground?" omg.

Not being hard on ATC, but. The approach controller asks 'did you declare an emergency?' He should have it up on his situation display big red 'EM' next to the aircraft, and it should have been briefed in the handover from departure controller.

Finally FO: "I think we got it"

Back to NTSB. Events (seconds clock)
12:33, cabin pressure drops 14.09 to 11.64 psi
14,830 ft, 271 knots. 'Cabin altitude > 10k ft warning' activates.

Ground level pressure about 14.7 psi. Cabin normally at 8000 ft altitude.

34, Master Caution, cabin pressure drops to 9.08 psi.

13:41, aircraft reaches peak altitude 16,320 ft begins descent

13:56, selected altitude changed 23,000 -> 10,000 ft.

17:00 descent below 10,000 ft
26:46 aircraft lands.

I'd not like to think what might have happened if this was after climb-out with the seatbelt sign off.

The door structure/function well explained by @greg_travis.


MED plug manufactured by Spirit AeroSystems 24 Mar 2023. Any opening of the door (for removal or to cant it outwards) requires removal of 4 retention bolts with castle nuts and cotter pins.

https://twitter.com/greg_travis/status/1744048209465372911

2 bolts on the upper guide rollers, 2 on the lower hinge fittings.

Opening the door requires a lift upwards. Once the roller pins clear the upper guide fitting, the door can cant out at the top, 15 degrees.

With the 4 bolts in place, it is impossible for the door to open.

The 2 automatic pressure controllers were removed for further study. Maintenance logs documented 3 pressure controller light illuminations on prior flights.

That could mean a failure in the controller, or that the differential pressure rates were exceeded (from memory).

They also retained the outflow valve. Further lab work here to assess if there is any issue with the pressurization system.

I do not expect they'll find anything. But, thorough investigation means you assess any potential causal system until it can be eliminated.

Watching the NTSB press events, you may recall early on they were surprised the flight deck door had blown open. Assumption (mine too) that this is supposed to be secured. But, diff pressure carries a lot of force, so makes sense it's design to open.

Why was this a surprise?

B/c it apparently wasn't in the Flight Crew Operations Manual. Thanks Boeing. Might recall this lack of information was also a factor in both Max crashes.

Boeing quickly amended the manual.

The adjacent seats to the door plug were rotated. 25A out and aft 10-20 degrees. 26A rotated forward and outboard.

The depressurization flow loaded these seats and twisted the frames toward the opening.

How would the dynamics change with 180 lbs of person strapped in?

NTSB will assess that imo.

They pulled all 12 stop pads from the airframe for inspection. There are clear witness marks showing where the door stops dragged across the pads as the door departed.

Witness marks help indicate contact points and movement, a key investigation indicator. The location confirms the door had moved up and then out, as designed.

Both the upper guide fittings were fractured, suggesting structural limit overload. Those two retention bolt holes were not deformed or witness marked.

If the bolts were in place, and the fitting failed under overload, there would be ample indication left behind.

Based on that, safe to conclude there were no retention bolts installed in the upper guide fittings.

The aft lower hinge fitting was bent and the attachment flange was fractured, "consistent with ductile overstress fracture".

Ductile overstress means the fitting was progressively loaded, deformed elastically, then plastically, until it reached an ultimate failure point.

Again, no elongation of damage to the retention bolt holes on the lower hinges.

"Overall, the observed damage patterns and

absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward

movement of the MED plug were missing before the MED plug moved upward off the stop pads."

Then looked at maintenance records from Boeing. 5 rivets adjacent to the door needed rework due to damage. Pretty routine stuff for a new airframe. To replace the rivets, the MED plug