There are now two separate but related investigations that have come about as a result of last week’s Alaska Airlines rapid depressurisation incident.

NTSB

The National Transport Safety Board (NTSB) is conducting the Alaska Airlines incident investigation. Their independence is congressionally mandated and they are not part of the FAA.

Their initial focus is determining what happened to the specific Alaskan Airlines aircraft involved. This is centred around the installation of the door plug, and the bolts that should have held it in place. Once those circumstances have been established, we can expect them to shift the focus onto the root causes of the event. If, for example, they ended up finding that the bolts weren’t fitted, they will be looking at how this came to be and why it was not identified during the final inspection process i.e. how did the aircraft get released to operational service in the first place.

The quality assurance aspect has the potential not only to generate findings related to Boeing but could in theory extend into the territory of how the FAA were discharging their approval and oversight responsibilities (hence the need for an independent body).

FAA

The Federal Aviation Administration (FAA) is the US government body that is responsible for oversight of the industry and amongst other things, for approving aircraft manufacturers.

Yesterday the FAA notified Boeing that it is conducting an investigation to determine if Boeing failed to ensure completed products [aircraft] conformed to their approved design and were in a condition for safe operation in compliance with FAA regulations.

Their investigation is a result of the Alaska Airlines incident but they also cite “additional discrepancies”, perhaps referring to the loose door plug bolts reported on some United Airlines 737-9 Max aircraft, or perhaps other issues that have not been made public as yet.

“This investigation is being performed to ensure compliance with Title 14 Code of Federal Regulations and your FAA approved quality system.” The FAA have alleged that Boeing may be non-compliant, which is a very serious development for any manufacturer. See the full letter to Boeing.

Currently all 737-9 Max aircraft fitted with door plugs remain grounded (via the FAA’s Emergency Airworthiness Directive) and its not clear how long it will take to resolve the situation. The directive calls for an approved inspection but looking at Alaska Airlines' recent announcement, they are yet to receive anything from the FAA which defines what the acceptable process is.

I'm not going to run about saying that the sky is falling down, this could all be pretty isolated, or perhaps more widespread, lets wait and see what the various investigations reveal.