2024 has already seen two notable aviation accidents.

The fiery loss of a Japan Airlines A350 during a collision with a Coastguard aircraft at Haneda airport – thankfully, with minimal loss of life due to a safe evacuation of the Airbus – and Friday 5 January's loss of a cabin door from a Boeing 737-9 MAX, have generated a considerable amount of concern and social media interest.

The Haneda accident is under investigation and there is little doubt that the efficient Japanese authorities will have an interim report touching on the many interlinked aspects of the accident chain out by the first week of February. For this reason it is perhaps appropriate to wait until more is known of the many variables at play during that event before delving deeper into that accident.

Before moving on, however, it is, perhaps, instructive to point out that passengers who evacuated the burning Airbus A350 did so under instruction of, and in obedience to, the crew and in a disciplined and orderly manner without trying to drag their cabin baggage down the slides. This played a large part in the positive outcome for all on board that aircraft.

Perhaps that is a point to consider by many members of the South African flying public who appear to delight, as directly observed on numerous local flights, in standing up while the seat belt signs are on, getting the huff on with cabin attendants when asked to await their turn to disembark and who generally appear intent on putting themselves first in all things. Given our local approach, it is sobering to consider that a similar positive result and orderly evacuation may not have been as easy to achieve had the accident taken place at a South African airport..

However, the accident to the 10 week old Alaskan Airlines aircraft appears, taken on available evidence in the public realm and from the NTSB (National transportation Safety Board of America), to have a more direct and identifiable, if somewhat concerning, cause.

For those who have not yet heard about the accident, a few minutes after taking off from Portland on a flight to Ontario, California, shortly after 5.00 pm Pacific time, while passing 16 000 feet, an exit door plug fell from an Alaskan Airlines Boeing 737-9 MAX aircraft. This led to rapid depressurisation of the cabin and the deployment of supplementary oxygen masks.

No fatalities resulted from the accident. While a distressing event to experience, all passengers appear to be un-injured and the lost door plug seems fortuitously to have landed harmlessly in a local school teacher's back yard. The Captain and her crew performed their emergency procedures well under difficult, noisy circumstances, and the aircraft returned safely for a landing at the departure airport.

Nervous members of the South African flying public, might be assured by the fact that no example of the 737-9 model involved in the accident operates in our airspace.

That said, I would not be too concerned about flying on one of these specific models as immediate inspections of the variant have been undertaken and most have been (or are being) cleared to be returned to service after these examinations. There appears to be a handful of aircraft where the locking bolts that secure this door plug needed additional tightening to specification but, the bolts and nuts with their pins were in place and secure and thus the door plugs were safe.

Bolts, nuts, pins? Perhaps it may be useful to understand how and why the door is plugged in the first place.

The Boeing 737-9 MAX model can, if desired, be certified to fly with 220 passengers as opposed to the usual, slightly more comfortable 189 seat configuration used by most airlines. To this end, the aircraft is fitted with an additional exit door just aft of the wings on either side. It is called a mid-cabin exit door.

Where airlines opt not to immediately use the 220 seat figure, as with Alaska Airlines and some others, the doorways are still be built into the aircraft in any event to permit a more cost-effective option to increase loads to maximum capacity in the future.

On 189 seat capacity aircraft, this mid-cabin exit doorway can be closed with a door panel that cannot normally be opened. This, effectively “plugged” exit, is then covered with interior panelling, making it all but un-detectable from inside the cabin - although it is readily identifiable from the outside.

When fitted at the factory the door plug panel is secured in place via a spring-assisted lower hinge assembly and frame mounted upper roller locking pins which engage in a track machined into the upper portion of the door plug.

When closed, the track and locking pins are secured with bolts that prevent the door plug moving upwards and outwards. Additional securing bolts are fitted to the lower section attached to the hinges to prevent upwards movement via the slider system on the spring assisted hinge mechanism. All four of these critical bolts are designed to be fitted with castellated nuts and secured with a cotter pin as shown in the accompanying illustration.

Finally, there are 12 stop-pads on the frame and matching stop fittings on the door to take the outward pressure loads around the frame. These stop pads and fittings slide into place to engage against each other as the door plug is slid downwards to secure it in position. This exact type of door plug arrangement has been in use on other aircraft model variants for decades without incident.

For the door plug to have fallen off the aircraft, therefore, all four lock bolts would have needed to fail in some manner, or to have been absent in order for the door to move upwards sufficiently to disengage the stop tabs and pads. That such an upward movement of the door plug took place on the accident aircraft has been confirmed by the NTSB investigators.

Given that the airliner was virtually new, speculation immediately after the accident has been directed against the sub-contractor which manufactures the fuselage, Spirit Aero Systems and Boeing who assembles the final product, for faulty assembly and poor quality control.

This may be understandable given the manufacturer's less than stellar record in admitting to design and manufacturing issues in recent years regarding its aircraft.

The production sequence of the fuselage begins at Spirit Aero Systems where the fuselage is manufactured and the door plugs are fitted. The entire fuselage is then shipped to Boeing for final assembly. During the final build process, all doors are opened and the door plugs are removed to permit easier access for the fitting of seats and other interior trimmings. The doors and door plugs are then re-fitted by Boeing and the fuselage pressure tested.

Digging a bit further into the short history of the accident airframe's service life, though, has revealed that it had no issues during initial service after delivery and completed 49 flights without incident.

On the available evidence, the first pressurisation warnings on the aircraft surfaced on 7^th of December 2023, the first day of commercial operations following its return from an after-market maintenance company called AAR Corporation. This company appears to perform work for more than one US 737-9 MAX operator and is thus, another common link in the chain on evidence to be examined.

The aircraft had spent 10 days at one of the company's facilities to have customer on-board wi-fi and other systems fitted.

AAR have, however, released a statement to the effect that they did not perform any maintenance on the door plug.

Nevertheless, in addition to the factory build record, the exact sequence of events within the Boeing and AAR facilities as well as the service records at the airline itself while dealing with the subsequent pressurisation warnings will, no doubt, form key parts of the investigation.

While a preliminary report will be available within a month, the full investigation report will likely only be available in a year from now. Nevertheless, this accident has little chance of being repeated.

The cause is limited in scope. The door plug was able to move upwards far enough to permit the stop-pads and stop-fittings to move past each other, as well as allowing the roller locking pins near the top of the frame to break off the securing track under the pressure loads which were being carried by the tracks. They were never designed to carry such loads. This sequence of events let the door plug open outwards under the growing pressure inside the fuselage, The high-speed airflow outside the aircraft then did the rest.

Ensuring that the four locking bolts are in place and properly fitted to all similar door plug assemblies, which is being undertaken in the mandated inspection process, will prevent a recurrence of this accident.

Why the bolts were not there on this aircraft, either totally or in part, or if they were there, why and how they failed, will be revealed in due course. However, this accident would appear to be an outlier and not a cause for concern about the aircraft type as the assemblies and designs involved have served safely for a long time. Certainly, it should not be any trigger for a mass grounding of the entire MAX family as it is only the 737-9 model that has these door plugs.

As with all airliner accidents, several inter-linked and sometimes seemingly unrelated events need to have aligned in order for the accident to take place.

Investigators look at every incident and accident, not to apportion blame, but to prevent a recurrence of the chain of events involved in the causes. The focus on learning the what, how and why permits regulators to develop procedures to trap and prevent the contributory causes in the future.

It is due to this diligence that airline safety has evolved to the current state of the art which permits us to take an airline flight almost as a matter of routine, confident that, even when dire incidents such as these do occur, we will, more likely than not, get home safely.

Mark D Young is a South African investigative journalist and flight safety author.

ILLUSTRATIONS:

A drawing of the door plug released by the NTSB.

Images placed in public domain by Captain Chris Brady from the Boeing 737 Technical Channel of the upper securing track with bolt, nut and cotter pin as well as the lower hinge assembly and locking bolt.

An image placed in the public domain by Captain Chris Brady from the Boeing 737 Technical Channel showing an actual door plug opened in a maintenance hangar.

Image from the NTSB of the accident aircraft showing the exit door from which the door plug detached.