Another dramatic event in aviation this week, with an Alaska Airlines 737-900 losing a door on climb and suffering a rapid depressurisation, thankfully not resulting in any injuries or fatalities. So I’ve been back on BBC News again and thought I’d post a few thoughts about the incident.

What could have caused this event?

The 737-900 has an optional emergency exit door which may or may not be required by individual airlines depending on the seat capacity for their particular cabin layout. Alaska Airlines don’t require this door so its permanently closed and the aircraft’s interior panels cover it up. From a passenger perspective, one wouldn’t even know that there is a door there, it just looks like a normal window.

The incident is pretty clearly related to an engineering problem and not something to do with flight or cabin crew actions. As the aircraft was climbing out on departure the pressurisation system would probably have been acting normally to increase the differential pressure between the cabin and the outside air. At about 16,000 feet, whatever was holding the door in place decided it had had enough and let go. The door looks to have separated cleanly (i.e. no jagged bits of metal from it being torn away).

I wouldn’t want to speculate as to the actual cause of this event but a few potential scenarios to consider would include:

• something having gone wrong during the manufacturing process e.g. the material strength not being to the correct specification;
• something having gone wrong during assembly or maintenance e.g. missing parts or misassembled parts;
• long term fatigue won't have been a factor here as this was a pretty new aircraft.

What would have happened in the cabin?

The rapid depressurisation would have been a dramatic and frightening event inside the cabin. The loss of the door and the rapid outrushing of air would have caused temporary misting in the cabin and a sudden pressure change on the passengers’ ears. At the same time, the oxygen masks would have dropped automatically from their overhead stowage and the aircraft would have commenced a steep descent as the flight crew responded. It’s hard to imagine how terrifying it would have been for anyone sitting next to, or near to, the new hole in the side of the aircraft.

Luckily no one was sucked out when the depressurisation occurred (a distinct possibility for someone seated adjacent to the area in an event like this), most likely because their seat belts were still fastened after take-off. Certainly a good reason to always keep seat belts fastened throughout a flight.

What would have happened on the flight deck?

On the flight deck, the pilots would have actioned the rapid depressurisation emergency procedure, donning their O2 masks and commencing a maximum effort descent to, initially, 10,000 feet, in order to  get the aircraft back to an altitude where supplemental oxygen is no longer required. It’s a procedure that’s well practiced in the simulator during initial and recurrent pilot training but none the less, the concerns over what was happing to their aircraft to cause the situation would have been very pressing.

Another piece of luck in this event is that at least it happened pretty early on in the climb, if the same thing had happened when the cabin was at its maximum pressure differential, at the cruising altitude of say 35,000 feet, it would have been an even more dramatic scenario.

I don’t imagine that the handling characteristics of the 737 will have been significantly affected and the crew would have proceeded to eventually make a ‘normal’ landing.

I've not seen reports of anyone on the ground having been injured by the door, which is really another piece of pure luck, I'm sure investigators will be keen to find it though.

That’s it for my take on the aviation news of the day. I hope that you've found it an interesting read.

I’ll look forward to the investigation report when it comes out.