commercial airplane crashes and the chain of events

I have a bit of a fascination with airplane crashes, and while feeding baby Nick in the middle of the night I’ve been watching a lot of Air Disasters (also known as Mayday/Air Crash Investigations/etc.). It’s pretty well done and does a good job of presenting the investigation as a mystery that needs solving, although I have a feeling things are dramatized a little bit to preserve this narrative.

After watching way too many episodes, one of the takeaways is that commercial aviation is incredibly safe, so when a plane crashes it’s because a bunch of things went wrong, not just one thing. This is know as the chain of events, and I recently watched an episode that really encapsulated this well.

The flight was Garuda Indonesia Flight 152. Here are all the links in the accident chain:
– The air traffic controller used the wrong callsign for the plane when giving instructions, because he was confused with an earlier flight that day (“Merpati 152” instead of “GIA 152”). When he realized his mistake, he repeated the instructions with the correct callsign, but forgot to say again that he was putting them on a course to approach the runway from the right side instead of the typical approach from the left side. (this was done because there was a plane departing from the runway at the time)
– Then when the plane was told to turn right, the captain (who was used to flying the normal approach) instead turned left, presumably out of habit.
– The first officer was distracted during the turn because the captain was complaining about the cockpit being hot. (In the US, this would be a violation of the Sterile Cockpit Rule, although I’m not sure if there’s a similar rule in other countries)
– As a part of the turn they were cleared to descend to 2000 feet. But the plane continued to descend lower than that – the assumption is that the captain set the autopilot wrong, although this is unclear.
– Because of forest fires in the area, the air was smoky so the pilots couldn’t see the ground.
– For some reason, their GPWS didn’t activate telling the pilots they were getting close to the ground. This was presumably a bug in the GPWS or something.

If any one of these had not been present, the accident would have been avoided. And during the investigation, safety agencies like the NTSB will try to address all the causes. For example, in this case a recommendation was made that flights going to the same destination should not have the same flight number, even across airlines. This is what makes commercial aviation so safe!

(this is similar to the computer security idea of defense in depth – you want to be protected at multiple layers)

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