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Comment by mrunkel | Hacker News Reader

This is entirely caused because autopilot (Flight Management) systems are complex beasts with minimal feedback and sometimes very subtle interactions.

Ultimately the problem here was:

ALT mode was selected accidentally ALT SEL mode was selected later, but ALT mode wasn't disengaged manually and engaging ALT SEL while in ALT mode doesn't disengage ALT mode.

A combination of other factors (rushed during takeoff, interruptions from ATC, unfamiliarity with the Airport) caused the workload to be such that neither pilot picked up on the numerous subtle visual clues that showed that the autopilot was not going to do what they were expecting.

Luckily, after descending 500ft in about 18 seconds, audible warnings in the cockpit alerted the crew that things were going awry and they very quickly remedied the situation.

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> Luckily, after descending 500ft in about 18 seconds, audible warnings in the cockpit alerted the crew that things were going awry and they very quickly remedied the situation.

If you read the UK Government report that is not what happened. The plane was at 1500 ft when it began to descend. At 1300 ft the warning sounds activated and the pilot reacted but it took another 300 ft of descent before the pilot was able to fully recover from the descent.

Poor reporting from the BBC.

Only losing another 300 ft with that rate of decent is a very quick remediation. Large aircraft don't turn on a dime.

Dash-8's are pretty small though.

The industry uses Light as under 7,000kg, medium 7,000 - 136,000kg, and heavy as 136,000kg or more for wake turbulence.

At ~15 Metric Tons (33,000lb) take off weight it's a long way from a light aircraft. However, if your walking around these things, my personal view is a medium is around a Cessna Citation Mustang, double that and their large, and aircraft quickly get into huge and monsters.

> Luckily, after descending 500ft in about 18 seconds, audible warnings in the cockpit alerted the crew that things were going awry and they very quickly remedied the situation.

Why did it require audible warnings? How could any pilot not notice that they were coming down that sharply?

500 feet in 18 seconds isn't that fast (around 20mph), especially if the decline was eased into. It would be visible on the altimeter, but "seat of the pants" wouldn't tell you much.

Despite the fix suggested, what you'd actually feel is acceleration, rather than velocity. 500ft descent over 18 seconds would be about -1.2ft/s acceleration.

Which, shortly after takeoff (i.e. shortly after ascending) could very well feel like a continuation of a leveling off maneuver.

Not to mention, in limited visibility you simply can't trust your sensation of G forces. In the absence of visual stimulation, your mind can hallucinate the feeling of movement that doesn't match your actual movement. You can simulate this by standing with your feet together and closing your eyes.

Do commercial jets have things like displays of graphs over time for the altimeter? Or is it all looking at instantaneous values?

Kinda. There's an ascent/descent rate display separate from the altimeter. It's not a graph, just an indicator.

500 feet in 18 seconds is approx 20mph.

You're right, fixing.

I think it might be even more subtle than that. If I'm understanding page 9-11 correctly, they selected GA, HDG and ALT SEL (the correct settings) before entering the altitude, and doing it in this order caused the autopilot to switch to ALT mode. Then when they subsequently re-selected ALT SEL mode, this was ignored because the autopilot was in ALT mode. Nasty bit of UI design.

> after descending 500ft in about 18 seconds

The article says a maximum decent rate of 4,300 ft/min, so almost 3 times faster than that. They were about 13 seconds from crashing into the ground if that rate held constant.

It sounds far worse than it is. 13 seconds is a huge amount of time. Any pilot would react and correct in a second at most. This isnt some slight drop noticed by scanning instruments. They would have felt this and reacted by instinct. Autopilots off, throttles up, arrest the drop and regain a climb. Over and done within moments. They may have even deliberately let it fall a bit to gain speed, if they thought this might be a stall.

>Any pilot would react and correct in a second at most.

I can only speak to my personal experience and I don't have any sort of advanced pilot's license: I just did the very first one - PP/ASEL. In that course even prior to instrument training I was taught to not fly by the seat of my pants.

The way my flight instructor explained it the human body is easily fooled and will quickly adapt to a new normal. So you had as an example a pilot getting used to a slight tilt and assuming this is normal, so they keep correcting for the tilt and worsening their situtation.

Even as a lowly PP/ASEL I'd still validate any reaction by reading the instruments before assuming a feeling of falling is accurate. Would that take thirteen seconds? Hopefully not, but it would take more than one.

Combined with the idea that airline pilots do less and less hand flying and also have many inputs in the cockpit trying to tell them what's happening and therefore may need a few seconds to integrate all that data and then react with a potentially rusty skill I'd feel comfortable saying a subsecond response seems unlikely.

Please re-read parent. They already had not felt, reacted, or corrected in 18 seconds. If the bells had gone off later, they would have reacted later.

That's what the parent says, and what the BBC inplies, but not what the UK report says. The pilot reacted at 1300 ft. It took another few seconds of falling to recover at which point they were at 928 ft.

Which is how i practiced things like apparent high speed stalls, where the indication is that you have speed enough but are still stalling, and so you dont trust the indicated airspeed. If you know your altitude and suspect a stall, you dont pull immediately for fear of triggering another stall. You biuld up some airspeed first. If you have it, you use the altitude.

It's not what you meant, but "moments"[0] is pretty a pretty long time in this circumstance.

[0]https://en.wikipedia.org/wiki/Moment_(time)

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