Comment by halter73 | Hacker News Reader

Comment by halter73 | Hacker News Reader

Parent Full thread

IIRC, the cockpit voice recording included a comment from one of the co-pilots about how pulling back on the yoke couldn't cause a stall. The assumption was that the Airbus's fly-by-wire system would prevent it and ensure the aircraft still climbed as long as the pilot held back on the stick.

The co-pilot apparently didn't realize that the sensor issue that disabled the autopilot also disabled the stall prevention. And that's despite an audible "STALL" warning being repeated in the background.

The captain was not in the cockpit when the whole situation started, but as he re-entered the cockpit during the stall he saw one of the co-pilots holding back on the yoke and told him to push the yoke forward to prevent the stall. The co-pilot followed the instructions, but only for a few seconds before pulling the yoke back again.

All of this is to say if the plane hadn't been known to ignore user inputs in most situations, the co-pilot might not have assumed the Airbus would do the right thing and climb no matter what when pulling back on the yoke. So in a sense, never ignoring user inputs might have also saved Air France 447.

Apparently (think I read in the Langewiesche feature) the plane ended up in such a deep stall that the flight control software started ignoring the AoA sensor data (as implausible) and the STALL warning stopped. But when the co-pilot stopped pulling back on the stick, the AoA decreased, and the STALL warning sounded again.

This might have convinced him that easing off on the stick was actually causing the stall, which was tragically misguided.

Exactly - the computer had switched contexts, but the pilot hadn't. And expecting pilots to switch their mental map of expected behaviour when the computer does (and did so with, from the accounts I read, very minimal indication that it had done so) during a high stress situation, is asking for trouble.

One of the best explanations I have read on that issue. Short and accurately summing it up.

The indications were different alerts continuously sounding. It’s a complex problem, there’s a lot that can go wrong at once.

This is my number one objection to over-reliance on automation.

Every piece of software is a mechanism. In order to truly be able to safely use something without outside aid, one must have a complete mental map of the mechanics of the system in question. Abstraction helps; but not when you start getting into high-risk contexts.

The copilot pulling the yoke back continued to do so, long after the other, much more experienced, copilot had formally assumed control and had attempted to bring the nose back down by pushing the yoke forward. Ultimately the inexperienced copilot fighting against his more experienced superior was what doomed the airplane. Both the senior copilot and the captain immediately identified the problem and attempted to take the correct action.

This is not a problem with how the system works, since this behaviour is explicitely communicated to pilots. It even says right on the instrument panel what control law the plane is in. There are only a handful of control laws and the differences aren't that complex. Anyone with sufficient experience in flying Airbus products knows this.

>The copilot pulling the yoke back continued to do so, long after the other, much more experienced, copilot had formally assumed control and had attempted to bring the nose back down by pushing the yoke forward.

At least according to the official accident report, neither of the pilots at the controls consistently made nose down stick inputs.

I don't know a whole lot about this, but I seem to remember that there was one design decision, that, while not wrong, was different from the generation before, and that is that the airplane yokes were not mechanically coupled to one another. If they were mechanically coupled, the experienced pilot could have felt the other pilot pulling back on the controls, but what was happening was that the two pilots were pulling the controls in different directions AND the plane was averaging the control inputs and giving no feedback to the pilots that what each was doing was wildly inconsistent or contradictory.

Reply on news.ycombinator.com