Comment by rlu | Hacker News Reader

Comment by rlu | Hacker News Reader

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When a mistake happens is normal to disconnect the autopilot and/or auto-throttle and take over on manual.

Is the mistake that they made here that they used the flight level change mode for final approach where they shouldn't have done that to begin with? Or what is the mistake they made they caused them to turn off autopilot?

My second question is the one I originally had when reading the article:

The pilot flying responded by disconnecting the autopilot and manually retarding the throttles to idle. In this configuration, the autopilot was not commanding the airplane, although the system made inputs to the flight directors because it was still in flight level change mode.

I don't understand this. What does it mean to turn off autopilot but still have the plane give input due to the AP mode? Is what the article saying that the pilot simply turned off the AP voice warnings but left the AP input on? - if so, I don't understand why that should even be possible.

What are the scenarios where you would want to leave the AP inputs on but switch off the audio warnings other than the pilot "finding them annoying" (!!) ?

They were flying with vertical speed, with that mode you tell exactly to the AP how fast you want to go up or down( 1000ft/min is anormal rate for approach). For that mode you don't need a lower capture altitude selected if you are descending (or the opossite if you are climbing). So they set the go around altitude on the selector, this is done to be prepared in case of a go around and the altitude is given by the standart approach procedure you are performing. So they are descending but the altitude selected is higher than them. Till this point there is nothing unusuall if you are flying a visual approach. Then they realize they are descending too slowly and they are high on the approach. This is not unusual nor dangerous. To loose that excess altitude they select the descend mode "Level Change", this means that the plane is going to capture the selected altitude, if it's higher it will apply power and climb, if it's lower it will set the engines to iddle and descend. As they had selected a higher altitude the plane set climb power and rised the attitude to climb. Not even this moment is dangerous, as I said it happens sometimes. They disconnected the autopilot and autothrottle and begun to descend manually. As they had excess altitude the speed is was not a problem initially you change altitude for speed, and descend at a faster rate to reach the ideal glide slope. All this is a quite usuall procedure that we perform quite often when high on the glide. Their mistake was that once they reached the desired visual glide path from above you need to apply a bit of power (around 50%N1 ) to reduce the descend and keep the speed. They correctly pulled the nose to follow the glide but forgot to apply power. Then the speed started to decrease slowly till it went too far down and they stalled into the ground. The reply to the second question is that they disconnected the autopilot and autothrottle, but the flight director was still on and trying to go up. The flight director are two bars located at the center of the artificial horizont that tell you were you need to put the airplane symbol to flight the selected path. It helps quite a lot, because manual flying inside the clouds requires a lot of concentration on the basic positions of the artificial horizont,speed and heading. The flight director makes it much easier so you can concentrate in other tasks. It's recommended that when you are flying visually and ignoring the flight director, to disconnect it so it doesnt give you misleading indications. This happened to them, they were descending but the FD was telling them to climb (as it had been selected eith level change). This maybe disconcerted them when they reached te visual glide. But the biggest problem was their failure to apply power manuallyto the thrust levers. Sorry for the wall of text, but I'm writing from my Iphon and I can't make the intro work and separate the text in paragraphs.

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