In this link I read about the crashed aircraft:
The airplane then pitched nose down over the next 18 seconds to about 49° in response to nose-down elevator deflection.
I feel that this is a common mechanism, unknown to non-expert, like me. What does it mean?
Interested in a rather intuitive, relaxed explanation, rather than an analytical one.
There are a few words in that description which might seem a bit technical to a lay-person.
The word "pitch" refers to the angle of the aircraft, either up or down.
pitch: (of a moving ship, aircraft, or vehicle) rock or oscillate around a lateral axis, so that the front moves up and down.
"49°" refers to the angle from the horizontal (0°). 49° is a very steep decent. Much more so than a normal descent profile.
The "elevator" is the control surface used on an aircraft to control the pitch, either up or down. So "nose-down elevator deflection" says that the elevator position were such that the aircraft is pointing down. The implication/possibility is that the controls in the cockpit were commanding that input, or that a mechanical fault caused the elevators to do so.
"Nose-down elevator deflection" is a long winded way to say the elevators moved in the direction that would push the aircraft into a dive. It's phrased this way because it isn't always clear from flight data recorders whether this was a commanded (pilot input) movement, or uncommanded.
In either case, the elevators were deflected downward, which pushes the tail up and the nose down, and over a fairly long time (in terms of a crash in progress) the aircraft was nosed down by 49 degrees.
