yxcumnb
The alt capture system on most aircraft works as follows: set desired altitude/flight level, point aircraft towards that altitude, let the system capture it. It will leave that altitude only if you deselect it or you capture the glide slope of an ils (having first captured the localiser).
Not so the mighty dash 400.
Firstly select desired altitude. then press the alt sel button, cross check to ensure that alt sel is displayed on the screens. point aircraft towards selected altitude. Monitor like a hawk to ensure that the alt sel does not drop out. Within one to go (1000' of desired altitude) touch nothing or you will inadvertenly deselect alt sel, monitor even more closely with hands on the controls poised for the system to fail to capture. System captures desired altitude, breath sigh of relief that you haven't had an alt bust. repeat for all level changes during working day.
(The system works for glideslope capture as above, and you can leave alt capture by engeging any vertical mode ( ie IAS, VS Pitch hold).
The problem is that without the alt sel selected the aircraft will fly through the selected altitude, the alt sel system is very prone to drop out and this can be missed in a busy environment. It is a fail unsafe system.
The autopilot can aparently also revert to basic modes from aparently stable conditions such as alt capture and glide capture all by itself.
The alt sel feature is not an option on the instrument fits unlike coupled heading bugs etc and aparently is a major software and expensive fix.