My second question: why we have auto call out 50 40 30 20...what is main reason for that? 20 ok 10 ok but why 50,40,30... is that some reminder for something...some trigger ?
Only guessing here. These auto call-outs are handy if landing in heavy rain where rain refraction on the windshield can give an optical illusion and this may lead to potentially hazardous flare height. Also if landing in fog whether autoland or semi-visual the call-outs are useful as an aid to expected flare height. But for a purely normal landing they can be a distraction especially in cases where the auto-callout is inoperative and the PM calls out these heights from his radio altimeter and is usually far too late and the aircraft bashes into the runway as he is singing out 50, 30 Oooops crash.
One final advantage of the auto-call-outs is that if the pilot is wearing his bifocals instead of his trifocals and thus depth perception is compromised, the call-outs are an aid to depth perception.
