Originally Posted by DJ77
The way we flew airplanes before FBW had been refined over decades and was satisfying almost everyone. Why change? Why did Airbus needed to invent a new way of flying airplanes? Until now, I fail to be convinced the reason was for the pilot’s better.
DozyWannabe's #451 describes very well my own thinking on the matter. I've had no professinal involvement with FBW but witnessed that evolution from the sidelines. I saw the evolution from fully mechanical controls, through mechanical with hydraulic boost, to hydraulic with mechanical backup plus AFCAS (Auto Flight Control and Augmentation System, IIRC) plus FMS. If I had to reply to this kind of question from a fellow-layman across the street, I would say that Direct Law gives you an airplane pretty much as a conventional airplane, but gets rid of the weight, maintenance burden and vulnerability of cables and pulleys. Alternate Law uses the opportunity that FBW offers to make the airplane easier to fly by ironing out the 'imperfections' that conventional airplanes have: it provides stability in roll and pitch, adds turn coördination and dutch roll damping. Normal Law then adds the envelope protection, and with it the age-old debate about "encroachment on a pilot's authority", expertly discussed in DW's post.
I believe that envelope protection enhances safety, even if it turns out to have features that can be improved in the light of service experience.
The question of when auto-trim should be on or off is not limited to FBW in my view, and I must admit that I don't understand the drama that some make of it. As I view it, when it operates, autotrim obeys the pitch-orders from sidestick or AP by following the elevator. If the order is nose-up, THS trims nose-up. If the order is nose-down, THS trims nose-down. At FL350 there was plenty of time for that to take effect. Maybe I got that all wrong.