All this discussion about mode reversions is a bit confusing and re-inforces the prejudiced one's bias aginst modern flight management systems.
Be it a Airbus, or an Embraer, a Boeing...or any brand one cares to mention, the principles are the same as those that have been used since flight controls have been used :
A pilot has at his disposal a pitch control and an engine. Full stop.
The rest is a matter of flight path management, which in turn breaks down to "
Energy Management". That is, exactly, what piloting is about.
From that principle, a pilot can elect to have :
- either a fixed engine output - idle, climb, max continuous, TOGA - and control the desired speed with the pitch control, or
- follow a fixed vertical path of his choice -vertical speed, Vnav, or an electric glide path of any sort, in which case the speed is controlled by the pitch control and the engine is used to control the vertical trajectory. (There is nothing new, nothing hidden lurking behind the pilot as that is the way we're taught to fly an ILS).
Now, a pilot is supposed to know what he wants, isn't he ?
- either he flies "raw data" and he does it exactly like he would a Cessna 172, a DC-8 or a 777,
- or he uses the accuracy brought by the flight director, he will get the help of the command bars, but first he has to sign a contract with the FD : "I want to fly with the thrust at hand, you help me control the speed I want with the stick/yoke. As a proof of your agreement I want you to show me , on the FMA, a fixed thrust mode (Idle / Clb / Max Cont / TOGA ) and the Open mode / FLT CHG.
- If I need a fixed path, I want to see on the FMA that you'll take care of the engine, therefore I want to see "THRust" and a confirmation of the path you'll help me follow : VNav, GP...
- I said there was a contract. "If I don't follow the pitch command bar, you are allowed to "revert" to a mode that will keep our energy management on the safe side, and tell me at once what you've done through an alert and a clearly stated mode change."
That is all there it is about Flight management, and, to me, these "reversions" are quite logical and straightforward.
I might also add that in circumstances where the "FD or A/P acts funny", the -very easy - solution is "BACK TO BASICS" AND REPROGRAM THE DAMN THING ! In most occasions, the pilot has been distracted and hasn't followed the terms of the original contract.