@gcal +1
@Pace
' Nothing that is done will make a future Lubitz impossible only less likely'
That is it in a nutshell.
The purpose of legislating the existing informal practice of having a CA in the cockpit for breaks is so that regulators can show they are doing something, to reassure the public.
As a mental health professional observed way back in the thread it also reduces the likelihood of a mentally unstable person committing this kind of mass murder.
But professional pilots know that even if there were four fully qualified pilots in the cockpit, if one of the pilots seated at the controls were determined he could easily crash the aircraft by applying extreme control inputs, 90+ roll and max positive or negative g and it is unlikely the others could stop him before bits started coming off the 'plane. All he has to do is wait until the other pilot gets up from his seat. The three not strapped in at the controls would be thrown around the cockpit or unable to reach.
The SilkAir aircraft broke up and went down in under a minute, didn't it?
As did the BAe 146 whose crew were shot by a deranged passenger.
Consider the Russian pilot's young son. He wasn't deranged, but still managed to crash the aircraft despite the presence of two fully qualified sane pilots who should have been able to overpower him, you would think.
The best (only) chance we have is not to allow such a person from strapping in at the controls.
I still say admit a CA to the cockpit, and let the pilots or senior CA decide who it is.