Locking the cockpit doors and beaching the social contract that no one gets hurt in a hijacking stopped it, not TSA.
The trade-off here is that there's at least one instance where a protocol gap and a locked door led to intentional CFIT (Germanwings 9525)
alt Hacker News
The trade-off here is that there's at least one instance where a protocol gap and a locked door led to intentional CFIT (Germanwings 9525)