[vkou]

The biggest thing that stopped 9/11 attacks since 9/11 was locks on cockpit doors.

The second biggest thing that stopped them since 9/11 is passenger awareness that they could just... Not let a few assholes with boxcutters fly a plane into a building.

[a2tech]

After 9/11 people are not going to let hijackers run the plane. They may hurt you in retaking the plane, but if they fly it into a building you'll be dead for sure.

[sokoloff]

Since mid-morning on 9/11, in fact.

https://en.wikipedia.org/wiki/United_Airlines_Flight_93

[spzb]

Recognising that domestic flights also need security and not treating them like flying Greyhound buses probably helped.