There are two improvements to airline security that have occurred since the world trade center attacks:
1. Blast-proof cockpit doors
2. Better passenger awareness
Let's put in perspective why this is: a terrorist with resources can easily get weapons on a plane. You blackmail and bribe a bunch of security people, tell them it's drugs, and you're in.
But then they run into problems. They can threaten passengers, but the passengers know that if they let the terrorists take over the plane they're dying anyway, so they're going to fight to the death: this realization was enough that by itself passenger awareness prevented whatever attack was intended with the plane that went down in central Pennsylvania.
And if somehow the terrorists kill all the passengers, they come up against the blast-proof door. It's probably possible to get past that if they smuggle some truly gigantic fireworks on board, but whatever they do to get through the door is probably going to take down the plane; not exactly a targeted attack.
So at the very worst, you're looking at them killing max 120 people on a plane? Let's be generous and say they kill 300.
Let's do some back-of-the-napkin math here:
There were 9,074,185 revenue departures on planes in 2015[1]. There were no crashes. So even if a terrorist hijiacking happened every year you'd have a 300 in 9,074,185 chance, or about 1 in 30,000 chance, of dying on a plane.
In 2013 there were 10.345 motor vehicle deaths per 100,000 people[2]. So just by living in a society where people drive, you have about a 1 in 10,000 chance of dying from a car. Not in a car, from a car. If you're in a car, your chances are probably significantly higher.
In other words, we could let a real hijacking attempt get as successful as one can get once a year and planes would be more than three times safer than cars. At that rate, we could probably get rid of TSA security completely and devote all that funding to car safety, and it would make us safer.
[1] http://www.rita.dot.gov/bts/acts
[2] https://en.wikipedia.org/wiki/List_of_motor_vehicle_deaths_i...