According to DNA Info's Murray Weiss, there's a stash of heat-seeking Stinger missiles hidden somewhere in Manhattan, ready to bring down any hijacked jet at a range of up to 15,700 feet (4.8 kilometers).
The FIM-92 Stinger is a portable shoulder-launched infrared homing surface-to-air missile. The 60-inch long projectile can hit a plane anywhere between 600 and 12,500 feet of altitude.
This stash will not be the first. According to Weiss there was a missile in the World Trade Center towers, which the Secret Service kept in case of a potential attack when the President was in town. [DNA Info via Gothamist]