Mid air collisions is one of the most frightening things for people to imagine especially if you already have a fear of flying. Although mid air collisions involving airliners are rare, they have happen in the past. The last one was in LAX Airport in California when a B-737 collided with a turbo prop in the early 90's. However, one of the worst mid air accidents happened in the 1970's in California when a B-727 collided with a small Cessna Aircraft. Everyone on board both planes died. The government, & industry came together to find solutions & new innovations to prevent similar mid air collisions from happening again. One of the first things the FAA did was to change the airspace & introduce a TCA Airspace which stood for Terminal Control Airspace. Today, it's called  Class B Airspace. The purpose of this airspace was to further restrict small recreational airplanes from large terminal airports where most airliners fly so to reduce the amount of small aircraft incursions into this airspace. Back in the 70's there were a lot of incursions within this airspace from small airplanes. Most of these incursions were unintentional, or just recreational pilots getting lost. However, these incidents made airline operations a little hazardous. Today, the Class B Airspace has one of the toughest restrictions when it comes to fly into it. The airspace boundaries are clearly defined for recreational pilots & any airspace incursions are not tolerated. A pilot who bust through this airspace without permission can face a license revocation & stiff fines. Another thing the industry did to cut down on mid air collisions was the innovative creation of the TCAS ,or Traffic Collision Avoidance System. This was probably the best safety device ever created in aviation. The TCAS is an instrument box in the cockpit. It's a build in computer that talks to the computer of other airplanes. It monitors all aircraft within 10 miles of the host aircraft. When it see's a potential threat of a collision it warns the pilot a minute in advance by pointing out the direction & distance of that threat aircraft so that you can see it & avoid it in time. If the aircraft is still on a collision course then the computer gives aural & verbal commands to avoid a collision. It will tell one aircraft to descend while telling the other aircraft to climb. Many potential accidents we never heard about because the TCAS save the day countless times. The TCAS is expensive & it's not common to see it in small recreational aircraft. The TCAS is mandatory on all commercial airliners, & is standard equipment on most business jets. I consider it to be my best friend.