Ask the Captain: What's a near-miss in the sky?

Question: What's considered a near-miss in the sky, and how do you go about reporting one, or to see if a captain did so? I was on a recent flight and a plane passed directly below us at what I would consider too close a horizontal separation. — Submitted by reader Tom Bresnan, N.J.

Answer: The horizontal separation standard ranges from 3 to 5 miles and drops to 1 mile under certain conditions near an airport with parallel runways.

The vertical separation is 1,000 feet for other instrument traffic and 500 feet for visual traffic.

If you see an airplane you believe to be too close, I would suggest discussing it with the captain at the end of the flight.

Usually there is more vertical separation than you think. If there is a real threat, there is a warning system (TCAS) in the airplane(s) that will command a descent or climb to ensure separation.

A USA TODAY investigation found that near collisions happen more than you might think. Watch the video below to hear the surprising accounts of real events.

Q: If TCAS says go down and flight control says go up, which do you do?

— JT Ryan, Las Vegas

A: You follow the TCAS.

Some years ago, a midair collision occurred in Europe because one pilot followed TCAS while the other followed the controller's instruction. Worldwide, the proper action is to follow the TCAS, then advise the controller.

Q: In your recent column on air-traffic control, you said that all aircraft have TCAS. Shouldn't you say that all air carriers have TCAS?

— Russ, S.C.

A: You bring up a good clarification. Airliners and business jets have TCAS, small general aviation aircraft may or may not. The TCAS on the airliners and business jets can see and avoid the transponder of small general aviation aircraft even if the other plane is not so equipped.

Q: While flying back from the Caribbean, I saw a small black private jet shoot by not more than a few hundred feet in the opposite direction. At what point does a pilot take evasive action?

— Paul, Arizona

A: The opposite-direction traffic was 1,000 feet below you, and both pilots were aware of the other aircraft. Had that not been the case, you would have felt the effects of a TCAS-directed evasive response.

John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.