Keeping a Close Eye on Close Calls

By Debbie Hersman

Image of several planes near each other in flight
TCAS warns flight crews about a possible collision in the air.

We are hearing a lot this week about the loss of separation between an Air National Guard Boeing 737, with First Lady Michelle Obama on board, and a military cargo plane near Andrews AFB. Fortunately, the error was caught, and corrections were made to prevent the 737 from encountering wake vortices from the cargo plane. Last January, a significant loss of separation occurred between a Boeing 777 bound for Brazil and two military C-17s just outside New York. At their closest, the aircraft were only separated by one mile.

“Loss of separation” occurs when the space — vertically or horizontally — between two airborne aircraft falls below the minimum allowable. Modern commercial and military aircraft are equipped with collision avoidance alerts from their Traffic Alert and Collision Avoidance System, or “TCAS.” These alarms sound inside the cockpit to warn flight crews about a possible collision in the air. The vast majority of these conflicts are resolved quickly, and passengers are never even aware of the event. Once in a while, the conflict is more serious and an aircraft must take sudden evasive action.

Sophisticated systems like TCAS require us to do more detailed reviews in order to learn new safety lessons from them. So a year ago, the NTSB began collecting information about all TCAS alerts. In certain situations, commercial aviation operators are required to let us know when their flight crews received one of these alerts. In the past year, we reviewed reports of 950 TCAS alerts. We found that many of the alerts were actually “nuisance alerts” in which an aircraft in a high rate of climb or descent, in take-off or landing, caused the alert to sound, and there never really was a collision risk. We followed up on 260 events by asking FAA for radio and radar recordings to better evaluate the circumstances of the conflicts, and we initiated formal investigations into nine of these events

By collecting this TCAS data and investigating selected events, we are learning key information about the circumstances that lead to aircraft conflicts, as well as FAA’s processes for responding to them. The information we’ve collected has provided the FAA the opportunity to further adjust internal controls. Under this program, we hope to achieve a better understanding about how circumstances evolve to create midair conflicts and losses of separation, and more importantly, how they can be avoided in the future.

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