F-16 Triggers Airliner Alarm

A few articles recently described how a formation of US Air Force F-16s

came so close to a commercial flight over the US this week that they triggered a cockpit alarm in the commuter plane

The incident was later described in this way:

The commercial plane “encountered two F-16s and they had a near-miss incident,” [Federal Aviation Administration spokesman Tony] Molinaro said.

While the term “near miss” in FAA parlance bodes ill, the incident is rarely as dramatic as it sounds.

While the reports don’t say how close the aircraft actually were, it is possible to draw some conclusions.  For example, the TCAS (Traffic Collision Avoidance System) on civilian airliners offers two warnings: a Traffic Advisory and a Resolution Advisory.  In a TA, the TCAS literally says the audio “Traffic!” to the pilots.  It is an announcement of other aircraft that are potential factors to the aircraft’s course.  In an RA, the TCAS actually directs the airline pilots to take evasive actions, normally “Climb” or “Descend.”  The pilots are required to obey these computer directives–which are considered essential to avoid a midair collision–as if they were given by a human air traffic controller.

In this case, it appears the airline pilots received a TA, and were able to see the F-16s maneuvering.  They asked for a climb “as a precaution.”

A spokesman for the Ohio National Guard, whose 180th Fighter Wing aircraft were reportedly involved, indicated that the F-16s may have been “above their ceiling,” but was unable to confirm any details.

It is worth noting that high performance aircraft can activate TCAS alerts in aircraft that are literally many miles away.  The TCAS warnings are predictive for an instant in time.  If a fighter pilot is pulling 8gs at nearly 600 knots, he’s flying a tight circle. However, a TCAS four or five miles away may perceive the instantaneous flight vector as a conflict (since the fighter could cover the miles of separation in seconds at that speed); the TCAS isn’t smart enough to know the fighter is turning and isn’t really a potential conflict.

Unlike aircraft whose primary function is to takeoff and land, fighter aircraft need specialized training that can only occur in a large volume of restricted airspace.  Sometimes the aircraft “spill out” of the boundaries; sometimes the TCAS just thinks they did.  In the vast majority of cases, the “near miss” just means the planes came closer than planned, not that there was a reasonable risk of collision.

Either way, it is likely the fighter wing will get a stiff reminder on how to stay out of the news, and the airline pilots, who “did a good job and handled the flight appropriately and got our passengers safely to Atlanta,” have a story they can tell their friends about how they “saved the day.”