Another Miss

I guess this story is going to get too big to ignore. It’s never a good sign when the headline screams;

Jumbo Jets Nearly Collide Outside New York City

”A packed American Airlines jumbo jet and two military C-17 cargo planes avoided catastrophe, after they were mistakenly sent to the same altitude by controllers at New York's Air Traffic Control Center. The planes -- both closing in on 22,000 feet -- came less than a mile from each other horizontally, and 200 feet vertically. A collision alarm in the cockpit of the American Airlines Boeing 777 sounded, warning pilots to descend. They did, averting a possible collision.”

Oy vey. Let’s not jump to any conclusions just yet folks. Oh..I see...we’re already too late.

”"The impact of a breakdown in communication can be very serious," said former air traffic control manager and safety consultant Dick Marakovits. "In this circumstance, technology stepped in within the aircraft and saved the day."”

There’s a reason people shouldn’t speculate about an incident in aviation. There’s a reason safety people in aviation don’t. I won’t speculate that Mr. Marakovits didn’t add in some qualifier or note of caution. He may (or may not) have.

Because this is the first thing that came to my mind...

The “technology” that “stepped in” is TCAS. TCAS only works when both aircraft have functioning transponders -- with the altitude encoding turned on. The problem in this situation is that you have 3 aircraft. (You might want to ask yourself how TCAS works when you’re in a military formation flight while we’re here.)

Back in my day as a controller, if I was working a military formation flight of two, I’d only have the lead flight squawking a beacon code. Otherwise the ATC “conflict alert” would go off continuously and annoy me to death. Sometimes, in a large formation, I’d have everybody on a beacon code but I’d have them turn off the altitude reporting portion. TCAS provides “resolution advisories” (in this case, telling the American to descend) based on altitude. If an aircraft isn’t “reporting” its altitude, TCAS can’t give a resolution advisory.

I’ll grant you it sounds as if TCAS saved the day. But it’s possible it made the situation worse. We’ll just have to wait for the NTSB to tell us what really happened.

There’s a reason TCAS is a last-line-of-defense type system. The consensus is that we’re better off with it than without it. But it isn’t perfect. And if it gets “tested” too often, it will fail. Just like the rest of the system. It’s best if we avoid “testing” the system too often.

Don Brown
February 8, 2011