If an overlap is undetected, one pilot might respond to a directive from the control tower intended for the other plane.
What happened in Phoenix was “very serious,” said John Cox, a former airline pilot who runs an aviation safety consulting firm. “It’s a process failure. My question is, How did it happen?”
Mr. Cox added that, in his decades as a commercial pilot, he had never seen two planes sharing a call sign in the same vicinity.
A collision was unlikely because of warning systems and training, he said, but the confusion pointed to at least a momentary glaring gap in procedure.
“The investigators are going to look at how the process failed, and that’s really of greater interest to me than the single event in Phoenix,” Mr. Cox said. “There is such a robust process, so that’s the thing I want to know: Where was the process breakdown?”