Squawk codes 7500, 7600 and 7700 can silently tell air traffic control that an aircraft is facing unlawful interference, a radio failure or a general emergency. Here is what each code means, what controllers see and what happens next.

Squawk codes 7500, 7600 and 7700 can silently tell air traffic control that an aircraft is facing unlawful interference, a radio failure or a general emergency. Here is what each code means, what controllers see and what happens next.