At Tampa International Airport on May 25 a voice that sounded like an air traffic controller cleared a jetliner on the runway for takeoff. A real controller, who heard the bogus instruction, immediately countermanded the order.
During the next three days, five mysterious messages were transmitted to planes around Tampa International. They ranged from an innocent "nice turn" compliment to a Delta Airlines pilot who had just turned off a runway onto a taxiway, to a dangerous order aborting a landing moments before a TWA jetliner was about to touch down.
For years radio enthusiasts have listened to messages going back and forth between flight controllers and commercial airline pilots. But for the first time, someone is cutting in on those messages, and it has the Federal Aviation Authority concerned.
Agents for the Federal Bureau of Investigation, Federal Aviation Administration, and the Federal Communications Commission rushed in to investigate after they were alerted by controllers in Tampa's tower. But FCC radio tracking equipment failed to locate the "phantom controller" before he became aware of the search and fell silent.
Five weeks later, a mysterious voice again interfered with a commercial flight. A Delta Airlines pilot, bound from Tampa to Indianapolis, heard a command to level off at 29,000 feet. He was under control of the Jacksonville air traffic center at the time, and a controller there could not understand why the pilot had not responded to his command to continue climbing to 31,000 feet.
The Delta pilot told him about the other command, and another commercial pilot in the area said he had heard it, too.
Investigators have not yet determined whether the person sending the false message to the Delta jet was the same one who had beset Tampa International's airwaves, but it was possible.
Flight controllers are taking precautions to guard against these phantom commands, but they are concerned that the potential for a dangerous situation remains.
The FAA can do little to guard against intruders on their radio frequencies because those frequencies must remain open to the flying public, said FAA spokesman Roger Myers.
"The radio frequencies used by pilots and towers are published on aeronautical and landing charts, and they can be purchased at any airport," he said."Someone could buy a tunable transmitter and tune to those published frequencies and make a broadcast and the only way we can guard against it is with regulations which impose civil and criminal penalties for someone who uses the frequencies without authorization."
But finding those violators is very difficult, according to the FBI, which is leading the investigation for the ghost controller. "It's a difficult violation to solve because no one sees the individual, and there is very little if any physical evidence," said Phillip McNiff, agent in charge of the FBI's Tampa office. "But we are extremely interested in obtaining a solution because we know of the danger involved in this kind of activity."
The latest incident was dangerous because flight controllers assign an aircraft to a particular attitude to make sure it is kept separate from any other planes, said William Allyn, assistant chief in the Tampa tower.
"Separation of aircraft is sacred to controllers," he said. "Their primary duty is to keep aircraft separate, and when someone starts messing with that, it scares them."
And trust between pilots and controllers is also key to keeping flying safe, said FAA spokesman Jack Barker.
"The air traffic control system is a mutual system between pilots and controllers," he said, "and they have to believe each other and act quickly on instructions given. Fortunately there are safeguards built into the system that can somewhat preclude problems from spurious messages, but the fact that someone can intrude like that creates a very grave safety problem."
The culprit or culprits would have to be very familiar with the air traffic controlling system and would have to have an aviation radio. Anyone with a rudimentary knowledge of air traffic controller language could have sent the first messages, Mr. Allyn said, but whoever sent the last message had to have had a sophisticated understanding of the system.
Interfering with the air traffic control system carries a prison term up to five years, Mr. Barker said, and if a fatal crash should happen bacause of a bogus command, the phantom controller could be charged with murder.