WASHINGTON (CNN) -- A student controller was directing planes during two runway mishaps in the past month at Cleveland Hopkins International Airport, apparently giving instructions that placed planes on possible collision courses, federal investigators say.
Federal transportation safety investigators say the exact causes of the mishaps are still unknown. But in both cases, potential accidents were averted only after pilots recognized that mistakes had been made, according to the National Transportation Safety Board.
In one instance, two commercial jetliners came within 500 feet of each other, the NTSB said.
Both incidents involved a "developmental" controller -- a controller who is not certified in every position in the control tower. The Federal Aviation Administration, which regulates aviation in the United States including air traffic control, said Tuesday the developmental controller was under the supervision of different trainers during the two incidents, and that it is the controller/trainers -- not students -- who are held accountable for mistakes.
FAA spokeswoman Laura Brown did not immediately know what action, if any, was taken against the student.
But a controllers' union representative said the controller was still on the job, and deserves to be.
"This particular trainee had a total of 11 hours of training in the entire month of June. That's less then an hour a day," said Bob Kerr of the National Air Traffic Controllers Association. "He's brand new; he's going to make mistakes."
The student had completed about 30 percent of his training hours at the position, Kerr said. "He has plenty of time to not only learn from the present situation, but to continue learning and develop into a fine controller in that position."
Kerr said staffing shortages are partly to blame, contending there aren't enough certified controllers to train the uncertified ones. "Forty-six percent of our workforce is trainees, which is insane," Kerr said, saying the FAA target is to have only 25 percent of the workforce in training.
Consequently, certified controllers are stretched and students get inconsistent training, frequently during periods with heavy workloads, Kerr said.
FAA spokeswoman Brown said the two incidents -- known as "runway incursions" -- were reported through a voluntary reporting system. The system is designed to encourage controllers to report mistakes, so the FAA can take corrective action. The system is not intended to be punitive, she said.
The first incident occurred the afternoon of June 3, when Southwest Airlines Flight 1080, a Boeing 737, was cleared to taxi onto a runway for takeoff. The controller also had given clearance to Continental Express Flight 2942, an Embraer 145, to enter the runway, the NTSB said. The Continental crew saw the Southwest jet and queried the tower controller, the NTSB said.
The two flights came within 500 feet of each other, the NTSB said.
Three weeks later, on June 26, Express Jet Flight 2426, an Embraer 145 regional jet, was cleared to cross runway 24-Left to depart from runway 24-Right. About 19 seconds later, before the plane had crossed the runway, the controller cleared CommutAir Flight 8717, a DH8, for takeoff on runway 24L. The Express Jet flight crew saw the departing airplane and advised the tower controller they would not cross the runway. CommutAir 8717 took off about 1,500 feet from where Express Jet 2426 was positioned.
The FAA's Brown said she was not aware what action had been taken against the trainers in the Cleveland incidents. But typically they are given additional training, she said.
The NTSB has launched an investigation, which could take three to nine months given the complexity of the issues involved, NTSB spokesman Peter Knudson said. The supervision of developmental controllers will be among the factors the NTSB will review, he said.