CHARLESTON, SC (WCSC) - The National Transportation Safety Board released the probable cause Tuesday in the deadly 2015 mid-air collision between an F-16 and a Cessna 150 over Moncks Corner, citing the approach controller's warning to the jet pilot.
The probable cause was "the approach controller's failure to provide an appropriate resolution to the conflict between the F-16 and the Cessna, contributing to the accident or the inherent limitations of the see and avoid concept, resulting in both pilots' inability to take evasive action to avoid the collision in time," NTSB Investigator-in-Charge Dennis Diaz said.
The controller, investigators say, expected the Cessna's would remain within the local traffic pattern at Moncks Corner. But as the Cessna climbed above the altitude of 1,100 feet and the airplane closed within 3.5 miles, she recognized the potential for a possible collision between the planes and transmitted the traffic advisory, he said.
Audio recordings of the controller's advisory to the F-16 pilot was played during the presentation.
"Traffic 12:00 to miles. Indicated type unknown," the controller said in the recording. "Turn left if you don't have that traffic insight -- in sight. If you don't have the traffic in sight, turn left immediately."
The planes were closing on each other at a rate of about 300 knots at that point, he said.
Avoiding a crash between the two planes would have required the F-16 pilot to first see and then take action to avoid the Cessna.
"While the controller tried to ensure separation, her attempt at establishing a visual separation at so close a range and with the airplanes converging at such a high rate of speed left few options and a visual separation could not be obtained," Diaz said.
Diaz said the controller's instruction to the F-16 pilot to "turn left" was not contrary to FAA guidance for air-traffic controllers, but that it was the "least conservative decision" and most dependent on the F-16 pilot's "most timely action" for its success.
In hindsight, the controller should have instructed the F-16 to turn before the planes came in close proximity to each other and and to turn right, which would have kept the F-16 from crossing in front of the Cessna's path, he said.
Investigators say the controller's conditional response may have resulted in "a slight delay" in the F-16 pilot beginning the turn.
"The controller repeated the instruction and appended the word immediately, expecting that the F-16 pilot would perform a high-performance turn," Diaz said. "While the F-16 pilot began the turn before the controller completed her radio transmission, the rate of the turn was slower than what she had expected."
Investigators say post-accident interviews revealed the word immediately held "different meanings" to the controller and the pilot.
"Although the controller's use of the term immediately was in keeping with the guidance established, further clarification of her expectation, such as directing the pilot to expedite the turn would have removed any ambiguity," Diaz said.
NTSB investigators say the "see and avoid" concept, which relies on the pilot to look through cockpit windows, identify other aircraft, decide if the aircraft could be a collision threat and then take appropriate action has "inherent limitations."
Those include, investigators say, limitations of the pilot to visually process the information, other required tasks that compete with the requirement to scan for traffic, a limited field of view from the cockpit and weather conditions.
The Cessna was not equipped with any technology in the cockpit that would have displayed or alerted the pilot of traffic conflicts, Diaz said. The F-16 pilot's ability to detect other traffic, meanwhile, was limited to the "see and avoid" concept supplemented with air tower control advisories.
"While the F-16 pilot could use his airplane's tactical radar system to enhance his awareness of the radar system, it is designed to acquire fast-moving enemy aircraft and not slow-moving aircraft," Diaz said. "The F-16 was not equipped to display or alert traffic conflicts."
Diaz said their investigation showed both pilots would have had difficulty detecting the other plane.
The planes collided at approximately 11:01 a.m. on July 7, 2015, over the west branch of the Cooper River and near Old Highway 52 in Moncks Corner.
Michael Johnson, 68, and his son, Joseph Johnson, 30, were killed in the crash, according to Berkeley County Coroner Bill Salisbury. The two were father and son and Joseph was piloting the plane at the time of the crash, he said.
The F-16 continued to fly approximately two-and-a-half minutes after the crash during which its pilot, later identified as Maj. Aaron Johnson, was able to eject from his aircraft before the jet crashed, investigators said. The F-16 crashed into land approximately six nautical miles from the collision location, Diaz said.
At the time of the collision, the F-16 was en route to Charleston, where the pilot planned to perform a practice instrument approach.
The Cessna had departed from a non-tower-controlled airport in Moncks Corner four minutes before the accident on visual flight rules, Diaz said.
"The Cessna pilot was not in contact with air traffic control, nor was he required to be," Diaz said.
In previous reports, investigators said the two pilots most likely did not see each other until it was too late.