A lack of flight test data was a contributing factor in an April 2015 accident that led to an AC-130J nosediving 5,000 feet in an inverted position before the pilots could recover, according to an Air Force Accident Investigation Board.

The flight datacould have helped the pilots better understand the limits of the aircraft’s handling while performing tight maneuvers. But defense contractor Lockheed Martin declined to provide the proprietary information without a contract, and the Air Force decided not to purchase it, according to the report.

The AC-130J was being tested by the 413th Flight Test Squadron at Eglin Air Force Base, Florida. The crew was performing various maneuvers to collect data on the flight envelope of the aircraft, the report said.

The pilot performed a “sideslip” maneuver by lowering one wing of the aircraft and applying opposite rudder. The action is usually used to aid aircraft landing in a strong crosswind.

The cause of the accident, the investigation board said, was that the pilot pushed the rudder too far while performing the sideslip, and then didn’t apply enough pressure to the rudder to stop the plane from rotating once it started to go out of control.

However, the report noted, performing the sideslip and pushing the aircraft to its limits was the point of the test flight, and the pilot had been given special permission to carry out the maneuver.

With the flight data, however, the pilot might have had a better idea of when  control of the plane would be lost.

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Image courtesy of US Air Force