What happened
During night operations, a DC-10 was climbing through 3,800 feet on a course converging with a DC-9. The DC-9 was descending out of 5,000 feet, and the two aircraft were approximately 7 miles apart. In response to a Traffic Collision Avoidance System (TCAS) alert, the captain of the DC-10 executed an aggressive pitch change. As the aircraft decelerated, the crew felt a slight buffet.
Air Traffic Control (ATC) instructed the DC-9 crew to maintain visual separation from the DC-10, an instruction which the DC-9 crew accepted. During the event, the controller had to repeat instructions to the DC-10 crew and other aircraft in the area. One crew reported to ATC that their radio transmissions were weak.
The investigation
The maneuver performed by the DC-10 resulted in the wrinkling of the elevator skins. An examination of Douglas Flight Operation Letter AOL, C1-E60-HHK-L033, dated January 21, 1988, provided technical context regarding the damage. The documentation explained that aircraft component damage could occur when speed was reduced to buffet speeds as indicated on the Cruise Buffet Onset Boundary Chart in the DC-10 Aircraft Flight Manual. The letter noted that such buffeting could result in wrinkled outboard elevator skins and emphasized that reducing speed to near the clean configuration stalling speed significantly increased the risk of elevator damage.