Amateur-built F 1 N158CF accident near Reno, Nevada

Probable cause

The pilot’s operation of the airplane above the established never-exceed speed and the incorrect installation of the trim tab control horn, which resulted in the flutter of the left elevator trim tab.

NTSB analysis

The accident airplane was participating in an air race, and, when the airplane was at 180 knots indicated airspeed (KIAS), the pace airplane released the flight of airplanes to enter the race course. The pilot reported that, shortly after, a "vibration" began at an unknown airspeed and that it then increased to a heavy flutter event. The pilot pitched the airplane upward and reduced the airspeed. As the airspeed decreased below 90 KIAS, the vibration stopped, and the pilot landed uneventfully.

Postaccident examination of the control horn assembly revealed fractures at the forward and aft trim tab control horn brackets and the rod end clevis. According to the airplane manufacturer's assembly manual, the aft trim tab control horn was supposed to be attached to the lower skin with four rivets. However, examination revealed that only three of the four rivets were installed; no evidence of a through hole for the fourth rivet was found. The lack of the fourth rivet allowed the aft bracket to flex, and the relative movement between the bracket and the trim tab skin during flight resulted in fretting damage. Examination of the aft bracket's fracture surface revealed features consistent with an overstress separation.

The published never-exceed speed for the airplane was 240 knots true airspeed (KTAS). The airspeed was calculated to be 266 KTAS at the time of the event. The airplane's excessive speed coupled with the incorrect installation of the trim tab control horn likely initiated the flutter in the left elevator trim tab during the start of the air race. The flutter subsequently produced dynamic loads in the trim tab control system that exceeded the material strength of the system components and led to the fracture of the forward and aft trim tab control horn brackets and the rod end clevis before the airplane was slowed sufficiently to stop the flutter.

Text from the NTSB. Open the NTSB docket for WPR12FA421 for the full record.