Glasflügel 304S N304AB accident near Briggs, Texas

Probable cause

The overstress fracture of the elevator control tube due to reduced wall thickness from water intrusion and subsequent corrosion. Contributing to the accident was the lack of a drain hole at the bottom of the elevator control tube, which allowed the tube to collect water, and the mechanic’s inadequate inspection of the elevator control system during the recent condition inspection.

NTSB analysis

The pilot was departing on a local flight in the experimental glider when the glider unexpectedly separated from the towline shortly after liftoff. The glider then entered a left turn and landed in a wings-level attitude. The tailboom was substantially damaged during landing. The tow airplane and the towline were not damaged during the event.

Postaccident examination determined that the elevator control tube installed in the vertical stabilizer was corroded along the entire length of its inner surface, reducing its wall thickness. Water likely entered the control rod, either through a witness hole near the upper end of the control tube or as moisture carried in by humid air. There was no drain hole at the bottom end of the control tube and, as a result, there was no way for liquid water to drain out of the control tube.

The wall thickness eventually thinned sufficiently to cause the tube to burst in the longitudinal direction near its upper end. After the control tube burst, the resulting hole on the side of the tube allowed for the easy ingress of water that made its way past the boot seal. The corrosion product and standing water at the base of the tube eventually reduced the tube wall thickness to a point where it could no longer withstand the typical operational loads and subsequently fractured in overstress near the clevis fitting during the accident flight. The overstress failure of the control tube prevented the pilot’s control of the elevator during the accident flight.

The last condition inspection of the glider was completed 24 days before the accident. The corresponding logbook entry noted that the flight controls were inspected, and that the glider was in an airworthy condition. The longitudinal fracture near the upper end of the elevator control tube would have been readily visible with the rubber boot removed and, as such, it is likely the mechanic did not remove the rubber boot to adequately inspect the elevator control tube during the last condition inspection.

Text from the NTSB. Open the NTSB docket for CEN21LA346 for the full record.