Amateur-built TWINSTAR TSP 1 N217JM accident near Hector, Minnesota

Probable cause

The pilot’s operation of the airplane outside of the center of gravity limits, which resulted in an adverse aerodynamic condition and subsequent hard landing. Contributing to the accident was the pilot’s incorrect calculations of the airplane’s CG limits.

NTSB analysis

The pilot was conducting a maintenance test flight and had been in the air about 15 minutes when airplane began to oscillate up and down. He had difficulty maintaining control, so he made a rapid descent toward the airport. During the rapid descent the oscillation and shaking reduced, but as he decreased the descent rate, the airplane oscillated violently. The airplane landed hard on the grass runway. The airplane sustained substantial damage to the wings, fuselage, and empennage.

After the accident, the pilot stated that he thought the tail boom had fractured inflight. A postaccident examination of the airframe revealed that the tubular empennage structure had fractured and was separated. An examination of the fractured section determined that the fractures were due to overstress as a result of the hard landing.

A review of the pilot’s calculated weight and balance information revealed that the pilot had incorrectly calculated the weight and balance and that the airplane’s center of gravity (CG) was aft of manufacturer limitations. After the accident, the pilot reported that he was adjusting his CG while airborne, and he placed a bag of bird seed in the passenger seat to assist during the inflight test. The bag was aft of the datum per the manufacturers specifications and would have moved the CG further aft.

This likely resulted in an adverse aerodynamic condition and an uncontrollable pitch oscillation.

Text from the NTSB. Open the NTSB docket for CEN19LA176 for the full record.