Lake LA4 N93AB accident near Rawlins, Wyoming

Probable cause

The pilot’s decision to depart despite a known mechanical trim malfunction and with a high-density altitude and the airplane at or above its maximum gross weight.

NTSB analysis

The pilot reported that, during a flight on the day before the accident, a trim anomaly caused the airplane to enter a nose-down pitch attitude and descend. The pilot had to manually reset the trim position about every 8 seconds to maintain level flight and prevent the airplane from prematurely descending. After landing, the pilot had a discussion with a mechanic to troubleshoot the mechanical issue, and no anomalies were identified.

Before the accident takeoff, one of the two passengers had the airplane fueled with 20 gallons of fuel, for total fuel of 30 gallons. The pilot thought that the airplane was fueled with 12 gallons, for a total fuel of 22 gallons. This miscommunication between the pilot and passenger resulted in the airplane being fueled more than the pilot expected, which likely resulted in the airplane being near or slightly over the maximum allowable gross weight at the time of takeoff.

The pilot performed a short-field takeoff during the accident flight. The pilot reported that, when the airplane reached an altitude of about 500 ft above ground level, he experienced an “abnormal heaviness” in the control wheel that was consistent with the pitch trim anomaly during the flight on the day before. The airplane would not climb any further, so the pilot made a left turn back to the runway, during which the airplane descended and subsequently collided with terrain.

Postaccident examination of the trim actuator revealed an anomaly of the O-rings, which prevented the hydraulic pressure from maintaining the desired trim position. Consequently, the trim surface could move without command and could thus not hold the required takeoff position.

In addition, the density altitude at the time of the accident takeoff was about 8,600 ft mean sea level, and the airplane was close to its maximum allowable gross weight. The effects of highdensity altitude conditions and a heavy airplane weight on takeoff performance include increased takeoff roll distance and reduced rate of climb. It is likely that while the pilot was dealing with a high-density altitude takeoff which may have affected the performance of the engine, he did not adequately monitor the trim that was not holding its required takeoff position and that he knew was malfunctioning before departure.

Text from the NTSB. Open the NTSB docket for WPR20LA301 for the full record.