History of Flight
On August 25, 2020, about 0700 mountain daylight time, an Aerofab Lake LA-4-200, registration N93AB, was substantially damaged during an accident near Rawlins Municipal Airport/Harvey Field (RWL) in Rawlins, Wyoming. One passenger sustained fatal injuries; the pilot and another passenger sustained serious injuries. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
The pilot reported that, during a cross-country flight from Searle Field Airport (OGA), Ogallala, Nebraska, to RWL the day before the accident, the hydraulic pitch trim system appeared to malfunction while the airplane was flying straight and level. The pilot recorded a cell phone video during that flight showing the nose pitching downward during straight-and-level flight. The pilot had to reset the trim position about every 8 seconds for the last half of the 166-nautical mile flight. Passengers monitored the trim position indicator and took turns applying aft pressure to the trim handle when the nose pitched down.
Upon arrival at RWL, the pilot continued troubleshooting the trim anomaly with a mechanic via telephone. The pilot searched for hydraulic leaks and saw no visual mechanical anomalies. During troubleshooting, the pilot asked the mechanic via text about a good mixture setting at 8,000 ft. The mechanic advised leaning the mixture and monitoring exhaust gas temperature, emphasizing that at that altitude with full rich mixture, the engine would likely be flooded.
The pilot reported that the airplane landed with 10 gallons of fuel and that one passenger refueled. The pilot thought 12 gallons were added for a total of 22, but the airplane received 20 gallons, totaling 30.
On the accident morning, the pilot completed the run-up performance checks and confirmed a standard configuration for takeoff. According to the Lake L-4-200 owner's manual, for standard takeoff, it is wise to check trim tabs visually for about 30° up trim, and to check mixture rich and propeller full RPM. The pilot completed the checklist and taxied onto runway 29.
The pilot held brakes, applied full power, and released brakes. He kept his right hand on the throttle control above his head and left hand on the control wheel. Once airborne, he immediately retracted the landing gear as the airplane flew over the last one-third of the runway. After establishing a positive rate of climb, he continued the takeoff. During initial climb at about 500 ft above ground level, he experienced an “abnormal heaviness” in the control wheel, consistent with the previous trim anomaly.
The pilot reported the airplane would not climb. He turned left toward the airport, and the airplane began to descend. During the descending left turn, the airplane impacted a 30-ft wide, 10-ft high mound of construction material in the northeast quadrant of the airport property. The initial impact point was about 355 ft southeast of runway 4 threshold.
Aircraft Information
According to the airplane maintenance logbook, the airframe total time was 2,735 hours when the most recent annual inspection was completed about 1½ years before the accident. The engine logbook indicated the engine was overhauled with 859 total hours on April 30, 2014. The last 100-hour inspection (during the annual) showed a tachometer time of 1,144.7 hours. Maintenance on August 6, 2020 included trim tab adjustment at a tachometer time of 1,162 hours and airframe total time of 2,752.8 hours. The pilot reported no known maintenance issues until the day before the accident.
The manufacturer stated the airplane's maximum gross weight was 2,600 pounds. The pilot's weight and balance calculations for the accident flight indicated a takeoff gross weight of 2,545 pounds. Adding 48 pounds of fuel added unknown to the pilot, the calculated takeoff gross weight was about 2,593 pounds. The IIC calculated the gross weight of the amphibious airplane using pilot details, yielding 2,631 pounds.
Meteorological Information
The pressure altitude was 6,512 ft, and the density altitude was 8,588 ft.
Wreckage and Impact Information
The airplane came to rest inverted with the nose on a heading of 010°. The left and right trim surfaces remained attached to the horizontal stabilizer with minor impact damage. The aft fuselage longerons sustained structural deformation, and the cabin showed uninhabitable space encroachment. The nose remained intact; the lower fuselage sustained impact damage to longerons, formers, and bulkheads. Main landing gear remained attached and stowed. Wings, ailerons, and flaps remained attached. Flight control continuity was established. The pusher-type two-blade propeller remained attached to the engine, which separated from the upper fuselage and came to rest underneath the right wing.
A postaccident examination of trim and hydraulic systems found no anomalies. The trim valve and actuator were removed for further examination. The trim valve showed no malfunction. The trim actuator's O-ring was noticeably harder than a new O-ring. A mechanic reinstalled the accident O-ring onto the piston and inserted it by hand; the fit seemed looser than normal with little resistance. With the cylinder outlet capped, the piston again showed no compression resistance. A new O-ring installed on the accident piston exhibited compressive resistance and could not be inserted by hand. The mechanic reported that the hardened O-ring could apparently leak during flight. Hydraulic simulation, which did not include restrictors, showed retention of pressure and position for days; according to the mechanic, a quick pressure application can force an O-ring to seal, but without a complete seal, flight loads would cause bleed toward neutral.
Additional Information
The Lake L-4-200 owner's manual sea-level and altitude engine performance chart indicates an IO-360-A1B engine produces 200 horsepower at sea level. Under accident takeoff environmental conditions, only 158 horsepower would have been available. According to engine manufacturer Service Instruction No. 1094D, for density altitude of 5,000 ft and above or high ambient temperatures, roughness or power reduction may occur with a full rich mixture.