Accident Overview
A test flight conducted at night ended with the aircraft impacting the ground after the pilot failed to maintain control and made an improper evaluation of the weather. The flight was intended to perform a Negative Torque Sensing (NTS) check on the left engine, which had been overhauled. The procedures required shutting down the left engine during the test flight.
Flight Details
The pilot was briefed on existing icing conditions, including moderate rime icing mixed below 15,000 feet in clouds and precipitation, with cloud bases between 2,500 to 2,900 feet above ground level (agl). After departure, the pilot reported to air traffic control (ATC) that they were clear and on top of the clouds at about 6,500 feet mean sea level (msl).
ATC assigned N5LN a 180-degree heading at an assigned altitude of 8,000 feet. Without notifying ATC, the aircraft turned to a southeast heading, descended from 7,700 feet to about 5,500 feet, and decelerated from about 182 knots to about 138 knots. ATC then assigned a block altitude of 6,000 to 8,000 feet and a VFR-On-Top clearance. ATC instructed N5LN to turn right to stay in the assigned airspace. The aircraft turned right but continued descending from about 5,500 feet to the last radar indication of 4,500 feet.
Wreckage Examination
The airplane impacted the ground in a steep attitude. Inspection of the wreckage revealed the landing gear was down, with full right rudder trim and about six degrees of nose-up trim. Examination of both engines indicated they were rotating and operating at the time of impact. The airframe and propellers showed no pre-existing anomalies that would have precluded normal operation.
Probable Cause
The pilot failed to maintain control of the aircraft and made an improper evaluation of the weather. Additional factors were flying a test flight at night with the icing conditions in the clouds.