What happened
A night test flight was being conducted to perform an in-flight Negative Torque Sensing (NTS) check on the left engine, which had recently undergone an overhaul. The required procedure for this check necessitated shutting down the left engine during the flight. At the time of the flight, pilots were briefed on weather conditions involving clouds and precipitation with moderate rime icing below 15,000 feet; cloud bases were reported between 2,500 and 2,900 feet AGL.
After departing, the pilot notified Air Traffic Control (ATC) that the aircraft was clear of the clouds and established on top of the layer at approximately 6,500 feet MSL. N5LN was assigned a 180-degree heading with an assigned altitude of 8,000 feet. However, without notifying ATC, the aircraft turned to a southeast heading and began a descent from 7,700 feet down to approximately 5,500 feet, while decelerating from 182 knots to 138 knots.
ATC subsequently assigned N5LN a block altitude of 6,000 to 8,000 feet and a VFR-On-Top clearance, instructing the crew to turn right to remain within the assigned airspace. Although the aircraft turned right as instructed, it continued to descend from 5,500 feet until the last radar indication showed the aircraft at 4,500 feet. The aircraft then impacted the ground in a steep attitude.
The investigation
An inspection of the wreckage revealed that the landing gear was in the down position. The aircraft was found with full right rudder trim and approximately six degrees of nose-up trim. Examination of the engines confirmed that both engines were rotating and operating at the time of impact. Furthermore, an examination of the airframe and propellers showed no pre-existing anomalies that would have prevented normal operation.