King Air Captain reported a yaw on takeoff along with a loss of power on the associated engine. The flight crew continued the climb and performed the Company SOP procedures. The flight crew discovered the Left Engine Friction Lock had loosened due to vibration and would not hold the throttle in place. The flight crew elected to perform an air turn back and precautionary landing at departure airport.

Narrative

During Vy climb; just after takeoff; the crew experienced what felt like a left engine roll back accompanied by the usual yaw and wing drop. Takeoff from ZZZ was standard; power on both engines advanced normally during the takeoff roll. Once we rotated and started the Vy climb; the 'gear up' call was made; the First Officer selected gear up; and I took my hand off the power levers to switch the taxi light off. (The light is in the gear well.) At that moment; the plane yawed to the left and the left wing dropped. The plane's movement signified a left engine failure/ a significant reduction in power. The crew identified there was a problem; reached safe altitude; and continued the climb to join the left downwind for Runway XX. The First Officer contacted Tower to advise that we were having an engine malfunction and would be joining the left downwind for Runway XX. As a crew; we worked through our company's standard operating procedure to identify the engine problem. In troubleshooting; the engine gauges all looked normal with exception of the left throttle; I moved the power lever forward; and was getting a consistent power increase. We noted that the power lever would not remain in position when advanced. The friction lock had loosened due to vibrations during takeoff. When the friction lock loosened; the left power lever moved to idle; feeling like an engine failure. On downwind; the throttle lock was tightened; and engine gauges monitored. The engine maintained power; and the friction lock remained tight during the rest of the landing sequence. A normal landing was made at ZZZ.