Kaman Aerospace Corp K-1200 N202WM accident near Sweet Home, Oregon

Probable cause

Improper installation of the hardware retaining the adjustment screw in the N2 topping governor lever and roller assembly, which resulted in a partial loss of engine power.

NTSB analysis

The accident occurred as the pilot was conducting a logging operation in a helicopter that had an overhauled engine recently installed. The helicopter sustained a loss of engine power when the pilot added power while laying a log into the log landing zone. Unable to maintain altitude, the pilot elected to make a forced landing onto open terrain. As the helicopter descended toward the open terrain, its landing gear impacted cut logs before the helicopter rolled down a hill and came to rest inverted.

On the day before the accident, the pilot experienced what seemed to be a slower than normal governor response (governor lag) to an increase in power requirement. The company mechanic made engine adjustments and the pilot continued with the logging operations. On a later flight, the pilot heard “rushing air” while landing a log. He immediately returned to the service landing area and again consulted the mechanic. They confirmed the bleed band function, and the mechanic made an adjustment. They performed a ground run to confirm the adjustment did what they wanted it to and ended operations for the night.

The engine was removed from the airframe and placed on a test stand following the accident. The power turbine governor (PTG) exhibited a random intermittency during functional testing, and a subsequent partial teardown revealed the locking feature of the adjusting screw of the N2 topping governor and roller assembly was not correctly clamped. A visual examination of the N2 topping governor lever and roller assembly revealed that the adjustment screw was not being retained by the locking tab and could easily be rotated. It was extended almost out of its threaded seat. The tab locking screw lockwire was undamaged and in its original location. Additionally, it was observed that the lock screw was free to turn within the lockwire hysteresis band, and no damage was noted on the threads of the screw and topping governor lever.

The PTG was inspected and installed on the accident engine in April of 2020; the PTG time since overhaul was about 1,875 hours.

The loose hardware retaining the adjustment screw in the N2 topping governor lever and roller assembly most likely caused an uncommanded rotation of the rockshaft and the subsequent opening of the bleed band. With the bleed band open air to the gas producer (GP) turbine was lost, resulting in a sudden loss of power. The loss of cooling air to the GP turbine also resulted in the thermal distress of the 2nd stage GP turbine blades.

Text from the NTSB. Open the NTSB docket for WPR23LA097 for the full record.