On September 4, 2022, about 1509 Pacific daylight time, a float-equipped airplane operated by West Isle Air as a commercial passenger air service flight abruptly pitched down and impacted the water in Mutiny Bay near Freeland, Washington, while en route to its destination. The pilot and nine passengers were fatally injured, and the airplane was destroyed.
Examination of the airplane revealed that the clamp nut that attached the top eye end and bearing assembly of the horizontal stabilizer trim actuator to the actuator barrel had unscrewed from the barrel. The examination also found that the circular wire lock ring, which was designed to prevent the clamp nut from unscrewing, was not present.
We found that if a lock ring is not present to secure the actuator barrel and the clamp nut together, they can become separated, and the actuator would not be able to control the position of the horizontal stabilizer, resulting in a loss of airplane pitch control. Additionally, a secondary locking feature is not required. Requiring such a feature could provide redundancy if a lock ring is not installed, fails, or separates from the clamp nut in flight.
We also found that, to prevent environmental elements from entering the actuator, maintenance personnel installed a moisture seal between the clamp nut and the eye bolt. The moisture seal, which was not approved by the airplane manufacturer in any documentation, has the potential to create interference in the clamp up of the top eye end and bearing assembly. In addition, the moisture seal increased the rotational friction between the clamp nut and eye bolt, which has the potential to increase the rate of separation between the clamp nut and barrel in the absence of the lock ring.
The investigation identified maintenance documents and guidance pertaining to the horizontal stabilizer trim actuator assembly that, although found not to be;causal in this accident, could lead to errors concerning the lock ring installation. For example, maintenance documents do not currently define how many holes are allowed to be drilled into a clamp nut, specify a torque requirement for the installation of the clamp nut, instruct personnel to inspect the clamp nut lock ring hole(s) for damage before installation, or ensure that the lock ring is installed properly.
We determined that the probable cause of this accident was the in-flight unthreading of the clamp nut from the horizontal stabilizer trim actuator barrel due to a missing lock ring, which resulted in the horizontal stabilizer moving to an extreme trailing-edge-down position rendering the airplane’s pitch uncontrollable.
 An eye bolt is located within the eye end and bearing assembly. One end of the eye bolt is
threaded, and the opposite end (the head) is formed into a ring or eye for lifting, pulling, or securing.
 The term “clamp up” refers to how the adjacent parts of the assembly match up to each other
when a compressive load (in this case caused by the tightening of the clamp nut) forces them together.
As a result of the investigation, we issued recommendations Federal Aviation Administration, Transport Canada and Viking Air.