DEHAVILLAND DHC-3West Isle Air
Freeland, WA, USA · 2022-09-04·N725TH
What happened
NTSB summary, verbatimHISTORY OF FLIGHTOn September 4, 2022, about 1509 Pacific daylight time, a float-equipped de Havilland DHC-3 (Otter), N725TH, was destroyed when it impacted the water in Mutiny Bay, near Freeland, Washington, and sank. The pilot and nine passengers were fatally injured. The airplane was owned by Northwest Seaplanes, Inc., and operated as a Title 14 Code of Federal Regulations (CFR) Part 135 scheduled passenger flight by West Isle Air dba Friday Harbor Seaplanes. The flight originated at Friday Harbor Seaplane Base (W33), Friday Harbor, Washington, with an intended destination of Will Rogers Wiley Post Memorial Seaplane Base (W36), Renton, Washington. Visual meteorological conditions prevailed at the time of the accident.
Photographs
7the aircraft, and what the investigators foundPhotographs are NTSB docket attachments — works of the US government, in the public domain. Captions are the Board’s own.
Probable cause
the Board's determinationThe 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.
Occurrence sequence
1 stepsNTSB coding · CICTT taxonomy- 1 · EnrouteLoss of control in flightdefining event
Findings
1 causal · 5 contributing- CAUSEAircraft › Aircraft systems › Flight control system › Stabilizer control system — Malfunction
- FACTORPersonnel issues › Experience/knowledge › Knowledge › Knowledge of equipment — Maintenance personnel
- FACTOROrganizational issues › Development › Manufacture/production › Document/info production — Manufacturer
- FACTORPersonnel issues › Task performance › Maintenance › Installation — Maintenance personnel
- FACTOROrganizational issues › Management › Policy/procedure › Adequacy of policy/proc — Maintenance provider
- FACTOROrganizational issues › Support/oversight/monitoring › Documentation/record keeping › Maintenance records — Maintenance provider
Safety recommendations
10 issuedwhat changed because of this accidentTO THE FEDERAL AVIATION ADMINISTRATION: Require all operators of De Havilland Canada DHC-3 airplanes to conduct an immediate one-time inspection of the horizontal stabilizer actuator lock ring in accordance with the instructions in Viking Air Limited Service Letter DHC3-SL-27-001 and report their findings to the Federal Aviation Administration. (Urgent)
TO TRANSPORT CANADA: Require all operators of De Havilland Canada DHC-3 airplanes to conduct an immediate one-time inspection of the horizontal stabilizer actuator lock ring in accordance with the instructions in Viking Air Limited Service Letter DHC3-SL-27-001 and report their findings to Transport Canada. (Urgent)
TO THE FEDERAL AVIATION ADMINISTRATION: Require operators of DHC-3 airplanes to install a secondary retention feature on the horizontal stabilizer actuator clamp nut to ensure it remains secured to the barrel in the event the lock ring is not installed or otherwise fails to retain the clamp nut.
TO THE FEDERAL AVIATION ADMINISTRATION: Require operators of DHC-3 airplanes to visually inspect both the horizontal stabilizer actuator lock ring and the associated torque seal at an interval that ensures safe flight or until a secondary retention feature is installed.
TO TRANSPORT CANADA: Require operators of DHC-3 airplanes to install a secondary retention feature on the horizontal stabilizer actuator clamp nut to ensure it remains secured to the barrel in the event the lock ring is not installed or otherwise fails to retain the clamp nut.
TO TRANSPORT CANADA: Require operators of DHC-3 airplanes to visually inspect the horizontal stabilizer actuator lock ring at an interval that ensures safe flight or until a secondary retention feature is installed.
TO TRANSPORT CANADA: Require operators of DHC 3 airplanes to install a torque seal on the horizontal stabilizer actuator clamp nut and lock ring and inspect the torque seal at an interval that ensures safe flight until a secondary retention feature is installed.
TO VIKING AIR: Develop inspection criteria for maintenance personnel to determine whether the horizontal stabilizer trim actuator lock ring is airworthy or needs to be replaced; the inspections should be performed at an interval that ensures safe flight.
TO VIKING AIR: Notify DHC-3 operators of the circumstances of this accident and instruct them to remove moisture seals that are installed within horizontal stabilizer actuators and are not approved by Viking Air or a regulatory agency.
TO VIKING AIR: Revise the DHC-3 aircraft maintenance manual to include inspection and overhaul procedures for the horizontal stabilizer actuator that specify how many holes can be drilled into the clamp nut and how to determine if the holes are airworthy, a proper torque value for installing the clamp nut, and how to install the lock ring and verify that it is properly seated and functions as intended.