The National Transportation Safety Board (NTSB) is urging aviation regulators to require De Havilland Canada DHC-3 Otters be equipped with secondary devices to prevent horizontal stabiliser actuators from separating.
“Secondary retention” devices are needed to prevent the type of failure that caused an Otter in September 2022 to plummet into Mutiny Bay in Washington state, killing all 10 people aboard, according to the NTSB’s final accident report, released on 5 October.
The report confirms some of the NTSB’s previously disclosed conclusions but warns that an unapproved “moisture seal” might have contributed to failure of the Otter’s horizontal stabiliser.
“The probable cause of this accident was the in-flight un-threading of the clamp nut from the horizontal stabiliser trim actuator barrel due to a missing lock ring, which resulted in the horizontal stabiliser moving to an extreme trailing-edge-down position, rendering the airplane’s pitch uncontrollable,” the report says.
It urges the Federal Aviation Administration and Transport Canada to take action. Regulators should require operators to “install a secondary retention feature on the horizontal stabiliser actuator clamp nut” – a backup in case the lock ring fails or goes missing.
The NTSB also recommends that Canadian manufacturer De Havilland’s subsidiary Viking Air, which owns the Otter’s type certificate, issue criteria to help mechanics determine if lock rings need to be replaced. The report also urges operators to remove moisture seals.
“The NTSB remains concerned that this hazard can be catastrophic and represents a single point of failure,” it says. “The NTSB concludes that a secondary retention feature for the DHC-3 horizontal actuator assembly clamp nut is necessary to mitigate the potential single-point-of-failure hazard.”
Neither the FAA, Transport Canada nor De Havilland immediately responded to requests for comment.
The 4 September 2022 accident involved an Otter floatplane (registration N725TH) operated by West Isle Air, doing business as Friday Harbor Seaplanes. The aircraft, carrying nine passengers and one pilot, took off from Friday Harbor in Washington bound for Renton, Washington.
Flying at about 1,000ft, the Otter suddenly pitched up and then “abruptly pitched down”, reaching a descent rate of 9,500ft per minute and “spinning, rotating and spiralling” before hitting the water, killing all aboard.
In an October 2022 investigation update, the NTSB said it had determined from the aircraft’s wreckage that the clamp nut in the horizontal stabiliser actuator had backed out, causing the actuator to separate into two pieces and leaving it ineffective. In that update, the NTSB said it had issued “urgent’ recommendations calling on the FAA and Transport Canada to require Otter operators to inspect actuator lock rings.
In response, the FAA mandated such inspections and Transport Canada recommended them.
The NTSB’s final report notes that in April 2022 a mechanic had installed a moisture seal in the crash Otter’s actuator clamp nut, a measure intended to “keep water out and prevent the bearings from seizing”.
”The unintended consequence was increased 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 report says. Still, the NTSB could not determine if the moisture seal contributed to the accident.
It notes that regulations in 1952, when the Otter was certificated, required only single locking mechanisms for fasteners such as actuator clamp nuts. Rule changes in 1996 require two locking devices, but only for new aircraft.