Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
2023-05
March 28, 2023
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) alerts owners,
operators, maintenance technicians, and inspectors of Viking Air
Limited (Type Certificate Previously Held by Bombardier Inc. and de
Havilland Inc.) Model DHC-3 (Otter) airplanes of information gathered
as a result of a fatal accident on September 4, 2022. The cause of the
accident has not yet been determined by the National Transportation
Safety Board (NTSB). However, some information is available from the
investigation that may benefit the safety of the 2023 Otter operational
season.
Additionally, the FAA is using this communication to gather and evaluate additional information from the fleet.
At this time, the Federal Aviation Administration (FAA) has not yet
determined if this airworthiness concern is an unsafe condition that
would warrant airworthiness directive (AD) action under Title 14 of the
Code of Federal Regulations (14 CFR) part 39. The actions specified by
this SAIB are not mandatory, however, the information requested by this
SAIB may help the FAA develop future rulemaking action should the issue
be identified as an unsafe condition.
Background
On September 4, 2022, a fatal accident of a Viking Model DHC-3 airplane
in Mutiny Bay near Freeland, WA occurred. NTSB investigation showed
that the lock ring of the horizontal stabilizer actuator was found
missing. The investigation revealed that the clamp nut that attaches
the top eye end and bearing assembly of the horizontal stabilizer
actuator to the actuator barrel had unscrewed from the barrel. The
investigation also found that the circular wire lock ring, which is
designed to prevent the clamp nut from unscrewing, was not present.
This condition, if not detected and corrected, could result in a
reduction or loss of pitch control during flight with consequent loss
of control of the airplane.
In response, the FAA issued AD 2022-23-08, which requires a visual
inspection of the stabilizer actuator to confirm that the stabilizer
actuator lock ring is present, correctly seated in the groove in the
upper housing, and engaged in the clamp nut. The AD also requires
application of a torque seal and reporting the inspection results to
the FAA. The AD is interim action and does not require repetitive
inspections.
The NTSB laboratory investigation subsequently identified the potential
for the circular wire lock ring tang, part number (P/N) AN996-28, to be
forced out of the clamp nut engagement hole with light to medium hand
rotational force, permitting the clamp nut to unscrew. To date, the
lock ring ha s not been recovered and it is unknown if this is relevant
to the accident sequence of events.
The NTSB laboratory investigation examination of the clamp nut
identified that five holes had been drilled into the clamp nut threads.
Multiple holes, especially adjacent to each other, may affect the lock
ring tang engagement.
The NTSB laboratory investigation identified an internal seal installed
on the horizontal stabilizer actuator clamp nut for the accident
airplane and an exemplar horizontal stabilizer actuator that was
examined. The internal seals are unauthorized and not part of the type
design, nor any known Supplemental Type Certificate (STC). It is
believed the seal was installed to prevent moisture intrusion. The
investigation has not yet determined if this internal seal was a
contributing factor to the accident.
The FAA issued STC No. SA02761SE to Kenmore Air Harbor, LLC on January
31, 2023. This STC provides a secondary locking method for the
stabilizer actuator clamp nut, helping to prevent the clamp nut from
unscrewing if the lock ring disengages or fails. STC No. SA0276SE was
not installed on the accident aircraft.
1. Instructions for Continued Airworthiness
The FAA recommends maintenance be performed following the
manufacturer’s instructions for continued airworthiness using approved
parts and assembled in accordance with the Maintenance Manual PSM 1-3-2
and the Supplementary Inspection and Corrosion Control Manual PSM
1-3-5. Operators of float planes (i.e., seaplanes), particularly on
salt water, may need to perform maintenance more frequently than
recommended in the manual to avoid internal corrosion.
2. Lock Ring and Clamp Nut
a. After lock ring installation, ensure the lock ring is engaged by
trying to unscrew the clamp nut using light to medium hand force. If
the lock ring disengages, contact Viking Air Limited for direction and
report the occurrence to the FAA Aerospace Engineer listed on this SAIB.
b. If the maintainer finds that the holes in the clamp nut and the
barrel do not align or that multiple holes exist they should contact
Viking Air Limited for direction.
c. If disassembled for any reason, ensure the lock ring tang edges are
not rounded over. Verify the hole in the clamp nut is not damaged and
the hole edge is not significantly chamfered due to wear.
3. Repetitive Inspections
The FAA recommends operators perform repetitive visual inspection at
intervals not exceeding 110 hours: Visually inspect both the horizontal
stabilizer actuator upper eye and lower fork end attachments for
security and the surrounding structure for damage and wear. Refer to
the applicable service information for access. Verify that the lock
ring is present, correctly seated in the groove in the upper housing,
and engaged in the clamp nut and that the torque seal is intact. If
movement is observed, report it to Viking Air Limited and the FAA
Aerospace Engineer identified on this SAIB.
4. Water Intrusion
The FAA is aware that the horizontal stabilizer actuator, as well as
the tail of the airplane, is subject to water intrusion, particularly
in float plane (i.e., seaplane) operations. The FAA seeks industry
input on prevention techniques for water intrusion that could address
this concern without introducing other issues. While the installation
of the unauthorized seal in the accident airplane is of concern because
of the effect it could have on the locking features, the motivation
behind it remains valid and a solution to the water ingression issues
is desirable. Please send water intrusion input to the FAA Aerospace
Engineer identified on this SAIB.
5. Secondary Retention Methods
The FAA recommends installation of an approved secondary retention
feature to retain the actuator lock ring or clamp nut to enhance
safety. Additionally, feedback from owners, operators, and maintenance
personnel on any additional known methods for secondary retention of
the clamp nut may provide additional information to address this safety
concern.
6. Other Feedback
The FAA welcomes any additional feedback on the topics discussed in this SAIB.
Paperwork Reduction Act Burden Statement
A federal agency may not conduct or sponsor, and a person is not
required to respond to, nor shall a person be subject to a penalty for
failure to comply with a collection of information subject to the
requirements of the Paperwork Reduction Act unless that collection of
information displays a currently valid OMB Control Number. The OMB
Control Number for this information collection is 2120-0731. Public
reporting for this collection of information is estimated to be
approximately 1 hour per response, including the time for reviewing
instructions, searching existing data sources, gathering and
maintaining the data needed, completing and reviewing the collection of
information. All responses to this collection of information are
voluntary; the nature and extent of confidentiality to be provided, if
any. Send comments regarding this burden estimate or any other aspect
of this collection of information, including suggestions for reducing
this burden to: Information Collection Clearance Officer, Federal
Aviation Administration, 10101 Hillwood Parkway, Fort Worth, TX
76177-1524.
For Further Information Contact
Gabriel Kim, Aerospace Engineer, 1600 Stewart Avenue, Suite 410,
Westbury, NY 11590; phone: (516) 228-7300; fax: (516) 794-5531; email:
gabriel.d.kim@faa.gov.
For Related Service Information Contact
Viking Air Ltd, 1959 de Havilland Way, Sidney, BC, V8L 5V5, Canada;
phone: (866) 663-8444; email: dh_technical.support@vikingair.com.
