Safety Recommendations

The NTSB issues safety recommendations to address specific safety concerns uncovered during investigations and to specify actions to help prevent similar accidents from occurring in the future. Safety recommendations are our most important product because they alert government, industry, and the public to the critical changes that are needed to prevent transportation accidents and crashes, reduce injuries, and save lives. We:

issue recommendations to the organizations best able to take corrective action, such as the US DOT and its modal administrations, the Coast Guard, other federal and state agencies, manufacturers, operators, labor unions, and industry and trade organizations.
issue safety recommendations at any point during the investigation of transportation accidents and in connection with safety studies.
monitor the progress of action to implement each recommendation until it is closed, which usually takes several years.

Find Our Recommendations

CAROL  (Case Analysis and Reporting Online) is our search tool for investigations and safety recommendations across all modes. CAROL includes all NTSB recommendations. See the Field Descriptions page for specific information about safety recommendations data fields.

Recommendation Spotlight

Each month, we shine the spotlight on a few recommendations that have been successfully implemented (closed acceptable action) and are helping to further safety. These recommendations span all modes of transportation and recommendation recipients. Visit the Recommendation Spotlight Archive to see previous safety wins.

We urge recommendation recipients to keep us informed of the progress on implementing recommendations. If you do, you may see your recommendation spotlighted here. Read more about responding to our safety recommendations.

NTSB Safety Recommendations Strengthen DHC-3 Aircraft Maintenance Procedures

A small maintenance or assembly error can have catastrophic consequences when it affects a critical flight control system. That is one of the key lessons from the NTSB's investigation of the September 4, 2022, crash of a de Havilland DHC-3 Otter seaplane near Mutiny Bay, Washington.

The investigation found that a missing lock ring (figure 1) in the aircraft's horizontal stabilizer trim system allowed the clamp nut that secured the horizontal stabilizer trim actuator to unscrew from the actuator barrel (figure 2) during flight.

As the actuator separated, the pilot lost pitch control. The airplane abruptly pitched nose-down while cruising and impacted the water, killing the pilot and all nine passengers aboard.

Figure 1 Exemplar lock ring

The investigation also identified maintenance practices and documentation that could allow critical assembly errors to go undetected.

In response, the NTSB issued several safety recommendations for the Federal Aviation Administration, Transport Canada and De Havilland Aircraft of Canada (formally Viking Air).

On April 15, 2026, the NTSB classified three recommendations (A-23-6, A-23-7, and A-23–8) as closed after determining De Havilland Canada's actions adequately addressed the identified safety issues.

Figure 2 Clamp nut and barrel separation on accident airplane

Better inspection guidance for critical hardware

Following Safety Recommendation A-23-6, De Havilland Canada revised the DHC-3 Aircraft Maintenance Manual to designate the horizontal stabilizer trim actuator lock ring as a single-use part. Rather than requiring maintenance personnel to determine whether a lock ring remains airworthy, the updated guidance requires it to be replaced

whenever the actuator is repaired or disassembled. This change helps prevent worn or damaged lock rings from being reused in service.

Removal of unapproved components

In response to Safety Recommendation A-23-7, De Havilland Canada issued an All-Operator Message to all DHC-3 operators and field service representatives highlighting the safety issues identified during the investigation. The notice informed operators that moisture seals installed on the horizontal stabilizer trim actuator are not approved as part of the DHC-3 type design and reminded them that only manufacturer-approved or regulator-approved parts may be installed on the aircraft. It also instructed operators to contact De Havilland Canada if any unapproved moisture seals were found.

The NTSB found that these unapproved moisture seals could make the actuator harder to assemble correctly. If the required lock ring was also missing, the seals could increase the risk of the actuator coming apart during flight.

Clearer maintenance procedures

To address Safety Recommendation A-23-8, De Havilland Canada revised the DHC-3 Aircraft Maintenance Manual with clearer instructions for assembling and inspecting the horizontal stabilizer trim actuator.

The updated manual now includes the following guidance for maintenance personnel:

Ensure the clamp nut has only one #40 drilled hole.
Ensure the clamp nut is fully seated against the bearing.
Verify that the top eye rotates freely after assembly.
Properly tighten the clamp nut.
Contact De Havilland Canada if the lock ring cannot properly engage the clamp nut during installation.

These revisions provide maintenance personnel with clearer assembly procedures and verification steps that help ensure the horizontal stabilizer trim actuator is properly installed and functioning as intended.

Why it matters

The horizontal stabilizer trim actuator is a critical flight control component that helps the pilot control the aircraft's nose-up and nose-down movement (pitch). As a result of the NTSB's recommendations, DHC-3 maintenance guidance now includes improved inspection criteria, encourages the removal of certain unapproved components, and provides clearer assembly procedures. These changes help maintenance personnel find and correct potential problems before they can lead to a catastrophic accident.