Investigators discovered no single catastrophic failure when a Sikorsky S-61N experienced a loss of tail rotor authority during flight. Instead, they found three distinct links in a chain:

1. The Machining Defect: The yaw pedal damper housing lug with a machined radius that had a sharp edge where the design called for a smooth curve. That deviation created a stress concentration point that was working against the part on every flight.
2. The Maintenance Oversight: Someone failed to properly torque and safety wire the attachment bolts during a routine maintenance interval.
3. The Operational Variable: The flight crew activated the rudder pedals to full throw in less than five seconds, violating a standing RFM caution.

Separately, each one was a latent hazard waiting for the others. Together, they caused a fatigue fracture of the damper housing, an auxiliary system losing fluid, and a complete loss of directional control. Sikorsky issued Alert Service Bulletin ASB 61B65-25 in October 2022, initiating the daily auxiliary servo assembly inspection immediately upon receipt. AD 2026-11-05 now makes that inspection a regulatory requirement across the entire S-61 family.

‍Field Notes: Starting immediately, operators must perform a daily preflight visual of the auxiliary servo assembly (the "broom closet") for fluid leakage, cracks, and compromised bolt security. Within 10 hours TIS after the effective date, operators must determine the remaining life of the yaw pedal damper check valve housing and record it in the helicopter logbook. The housing carries a new service life limit of 30,000 hours TIS. Within 150 hours TIS or 4 months, whichever comes first, a Fluorescent Penetrant Inspection (FPI) of the housing is required. Once the FPI is complete, the daily check is replaced by a visual inspection every 15 hours TIS going forward. Both the existing RFM and the airworthiness limitations section of the maintenance manual require updating to reflect the AD's requirements.

Why It Matters: A loss of tail rotor authority is about as serious as helicopter emergencies get. What makes this accident so instructive for the community is how ordinary the ingredients were:

• A machined edge, not conforming to its radius blending specification, was the source of metal fatigue.
• In real hangars, maintenance mistakes like un-torqued bolts and skipped safety wires do happen.
• The RFM caution that the crew didn't follow was there for exactly this reason.

Need the complete AD? Download the complete AD here.

Research thousands of ADs in seconds. Start your free 10-day TBX trial today!

‍