Follows Several Reports Of The Failure Of Tail Boom Attachments

The FAA has proposed a new AD for Model EH-1H, EH-1X, HH-1H, HH-1N, UH-1D, UH-1M, UH-1N, and UH-1V helicopters operating under experimental airworthiness certificates; and for restricted category type certificated Model HH-1K, TH-1F, TH-1L, UH-1A, UH-1B without STC No. SR00026DE installed, UH-1E, UH-1F, UH-1H, UH-1L, and UH-1P helicopters. The FAA plans to publish separate rulemaking to address Model UH-1B with STC No. SR00026DE installed.

The type certificate holders for the restricted category models are Arrow Falcon Exporters Inc.; AST, Inc.; Bell Helicopter Textron, Inc.; California Department of Forestry; West Coast Fabrications; Global Helicopter Technology, Inc.; Hagglund Helicopters, LLC; International Helicopters, Inc.; JJASPP Engineering Services, LLC; JTBAM, Inc.; Northwest Rotorcraft, LLC; Red Tail Flying Services, LLC; Richards Heavylift Helo, Inc.; Robinson Air Crane, Inc.; Rotorcraft Development Corporation; San Joaquin Helicopters; Smith Helicopters; Southwest Florida Aviation International, Inc.; and Tamarack Helicopters, Inc.

This proposed AD would require revising the RFM for your helicopter to incorporate pre-flight checks; removing paint and sealant, and cleaning structural components that attach the tail boom to the fuselage; repetitive inspections of the cleaned structural components; repairing scratches, nicks, gouges, tears, and corrosion within allowable limits; replacing structural components with non-repairable damage, cracks, buckling, or distortion; replacing loose or missing rivets; re-bonding structures with dis-bonds; and removing loose bolts and self-locking nuts from service and replacing them with new bolts and new self-locking nuts.

This proposed AD was prompted by a series of accidents and incidents involving failure of the tail boom attachment structure on several restricted category military surplus helicopters. This condition, if not addressed, could result in separation of the tail boom from the helicopter, and subsequent loss of control of the helicopter.

In January 1982, a tail boom separated from a UH-1B helicopter engaged in logging operations, resulting in a fatal accident. The National Transportation Safety Board's (NTSB) final report identified structural fatigue and inadequate maintenance as probable causes. In September 2013, a tail boom separated from another UH-1B helicopter engaged in logging operations, resulting in another fatal accident. The NTSB's final report for that accident identified the cause as fatigue failure of the upper two tail boom attach points. Contributing to this accident was poor maintenance throughout the helicopter's operational life. In addition to these accidents, the FAA is aware of three forced landings due to tail boom attachment structure failures, one in May 2014 on a UH-1H helicopter, one in August 2016 on a UH-1H helicopter, and one in August 2018 on a UH-1F helicopter. The helicopter involved in the May 2014 forced landing was engaged in construction operations. The operations the helicopter was engaged in during the August 2016 forced landing are unknown. The helicopter involved in the August 2018 forced landing was engaged in firefighting operations.

In the first fatal accident and two of the forced landings, a loud pop or bang was heard in the rear of the aircraft at the moment of failure. In the second fatal accident, the pilot indicated before the flight that the helicopter felt like it “shuffled” during translational lift. Four of the five incidents involved a failure of the upper left hand tail boom attachment structure. In three cases it was the attach fitting on the tail boom side. In one case it was the longeron on the tail boom side. The upper left hand tail boom attach point is the most heavily loaded of the four attach points.

The FAA issued Special Airworthiness Information Bulletin SW-18-29 (SAIB SW-18-29) on October 1, 2018 to alert owners and operators of Restricted Category Bell Model HH-1K, UH-1A, UH-1B, UH-1E, UH-1F, UH-1H, UH-1L, UH-1P, TH-1F, and TH-1L helicopters of failure of the tail boom attachment structure.

SAIB SW-18-29 recommends adhering to the helicopter's Instructions for Continued Airworthiness which includes a repetitive 100 hour time-in-service (TIS) inspection of the tail boom attachment structure on both sides of the four attachment points. SAIB SW-18-29 also specifies the following supplemental recommendations: Keeping the fittings on both sides of all four attachment points, the cap angles running forward from the fuselage side fitting, and the longerons running aft from the tail boom side fitting, clean and free of paint and any non-faying sealant; and inspecting for cracks in the attachment structure with a borescope since the tail boom side structure is difficult to access. On the fuselage side, SAIB SW-18-29 recommends paying particular attention to the most forward fitting fasteners, the cap angle and the cap angle rivets just forward of the fitting as failures in these areas are more common. On the tail boom side, SAIB SW-18-29 recommends paying particular attention to the most aft fitting fasteners as failures in this area are more common. SAIB SW-18-29 also advises that smoking rivets are an indicator of attachment point issues. For helicopters that perform heavy lift operations, SAIB SW-18-29 recommends performing a repetitive 20 hours TIS inspection of both the fuselage and tail boom upper left-hand (LH) attach fittings, fuselage side cap angle, and tail boom side longeron. Lastly, for all affected helicopters, if a loud noise (typically a “pop” or “bang”) is heard in the rear of the aircraft during flight, SAIB SW-18-29 recommends minimizing hovering, slow flight, and pedal turns; reducing power when possible to avoid left pedal input; jettisoning external loads (if present) as soon as possible; and landing the aircraft as soon as possible. If an external load must be landed, SAIB SW-18-29 recommends performing a forward descent until the load touches down.

Based on input from a repair station, the FAA revised SAIB SW-18-29 to SAIB SW-18-29R1, dated February 19, 2019 (SAIB SW-18-29R1), to alert all owners and operators of clarified paint and sealant removal procedures and simplify the wording of recommendations to provide clarity. In particular, it was necessary to correct the emphasized inspection areas. The fuselage side fitting emphasis was revised to focus on the fitting section through the rivets closest to the attach bolt (the aft fitting fasteners) rather than through the most forward fitting fasteners. The tail boom side fitting emphasis was revised to focus on the fitting section through the rivets closest to the attach bolt (the forward fitting fasteners) rather than through the most aft fitting fasteners.

(Source: FAA. Image from file)

FMI: Proposed AD