AD 2020-19-08 Prompted By Reports Of Corrosion On Main Rotor Hub Tension-Torsion Strap (TT strap) Assemblies

The FAA is adopting a new airworthiness directive (AD) for Bell Textron Inc. (Type Certificate previously held by Bell Helicopter Textron Inc.) (Bell), Model 204B, 205A-1, and 212 helicopters. This AD was prompted by reports of corrosion on main rotor hub tension-torsion strap (TT strap) assemblies.

This AD requires reducing the life limit of a certain part-numbered TT strap assembly and prohibits installing this TT strap assembly on any helicopter. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective October 21, 2020.

Supplementary Information: The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to Bell Model 204B, 205A-1, and 212 helicopters with TT strap assembly part number (P/N) 204-012-112-005 installed. The NPRM published in the Federal Register on June 9, 2020 (85 FR 35227). The NPRM was prompted by three incidents of fatigue cracking in TT strap assembly P/N 206-010-105-3 installed on Model 206 helicopters. These TT strap assemblies have stainless steel filament windings (wires) encased in a urethane cover, which was manufactured using Caytur 21 (also known as Cature 21) as the urethane-curing accelerator. Caytur 21 contains chlorides, which are retained in the urethane cover after curing and result in premature failure of the urethane cover and subsequent corrosion and failure of the encased wires of the TT strap assemblies. As a result, Bell changed the curing accelerator in the manufacturing process.

Due to manufacturing process similarities of the urethane cover, TT strap assembly P/N 204-012-112-005, which is installed on Model 204B, 205A-1, and 212 helicopters, is affected by the same unsafe condition.

Accordingly, the NPRM proposed to require reducing the life limit of the TT strap assembly from 2,400 total hours time-in-service (TIS) to 1,200 total hours TIS or 18 months since initial installation on any helicopter, whichever occurs first, and creating a component history card or equivalent record. The NPRM also proposed to prohibit installing the affected TT strap assembly on any helicopter.

The actions of this AD are intended to prevent the TT strap assembly from remaining in service beyond its fatigue life. This condition, if not addressed, could result in failure of a TT strap, loss of a main rotor blade, and subsequent loss of control of the helicopter.

FMI: www.regulations.gov