AD 2022-21-13 Prompted By Reports Of Cracks At Different Positions On Two Affected Propeller Hubs

The FAA is superseding Airworthiness Directive (AD) 2021-23-17 for all Hoffmann GmbH & Co. KG (Hoffmann) model HO-V 72 propellers.

AD 2021-23-17 required amending the existing aircraft flight manual (AFM) by inserting abnormal propeller vibration instructions, visual inspection and non-destructive test (NDT) inspection of the propeller hub and, depending on the results of the inspections, replacement of the propeller hub with a part eligible for installation. Since the FAA issued AD 2021-23-17, further investigation by the manufacturer revealed that cracks found on propeller hubs likely resulted from propeller blade retention nuts that were not tightened using published service information during blade installation. This AD is prompted by reports of cracks at different positions on two affected propeller hubs. This AD retains the required actions of AD 2021-23-17. This AD also requires a maintenance records review and, depending on the results of the maintenance records review, tightening of each propeller blade retention nut to specific torque values. Depending on the results of the maintenance records review, this AD requires physically inspecting the propeller blade for shake. If any axial play is detected during the performance of the inspection, this AD requires the removal of the propeller from service and the performance of an NDT inspection of the propeller hub, and depending on the NDT inspection results, replacement of the propeller hub with a part eligible for installation. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective December 6, 2022.

Supplementary Information: The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to supersede AD 2021-23-17, Amendment 39-21815 (86 FR 68905, December 6, 2021) (AD 2021-23-17). AD 2021-23-17 applied to all Hoffmann GmbH & Co. KG model HO-V 72 propellers. AD 2021-23-17 required amending the existing AFM by inserting abnormal propeller vibration instructions, visual inspection and NDT inspection of the propeller hub and, depending on the results of the inspections, replacement of the propeller hub with a part eligible for installation. The FAA issued AD 2021-23-17 to prevent failure of the propeller hub.

The NPRM published in the Federal Register on August 01, 2022 (87 FR 46903). The NPRM was prompted by EASA AD 2022-0061, dated April 4, 2022 (referred to after this as “the MCAI”), issued by the European Union Aviation Safety Agency (EASA), which is the Technical Agent for the Member States of the European Union.

The MCAI states:

• Cracks have been reported at different positions on two affected parts, both installed on Slingsby T67 “Firefly” aeroplanes. One crack was found during scheduled inspection, the other crack during an unscheduled inspection after abnormal vibrations occurred. Subsequent investigation determined that improper tightening of blade nuts has caused or contributed to those events.
• This condition, if not detected and corrected, could lead to in-flight propeller detachment, possibly resulting in damage to the aeroplane and/or injury to persons on the ground.
• To address this potential unsafe condition, Hoffmann Propeller issued the SB, providing applicable instructions, and EASA issued Emergency AD 2020-0226-E (later revised [to EASA AD 2020-0226R1]) to require inspections of affected parts and, depending on findings, replacement. That AD also required, for certain aeroplanes, amendment of the applicable Aircraft Flight Manual (AFM).
• Since that [EASA] AD was issued, further investigation revealed that not all propeller blade nuts were tightened in accordance with the Hoffman Propeller blade nut tightening procedure B2.23 which requires a certain over-torquing and loosening of the blade nut to limit a preload reduction due to material settlement. Prompted by this development, Hoffmann Propeller issued SB057 (incorporating blade nut tightening procedure B2.23) providing torquing instructions, and SB58 providing instructions for setting correct counterweight angles. Additionally, Hoffmann Propeller issued the torque tightening SB (referencing SB57 and SB58) providing inspections and corrective action instructions.

For the reasons described above, this [EASA] AD retains the requirements of EASA AD 2020-0226R1, which is superseded, and requires additional blade checks, inspections, and re-tightening of the propeller blade nuts

You may examine the MCAI in the AD docket at regulations.gov under Docket No. FAA-2022-0980.

In the NPRM, the FAA proposed to retain all of the requirements of AD 2021-23-17. In the NPRM, the FAA also proposed to require a maintenance records review and, depending on the results of the maintenance records review, tightening of each propeller blade retention nut to specified torque values. Depending on the results of the maintenance records review, the NPRM also proposed to require physically inspecting the propeller blade for shake. If any axial play is detected during inspection, the NPRM proposed to require the removal of the propeller from service and the performance of an NDT inspection of the propeller hub, and depending on the NDT inspection results, replacement of the propeller hub with a part eligible for installation.

FMI: www.regulations.gov