Prompted By Discovery Of A Crack On A Main Rotor Blade

The FAA has issued an Emergency AD (2015-20-51) for Model UH-12-series helicopters, with a main rotor blade (MRB) fork, part number 52110-3, with a serial number 11502P through 11537P, or 11551P through 11579P, excluding 11577P, certificated in any category.

The Emergency AD was prompted by a pilot report of severe lateral vibrations on a Model UH-12D helicopter. After that incident, the operator of the helicopter discovered a crack in the main rotor blade (MRB) fork at the tension-torsion (TT) retention pin holes. The operator then inspected the rest of its UH-12 fleet and discovered another helicopter with a cracked MRB fork. The preliminary investigation suggests the failure of the forks may be a production issue with certain batches of forks. However, the investigation into the root cause of the failure of the forks is on-going. The crack in one of the forks was not detectable by the dye-penetrant inspection required by AD 86-17-02, Amendment 39-5367 (51 FR 28062, August 5, 1986), which addresses a similar unsafe condition.

Accordingly, this Emergency AD requires a one-time magnetic particle inspection of the MRB fork at the TT retention pin hole. If there is a crack, this Emergency AD requires replacing the fork before further flight. This Emergency AD also requires reporting certain information to the FAA to enable us to obtain better insight into the cause of the cracking. These Emergency AD actions are intended to detect a crack in the MRB fork and prevent blade separation and subsequent loss of control of the helicopter. This Emergency AD does not supersede or affect the requirements in AD 86-17-02.

The FAA is issuing this Emergency AD after an evaluation of all the relevant information and determined the unsafe condition described previously is likely to exist or develop on other helicopters of the same type designs.

The AD requires a one-time magnetic particle inspection of a 1-inch strip around the edge of each MRB fork TT retention pin hole. The Preliminary SB requires a visual inspection and a magnetic particle inspection of the fork. If there is a crack, we require replacing the MRB fork, while the Preliminary SB requires replacing the TT pin and a fluorescent penetrant inspecting the main rotor hub.

It is estimated that this AD will affect 64 MRB forks installed on helicopters of U.S. Registry and that labor costs average $85 a work-hour. Based on these estimates the agency expects the following costs:

• Magnetic particle inspecting each affected MRB fork will require 3 work hours for a labor cost of $255. No parts are needed for a total U.S. fleet cost of $16,320.
• Reporting the information requested in Appendix 1 requires 1/12 work-hour for a labor cost of about $7 per MRB fork and $455 for the U.S. fleet.
• If required, a replacement fork costs $14,003.

Before further flight, unless done within the last 100 hours time-in-service, prepare the area for a one-time magnetic particle inspection by stripping any paint in the interior and exterior of each MRB fork. The inspection area is a 1-inch strip around the edge of each MRB pin hole. Magnetic particle inspect for a crack in the MRB fork at the pin holes, following the magnetic particle examination process and qualifications found in American Society for Testing and Material (ASTM) E1444 or equivalent. If there is a crack, replace the MRB fork with an MRB fork that has been inspected per the requirements of this paragraph or with an airworthy MRB fork that is not listed in the applicability paragraph of this Emergency AD.

FMI: Emergency AD