Condition Could Result In Loss Of Tail Rotor Drive

The FAA has issued an emergency AD (2011-22-51) for Sikorsky Aircraft Corporation (Sikorsky) Model S-70, S-70A, S-70C, S-70C(M), and S-70C(M1) helicopters.

Sikorsky S-70 File Photo

This emergency AD was prompted by an accident that resulted from the blockage of the internal oil passages of the intermediate gearbox (IGB). The blockage occurred when a plug was left in the IGB during coating of the IGB housing. The blockage caused insufficient lubrication of the IGB, which overheated, seized-up, and caused a fracture in the output shaft that drives the tail rotor. This condition, if not corrected, could result in loss of tail rotor drive and subsequent loss of control of the helicopter.

The FAA reviewed Sikorsky Alert Service Bulletin No. 70-06-29A, dated October 11, 2011 (ASB). The ASB specifies:
A one-time borescope inspection of the lubrication passages from the oil scupper to the input and output housing.
Disassembling the IGB for inspection as an alternative to the borescope inspection.
Adding an “A” suffix to the serial number of all IGBs that have been inspected.

The FAA are issuing this AD because it evaluated all the relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the same type designs.

This AD requires borescope inspecting the IGB for any obstruction in the oil passages. As an alternative to the borescope inspection, this AD allows disassembling the IGB and inspecting the oil passages for any obstruction. If there is any obstruction in any oil passage, replace the IGB with an airworthy IGB before further flight. These actions are required before further flight and must be accomplished in accordance with specified portions of the ASB described previously.

This AD does not apply to the Model H-60 helicopter as it does not have a U.S. type certificate. Also, this AD does not require returning any parts to Sikorsky nor does it require marking the IGB after inspection.

FMI: AD 2011-22-51