NPRM Focuses On Tail Rotor Drive Shaft Yoke Assembly

The FAA has proposed an Airworthiness Directive (AD) for Robinson Model R66 helicopters. The proposed AD would require replacing the tail rotor drive shaft yoke assembly and inspecting for sealant. This proposed AD is prompted by reports of tail rotor driveshaft failures.  

The proposed AD would require, within 100 hours time-in-service (TIS), replacing the forward yoke assembly of the tail rotor drive shaft unless already accomplished, visually inspecting for sealant, and applying sealant if needed to prevent seal rotation.

The NPRM follow two incidents of bearing failure that stem from a bearing assembly that included a bearing that was undersized for its housing. Consequently, the bearing was spinning at a speed that caused excessive heating of the bearing operation and led to the breakdown of the bearing's grease and ultimately seizure of the C647-16 bearing.

To correct this condition, Robinson initially specified installing a temperature recorder on the tail rotor driveshaft bearing assembly (bearing assembly) and inspecting the temperature recorder during preflight checks and during each 100-hour inspection. If the bearing was found running hot, then Robinson advised upgrading the bearing to a newer design.

Robinson later specified through R66 Service Bulletin SB-20, dated November 7, 2016, modifying the forward D224-3 and D224-4 tail rotor drive shaft assemblies by using a kit that has an improved, larger bearing that spins with less friction. The bulletin also specified inspecting whether 0.5 inch of sealant was on the junction of the black seal and bearing outer race and installing sealant if there was less than 0.5 inch of sealant. Robinson clarified R66 Service Bulletin SB-20 with R66 Service Bulletin SB-20A, dated June 6, 2017, that helicopters equipped with D224-4 tail rotor drive shaft assemblies and certain modified D224-3 assemblies do not require being upgraded with the kit.

The actions specified by this AD are intended prevent failure of the tail rotor driveshaft forward bearing and subsequent loss of helicopter control.

The FAA is proposing this AD because we evaluated all known relevant information and determined that an unsafe condition exists and is likely to exist or develop on other products of these same type designs.

The proposed AD would affect 249 helicopters of U.S. Registry, according to the FAA. At an average labor cost of $85 per work-hour.

Based on these estimates, the FAA expects the following costs:

• Replacing the yoke assembly would require 6 work-hours and $798 for parts, for a cost of $1,308 per helicopter.
• Inspecting for and applying sealant would require 1 work-hour and $30 for parts, for a cost of $115 per helicopter.
• The total cost of compliance is estimated to be $1,423 per helicopter, and $354,327 for the U.S. operator fleet.

Comments on the NPRM must be submitted before May 29, 2018.

(Source: Federal register. Image from file)

FMI: NPRM