18 September 2003
Blade retention bearings
Transport Canada has been informed via the Service Difficulty Reporting System (SDRS) of numerous problems associated with blade thrust bearings (P/N C-792-1). A two-piece (aluminum) hub retains each propeller blade on a thrust bearing.
During propeller overhauls, numerous thrust bearings have been found severely corroded, and in some cases, the bearing races were completely broken in half. On several occasions, hub assembly replacement was necessary due to severe corrosion and/or damage from bearing movement. The SDR submitter stated that approximately one of every three thrust bearings had to be replaced during normal overhaul requirements. Although no significant blade vibrations were noted prior to blades being routed for overhaul, there was a real potential for loss of pitch control. This damage, if not detected, could lead to blade separation with catastrophic results.
While the propeller is operating, the following forces are constantly present:
- spring force;
- counterweight force;
- centrifugal twisting moment of each blade; and
- blade aerodynamic twisting forces.
Operators are reminded that although blades are intended to be tight in the propeller hub, slight movement is acceptable if the blade returns to its original position when released. Blades, with excessive movement or that do not return to their original position when released, may be an indication of internal wear or damage.
The original C-792 bearing was plagued with corrosion problems. The C-792-(1) bearing was then introduced with a thin dense chrome material which added better corrosion protection. This unfortunately made the bearing harder and therefore brittle, reducing longevity. The new generation D-7745 blade retention bearing adds a total new material designed to combat these past problems experienced with both of the aforementioned C-792 series bearings.
For your attention, Hartzell now introduces Revision 1, dated 2 June 2003, to Service Bulletin (SB) HC-SB-61-258. Revision 1 introduces a new D-7745 blade retention bearing replacing C-792-(1) at the next propeller overhaul or disassembly. Propeller models HC-E4A-3(A,I,J)/E10950(P)(C) (B,K) are affected by this SB.
Additionally, Hartzell now has all the latest manufacturer's service information on compact disc with revision notification via e-mail. For more information about service bulletin HC-SB-61-258 and/or Hartzell documents on CD, you may contact Hartzell Product Support via e-mail: email@example.com or call (937) 778-4379.
Any defects or further occurrences should be reported by sending a Service Difficulty Report to Transport Canada, Continuing Airworthiness, Ottawa.
For further information, contact a Transport Canada Centre, or Mr. Barry Caldwell, Continuing Airworthiness, Ottawa, telephone (613) 952-4358, facsimile (613) 996-9178 or e-mail firstname.lastname@example.org.
For Director, Aircraft Certification
R. A. Raoux
Acting Chief, Continuing Airworthiness