The upgrade converts the double shoulder design of the X and V shank blades to a single shoulder design, similar in configuration to Hartzell's proven steel hub turbine propellers. This is to be accomplished by either reworking existing blades according to the requirements of Service Letter 182 or replacing the blades with those manufactured in the MV configuration as new.
Service Letter 182, also issued April 17, 1998, was developed specifically to create a method to modify existing blades in the field; however, the process of recontouring the outboard shoulder is difficult and will be limited to selected propeller repair stations utilizing highly- skilled machinists. Hartzell will begin working with these shops to develop this capability. On modified blades, the inboard shoulder, where most of the cracks have been discovered, will no longer be load bearing.
In order to accommodate the single shoulder MV blade, new blade retention clamps are required. The wall and outboard flange thickness of the new clamps has been increased to retain the single shoulder blade. In addition, this upgrade will only be applicable to HC-A or -D style hubs. Operators with HC-1 or -8 style hubs will need to purchase the A or D style hub and associated parts. Due to the design improvements provided by this upgrade, not only are the requirements of the AD eliminated, but also the recommended time between overhaul (TBO) has been increased to 2,000 hours or 5 years. Operators are urged to adhere to these recommendations to maintain the benefits of this new design.
General Maintenance Guidelines
As stated earlier, the criticality of the propeller in the continued safe operation of the aircraft demands that it be properly maintained. In addition, thorough maintenance can help to maximize its useful life. Proper care and maintenance of propellers even carries the potential of preventing the need for future service bulletins and airworthiness directives.
Nicks and gouges in the blade create stress risers that can lead to fatigue cracking and ultimately to failure of the blade. Any damage to the blade larger than about 1/32 inch should be dressed out by an A&P mechanic.
If there's grease or oil present, investigate the source. Although rare, grease and/or oil leaks can result from cracks in the hub assembly. Also, look for cracking in the spinner and make sure that the propeller mounting bolts/nuts and the spinner are secure. Advise pilots to stay alert for unusual vibrations felt in the airplane. Many propeller failures are preceded by unusual vibrations felt in the aircraft.
During annual inspections, make sure the propeller is greased properly. In steel hub propellers, this usually requires removing one of the two grease fittings at each blade and pumping grease in the opposite grease fitting until good, clean grease flows from the removed fitting hole. Some steel hub propellers only have one grease fitting per blade. These and compact propellers require that an ounce of grease be injected into each fitting.
Check the tachometer for calibration. Mechanical tachometers are notoriously inaccurate and operation above red line or in a placarded area can be damaging to the propeller. If placards or restricted ranges are present, this means that the blade stresses during the vibrational stress survey were found to exceed design limits for continuous operation in these ranges.
With the spinner removed, check for external corrosion on the propeller. Corrosion pitting has the affect of concentrating stresses and can also foretell internal corrosion. If corrosion is present, the propeller must be removed and sent to a propeller shop for inspection and repair.
Before dismissing the need for propeller overhauls, remember the discussion earlier concerning the potentially catastrophic implications of a propeller failure. The recommended interval for overhaul of Hartzell propellers varies from 1,000 to 4,000 hours of service or five to six years of calendar time. The interval based upon hours is to inspect for wear and fatigue. The interval based upon calendar times is to catch corrosion in its infancy, hopefully before it damages parts beyond repair. Refer to Hartzell Service Letter 61 for the overhaul intervals of specific model propellers.