Minimum service limits
Each propeller blade has a minimum permissible blade width and thickness at each station of the blade which is critical for maintaining the strength of the blade after it has had minor damage removed. AC 43.13-1B, Section 8-73, cautions the mechanic that the removal of damage must “not materially affect the strength [italics added], weight, or performance of the blade.” For any given blade station, this minimum blade width and thickness, which is critical for the strength of the blade, varies with the manufacturer, propeller model number, blade number, and in some cases, even the diameter of the propeller.
Also, this information is not always easily accessible to the aircraft technician or “line mechanic.”
McCauley states in its Service Manual for McCauley Met-L-Prop Fixed Pitch Propellers: Manual No. 730720 (1973, page 2-8), “Reject blades that will require removal of more material than the minimum permissible widths and thickness in applicable table ...” Similar cautions or warnings may be found in other McCauley overhaul manuals.
Hartzell’s Propeller Owner’s Manual & Logbook: Manual No. 115N (2003) illustrates how to remove damage from its blades. Rather than just a note to customers to not exceed the tolerances for blade repairs, Hartzell now states the caution on page 6-15, “Blades that have been previously repaired or overhauled may have been dimensionally reduced. Before repairing significant damage or making repairs on blades that are approaching serviceable limits, contact an appropriately licensed propeller repair facility or the Hartzell product support department for blade dimensional limits.” So, how does the aircraft mechanic know how much material he or she has to work with?
A very critical inspection that line personnel should be making after performing minor nick and scratch repairs on blades is measuring the blade width and/or thickness at the blade station where the repair was made to make certain there is still the minimum width and/or thickness of the blade at that station. McCauley, Hartzell, and Sensenich, for example, publish minimum blade dimension tables in their appropriate overhaul and/or repair manuals. The mechanic will not find this information in the propeller owner’s manuals. For this reason it is wise to contact an approved propeller repair/overhaul facility, as Hartzell recommends, when repairing questionable blades that have been previously repaired.
Minimum service limit examples
For each propeller make, model, and blade number of modern propellers, the manufacturer publishes what the minimum repaired blade width and thickness is at each blade station. Using the Sensenich table in Figure 2 as an example from Metal Propeller Repair Manual (SPRM) 590, page 10, the technician will find that after filing and dressing out a nick at station (blade radius) 25.88 on a Sensenich 69C fixed pitch propeller blade, the final width must be no less than 4.15 inches. Anything less renders the propeller, in this case, beyond service limits.
Sometimes the variation of width and thickness limitations between blade stations depends upon the diameter of the propeller, even when it is the same blade part number. McCauley, for example, may have different minimum repair limitations at the same blade station of the same blade part number, depending upon what the diameter of the propeller might be. Consider the width difference at station 30 between a 72-inch and 76-inch diameter propeller in McCauley’s Table 4-32 in Figure 3. On the 72-inch diameter propeller the minimum width is 3.83 inches at station 30, and on the 76-inch diameter propeller the minimum width is 3.91 inches at station 30.