How Much Is Too Much?

Observing propeller blade service limits after minor repairs

Recently the Federal Aviation Administration (FAA) published two Special Airworthiness Information Bulletins (SAIB) for propellers, SAIB # NE-08-20 and SAIB # NE-08-22. These bulletins are not mandatory but are nevertheless very informative. Every certified aircraft maintenance technician should be familiar with these documents, as well as Advisory Circular (AC) 43.13-1B and AC 20-37E, the latter being published specifically for aircraft propeller maintenance, when performing minor repairs on propeller blades.

It would be easy for the casual reader of SAIB NE-08-22 to breeze past an important point on page 2 which calls for “maintenance action or further evaluation prior to return to service: Unusual wear of either unexpected severity or in an unexpected location that might be beyond the manufacturer’s service limits [italics added].” Manufacturer’s service limits are often overlooked by the technician or the “line mechanic” who may be in a hurry to get the minor damage “dressed out” of a blade. The technician is familiar with the diagrams and instructions of the repair limitations such as shown in Figure 1, which is from AC 20-37E. However, individual manufacturers may publish more conservative tolerances to which the technician must adhere. But is this all there is to the manufacturer’s service limits?

Common errors
A technician may look at these repair diagrams and consider the depth of the impact to the blade as the most important criteria when dressing out damage. However, propeller manufacturer product support representatives and field service engineers have stated that the manufacturer’s repair limitations or service limits are often overlooked by mechanics when doing minor repairs such as filing and dressing out nicks on propeller blades.

Often mechanics will fail to take into consideration normal wear and previous blade filing when removing damage. For example, if a manufacturer has a blade repair limit of 0.250 inch maximum material removal on a new blade and a customer gets a nick in the blade requiring that 0.200 inch of material has to be removed as a result of damage, there is only 0.050 inch left that can be removed in the event of future damage. If the blade receives damage again in the same area, it is not enough to simply follow the diagrams that indicate the radius of the repair and the maximum material that can be removed. Even a slight repair may take the blade beyond the service limits.

Another factor the technician needs to take into consideration is whether or not the propeller has previously been reconditioned by an approved propeller overhaul facility. Even though you have a fixed pitch McCauley or Sensenich propeller, for example, it may have been reconditioned or overhauled at some point. When a McCauley fixed pitch propeller is inspected at overhaul, the blades are etched or dressed, having anywhere from 0.002 to 0.003 inch of material removed from the entire surface of the blades. A Sensenich propeller may have as much as 0.005 to 0.007 inch removed from the entire surface of the blades at overhaul. Even though the overhauled propeller looks new, there is less material that can be removed when doing future repairs. Granted, 0.007 inch is not much material but it could still mean the difference between an airworthy or unairworthy propeller.

Figure 2: Repair Specifications 69C Series Propellers
Weight: 24 pounds (10.9 kilograms)69CK attaching kit adds approximately 0.75 pounds to installed weight.
Maximum Out-of-Balance Moment: 0.048 Pound-Inches (55 Gram-Centimeters)
Blade Radius (inches) 6.90 10.35 15.53 20.70 25.88 31.05 34.50
  (cm) 17.53 26.29 39.43 52.58 65.72 78.87 87.63
Minimum Blade Dimensions
Chord (inches) 4.56 4.79 4.89 4.69 4.15 3.17 2.10
  (cm) 11.58 12.17 12.42 11.91 10.54 8.05 5.33
Thickness (inches) 1.35 0.97 0.68 0.50 0.37 0.24 0.14
  (cm) 3.43 2.47 1.73 1.27 0.94 0.61 0.36
Figure 2: Minimum Repair Limits for Sensenich 69C Series Propellers. (Sensenich Metal Propeller Repair Manual: SPRM 590, page 10.) For Educational Purposes Only, Used By Permission.
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