56-22-01 HAMILTON STANDARD: Applies to all Hamilton Standard Aluminum Alloy Propeller Blades.
Compliance required for 6507 blades before July 1, 1957; all other blades before January 1, 1958.
Omission of EC776 cement as a prime coat under BB1007 in the cement sequence for adhering rubber accessories to Hamilton Standard aluminum blades as required by their Service Bulletins 210 and 217 has resulted in corrosion of the blade area under these parts. Although this corrosion does not necessarily appear severe to the naked eye, two instances of fatigue cracking of 6,507 blades on DC-4 type aircraft have been encountered and are believed to have originated from such corrosion. Remove the adhered rubber parts from all aluminum blades having fluid deicing boots, electric blade heaters, deicer troughs, or slip ring mounts excepting only those blades which have known histories of positive application of EC776 adjacent to the blade surface under BB1007 throughout the blade's entire life, or for at least the last 5,000 hours of operation. Inspect and rework uncovered areas in accordance with Hamilton Standard Service Bulletin No. 385. Where anodize inspection facilities are available, non-surface-treated blades and non- surface-treated areas of surface treated blades may be inspected and reworked in accordance with Hamilton Standard Service Bulletin No. 385B. Blades with excessive corrosion and/or cracks should be returned to Hamilton Standard for further evaluation.
This directive is also applicable to aluminum blades which do not currently include the aforementioned adhered rubber parts, but formerly incorporated them without the EC776 application.
(Hamilton Standard Service Bulletins Nos. 210, 217, 385, 385A, and 385B cover this same subject.)