A recent Maintenance Alert issued by Twin Commander Aircraft addresses the potential for corrosion to develop as a result of moisture that becomes trapped under the dorsal fin on Model 690A and B Twin Commanders. The original dorsal fin design for the 690A/B does not allow for dispersion of moisture that builds up on the fuselage beneath the dorsal, and this can lead to corrosion on the fuselage skin.
The trapped moisture problem can be eliminated by installing Custom Kit (CK) 193, which is comprised of a weep-hole system of parts, including shims, to be installed under the existing dorsal to provide channels for moisture to drain out of the cavity. Or, better yet, replace the original plastic dorsal fin with one constructed from stronger, more durable composite laminate material. The new dorsal fin also incorporates channels to disperse moisture. CK 195 is the new dorsal fin for Twin Commander models 500S, 681, 685 and 690; CK 196 is for the 690A/B. Contact your Twin Commander-authorized Service Center for more details.