One of the many interesting variations among the various Twin Commander models produced over the years is the dorsal fin. Some models have a short fin, some long, some in-between. The fins look good and accentuate the lines of the airplane, but there can be a problem with moisture retention.

Over the years, some Twin Commander models with the longer dorsal fin have developed excessive corrosion of the skin under the fin. It’s been determined that the affected aircraft had the fin installed using a fay and/or fillet seal around the periphery of the fin.

In some cases the sealant application caused moisture to be trapped inside the dorsal fin, which led to corrosion of the aluminum skin. A factory maintenance alert has been issued to ensure that aircraft at risk are checked.

The alert classifies corrosion damage in the affected area as either light, moderate, or severe. The formal definitions are:

LIGHT—Characterized by discoloration or pitting to a depth of approximately 0.002-inch maximum.

MODERATE—Appears similar to light corrosion except there may be some blisters or evidence of scaling and flaking. Pitting depths may be as deep as 0.010-inch.

SEVERE—General appearance may be similar to moderate corrosion with severe blistering exfoliation and scaling or flaking. Pitting depths will be deeper than 0.010-inch.

The alert specifies that, at the next scheduled maintenance event, remove the dorsal fin and inspect the fuselage skin underneath for corrosion.

If only light corrosion is found, follow these steps to correct the condition:

  • Remove corrosion by light hand-sanding. Do not exceed the depth of defect.
  • Corrosion Convert (Alodine) reworked area.
  • Prime complete area under the dorsal with epoxy-based primer per manufacturer’s instructions.
  • Reinstall the dorsal fin and fillet seal outside edge to aircraft skin using cabin pressure-type sealant as required to conceal any gaps.
  • If moderate or severe skin corrosion is found, replace the skin and then follow steps 3 and 4 above.

Be sure to notify Twin Commander Aircraft LLC, and please provide the total time on the aircraft. The information provided will assist in establishing an inspection interval for dorsal fin corrosion.