AD FAA-2015-0684

We are adopting a new airworthiness directive (AD) for all Viking Air Limited (Type Certificate Previously Held by Bombardier, Inc.) Model DHC-7-1 and DHC-7-100 airplanes. This AD was prompted by reports of cracks that were discovered in the outboard nacelles upper longeron channels and angles. This AD requires a one-time detailed visual inspection for cracking in the outboard nacelles upper longeron channels and angles; and repair if necessary. We are issuing this AD to detect and correct cracks in the outboard nacelles upper longeron channels and angles, which could lead to the loss of stiffness in the forward engine mount; and possible catastrophic failure.