The FAA is adopting a new airworthiness directive (AD) for certain Twin Commander 690s, 690As and 690Bs. It requires inspection for cracking of the outer fuselage attachments, the lower wing main spar, the vertical channels, the upper picture window channels, aft cabin pressure web, external wing-to-fuselage fillets and fasteners; repair or replacement of damaged parts as necessary; and modification of the structure with reinforced parts. It is prompted by cracks found in the upper picture window frame channels, left- and right-hand wing main spar frame support channels and aft pressure bulkhead web. This condition, if not corrected, could result in structural failure of the airplane.