Textron Aviation issued a service letter requiring inspection of the forward lower carry-through spar caps on certain 400-series models. Undetected cracks in these caps could result in flight instability or loss of control.
The service letter states that inspection times are based on the model and the time in service. Aircraft with less than 15,000 hours are not required to perform the inspection until that threshold is reached at which time the inspection must occur within 100 hours.
Inspection of the spar cap must include a thorough cleaning and an eddy current test. If a crack is found, no further flight is permitted and Textron Structures & Repairs must be contacted. The inspection results form attached to the service letter should be completed and returned to Textron Structures & Repairs regardless of whether a crack is found. Recurring inspections must be completed at regular intervals as listed in the service letter.
For a complete list of affected models and flight hours for inspection, read the service letter here.