The FAA received numerous reports of empennage fuselage station (FS) and vertical stabilizer skin cracking on numerous models of Twin Commander Aircraft. The FAA previously issued Airworthiness Directive (AD) 95-13-02 to require inspections of the empennage FS 409 frame and adjacent vertical stabilizer exterior skin to prevent failure of the vertical stabilizer due to cracks, which if not detected and corrected, could result in loss of control of the airplane. Twin Commander Aircraft Corporation Service Bulletin 218 provided terminating action to AD 95-13-02 through modifications to the airplane empennage FS 409 frame and adjacent vertical stabilizer exterior skin.
The FAA is aware of continued crack growth in FS 409 as well as the vertical stabilizer skin extending beyond the original areas of concern identified in AD 95-13-02. Operators have also reported additional structural cracking on FS 386 and FS 429 frames, with several instances of cracking being identified in visually inaccessible empennage locations. Service Letter 345 installed access panels which may allow limited observation of the areas of concern, but other areas require more extensive work to gain access.
The FAA is distributing this ACS to both notify owners, operators, and maintainers of these cracks and request feedback and additional in-service data within 10 days to assist the investigation of the potential safety concern.
To view this Airworthiness Concern Sheet click on this link: 
https://www.faasafety.gov/files/notices/2025/May/ACS_Twin_Commander_Empennage_Cracking.pdf
For questions or to reply to this message contact:
Lekebis Russell
Aviation Safety Engineer (Airframe)
East Certification Branch
1701 Columbia Ave
Atlanta, Ga 30337
(404) 474-5510
Lekebis.M.Russell@faa.gov