Other / Unknown; Communication Breakdown; Troubleshooting
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Aircraft; Human Factors
An A&P or IA who changed out the tail-boom on Helicopter X in 1995 made an arithmetic error when he calculated the number of hours until the tail-boom needed to be replaced again. AFTT = 2385.6; Retire Hours = 2100; 2385.6 + 2100 = 4485.6. His entry was 4685.6. This went undetected for over 25 years until it was time in November of 2021 to change it out again. The error was noticed by the owner but not until he was going over the number of hours remaining until it needed to be changed out. By the time the error was found; it was 156.3 hour past the retire time of 4485.6. A ferry permit was obtained; an IA inspected and signed off the permit; an entry was made in the logbook and the helicopter was successfully ferried to ZZZ on Date.
Helicopter owner reported during an inspection of the aircraft's maintenance records an arithmetic error was found concerning the replacement of the Tail-Boom.