The system calibration determines the position and orientation between the sensor and the navigation systems, such as boresight misalignment. Although there is much research about boresight calibration, there are not sufficient studies on the frequency of the calibration performance. The shortterm stability of boresight misalignment was investigated in previous studies, but long-term stability research could not be done. It is important to emphasize that long-term stability is still open to questions. In this study, an Ultracam Eagle digital aerial camera's data from 2012 to 2016 were analyzed and the question of how often calibration should be performed was investigated. Boresight misalignment does not remain constant on a yearly basis and should be calibrated every year before the flight season. It was observed that the boresight misalignment changed dramatically when the inertial measurement unit or camera was removed from the aircraft and sent to the manufacturer for factory calibration.