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Cracks in planetary gearbox casings generate vibration responses, which, when properly isolated and analyzed, can be used for monitoring structural degradations. This paper provides a signal processing framework to effectively track casing crack related features in planetary gearboxes using carrier synchronous signal average (C-SSA). The proposed algorithm is based on processing the hunting-tooth synchronous signal average (H-SSA) to extract the C-SSA which contains the cyclic interaction between the gear loadings and the corresponding casing response. The root mean square (RMS) of the C-SSA signal can then serve as a health condition indicator (CI) to track crack propagation. Further enhancement can be achieved by applying the Hilbert transform (HT) on the C-SSA using the full bandwidth to derive squared envelope signal, which clearly shows the modulations from the crack response and further enhances the trending capability. To remove cyclic temperature influences observed in the trends, singular spectrum analysis technique (SSAT) has been used, ensuring the trend reflects the changes purely due to the damage progression. Experiments using three casing-mounted sensors show good capability to track crack progression. Tests under 100%, 125%, and 150% load levels show consistent performance across these operating conditions, with better results seen at higher loads. The results demonstrate that C-SSA and its squared envelope signal effectively enhances the sensitivity and reliability of vibration-based crack detection, providing a practical tool for long-term structural health monitoring of planetary gearbox.