In aircraft life-cycle management, why is 'heavy maintenance' scheduled based on specific flight cycles or hours rather than just calendar time?

Heavy maintenance, also known as a C-check or D-check, is scheduled based on flight cycles and flight hours because these metrics directly represent the physical wear and fatigue experienced by the aircraft structure and systems. A flight cycle is defined as one complete take-off and landing sequence, which causes significant stress on the airframe due to pressurization and de-pressurization cycles. These pressure changes cause the ....