What does "service life" refer to in the context of aircraft structures?

In the context of aircraft structures, "service life" refers to the period during which an aircraft structure can operate safely without major repairs. This definition encompasses not only the physical integrity of the materials used in construction but also their ability to withstand the stresses and strains encountered during operation, including environmental effects like corrosion and fatigue. Understanding service life is crucial for maintenance and ensuring the safety and reliability of the aircraft throughout its operational life.

The other options do not accurately capture the full meaning of "service life." For example, focusing solely on the airplane engine misses the broader implications of structural integrity and maintenance. Similarly, limiting the definition to the duration of flight hours or the years until retirement does not account for the need to assess an aircraft's condition continuously during its operational period.