Which of the following is NOT a type of unexpected event?

Scheduled maintenance is not classified as an unexpected event because it is a planned and anticipated activity within the aircraft operations cycle. It is regularly scheduled to ensure the aircraft's airworthiness and safety, allowing aircrews and maintenance personnel to prepare for it in advance. In contrast, unexpected events are those that occur without prior notice and can disrupt operations, such as malfunctions, weather condition changes, and inadvertent instrument meteorological conditions (IMC). These events require immediate response and adaptation from the aircrew, highlighting the importance of effective communication and coordination during unforeseen circumstances.