Airline Transport Pilot Written Knowledge Practice Test

The correct definition of "Night" for aviation purposes refers to the period from the end of evening civil twilight until the beginning of morning civil twilight. This definition is established to provide a clear and standardized operational framework for various regulations pertaining to flight operations, including those related to visibility, lighting requirements, and certain flight restrictions.

Civil twilight is the time when the center of the sun is between 6 degrees below the horizon and the horizon itself. During this period, there is still enough light for the surroundings to be largely visible, but it is not fully dark. Once civil twilight ends in the evening, the conditions represent full darkness, which is when night begins in aviation terms. Similarly, night ends in the morning at the start of civil twilight, marking the return of increasing light conditions.

This understanding is vital for pilots, as they must be aware of these definitions to comply with regulations and adjust their flying operations appropriately during darker periods when visibility is reduced and different rules apply.