If no altimeter setting is available before flight, to which altitude should the pilot adjust the altimeter?

When an altimeter setting is not available prior to flight, adjusting the altimeter to the field elevation of the departure airport is crucial for ensuring that the aircraft operates safely and accurately. Doing this allows the pilot to have a consistent reference to the actual elevation of the airport, thus avoiding potential navigation errors that may arise from flying at an incorrect altitude.

Using the field elevation helps to provide an accurate altitude readout and aids in avoiding obstacles during takeoff and initial climb. It is particularly important in mountainous or hilly areas where the terrain may rise abruptly.

While standard pressure settings and mean sea level are important references in aviation, they are typically used in specific contexts such as flying at higher altitudes or when transitioning from a climb to cruise. The highest terrain option is not a practical choice as it could lead to dangerously low altitude levels if the terrain is significantly elevated in relation to the airport elevation.

Overall, setting the altimeter to the departure airport's field elevation enhances safety by providing a clear baseline for altitude and terrain awareness during takeoff and initial flight operations.