What effect does the QNH setting have during flights?

The QNH setting is a key reference for altimetry in aviation, as it provides the altimeter setting that corrects for variations in atmospheric pressure to ensure accurate altitude readings relative to sea level. When pilots set their altimeter to the QNH value, they are calibrating their altimeter to reflect the altitude above mean sea level, which is essential for maintaining safe vertical separation from terrain and other aircraft.

This setting is particularly important in areas with fluctuating weather conditions, where atmospheric pressure can change significantly. By using the QNH, pilots ensure that their altitude readings are precise, allowing for safer navigation and accurate communication with air traffic control regarding flight levels.

Understanding this concept is critical for flight safety, as inaccurate altitude readings can lead to situations such as unintended encounters with terrain or other aircraft, especially in crowded or mountainous regions.