Why is local altimeter setting important and when should you update it?

• A. It ensures your altitude corresponds to the actual field elevation; update when entering or transitioning to a new area and on approach.
• B. It changes airspeed.
• C. It has no effect on altitude.
• D. It is only used on long flights.

Answer: (A)

Setting the local altimeter pressure is essential because it synchronizes your altitude reading with the actual height above the terrain for your current location. The altimeter measures ambient pressure and needs a local reference (the local QNH) to show true altitude relative to mean sea level or the field elevation. When you move into a different area with a new pressure, updating the setting keeps your indicated altitude accurate; otherwise you could be higher or lower than you think, which can affect terrain clearance and the accuracy of an approach. You should update whenever you enter a new area with different local pressure and again before an approach so your altitude readings correspond to the field and the published altitudes. The setting only affects altitude readings, not airspeed, and it’s relevant for all flights, not just long ones.