Why is correct altimeter setting essential before flight?

• A. It changes engine performance.
• B. It improves airspeed reading.
• C. It ensures altitude readings reflect true altitude above the reference datum, improving terrain clearance and separation.
• D. It determines fuel flow.

Answer: (C)

Setting the altimeter to the local pressure calibrates the instrument to the actual atmospheric pressure field at your location. The altimeter is a sensitive barometer; by dialing in the local pressure (the QNH or QFE as appropriate), the altitude it displays becomes the true height above a reference surface, usually mean sea level. This alignment is what you rely on for terrain clearance and obstacle separation, because your decisions about altitude depend on knowing how high you actually are above the ground. If the setting is off, the altimeter can show you being higher or lower than you really are, creating a real risk of hitting terrain or violating airspace limits. The setting doesn’t change engine performance, it doesn’t directly improve airspeed readings, and it doesn’t determine fuel flow. It simply ensures your altitude information is accurate for safe flight.