Why altimeter setting is necessary?

• A. To correct for pressure changes
• B. To calibrate the dial
• C. To account for temperature
• D. To measure airspeed

Answer: (A)

Altitude is defined in relation to sea level, but atmospheric pressure changes from place to place and with the weather. The altimeter translates ambient pressure into an altitude reading, so you must tell it what sea-level pressure to use as the reference. Entering the local barometric pressure (the altimeter setting, often labeled QNH) lets the instrument convert the current pressure into the correct altitude above mean sea level. When this setting is correct, the altimeter will read the field elevation on the ground and, in flight, will provide accurate altitude relative to sea level. If the setting isn’t updated, pressure changes from weather or different locations will throw off the reading, making your indicated altitude too high or too low. The other options don’t fit because the setting isn’t about calibrating the dial, accounting for temperature, or measuring airspeed—the altimeter setting specifically compensates for real-time pressure variations to give the correct altitude.