How does pressure setting (QNH) affect altitude readings on the altimeter?

• A. It has no effect
• B. It only affects vertical speed
• C. Setting the local QNH adjusts the altimeter so readings correspond to altitude above mean sea level; incorrect settings cause elevation errors
• D. It changes the altimeter's units

Answer: (C)

Setting the pressure reference on the altimeter acts as the datum for converting ambient pressure into height. When you dial in the local QNH, you’re telling the altimeter the current mean sea-level pressure, so it translates the pressure you measure into your altitude above mean sea level. If the QNH is set too high, the instrument will read a lower altitude than you actually have; if the QNH is set too low, it will read a higher altitude. The error is basically the difference between the actual sea-level pressure and what you set, carried into the height calculation on the pressure–height curve. This is why proper QNH ensures the altimeter indicates altitude above MSL, while incorrect settings cause elevation errors.