What occurs to the aneroid wafers in an altimeter when barometric pressure decreases?

The correct answer is that the aneroid wafers in an altimeter expand when barometric pressure decreases. This occurs because an altimeter is designed to measure changes in atmospheric pressure, which varies with altitude.

As an aircraft ascends into areas of lower atmospheric pressure, the external pressure acting on the aneroid wafers is reduced. The aneroid wafers, which are sensitive to pressure changes, respond to this decrease in pressure by expanding. This expansion is mechanically linked to a needle or indicator that moves on the altimeter dial, therefore indicating a rise in altitude.

Understanding this mechanism is essential for pilots, as accurate altitude readings are critical for safe navigation, obstacle clearance, and adherence to air traffic control requirements. The design of the altimeter ensures that it provides a reliable indication of altitude by translating variations in pressure into a visual display of altitude.