Private Pilot Private Exam

What change in indication occurs when a pilot adjusts the altimeter from 30.11 to 29.96?

1. The altitude will be indicated as 300 feet higher

2. The altitude will be indicated as 150 feet lower

3. The altitude will be indicated as 500 feet lower

4. No change will occur in the indication

The correct answer is: The altitude will be indicated as 150 feet lower

When a pilot adjusts the altimeter from 30.11 to 29.96 inches of mercury, the indication on the altimeter will reflect a lower altitude because the altimeter measures the atmospheric pressure and converts that into altitude. Specifically, a decrease in barometric pressure setting from 30.11 to 29.96 inches indicates a lower atmospheric pressure at the location of the aircraft. Since the altimeter is set to a higher pressure than the actual atmospheric pressure, it will show an altitude that is higher than the actual altitude. Therefore, when the setting is adjusted to a lower pressure, it will cause the altimeter to read a lower altitude. The standard rule in aviation is that for each 0.01 inch of mercury that you decrease, the altimeter indicates an altitude change of approximately 10 feet. In this case, the change from 30.11 to 29.96 reflects a difference of 0.15 inches, which results in an approximate indication change of 150 feet lower. This understanding of the relationship between atmospheric pressure and altitude readings is key for pilots in ensuring accurate altitude readings during flight operations.