If the altimeter setting changes from 29.15 to 29.85, how will this affect the indicated altitude?

When the altimeter setting increases from 29.15 to 29.85, it indicates that the pressure at sea level has increased, which can happen when climbing from a lower pressure area to a higher pressure area. The altimeter is calibrated to standard atmospheric pressure, which is defined as 29.92 inches of mercury (inHg).

In this situation, when the altimeter setting increases, the aircraft's indicated altitude will read lower than its actual altitude. This is because the altimeter works by measuring the ambient pressure and comparing it to the standard pressure. Therefore, if the altimeter is set to a higher pressure than the actual atmospheric pressure at the aircraft's current altitude (due to an increase in the altimeter setting), it will indicate that the aircraft is at a lower altitude than it really is.

The difference of 0.70 inches of mercury between the two settings is significant enough to affect the indicated altitude. A change of about 1 inch of mercury corresponds to approximately a 1,000-foot change in altitude in the vertical air column. Therefore, a 0.70-inch increase in pressure from 29.15 to 29.85 would lead to an approximate 700-foot decrease in indicated altitude.

Thus