When the altimeter subscale settings are incorrect, what method is used to determine altimeter indications?

To determine altimeter indications when the subscale settings are incorrect, the average pressure lapse rate is significant because it provides a reliable means of estimating altitude changes based on pressure variations. The average pressure lapse rate of approximately 30 feet per hectopascal represents the way altitude affects air pressure in the atmosphere — as altitude increases, pressure typically decreases at a consistent rate.

When the altimeter is not set correctly, knowing the average lapse rate allows pilots to interpret the changes in pressure they receive. If the altimeter indicates that the pressure is lower than it should be at a specific altitude, or vice versa, the pilot can use the lapse rate to estimate what the actual altitude is relative to the corresponding pressure reading. This understanding becomes especially vital in varying weather conditions where pressure systems might fluctuate, requiring pilots to account for these changes in determining their altitude.

This method is useful for maintaining situational awareness and enhancing safety during flight, ensuring that pilots can make informed decisions even when the altimeter settings do not reflect the actual atmospheric pressure accurately.