What happens to the altimeter setting when flying at FL 290 and not resetting it during descent?

When flying at Flight Level 290 (FL 290), the altimeter is set to a standard pressure setting of 29.92 inches of mercury (inHg). As the aircraft descends and if the altimeter is not reset to the local altimeter setting (reported in inches or hectopascals), it will continue to read based on the standard setting.

As a result, when the aircraft descends without resetting the altimeter for the local airport pressure, the altimeter will read lower than the actual altitude. This discrepancy occurs because the local atmospheric pressure, especially at sea level, is usually different from the standard pressure used at flight levels. If the local altimeter setting is, for example, 29.30 inHg, the altimeter may indicate an altitude that does not correspond to the actual height above the ground, often leading to an undervaluation of the altitude.

This situation can lead to dangerous scenarios, especially during landing when accurate altitude readings are crucial to avoid obstacles and ensure a safe approach. Therefore, it is essential to reset the altimeter to the local setting during descent to maintain an accurate altitude reading.