How can a pilot obtain pressure altitude on flights below 18,000 ft?

To obtain pressure altitude on flights below 18,000 feet, the correct method is to set the altimeter to 29.92 inches of mercury (inHg). This practice is based on the standard atmosphere, where pressure altitude is defined as the height above the standard datum plane, which is set at 29.92 inHg. When a pilot adjusts the altimeter to this setting, the altimeter reads the pressure altitude directly, allowing for navigation and performance calculations to be made with respect to the standardized reference of sea level pressure.

In flight operations, understanding pressure altitude is crucial, particularly for understanding aircraft performance, safe terrain clearance, and ensuring proper separation from other aircraft. This method works because, regardless of the local altimeter setting, setting it to the standard pressure creates a consistent frame of reference for altitude.

Using a computer to convert indicated altitude or contacting an FSS for pressure altitude may provide information, but these methods are indirect and not as straightforward. Likewise, calculating pressure altitude manually based on oxygen levels is unnecessary at altitudes below 18,000 feet, as the altimeter adjustment effectively provides the required information directly and accurately.