What measures altitude in hundreds of feet for aircraft flying above 18,000 feet?

Flight Level (FL) is the term used to designate altitude in hundreds of feet for aircraft flying above 18,000 feet. When an aircraft operates above this altitude, its altitude is typically measured relative to standard atmospheric pressure (29.92 inHg or 1013.2 hPa), which simplifies altitude reporting and allows for a standard reference point across different aircraft and environments.

At higher altitudes, the air pressure decreases, making it more practical to use a standard pressure instead of the local altimeter setting. This practice helps to maintain safe vertical separation between aircraft operating at different altitudes in busy airspace. Each flight level is denoted by the altitude in hundreds of feet, so a Flight Level of 240 would indicate an altitude of 24,000 feet.

The other options represent different types of altitude measurement but do not specifically apply to the context of altitude reporting in hundreds of feet above 18,000 feet in the way that Flight Level does. For instance, pressure altitude refers to altitude measured using the current atmospheric pressure and can be used, but it doesn’t have the specific designation in hundreds of feet format like Flight Level does. True altitude measures the actual height above mean sea level and is not standardized in the same way. Alt