When an aircraft altimeter is set at 29.92" Hg on the ground, the altimeter will read what?

When an aircraft altimeter is set to 29.92 inches of mercury (Hg) at sea level, it is measuring pressure altitude. Pressure altitude is defined as the height above a standard datum plane, which is established at 29.92 inches of mercury. This setting provides a baseline for atmospheric pressure, allowing the altimeter to calculate height based solely on the pressure in the atmosphere at that given location.

When the altimeter is calibrated to this standard atmospheric pressure, it gives a reading that corresponds directly to pressure altitude, which is essential for pilots during various phases of flight. It simplifies altitude reporting and is particularly useful for aircraft operating at high altitudes and in controlled airspace, where uniformity in altitude measurement is critical.

In summary, with the altimeter set at 29.92 inches of mercury on the ground, it reflects the aircraft's pressure altitude, equating to the altitude reading that would be displayed if the aircraft were at a level where the atmospheric pressure matches the standard setting of 29.92 inches of mercury.