Surface Pressure Error

A simple altimeter will have no method of correcting for non-standard pressure. While permitted for flight in some instances its use is limited since a simple altimeter will only display the height above the standard datum plane and does not allow a pilot to correct for non-standard pressure. Refer to the image below, the aircraft on the left has climbed from 0 feet (29.92 in. Hg) through an ISA atmosphere to a pressure of 29.86 in. Hg which corresponds to a pressure altitude of approximately 1,000 feet. In this particular case, the actual altitude is equal to the actual altitude of the aircraft.

Now refer to the aircraft in the centre. The surface pressure is 30.02 in. Hg and since, 0.1 in. Hg corresponds to approximately 100 feet the standard datum plane (29.92 in. Hg) is 100 feet above the surface. Therefore, when the aircraft is on the surface the altimeter will indicate -100 feet. Upon climbing to 28.86 in. Hg, despite the altimeter indicating 1,000 feet, the aircraft will in fact be 1,100 feet above the ground. When the surface pressure is higher than standard the aircraft’s actual height will be greater than the altitude indicated on the altimeter.

Finally, if the atmospheric pressure is lower than standard, as shown in the aircraft on the right (29.82 in. Hg), the standard datum plane will be below the surface. In this particular case, while the aircraft is on the ground it will indicate an altitude of 100 feet. If it was to then climb to an indicated altitude of 1,000 feet, the actual height above the ground would be 900 feet – less than what is indicated on the altimeter.

Correcting for Non Standard Pressure

Changes occurring in surface pressure are compensated by means of a pressure adjustment knob found on an altimeter. The altimeter setting is the pressure that when set on the altimeter will cause the altimeter to read station elevation when the aircraft is on the ground, it is used throughout the altimeter setting region. The altitude obtained using the altimeter setting is called the indicated altitude.

The altimeter setting is used during takeoff and landing and inflight within low-level airspace. For flight within high-level airspace and in the standard pressure region 29.92 in. Hg is set on the altimeter. The resulting altitude then shown on the altimeter is the “Pressure Altitude”.

Pilots must be very cautious with regards to maintaining the proper altimeter setting and when operating from an area of high pressure to an area of low pressure without frequent updates to the altimeter setting. “From high to low look out below” is a useful way to remember this concept.