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When determining whether to add or subtract the amount of altimeter error, remember that when the actual pressure is lower than what is set in the altimeter window, the actual altitude of the aircraft is lower than what is indicated on the altimeter. 

The following is another method of computing the altitude deviation. Start by subtracting the current altimeter setting from 29.94 "Hg. Always remember to place the original setting as the top number. Then subtract the current altimeter setting. 

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(Since 1 inch of pressure is equal to approximately 1,000 feet of altitude, 0.25 × 1,000 feet = 250 feet.) Always subtract the number from the indicated altitude. 

2,600 – 250 = 2,350 

Now, try a lower pressure setting. Adjust from altimeter setting 29.94 to 30.56 "Hg. 

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(Since 1 inch of pressure is equal to approximately 1,000 feet of altitude, 0.62 × 1,000 feet = 620 feet.) Always subtract the number from the indicated altitude. 

2,600 – (–620) = 3,220 

The pilot will be 620 feet high. 

Notice the difference is a negative number. Starting with the current indicated altitude of 2,600 feet, subtracting a negative number is the same as adding the two numbers. By utilizing this method, a pilot will better understand the importance of using the current altimeter setting (miscalculation of where and in what direction an error lies can affect safety; if altitude is lower than indicated altitude, an aircraft could be in danger of colliding with an obstacle). 

## Altimeter Operation 

There are two means by which the altimeter pointers can be moved. The first is a change in air pressure, while the other is an adjustment to the barometric scale. When the aircraft climbs or descends, changing pressure within the altimeter case expands or contracts the aneroid barometer. This movement is transmitted through mechanical linkage to rotate the pointers. 

A decrease in pressure causes the altimeter to indicate an increase in altitude, and an increase in pressure causes the altimeter to indicate a decrease in altitude. Accordingly, if the aircraft is sitting on the ground with a pressure level of 29.98 "Hg and the pressure level changes to 29.68 "Hg, the altimeter would show an increase of approximately 300 feet in altitude. This pressure change is most noticeable when the aircraft is left parked over night. As the pressure falls, the altimeter interprets this as a climb. The altimeter indicates an altitude above the actual field elevation. If the barometric pressure setting is reset to the current altimeter setting of 29.68 "Hg, then the field elevation is again indicated on the altimeter. 

This pressure change is not as easily noticed in flight since aircraft fly at specific altitudes. The aircraft steadily decreases true altitude while the altimeter is held constant through pilot action as discussed in the previous section. 

Knowing the aircraft’s altitude is vitally important to a pilot. The pilot must be sure that the aircraft is flying high enough to clear the highest terrain or obstruction along the intended route. It is especially important to have accurate altitude information when visibility is restricted. To clear obstructions, the pilot must constantly be aware of the altitude of the aircraft and the elevation of the surrounding terrain. To reduce the possibility of a midair collision, it is essential to maintain altitude in accordance with air traffic rules. 

## Types of Altitude 

Altitude in itself is a relevant term only when it is specifically stated to which type of altitude a pilot is referring. Normally when the term “altitude” is used, it is referring to altitude above sea level since this is the altitude which is used to depict obstacles and airspace, as well as to separate air traffic. 

Altitude is vertical distance above some point or level used as a reference. There are as many kinds of altitude as there are reference levels from which altitude is measured, and each may be used for specific reasons. Pilots are mainly concerned with five types of altitudes: 

1. Indicated altitude—read directly from the altimeter (uncorrected) when it is set to the current altimeter setting. 

2. True altitude—the vertical distance of the aircraft above sea level—the actual altitude. It is often expressed as feet above mean sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes. 

