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Hot to cold, look out below!


The altimeter installed in the aircraft you fly is designed to sense the atmospheric pressure outside of the aircraft. As a barometer it is very accurate. Ironically, as a gauge to assess our altitude, it has some serious shortcomings. Not all is lost, however. Fortunately, all aircraft use the same basic altimeter and therefore all have the same errors. However, these errors don't typically cause a problem until the temperature is very cold. Let's explore this error a bit further. One of the more common altimeter errors is due to non-standard pressure. When the weight of atmosphere above our position changes, the atmospheric pressure also changes. Less weight means lower pressure; more weight means higher pressure. Such a change in pressure is not a problem since we can adjust our altimeter for non-standard pressure. This correction is done through the altimeter setting and is adjusted in the Kollsman window (which could be an analog or digital gauge depending on the installed equipment). We update the altimeter setting periodically as required by FAA regulations. The density of the air below our flight level also plays an important role. The density of air is a function of both temperature and moisture. An increase in moisture and/or temperature decreases the air density. Conversely, a decrease in moisture and/or temperature increases the air density. Since moisture has a much less significant role, we will focus on the role of non-standard temperature in this post. Adjustments to account for non-standard pressure, however, do not compensate for non-standard temperature. In fact, pilots don't specifically account for non-standard temperature. Air is a mixture of many gasses. When the temperature of air is increased, for example, the molecules in this mixture will achieve a higher kinetic energy and space themselves farther apart resulting in a decrease in air density. As air temperature is decreased, the molecules in the mixture will achieve a lower kinetic energy and the molecules in the mixture are spaced closer together resulting in an increased air density. Let's assume that you are flying at a constant pressure level of 850 mb resulting in an indicated altitude of 5,000 feet. Now, imagine that the temperature the air below you suddenly increases. How will this change your indicated altitude?