Flight Instruments

When determining whether to add or subtract the amount of A decrease in pressure causes the altimeter to indicate an altimeter error, remember that when the actual pressure is lower increase in altitude, and an increase in pressure causes the than what is set in the altimeter window, the actual altitude altimeter to indicate a decrease in altitude. Accordingly, if of the aircraft is lower than what is indicated on the altimeter. 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 The following is another method of computing the altitude altimeter would show an increase of approximately 300 feet deviation. Start by subtracting the current altimeter setting in altitude. This pressure change is most noticeable when the from 29.94 "Hg. Always remember to place the original setting aircraft is left parked over night. As the pressure falls, the as the top number. Then subtract the current altimeter setting. altimeter interprets this as a climb. The altimeter indicates an altitude above the actual field elevation. If the barometric Mineral Wells altimeter setting 29.94 pressure setting is reset to the current altimeter setting of 29.68 Abilene altimeter setting 29.69 "Hg, then the field elevation is again indicated on the altimeter. 29.94 – 29.69 = Difference 0.25 This pressure change is not as easily noticed in flight since (Since 1 inch of pressure is equal to approximately 1,000 feet aircraft fly at specific altitudes. The aircraft steadily decreases of altitude, 0.25 × 1,000 feet = 250 feet.) Always subtract true altitude while the altimeter is held constant through pilot the number from the indicated altitude. action as discussed in the previous section. 2,600 – 250 = 2,350 Knowing the aircraft’s altitude is vitally important to a pilot. The pilot must be sure that the aircraft is flying high Now, try a lower pressure setting. Adjust from altimeter enough to clear the highest terrain or obstruction along the setting 29.94 to 30.56 "Hg. intended route. It is especially important to have accurate Mineral Wells altimeter setting 29.94 altitude information when visibility is restricted. To clear Altimeter setting 30.56 obstructions, the pilot must constantly be aware of the altitude of the aircraft and the elevation of the surrounding terrain. To 29.94 – 30.56 = Difference –0.62 reduce the possibility of a midair collision, it is essential to maintain altitude in accordance with air traffic rules. (Since 1 inch of pressure is equal to approximately 1,000 feet of altitude, 0.62 × 1,000 feet = 620 feet.) Always subtract Types of Altitude the number from the indicated altitude. Altitude in itself is a relevant term only when it is specifically 2,600 – (–620) = 3,220 stated to which type of altitude a pilot is referring. Normally when the term “altitude” is used, it is referring to altitude The pilot will be 620 feet high. above sea level since this is the altitude which is used to depict obstacles and airspace, as well as to separate air traffic. Notice the difference is a negative number. Starting with the current indicated altitude of 2,600 feet, subtracting a negative Altitude is vertical distance above some point or level used as number is the same as adding the two numbers. By utilizing a reference. There are as many kinds of altitude as there are this method, a pilot will better understand the importance of reference levels from which altitude is measured, and each using the current altimeter setting (miscalculation of where may be used for specific reasons. Pilots are mainly concerned and in what direction an error lies can affect safety; if altitude with five types of altitudes: is lower than indicated altitude, an aircraft could be in danger 1. Indicated altitude—read directly from the altimeter of colliding with an obstacle). (uncorrected) when it is set to the current altimeter setting. Altimeter Operation There are two means by which the altimeter pointers can 2. True altitude—the vertical distance of the aircraft be moved. The first is a change in air pressure, while the above sea level—the actual altitude. It is often other is an adjustment to the barometric scale. When the expressed as feet above mean sea level (MSL). Airport, aircraft climbs or descends, changing pressure within the terrain, and obstacle elevations on aeronautical charts altimeter case expands or contracts the aneroid barometer. are true altitudes. This movement is transmitted through mechanical linkage to rotate the pointers. 8-6