Flight Training - August 2012 - 45

ORDINARY AIRPLANE ALTIMETERS are called “pressure altimeters” because they use air pressure to measure height above sea level. This gives accurate altitudes—as long as needed adjustments are used— because atmospheric pressure decreases with height, but not at a steady rate. The abbreviated Standard Atmosphere chart shows how. Nevertheless, when you’re near the ground, the rule of thumb that says atmospheric pressure decreases by one inch of mercury for each 1,000 feet in altitude gained works pretty well. In fact, you should use this figure for altimeter questions on FAA knowledge exams. An altimeter is a barometer that’s calibrated to read in feet above sea level instead of inches or mercury or millibars as barometers do. Altimeters have traditionally been sensitive aneroid barometers. An aneroid is a flexible metal bellows from which some air was removed before it was sealed. Pressures lower than the inside pressure allow the bellows to expand; higher pressures squeeze it, making it shorter. A lever attached to one end of the bellows moves as the pressure changes to point to a number on a dial indicating the surrounding air pressure. Altimeters are available that replace mechanical aneroids with pressure transducers, which converts pressures into

5,000 feet

25.74 inches of mercury

4,000 feet

5,000 feet indi

cated altitude

25.74 inches of mercury

Airport A 30.74 altimeter setting

Sea level

Airport B 29.74 altimeter setting

IF YOU FLY MORE THAN a few miles from your takeoff point, you need to set your altimeter to the nearest airport both en route and at your destination.

STANDARD ATMOSPHERE
Altitude (feet) Surface 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 Pressure (in. Hg) 29.92 28.86 27.82 26.82 25.84 24.89 23.98 23.09 22.22 21.38 20.57 Temperature (F) 59.0 55.4 51.9 48.3 44.7 41.2 37.6 34.0 30.5 26.9 23.3

Source: Aerodynamics for Naval Aviators

electrical signals that can be used to give altitude in digital form instead of with the traditional pointers. Atmospheric pressures lower than the surrounding pressure are associated with foul weather. The centers of storms are areas of low atmospheric pressure, while areas of high pressure are associated with clear, calm weather. For an altimeter to give accurate indications, adjustments must be made for these pressure changes. To do this you need a figure that all weather stations report called the altimeter setting. Before taking off for a flight your checklist should include altimeter setting, which you can obtain via radio if the airport has automated weather reporting or a tower with automated terminal information service—ATIS. An altimeter setting from a nearby airport would work fine. Except in the centers of strong storms, atmospheric pressures don’t change much over a few miles. When you have the setting you turn the knob to the setting number next to the pointer in the Kollsman window. When you set your altimeter to the correct setting it should read the elevation of where you’re sitting. If it doesn’t, your altimeter might need calibration. If a nearby altimeter setting isn’t available, set your altimeter to show the airport elevation when you are still on the ground. The figure above shows what happens when you don’t set your altimeter when

flying into an area of low atmospheric pressure. At Airport A, you set your altimeter to 30.74 and since you’re at sea level, it reads O altitude. As you climb, the altimeter senses lower and lower pressure until the pressure is 25.74 inches of mercury. The altimeter reads 5,000, and you level off and head for Airport B. Since the surface pressure is lower at B than at A, all of the levels aloft at a particular pressure slope down from A to B following the line labeled “Indicated 5,000 feet.” You’re so busy holding an indicated altitude of exactly 5,000 feet and enjoying the view that you neglect to obtain the altimeter setting for Airport B and do not reset your altimeter. As you arrive over B, your altimeter still reads 5,000 feet. But, how high are you actually above the sea level airport? When you reset your altimeter to Airport B’s 29.74 reading, you see that you are actually 4,000 feet above the ground. You can remember this example with the saying, “high to low, look out below.” If you fly toward an area of lower pressure you need to look out below because things such as mountaintops and television towers could be closer than you think. If in this example you landed, and then reset your altimeter to the airport’s setting of 29.74 and flew back to Airport A without setting your altimeter at an indicated altitude of 4,000 feet, you would again follow the orange line to be higher than indicted at A. You could

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Flight Training - August 2012

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