Principle of Operation The pressure altimeter is simply an aneroid barometer that measures the pressure of the atmosphere at the level where the altimeter is located, and presents an altitude indication in feet. The altimeter uses static pressure as its source of operation. Air is more dense at the surface of the Earth than aloft, therefore as altitude increases, atmospheric pressure decreases. This difference in pressure at various levels causes the altimeter to indicate changes in altitude. The presentation of altitude varies considerably between different types of altimeters. Some have one pointer while others have more. Only the multipointer type will be discussed in this handbook. The dial of a typical altimeter is graduated with numerals arranged clockwise from 0 to 9 inclusive as shown in figure 3-2. Movement of the aneroid element is transmitted through a gear train to the three hands which sweep the calibrated dial to indicate altitude. The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand in thousands of feet; and the longest hand in hundreds of feet, subdivided into 20-foot increments. This indicated altitude is correct, however, only if the sea level barometric pressure is standard (29.92 in. Hg.), the sea level free air temperature is standard (+15° C or 59° F), and furthermore, the pressure and temperature decrease at a standard rate with an increase in altitude. Since atmospheric pressure continually changes, a means is provided to adjust the altimeter to compensate for nonstandard conditions. This is accomplished through a system by which the altimeter setting (local station barometric pressure reduced to sea level) is set to a barometric scale located on the face of the altimeter. Only after the altimeter is set properly will it indicate the correct altitude. Effect of Nonstandard Pressure and Temperature If no means were provided for adjusting altimeters to nonstandard pressure, flight could be hazardous. For example, if a flight is made from a high pressure area to a low pressure area without adjusting the altimeter, the actual altitude of the airplane will be LOWER than the indicated altitude, and when flying from a low pressure area to a high pressure area, the actual altitude of the airplane will be HIGHER than the indicated altitude. Fortunately, this error can be corrected by setting the altimeter properly. Variations in air temperature also affect the altimeter. On a warm day, the expanded air is lighter in weight per unit volume than on a cold day, and consequently the pressure levels are raised. For example, the pressure level where the altimeter indicates 10,000 feet will be HIGHER on a warm day than under standard conditions. On a cold day, the reverse is true, and the 10,000-foot level would be LOWER. The adjustment made by the pilot to compensate for nonstandard pressures does not compensate for nonstandard temperatures. Therefore, if terrain or obstacle clearance is a factor in the selection of a cruising altitude, particularly at higher altitudes, remember to anticipate that COLDER-THAN-STANDARD TEMPERATURE will place the aircraft LOWER than the altimeter indicates. Therefore, a higher altitude should be used to provide adequate terrain clearance. A memory aid in applying the above is “from a high to a low or hot to cold, look out below.” Setting the Altimeter To adjust the altimeter for variation in atmospheric pressure, the pressure scale in the altimeter setting window, calibrated in inches of mercury (in. Hg.), is adjusted to correspond with the given altimeter setting. Altimeter settings can be defined as station pressure reduced to sea level, expressed in inches of mercury. The station reporting the altimeter setting takes an hourly measurement of the station’s atmospheric pressure and corrects this value to sea level pressure. These altimeter settings reflect height above sea level only in the vicinity of the reporting station. Therefore, it is necessary to adjust the altimeter setting as the flight progresses from one station to the next. 14 CFR part 91 provides the following concerning altimeter settings: The cruising altitude of an aircraft below 18,000 feet mean sea level (MSL) shall be maintained by reference to an altimeter that is set to the current reported altimeter setting of a station located along the route of flight and within 100 nautical miles (NM) of the aircraft. If there is no such station, the current reported altimeter setting of an appropriate available station shall be used. In an aircraft having no radio, the altimeter shall be set to the elevation of the departure airport or an appropriate altimeter setting available before departure. Many pilots confidently expect that the current altimeter setting will compensate for irregularities in atmospheric pressure at all altitudes. This is not always true because the altimeter setting broadcast by ground stations is the station pressure corrected to mean sea level. The altimeter setting does not account for the irregularities at higher levels, particularly the effect of nonstandard temperature. It should be pointed out, however, that if each pilot in a given area were to use the same altimeter setting, each altimeter will be equally affected by temperature and pressure variation errors, making it possible to maintain the desired separation between aircraft. When flying over high mountainous terrain, certain atmospheric
conditions can cause the altimeter to indicate an altitude of 1,000 feet, or
more, HIGHER than the actual altitude. For this reason, a generous margin of
altitude should be allowed—not only for possible altimeter error, but also for
possible downdrafts which are particularly prevalent if high winds are
encountered. When over Mineral Wells, assume the pilot receives a current altimeter setting of 29.94 and applies this setting to the altimeter. Before entering the traffic pattern at Abilene Municipal Airport, a new altimeter setting of 29.69 is received from the Abilene Control Tower, and applied to the altimeter. If the pilot desires to fly the traffic pattern at approximately 800 feet above terrain, and the field elevation of Abilene is 1,778 feet, an indicated altitude of 2,600 feet should be maintained (1,778 feet + 800 feet = 2,578 feet rounded to 2,600 feet). The importance of properly setting and reading the altimeter cannot be overemphasized. Let’s assume that the pilot neglected to adjust the altimeter at Abilene to the current setting, and uses the Mineral Wells setting of 29.94. If this occurred, the airplane, when in the Abilene traffic pattern, would be approximately 250 feet below the proper traffic pattern altitude of 2,600 feet, and the altimeter would indicate approximately 250 feet more than the field elevation (2,028 feet) upon landing. Actual altimeter setting 29.94 The above calculation may be confusing, particularly in determining whether to add or subtract the amount of altimeter error. The following additional explanation is offered and can be helpful in finding the solution to this type of problem. There are two means by which the altimeter pointers can be moved. One utilizes changes in air pressure while the other utilizes the mechanical makeup of the altimeter setting system. When the aircraft altitude is changed, the changing pressure within the altimeter case expands or contracts the aneroid barometer which through linkage rotates 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. It is obvious then that if the aircraft is flown from a pressure level of 28.75 in. Hg. to a pressure level of 29.75 in. Hg., the altimeter would show a decrease of approximately 1,000 feet in altitude. The other method of moving the pointers does not rely on changing air pressure, but the mechanical construction of the altimeter. When the knob on the altimeter is rotated, the altimeter setting pressure scale moves simultaneously with the altimeter pointers. This may be confusing because the numerical values of pressure indicated in the window increase while the altimeter indicates an increase in altitude; or decrease while the altimeter indicates a decrease in altitude. This is contrary to the reaction on the pointers when air pressure changes, and is based solely on the mechanical makeup of the altimeter. To further explain this point, assume that the proper altimeter setting is 29.50 and the actual setting is 30.00 or a .50 difference. This would cause a 500-foot error in altitude. In this case if the altimeter setting is adjusted from 30.00 to 29.50, the numerical value decreases and the altimeter indicates a decrease of 500 feet in altitude. Before this correction was made, the aircraft was flying at an altitude of 500 feet lower than was shown on the altimeter. Types of Altitude Knowing the aircraft’s altitude is vitally important to the pilot for several reasons. The pilot must be sure that the airplane is flying high enough to clear the highest terrain or obstruction along the intended route; this is especially important when visibility is restricted. To keep above mountain peaks, the pilot must note the altitude of the aircraft and elevation of the surrounding terrain at all times. To reduce the possibility of a midair collision, the pilot must maintain altitudes in accordance with air traffic rules. Often certain altitudes are selected to take advantage of favorable winds and weather conditions. Also, a knowledge of the altitude is necessary to calculate true airspeeds. Altitude is vertical distance above some point or level used as a reference. There may be 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 usually concerned, however, with five types of altitudes: • Absolute Altitude—The vertical distance of an aircraft above the terrain. • Indicated Altitude—That altitude read directly from the altimeter (uncorrected) after it is set to the current altimeter setting. • Pressure Altitude—The altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92. This is the standard datum plane, a theoretical plane where air pressure (corrected to 15° C) is equal to 29.92 in. Hg. Pressure altitude is used for computer solutions to determine density altitude, true altitude, true airspeed, etc. • True Altitude—The true vertical distance of the aircraft above sea level—the actual altitude. (Often expressed in this manner; 10,900 feet MSL.) Airport, terrain, and obstacle elevations found on aeronautical charts are true altitudes. • Density Altitude—This altitude is pressure altitude corrected for
nonstandard temperature variations. When conditions are standard, pressure
altitude and density altitude are the same. Consequently, if the temperature is
above standard, the density altitude will be higher than pressure altitude. If
the temperature is below standard, the density altitude will be lower than
pressure altitude. This is an important altitude because it is directly related
to the aircraft’s takeoff and climb performance.
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