AP3456 – 5-1 - Introduction to Barometric Height Measurement CHAPTER 1 - INTRODUCTION TO BAROMETRIC HEIGHT MEASUREMENT Introduction 1. The atmospheric pressure at a point on the Earth’s surface is equivalent to the weight of the whole column of air standing on the area of that point. As distance increases from the Earth, the weight of the air above will be less, therefore atmospheric pressure decreases (Fig 1). Pressure altimeters operate on this principle, and indicate aircraft height relative to a selected pressure datum. Pressure altimeters are, in fact, aneroid barometers graduated to indicate height rather than pressure. In order for such an instrument to be calibrated, certain assumptions must be made concerning the manner in which air pressure decreases with height and this has given rise to a number of model atmospheres. 2. Units of Measurement. The units used in pressure measurement are: a. Hectopascal. The hectopascal (hPa) is the unit of measurement of pressure in common use. At mean sea level (MSL), the atmospheric pressure is of the order of 1,000 hPa; at 50,000 ft it is of the order of 100 hPa. b. Inches of Mercury. Some countries (notably the USA), measure pressure in inches of mercury (Hg). At MSL, atmospheric pressure is about 30 inches Hg. c. Millibar. Although the hPa is now in common usage, the millibar (mb) is still used by many aircrew. The hPa and the mb have equivalent values and so can be considered to be identical for all practical purposes. Both the hPa and the mb will be found in AP3456, but references to the mb will be gradually replaced by the hPa as chapters are routinely checked and amended. 5-1 Fig 1 Decrease in Atmospheric Pressure with Height Limit of Atmosphere Column of Air Weight of air in upper portion of column applies at height Weight of air in whole column applies at surface Revised Mar 12 Page 1 of 4 AP3456 – 5-1 - Introduction to Barometric Height Measurement The Atmosphere 3. The atmosphere is described in detail in Volume 1, Chapter 1. It is a relatively thin layer of gases surrounding the Earth, becoming more diffuse with increasing height. Water vapour is present in variable amounts, particularly near the surface. 4. The atmosphere can be divided into a number of layers, each with a tendency to a particular temperature distribution. The names, heights and characteristics of these layers may vary according to which standard atmosphere is being defined. However, in all cases the lower layer, the troposphere, is characterized by a fairly regular decrease of temperature with height. The upper limit of the troposphere is named the 'tropopause'. The height of the tropopause varies with latitude, season and weather. In general, it is lowest at the Earth’s poles (around 25,000 ft) and highest over equatorial regions (up to 54,000 ft). The layer above the tropopause is known as the stratosphere. Within this layer, the temperature is assumed to remain more or less constant, although, in reality, there is a noticeable increase near the top of the layer. The upper boundary of the stratosphere is called the 'stratopause', the height of which varies depending on which definition is being employed, but can be taken to be about 30 miles (166,000 ft). 5. Pressure Lapse Rate. As height increases, pressure decreases. However, this decrease is not proportional to the increase in height because the density of air varies with height. It is possible to deduce an expression for the pressure lapse rate at a constant temperature and thus establish a relationship between pressure and height. A practical approximation for the lower levels of the atmosphere, close to sea level, is that a decrease in pressure of one hPa equates to an increase in height of 30 feet. 6. Temperature Lapse Rate. Temperature varies with height in a complex manner. The temperature lapse rate depends on the humidity of the air, and is itself a function of height. This variation greatly affects the relationship between pressure and height. To calibrate an altimeter to indicate barometric height it is necessary to make some assumptions as to the temperature structure of the atmosphere. The relationship can be expressed in mathematical form for each of the various layers of the atmosphere and the instrument can then be calibrated accordingly. 7. Height Assumptions. The correlation between indicated barometric altitude and actual altitude is poor because of: a. Variations in conditions of temperature and pressure. b. The assumptions used in altimeter calibration. c. Real errors induced by the instrument itself. A barometrically derived height must therefore be used with extreme caution as a basis for terrain clearance. However, provided that all aircraft use the same datum (and the same assumptions in the calibration of their altimeters), safe vertical separation between aircraft can be achieved. Standard Atmospheres 8. A standard atmosphere is an arbitrary statement of conditions which is accepted as a basis for comparison of aircraft performance and calibration of aircraft flight instruments. Because of the extreme variability of conditions in the atmosphere, the standard can only represent the average conditions over a limited area of the globe. Most standards so far adopted are related primarily to the mean atmospheric conditions in temperate latitudes of the northern hemisphere. Revised Mar 12 Page 2 of 4 AP3456 – 5-1 - Introduction to Barometric Height Measurement 9. The first widely accepted standard was proposed by the International Commission on Air Navigation (ICAN) in 1924. Between 1950 and 1952 the International Civil Aviation Organization (ICAO) proposed and adopted another standard which varied only slightly from the ICAN model. Equations were formulated for determining height from barometric pressure which were valid up to 65,617 ft. The ICAO standard atmosphere is taken as the International Standard Atmosphere (ISA) and the assumed characteristics are: a. The air is dry and its chemical composition is the same at all altitudes. b. The value of g is constant at 980.665 cm/sec2. c. The temperature and pressure at mean sea level are 15 ºC and 1013.25 hPa, respectively. d. The temperature lapse rate is 1.98 ºC per 1,000 ft up to a height of 36,090 ft above which the temperature is assumed to remain constant at –56.5 ºC. 10. A number of other standard atmospheres have been formulated, mainly in response to the need to extend the height limit of the model beyond 65,617 ft to accommodate the requirements of missiles and certain high performance aircraft. The assumptions of these models are very similar to the ICAO standard and the differences in the relation of height to pressure are minimal in the lower altitudes. However, in the stratosphere and beyond, heights, lapse rates and layer names differ markedly. A comparison of Fig 2, which depicts the Wright Air Development Centre (WADC) Standard Atmosphere with Fig 1 in Volume 1, Chapter 1 will reveal some of the differences. Revised Mar 12 Page 3 of 4 Relative Density % Density Relative hPa Pressure C Deg Temp Altitude in Feet 1000 135 105 125 115 15 25 35 45 55 65 75 85 95 5 AP3456 – 5-1 AP3456 - Introduction– 5-1 to Barometric Measurement Height 8.242 hPa 8.242 1.97 hPa 1.97 –50 –40 –30 –20 –10 0 5-1 Fig 2 WADC Standard Atmosphere Standard Atmosphere 5-1 FigWADC 2 –56.5° 54.26 hPa 54.26 Revised Mar 12 12 Mar Revised 200 20 T e Upper Limit of ICAO ISA - 65,617 ft (20 km) (20 ft 65,617 - ISA ICAO of Limit Upper m hPa 226.65 Upper Limit of Wright Air Development Air Wright of Limit Upper p P e r r a ft 140,000 Atmosphere Centre e t s 400 u s 40 r u e R r e e km) (32 ft 104,987 Stratopause l a t i v km) (11 ft 36,090 Tropopause e D +15° e Page Page 4 of 4 n 600 60 s i t y 10 20 30 800 80 +22.473° 1000 100 Troposphere Stratosphere Chemosphere AP3456 - 5-2 - Altimeters CHAPTER 2 - ALTIMETERS PRINCIPLE OF OPERATION The Simple Altimeter 1. Air pressure is linked to height and this relationship is exploited by altimeters, which measure pressure but display height. 2. Fig 1 is a schematic diagram showing the main components of a basic, or 'simple' altimeter. Changes in air pressure are detected by an aneroid capsule (constructed from thin, corrugated metal) which is sealed and partially evacuated, and mounted inside a case. The case is fed with static pressure from the aircraft’s static tube or vents. As the aircraft climbs, the static pressure in the case will reduce, and the aneroid capsule will expand, assisted by a leaf spring. The linear movement of the capsule face is magnified and transmitted, via a system of gears and linkages, to a pointer moving over a scale (graduated in feet according to one of the standard atmospheres). Conversely, as the aircraft descends, the static pressure increases, and the capsule is compressed. An equilibrium is maintained between the pressure of the atmosphere on the face of the capsule and the tension of the spring. 5-2 Fig 1 Simple Altimeter – Schematic 3. A simple altimeter will normally be calibrated according to the International Standard Atmosphere (ISA), and will, therefore, normally be set to indicate height above the 1013.25 mb pressure level. The dial setting knob allows the indicator needle to be moved away from the normal datum. Thus, for example, before take- off, the altimeter could be set to read airfield elevation, so that it will thereafter indicate height above mean sea-level (providing that the prevailing sea-level pressure does not change).