C LIMATE AND A TMOSPHERIC T HERMODYNAMICS
3.4 Pressure
In an aircraft, static air pressure is measured by a static pressure system connected to a static port, and it is used in the pitot-static system connected to the altimeter, Vertical Speed Indicator (VSI), airspeed, and gyroscopic instruments—Figure 7. The static port is a small hole located on the main body of the fuselage [62].
FIGURE 7 Pitot-static systems (drawings creat ed using Solid Edge CAD tool).
3.4.2 Stagnation Pressure
The stagnation pressure, also known as the total pressure (Pt), is the pressure caused by a moving fluid. The pitot tube—connected to the exterior surface of an aircraft fuselage with its open end pointing toward the direction of the aircraft motion—measures the airspeed by measuring the dynamic pressure, the difference between the total pressure from the incoming ram air and the static pressure from the static port [63].
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The total pressure (Pt) is defined based on Bernoulli’s law presented by equation (11), where Ps is the static pressure, is the density of the fluid in which the measurement is done (air in this case), and v is the airspeed.
For an isentropic process (i.e., reversible and adiabatic), equation (12) is obtained. In this equation, Tt is the total temperature, Ts is the static temperature, M is Mach number, and is the ratio of the specific heat capacity at constant pressure (cP) to the specific heat capacity at constant volume (cv)— cp/cV
2 t s 2 P P v
(11)
2 1
1 1
1 2
t
t s s
s
T M
P P P
T
(12)
3.4.3 Pressure Altitude
The average elevation of the surface of multiple bodies of water (e.g., oceans) from which the terrain elevation is to be measured is known as the Mean Sea Level (MSL)—it is a vertical reference datum. Terrain elevation where the dry surface is present is called the Ground Level (GL). Elevation measured from the GL is referred to as Above Ground Level (AGL). Flight Level (FL) is the vertical altitude expressed in hundreds of feet as indicated on the altimeter when it is set to the standard pressure (101,325 Pa, 1 atm, 29.92 inHg, 14.67 psi) [64,65]. Thus, 21,000 feet would be expressed as FL210.
QNE is the altitude indicated on the altimeter when one is located at the runway (or another location of interest) and the altimeter is set to the standard pressure [66]. Quasi Non-Hydrostatic (QNH) is related to the pressure altitude adjusted for the MSL [67]. When set on the altimeter, it should give the correct altitude above the MSL. Above the transition altitude (above 18,000 ft in Canada and the United States, 13,000 ft in Australia, and as low as 3,000 ft in some parts of Europe), the altimeter is set to the international standard atmosphere at MSL (101,325 Pa, 1 atm, 29.92 inHg, 14.67 psi).
Pressure altitude (PA) is defined as the altitude observed in the International Standard Atmosphere (ISA)—when the pressure at the MSL is equal to 29.92 inHg (101,325 Pa, 1 atm, 14.67 psi)—at which the pressure is equal to the pressure under the current atmospheric conditions. Thus, if the current atmospheric pressure is lower than the standard pressure, the pressure altitude will be higher than the actual one. Pressure altitude (PA) is calculated using equation (13) as the actual elevation (H) plus the
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correction, which is proportional to the difference between the standard pressure (101,325 Pa, 1 atm, 29.92 inHg, 14.67 psi) and the actual current pressure at MSL (PinHg). The latter pressure would be the one set on the aircraft’s altimeter.
PA H + 1000(29.92 PinHg) (13) The highest barometric pressure, as reported in Guinness Book of Records (1,083.8 mb- 32 inHg), was recorded at Agata, Siberia, Russia, at an altitude of 862 ft (262 m), on December 31, 1968. This pressure is equivalent to the standard pressure at an altitude of 2,080 ft (600 m) below the MSL. The highest barometric pressure reported in the United Kingdom was 1,054.7 mb (31.15 in), in Aberdeen on January 31, 1902 [68].
This pressure is equivalent to the standard pressure at an altitude of 1,230 ft (374.9 m) below the MSL. The lowest barometric pressure, as reported in Guinness Book of Records (870 mb- 25.69 inHg), was recorded on October 12, 1979 by the United States Air Weather Service at 483 km (300 mi) west of Guam in the Pacific Ocean in the eye of Super Typhoon Tip. Wind speeds of 165 kt (305 km/hr- 190 mph) were reported [69]. This pressure is equivalent to the standard pressure at an altitude of 4,230 ft (1,289.3 m) above the MSL. You may use said data in order to solve for the pressure altitude by adding the altitude at standard pressure to the elevation figure.
For example, for case of Agata, Siberia, Russia, you will obtain a pressure altitude equivalent to a location that is 1,228 ft below the MSL.
The barometric pressure is how the height above the MSL is measured by an aircraft’s altimeter, which is part of the pitot-static system—Figure 7.
The sealed (airtight) case includes a vent to the static port. The altimeter is an aneroid-based barometer. The altimeter is a sensitive instrument (traditionally vacuum-based with some advanced electric ones with analog displays) that may over-read or under-read if there is any blockage of the nozzles. As the aircraft increases its altitude, the air pressure decreases.
The exterior pressure must be balanced by the capsule’s interior pressure;
therefore, the capsule expands to equally decrease its own interior pressure, thus balancing the forces. With decreasing altitude the air pressure increases, causing the capsule to reduce in volume, therefore making the wafer contract. The expansion or contraction of the aneroid wafer causes the connected rocking shafts and associated gears to move, and therefore the end-pointer rotates, indicating numbers on the instrument’s dial gauge.
If the static port is blocked for whatever reason, such as icing, the case is isolated from the surroundings, and therefore with increasing altitude, the altimeter reading does not change—it remains at the last valid reading—
Figure 7.
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