Off-standard and design atmospheres
Although ISA provides an internationally agreed model for the atmosphere it does not take into account actual variations in pressure and temperature due to geographical or seasonal effects. This variability is engineered by means of off-standard or design atmospheres [6.8]. Within the actual atmosphere the temperature and pressure at any given altitude is highly variable and therefore off-standard atmospheres could have been produced with non-standard variations of both pressure and temperature with geopotential height. This is not in fact done as all published off-standard and design atmospheres assume an ISA variation of pressure with height and simply define differing temperature profiles. The off-standard atmospheres also assume that the lapse rate in the troposphere is the same as ISA (-0.0065 K/m) and simply specify a different sea-level temperature. Thus for example property tables for atmospheres ranging from ISA — 15°C to ISA + 15°C are available. Another approach is the so – called design atmosphere that is intended to replicate more closely the atmospheric conditions pertaining to a particular climatic region of the earth. Four standard design atmospheres have been defined: tropical maximum; temperate and arctic maximum; tropical and temperate minimum; and arctic minimum. These extremes were drawn up on the basis of conditions unlikely to be exceeded more often than one day per year.
The tropical maximum atmosphere has a sea-level temperature of 318.15 K (45°C) and uses the ISA lapse rate. The troposphere is assumed to extend up to just above 13 km (13 077 m). In the temperate and arctic maximum atmosphere the temperature starts at 303.15 K (30°C) and reduces by the ISA lapse rate until 10769 m is reached. Above this the lower stratosphere with its constant temperature is assumed to start. The tropical and temperate minimum atmosphere features a layer of constant temperature (253.15 K or — 20°C) starting at sea-level and ending at 1219 m (4000 ft). Above this a lapse rate of — 0.005 2917 K/m is assumed to apply up to 10 667 m (35 000 ft). The arctic minimum atmosphere is the most complex with three layers defined in the troposphere. Between sea-level and 1524 m (5000 ft) the temperature increases from 223.15 K ( —50°C) at lapse rate of 0.0097425 K/m (approximately 3°/ 1000 ft). In the next layer ending at 3047 m (10000 ft) the temperature is assumed to be constant at 238.15 K ( —35°C). Above this and up to the tropopause, set at 10 667 m (35 000 ft), a lapse rate of — 0.004 5932 K/m (1.4°/1000 ft) applies.
6.2.2 Measurement of air data
As implied above the measurement of altitude requires static pressure information. This is obtained from a port located on the outer skin of an aircraft. The static pressure is converted into pressure altitude by the altimeter using a calibration law based on Equation (6.3). Altimeters feature a sub-scale marked in milli-bars (mb), or inches of mercury, which the pilot can adjust thereby altering the meaning of the height information presented on the altimeter. Since the pressure altitude (Hp) of a point in any atmosphere is defined as the geopotential height in the Standard Atmosphere giving the same pressure, it follows that an altimeter will give a pressure altitude reading provided it has been set to show zero at a pressure equal to P0 (1013.25 mb). In flight testing references to height are usually in terms of pressure altitude and it is therefore very common to set the altimeter sub-scale to ‘1013’ before commencing data gathering.
An altitude that is rarely displayed but often calculated is density altitude (HD). This is the geopotential height in a standard atmosphere with the same density as that being experienced by the air vehicle. Since the forces acting on a wing, blade or body are a direct function of air density the behaviour of a helicopter will depend on its density altitude.