Density height

Diagram for determining the density height

The density altitude (English: Density altitude ) describes the current density of the air at a specific location, that is the air density, for example, on an airfield or on a particular flight level . Instead of specifying the density of the air, which is dependent on altitude, temperature and air pressure, as usual, in aviation the current air density is described as the altitude that corresponds to this air density in the standard atmosphere . ${\displaystyle kg/m^{3}}$

The density altitude can deviate upwards and downwards from the actual flight altitude. For example, an airplane flying 5000 feet above sea level will encounter an air density equivalent to an altitude of 6000 feet in the standard atmosphere. The aircraft's density altitude in this case is 6000 feet, even though it is flying at 5000 feet. The efficiency of the engines, the wings and the propellers is as if the aircraft were flying in the standard atmosphere at 6000 feet above sea level.

Derivation and meaning

Air density is the particle density of the gas mixture air, which essentially depends on pressure, water vapor content and temperature. The reference value for the density level is therefore initially the so-called standard atmosphere. It stipulates that the air pressure at sea level is always 1013.25 mbar and the air temperature is 15 ° C. Pressure and temperature decrease with increasing altitude according to physical laws. So every known density value of the air can be assigned a certain, fixed height. The performance data of an aircraft depend largely on this. Meteorological factors such as humidity and especially temperature now vary in time and place depending on the weather. As a result, density and density altitude are not constants, but must always be recalculated in relation to the standard atmosphere and must be taken into account during flight planning. On a hot day, the particle density is lower, so the air is "thinner". As a carrier medium, it generates less buoyancy, which instead has to be generated by higher speed. The runway length required for the aircraft to take off is therefore longer. The aircraft therefore takes off at a higher density altitude than the actual altitude of the airfield. The same applies to the climb performance of the aircraft. The take-off at an airfield with an actual height of 1500 feet above sea level normal height must therefore be planned with lower air density as if the airfield were higher, e.g. B. at 2000 feet. On a cold day, however, the air becomes denser and therefore more stable. For the same aircraft to take off at the same airfield, the take-off distance is then shorter - it takes off earlier and climbs faster. With the same runway length at the same altitude above sea level, the aircraft can also be heavily loaded at a lower density altitude, while conversely, at a high density altitude, the aircraft cannot be completely refueled or loaded.

Concrete consequences of the density level

As mentioned above, take-off and landing distances depend on the density altitude. If the density is high, loading and refueling must be reduced. However, less fuel may make undesired stopovers necessary. In addition, pilots tend to move their flights to the cool morning or evening hours at high altitude.

Mountains and passes can only be safely flown over if the density altitude allows sufficient power reserves. The density altitude is also the reason why aircraft sometimes cannot reach the so-called service ceiling - while on very cold days it can be exceeded without any problems.

The use of the landing flaps is tempting . With the flaps extended more, a higher lift can be achieved at the same speed, a shorter take-off roll distance and a larger climb angle , but this is at the expense of the rate of climb , which is particularly important in mountain flying. A high angle of climb, on the other hand, is aimed for when obstacles close to the airport have to be overflown. The use of the landing flaps also requires a higher engine output, which is in turn hampered by the density altitude.

A diagram - the so-called Koch chart - can be used to determine, based on the pressure altitude and the air temperature, by how many percent the take-off distance increases and by how many percent the rate of climb decreases.

Calculation of the density height

The graphic above shows how the density altitude is derived from the pressure altitude and the currently prevailing temperature. The density altitude changes by 120 feet for each degree of deviation from normal temperature for the given print altitude. The pressure altitude is the altitude in the standard atmosphere, at which the same pressure prevails as at the place of our observation. The pressure altitude changes by 30 feet for every hPa pressure deviation from the standard pressure at sea level. To determine the density height, the pressure height (from the geographical height and pressure deviation from normal pressure) must first be determined. The density level (from the pressure level and the deviation from the standard temperature) can then be determined.

${\displaystyle {\text{Druckhöhe}}(ft)={\text{Geografische Höhe}}-30{\frac {ft}{hPa}}\cdot {\text{Abweichung vom Normaldruck}}(hPa)}$
${\displaystyle {\text{Abweichung von Normtemperatur}}(^{\circ }C)={\text{OAT}}(^{\circ }C)-{\text{ISA-Normtemperatur}}(^{\circ }C)}$
${\displaystyle {\text{Dichtehöhe}}(ft)={\text{Druckhöhe}}(ft)+120{\frac {ft}{^{\circ }C}}\cdot {\text{Abweichung von Normtemperatur}}(^{\circ }C)}$

• The pressure altitude can be read when an aircraft is on the ground by setting the altimeter to 29.92 inches of mercury (= 1013 hPa ).
• OAT, the actual outside air temperature in degrees Celsius (outside air temperature)
• The ISA temperature is 15 ° C at sea level and it drops 2 ° C for every 1000 feet of altitude. So at 9,000 feet, the ISA temperature is −3 ° C.

example

The Samedan airfield is located at 5600 feet or 1707 m above sea level. The standard temperature for this altitude is 4 ° C. The current temperature is 25 ° C. The air pressure at sea level (QNH) at Samedan is currently 1000 hPa.

The current air pressure is 13 hPa below normal pressure. The pressure height is thus above the geographic height. The pressure altitude is thus 5990 feet. The "new" standard temperature for this altitude is now only 3 ° C. (5990 feet / 1000 feet per 2 ° C is 12 ° C, so 15 ° C - 12 ° C is 3 ° C). The current temperature is 22 ° C above the standard temperature. The density level is thus above the pressure level. The density altitude is thus 8630 feet. ${\displaystyle 13hPa\cdot 30{\frac {ft}{hPa}}=390ft}$ ${\displaystyle 22\cdot 120ft=2640ft}$

So the pilot has to plan the flight as if he were not taking off at 5600 but at 8630 feet.

Web links

• Notes on Density Altitude , Aircraft Owners and Pilots Association .
• Educational film about the density height. FAA , 1966 (English).

Individual evidence

1. ^ Density Altitude. In: AOPA. Retrieved August 7, 2018 . 
2. ↑ See the English Wikipedia page Pressure altitude