Effects of Air Density
Most aircraft accidents occur during the take off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then.
In this section of the site we will take a look at the various factors contributing to the performance of the aircraft during take off and landing. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
Our first performance factor, air density, should be of great interest to the pilot. For example, aircraft all up weight is something we can do about but air density depends on more factors, as we will see, and most of them we can not influence at all.
If your aircraft is equipped with a turbo- or supercharged engine then the influence of air density doesn't really affect the engine until it reaches a certain altitude where even the turbo can not compensate anymore for the loss in air density.
Remember that the engine can be compensated for a loss in air density, but this will not apply for the propeller and wings.
Air Density & Pressure Altitude
When air density decreases both engine and aerodynamic performance decreases. The reason being is that air molecules are further apart from each other (thus there are less air molecules per m3 or ft3).
A number of factors (altitude/pressure, temperature and humidity) influence the air density. A higher altitude, low pressure area, higher temperature and high humidity all have one result: they lower the density of the air. And as a result of that: reduced aircraft performance.
International Standard Atmosphere
A standard atmosphere has been established to enable comparison of aircraft performance, calibration of instruments (altimeters, transponders encoders, etc) and for meteorological purposes. There are a number of hypothetical atmospheric conditions set: sea level pressure is 1013,25 hPa (29.92 inHg), a temperature of 15°C, pressure lapse rate is 1 hPa per 27 feet at lower altitudes (1" per 1000 feet), mass of 1,225 kg/m3, temperature drop of 1,98°C per 1000 feet (300 m) up to 36000 feet and the speed of sound is 340,3 m/s (1225 km/h or 661,4 kts).
Pressure Lapse Rate
This formula: PLR = 96 × ( T in kelvin) / QNH (in hPa) is used to calculate the exact pressure lapse rate. For example at 15°C and ISA QNH: 96 × (15+273) / 1013,25 = 27,3 ft/hPa.
Density Altitude Chart
Pressure Altitude, PA
This is altitude or elevation corrected for non standard pressure. In short: set the Kollsman window on the altimeter to 1013 hPa / 29,92 inHg and you will have your PA. Easy when you have an aircraft or altimeter nearby, but what to do when you only have the regional QNH and local elevation or altitude?
With a small but effective formula it is easy:
Density Altitude, DA
This is the combined effect of the factors mentioned above. It is defined as pressure altitude (PA) in the standard atmosphere corrected for non standard temperature. The performance of your aircraft depends on air density: which has a direct effect on lift, drag, engine performance and the propeller. Thus you can say that when air density decreases, aircraft performance decreases.
Rule of thumb: DA can be calculated by taking PA and adding (or subtracting) 120 feet for each 1°C difference above (or below) the standard atmosphere at that altitude.
The formula for dry air conditions is:
When taking off at any DA above ISA sea level, you will still need the same indicated airspeed. But because air density is lower the true airspeed will be higher and thus your groundspeed is higher. To get to the same indicated airspeed you will also need more runway. And as the engine is also reduced in power output, this will increase the need for more runway.
Note: that your indicated stall speed is always the same regardless when DA is higher or lower. DA influences the true airspeed / groundspeed and not indicated airspeed of the aircraft.
Normally aspirated engines
When your engine is not equipped with a turbo- or supercharger it will also suffer from the less dense air. Each intake stroke (which is by volume) will contain less air molecules and thus less power can be developed by the engine (due to the fixed fuel / air ratio). Propeller (and wing) efficiency is also reduced at higher density altitudes (for both fixed and controllable types).
High elevation airports
You will run into high density altitudes at high elevation airports in combination with high ambient temperatures. When there is also a low atmospheric pressure system this will accentuate the effect even more. Taking off in these conditions can be dangerous, make sure to check all related performance charts for your aircraft.