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 part of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. 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, is of great interest to the pilot (as it should be). 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 equiped 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.
Do not forget 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 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 they lower 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 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 Altitude
Pressure altitude 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 pressure altitude. That's easy when you have an aircraft or altimeter nearby, but what when you only have the regional QNH and local elevation/altitude?
With a small but effective formula it is easy: Pressure Altitude = Altitude + (1013 - QNH) × 27. Its obvious that when QNH is higher than 1013 (QNE) that result will be negative and deducted from altitude, or, put another way: add 27ft for every hPa below 1013 hPa (common in low pressure area's) or when QNH is lower than 1013 hPa -> Pressure Altitude will be higher (27 ft/hPa) and vice versa.
Pressure Lapse Rate
To calculate the exact pressure lapse rate use this formula: PLR = 96 ( T in kelvin) / local pressure (in hPa). For example at 15°C and ISA QNH: 96 (15+273) / 1013,25 = 27,3 ft/hPa.
Density Altitude
Density Altitude Chart
Density altitude is the combined effect of the factors mentioned above. It is defined as pressure altitude 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. When air density decreases, aircraft performance decreases.
Rule of thumb: Density altitude can be calculated by taking pressure altitude and adding (or subtracting) 120 feet for each 1 °C difference above (or below) the standard atmosphere at that altitude. Formula: Density Altitude = Pressure Altitude + (OAT - ISAT) × 120. OAT is actual outside air temperature and ISAT is ISA temperature at this pressure altitude. Or put another way: if OAT is lower than ISA -> Density Altitude will be lower than Pressure Altitude (120ft/°C) and vice versa.
For a larger density altitude chart click here or on the small image. Usage: first set a horizontal line representing the temperature, then draw pressure altitude and drop down vertical to obtain density altitude.
When taking off at a density altitude above ISA sea level, you still will see the same indicated airspeed. But because air density is lower the true airspeed will be higher and thus ground speed is higher. To get to the same indicated airspeed with the same engine power you will need more runway.
Note: Aircraft indicated stall speed is always the same when density altitude is higher or lower. Density altitude influences the true airspeed / ground speed and not the indicated airspeed.
Try this one day (only do this when you have adequate runway available): To simulate a take-off from 3000 feet field elevation, ie full throttle (30" manifold pressure) open the throttle to maximum of 27" manifold pressure and note how much more runway you will need and feel the difference in acceleration 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. Propeller (and wing) efficiency is also reduced at higher density altitudes (for fixed and controllable types).
In conclusion
The effects of density altitude on take-off and landing are:
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High density altitudes are most commonly found 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.
