Density altitude is a term used to describe the altitude in the International Standard Atmosphere (ISA) at which the air density would be equal to the actual air density at a certain location.
If you’re not familiar with the term then you probably had to read that twice. It’s all to do with the temperature of the air and the effect a higher or lower temperature has on your aircraft’s performance.
It is a pressure altitude corrected for non-standard temperature. Higher density altitudes mean thinner air and reduced engine and wing performance, affecting aircraft takeoff, climb, and cruise performance.
Why is it important for pilots to know what the density altitude is?
Pilots need to determine the density altitude because it directly impacts the performance of an aircraft. As the density altitude increases, the air becomes less dense and provides less lift. It affects the engine’s power and therefore the thrust. This means that the aircraft will require longer runways for takeoff, will have a slower climb rate, and reduced engine performance during flight.
As you can see, knowing the density altitude is crucial for safe and efficient flight operations, especially in hot and high altitude environments. It helps pilots to make informed decisions about the takeoff and landing distances, and to adjust their flight plan to compensate for the reduced performance of the aircraft.
What is meant by the expression ‘hot and high’?
“Hot and high” refers to airport environments that are characterised by high temperatures and high altitudes. High temperatures can cause the air to become less dense, making it more difficult for aircraft to generate lift and attain takeoff and landing speeds. High altitude can exacerbate the effect of high temperatures as the air becomes even less dense with increasing altitude.
This results in a decrease in engine performance, making it more challenging for aircraft to generate enough power to maintain altitude. When operating in hot and high environments, pilots must be aware of the impact of temperature and altitude on aircraft performance and adjust their flight plans accordingly.
How is density altitude calculated?
Density altitude is calculated using a combination of pressure altitude and temperature. Pressure altitude is determined by setting an altimeter to a standard pressure setting of 29.92 inches of mercury (1013.25 millibars). Then, the pressure altitude is corrected for temperature using a standardised temperature deviation table or a flight computer.
The formula for calculating density altitude is as follows:
Density Altitude = Pressure Altitude + (120 x (OAT – ISA Temperature))
- Pressure Altitude is the altitude indicated on an altimeter set to 29.92 inches of mercury
- OAT (Outside Air Temperature) is the actual temperature at the airport or location
- ISA Temperature is the standard temperature for a given altitude in the International Standard Atmosphere (ISA)
- The factor of 120 is used to convert the temperature deviation from degrees Celsius to feet
Tools For Calculating Altitude Density
There are several tools available for pilots to calculate the density altitude both during preflight planning and while flying:
- Density Altitude Calculators: Pilots can use online density altitude calculators or mobile apps to calculate the density altitude based on the pressure altitude and outside air temperature.
- Flight Planning Software: Many flight planning software programs have built-in density altitude calculators that allow pilots to easily determine the density altitude for a given flight.
- Flight Computers: Pilots can use flight computers or electronic flight bags (EFBs) to calculate the density altitude in real-time during a flight.
- On-board Weather Systems: Some aircraft are equipped with on-board weather systems that can calculate the density altitude in real-time and provide this information to the pilot.
- Charts and Tables: Pilots can also use density altitude charts and tables, which provide pre-calculated values for a given pressure altitude and outside air temperature.
As with any tool options, the important thing is to know how to use it correctly and to be confident that the results it generates are accurate, and that they can be applied accordingly in any decision making process.