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Density Altitude -The Silent Killer

22 Oct 2019

We have published this before but as we in the Northern Provinces of South Africa are currently experiencing heatwave conditions it is of vital importance to keep Density Altitude in mind with every flight. On Saturday Brakpan with an Elevation of 5300ft had a density altitude of well over 8500ft we all know that this will wreak havoc on the performance of your aircraft.

 

Indicated Altitude, True Altitude, Absolute Altitude, Pressure Altitude and Density Altitude, enough to make one’s head spin, especially for someone just starting out. Let me start by defining each.

 

 

Indicated Altitude is the altitude shown on the altimeter.

 

True Altitude is height above mean sea level (MSL).

 

Absolute Altitude is height above ground level (AGL).

 

Pressure Altitude is the indicated altitude when an altimeter is set to 1013 hPa. It is primarily used in aircraft performance calculations and in high-altitude flight.

 

 

 

NOW WE COME TO “DENSITY ALTITUDE”.

 

Density Altitude is the altitude relative to the standard atmosphere conditions (International Standard Atmosphere) at which the air density would be equal to the indicated air density at the place of observation, or the height when measured in terms of the density of the air rather than the distance from the ground.” Now that’s a mouthful, but let me explain.

Pattern altitude, cruising altitude and field elevations are “True Altitudes”. These altitudes do not change. If your airport elevation is 700ft in the morning, it will still be 700ft at high noon, regardless of pressure or temperature. Now let’s look at what can change, Pressure altitude and Density altitude. Pressure altitude changes with air pressure, it is not a hard altitude but an altitude based on barometric pressure. Density altitude, which takes into account several factors, including Pressure altitude, temperature and humidity, is also an altitude number that changes.

 

Pressure altitude is directly related to air pressure. As air pressure increases so does the density of the air. This is due to the fact that pressure increases when more and more molecules of air are packed into the same area making the air denser.

 

Temperature has a big effect on air pressure and consequently density altitude. We have all learned in science class that heat expands and cold contracts, when we heat an item such as a piece of metal it expands and the molecules spread out, and on the flip side, as we cool the metal it contracts, the molecules draw together and become more dense. This holds true for air molecules as well, as the outside temperature rises the molecules spread out and the air becomes less dense and as the temperature drops the air compresses and becomes denser.

One more factor, though not as big of a factor is humidity. Moisture in the air further displaces air molecules, making the air less dense and increasing density altitude. Most calculators do not factor in humidity. A rule of thumb on highly humid days is to add 10% to the density altitude.

THE EFFECT OF DENSITY ALTITUDE

As the air becomes less dense the density altitude increases, as a result the aircraft performance will diminish. The wings produce less lift, the propeller loses forward thrust and the engine power output is reduced. This is because there is less air to flow over the wings, less air for the propeller to bite into and less air for the engine to mix with fuel and ignite. Take-off distance will be increased and vertical climb decreased making it more difficult to clear obstacles. In some cases you may not be able to climb at all. A low density altitude as we might expect does the opposite, denser cooler air allows for better performance, more airflow over the wings creates more lift, we get increased vertical climbs and better engine performance

To help in understanding density altitude let’s do a little math.

density altitude = pressure altitude + [120 x (OAT – ISA Temp)]

Now I know what you are saying, the R40 Sporty’s E6B app will figure this for me. Well you are right, and I use the same app, but hopefully this will help you understand density altitude. This is a rough estimate, however, it will get you close.

 

First, we need pressure altitude which can be easily calculated. OAT is “Outside Air Temperature” in degrees Celsius and ISA Temp is always 15 °C at sea level. To find ISA standard temperature for a given altitude, double the altitude, subtract 15 and place a – sign in front of it.

 

Example:

For ISA Temp at 8,000 feet, we multiply the altitude by 2 to get 16, subtract 15 from that and you get 1, then add a – sign and you get get -1.

 

LET’S CALCULATE DENSITY ALTITUDE

So, let’s say that your airport elevation is 1,000ft MSL, the outside air temp is 30ºC and your pressure altitude is 950ft.

 

Next we need to figure the ISA standard temperature.

Double your altitude of 1,000ft or 1 x 2 = 2   then we subtract 15   2-15 = -13   next add a minus sign. When you add a minus to a negative number it becomes positive so we get 13.

Now we will put this information into the formula:

density altitude = pressure altitude + [120 x (OAT – ISA Temp)]

 

density altitude = 950 + [120 x (30 – 13)]

30-13 = 17 x 120 = 2040 + 950 = Density altitude of 2,990.

 

The Sporty’s E6B app comes up with a density altitude of 2865. As I said, this is a rough estimate but it gets you close.

 

Based on this information, even though our example airport is at an altitude of 1,000ft MSL our aircraft will perform as if it were at 3,000ft MSL. For many aircraft this is a significant difference. You will need to use density altitude to determine if you have enough runway length to safely take off, or that your vertical climb will allow for a safe climb out. On hot humid days you will want consider your gross weight and maybe reduce it to ensure proper performance from the aircraft. While density altitude is important for pilots located in the lower altitude airports of the Lowveld or coastal areas, it is critical for areas of higher elevations such as the Highveld.

This post is for informational purposes only. If you would like to know how it applies to your flying, please consult a certified flight instructor.

 

Please fly SAFE 

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