Flight School Newsletter - April/May 2018
It's Getting hot Hot HOT
What is Density Altitude?
It is getting warmer outside, and many of our students are noticing the plane doesn’t seem to climb as well as they expect. The reason for this, is air density (or rather a lack of it).
What is Density Altitude?
Density Altitude is a tool we use in aviation to predict aircraft performance. It is the pressure altitude adjusted for non standard temperature and pressure, it s a non-real altitude at which the airplane is effectively flying at. If the airport is at 1000 feet msl, but the density altitude is 6000 feet msl, then the airplane will perform as if it were flying at 6000 feet instead of 1000 feet. Many aircraft operating handbooks will reference density altitude directly in performance charts, expecting you to be capable of figuring out what it is.
How do you calculate it?
Density Altitude = Pressure Altitude + (120 * (Outside Air Temp - Standard Temp))
You can also use your E6B’s function to do this for you. But lets run through a problem together.
The altimeter setting is 30.19, the temperature is 21C. The airport is at 500’ MSL.
Step 1: What is Pressure Altitude?
Pressure altitude is the altitude above the standard pressure, 29.92. When you use the local altimeter setting, you are adjusting the instrument to read true altitude, or altitude above sea level.
To calculate difference between true and pressure altitude you take the difference between the altimeter setting and 29.92 (standard pressure) and multiply it by 1000. Since the setting is higher than standard, pressure altitude must be lower. In this case it is 270 feet lower; (30.19 - 29.92)*1000=270. For this example, pressure altitude is then 500-270 or 220 feet.
Step 2. What is standard temperature?
This is the standard temperature at the pressure altitude we are referring to. At sea level you might know this number to be 15C. It goes down every 2C for each 1000 feet. At 500 feet that is 1 degree less. Or 14C.
Step 3. Plug it into the formula
Density Altitude = Pressure Altitude + (120 * (Outside Air Temp - Standard Temp))
220 + (120 * (21 - 14)) = 840
Not that much difference at a temperature of 21C. But if it were a hot summer day at 31?
220 + (120 * (31 - 14)) = 2040
What is the big deal?
As air density goes down (and density altitude goes up) there is less air for both the wing and propeller to work with. Which means that you need more true airspeed to get the same amount of lift from the wing, and more RPM to get the same thrust from the prop.
This results in longer takeoff rolls and slower climbs. The rule of thumb for small aircraft like our is that take off roll increases by 25% for each 1000 feet of density altitude. So on a hot summer day, that is 50% more runway needed. In addition to the lifting surfaces being less effective, there is less oxygen in the air for the engine to mix with fuel, which results in less power. A double whammy. Watch out for those trees.
Other effects of warm days is that the cabin can get very warm, make sure you know where and how to use the vents, and keep an eye on your self for signs of heat exhaustion or dehydration. The airport also tends to become busier as more people are out flying, keep your eyes outside.
Student Achievements
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Will Moore, Robbie Quinn, and Shannon John passed their Private Pilot Written
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Grant Fisher did his first Solo
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Brendan Astley passed his Private Pilot Checkride
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