Sequim

L. Scott Brooksby, DDS, CFII,ATP,A&P, MEI

2022 Taylor Cutoff Rd

Sequim, WA 98382

702-274-6700

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Private Pilot Ground School

Sequim Advanced Dental Office

680 West Washington #E102 (Next to Subway)

Sequim, WA 98382

Tuesday and Thursday from 5-7 p.m. Starting November 8, 2016

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Flight Planning at SkyVector.com

Density Altitude

Density Altitude Density Altitude is the altitude the airplane thinks that it is flying in as a result of changes in the temperature of the air. When air molecules are heated they move farther apart from each other. The wings need a certain number of air molecules moving over them per unit time to create lift. This means that if the air is hot and the molecules are further apart the airplane must go faster to get sufficient air molecules flowing over the wing. This usually requires longer runways.

The second consequence of the air molecules being farther apart is that the engines are not able to produce as much power as they can when they get more air. Turbo chargers or superchargers work by compressing the air so that the air molecules are forced closer together. This allows the engine to produce the same or more power than it would if at sea level.

Once again with the air molecules farther apart less power is produced and this also requires a longer runway for the airplane do develop the speed necessary to produce lift. If the air molecules are too far apart the airplane may never generate enough lift and the airplane will not be able to get off of the ground.

In Fort Collins, Colorado, an airport that is at 5016 feet above sea level, Allegiant Airlines must restrict the number of passengers they can carry during the summer months so that they will still be able take off with the higher density altitudes.

So how do you calculate the density altitude? The are formulas that are incorporated into an E6B flight calculator where you just enter the pressure altitude and the temperature and the density altitude is computed for you. So here is how you calculate it manually.

First calculate standard temperature at your elevation in degrees fahrenheit. Standard temperature at sea level is 59 degrees F. The air cools at an average rate of 3.5 degrees F per 1,000 feet. If you are at North Las Vegas (VGT) the airport is at 2205 feet above sea level. Therefore 2205/1000 x 3.5 means the temperature atVGT is 7 degrees cooler than at sea level on a standard day. This means that the standard temperature at VGT is 52 degrees F.

The density of the air changes 600 feet per 10 degrees F.

If the temperature at VGT is 82 degrees F, you subtract the standard temperature of 52 degrees F and you get a temperature that is 30 degrees hotter than standard. 30/10=3 and if the change is 600 feet per 10 degrees then 3 x 600 = 1800. This means that we add 1800 feet to the current elevation at VGT or 2205 and we get a density altitude of 4005 feet.

If the temperature at VGT is 22 degrees F then 52-22 = 30/10 = 3 x 600 is 1800 feet lower than the field eleveation of 2205. Therefore the airplane will perceive that the airplane is at an altitude of 2205-1800 which is 405 feet above sea level.

An alternative way to calculate density altitude uses degrees Centigrade.  

First calculate standard temperature at you elevation in degrees Centigrade.  Standard temperature at sea level is 15 degrees C.  The air cools at an average rate of 2 degrees C per 1,000 feet. If you are at North Sequim (VGT) the airport is at 2205 feet above sea level. Therefore 2205/1000 x 2 means the temperature atVGT is 4 degrees cooler than at sea level on a standard day. This means that the standard temperature at VGT is 11 degrees C.

If the temperature at VGT is 26 degrees C, you subtract the standard temperature of 11 degrees C and you get a temperature that is 15 degrees hotter than standard. Multiply the 15 degrees by 120 and you get 1800 feet.  Add 1800 feet to the current elevation at VGT or 2205 and we get a density altitude of 4005 feet.

Both of these are approximations, but are easy to make and get you really close. Use the calculated density altitude to calculate your takeoff performance.

You can see that at 80 degrees the line in blue shows 2000 feet required for takeoff. At 20 degrees the red line shows a takeoff distance of 1400 feet. These distance must be calculated to make sure that you have enough runway for takeoff. I recommend that we add about 50% to these values as a safety measure since not all pilots are at the same level of skill as the pilots used to create these charts.