Sunday, July 22, 2012

Density Altitude, Partial Pressure of Oxygen, Mountain Flying

DENSITY ALTITUDE, OXYGEN USE AND MOUNTAINS 

All of the above play a role outside of our normal environment.

Density Altitude in Mountains

  • In the eastern plains of Colorado, for example, with Denver called "The Mile High City" and normal temperatures may have a Density Altitude of 8,500 feet during the summer. Increases in temperature and humidity cause an increase in density altitude above true altitude at a specific location.
  • Flying  underpowered airplanes in mountain areas is not recommended. Density altitude will affect your rate of climb adversely. Leaning your mixture, for full power, is a must at high altitudes with low concentrations of oxygen.
  •  Aircraft gross weight will decrease climb rate. Couple this with underpowered aircraft you have a very dangerous situation in high turbulence, strong updrafts and downdrafts found, on occasion, in mountain flying.
  • A general rule of thumb, for performance standards, in mountainous areas with low density altitudes, is approximately 55 to 65 HP per passenger. Four occupants requires a plane with at least 250 HP to fly safely. Fuel injected aircraft will eliminate carburetor icing. Turbocharged aircraft will correct the density altitude. Even then, maximum altitude may compromise tour flight.
  • Keep in mind the adiabatic lapse rate in degrees Fahrenheight is 4.5 degrees per 1000 feet. Therefore, as you climb, the temperature decreases 4.5 degrees each 1,000 from Sea Level. At an altitude of 10,000 feet the temperature has declined 45 degrees to 24 degrees. If you compare this to a Standard Temperature of 59 degrees at Sea Level the density altitude is above an airport located at 9,000 feet above sea level. Your airplane, at a 9,000 foot airport in the mountains is acting like it is at 10,000 feet at a temperature of 24 degrees. Even though the temperature is dropping the density altitude is 10,000 feet.
  • Reduction in air density reduces the force of lift generated by the planes wings when flying through the less dense air.
  • The propeller on a plane is like a vertical airfoil and the thrust generated by the propeller is reduced when the air density is reduced.


You must use oxygen at altitudes above 10,000 feet. Why? Density altitude means the partial pressure of oxygen declines with altitude. When the partial pressure of oxygen declines low enough the oxygen reaching your red blood cells declines to a point where you become hypoxic. please see my post on hypoxia. 

Density altitude is a killer if you don't recognize the effect density altitude can exert on you and your plane.