Lecture on Pressure and Density Altitude

Importance in Performance Calculations

• Key factors in aircraft performance calculations.
• Common weak area on tests.

Pressure and Air Density

• Low Pressure Air: Air molecules are spread out, less air in a given space.
• High Pressure Air: Air molecules are tightly packed, denser air.
• Aircraft Performance:
  • Propeller is more effective with denser air (more thrust).
  • Wing generates more lift with denser air.
  • Engine produces more power with denser air (more air molecules for combustion).

Altitude and Pressure

• Lower Altitudes: Higher pressure due to the compacting force of air molecules.
• Higher Altitudes: Lower pressure as molecules are more spread out.
• Aircraft performance decreases at higher altitudes.

Temperature and Air Density

• Heating Air Mass: Molecules move faster and spread out.
• Higher Temperatures: Create lower pressure air, decreasing aircraft performance.
• Weather changes affect pressure systems (high and low pressure air systems).

Humidity and Air Density

• Water Vapor: Reduces air density on humid days (water molecules crowd out air molecules).
• Aircraft performance decreases on days with high humidity.

Pressure and Density Altitude

• Used to predict aircraft performance under varying conditions.
• Pressure Altitude: True altitude corrected for non-standard pressure.
• Density Altitude: Pressure altitude corrected for non-standard temperature.

Factors Affecting Density Altitude

• Increase in altitude increases density altitude.
• Decrease in pressure increases density altitude.
• Increase in temperature increases density altitude.

Calculating Pressure and Density Altitude

• Use charts to determine these altitudes.
• Pressure Altitude Calculation:
  • Altimeter setting conversion factor added to field elevation.
  • Example: Altimeter 29.92 = conversion factor 0, field elevation 1000 feet = pressure altitude 1000 feet.
  • Example: Altimeter 28.90 = conversion factor 957, field elevation 1000 feet = pressure altitude 1957 feet.
• Density Altitude Calculation:
  • Use temperature and pressure altitude on a chart.
  • Example: Temperature 20°F and pressure altitude 3500 feet = density altitude 2000 feet.

Tips for Test Day

• Charts can be difficult to read, use a straight edge.
• Remember: A 15°F increase = approximately 1000-foot increase in density altitude.