14.100 Performance Factors

14.100.2 State the general effect of variation in the following on helicopter performance:

(a) QNH;

• Lower QNH (lower pressure):
  • Increases density altitude
  • Reduces helicopter performance (less lift, more power required)
• Higher QNH (higher pressure):
  • Decreases density altitude
  • Improves performance
   

Summary:
Lower QNH → worse performance; Higher QNH → better performance.

(b) ambient temperature;

• Higher temperature:
  • Decreases air density
  • Reduces lift and engine performance
  • Increases power required
• Lower temperature:
  • Increases air density
  • Improves lift and performance
   

Summary:
Higher temperature → worse performance; lower temperature → better performance.

(c) humidity.

• Higher humidity:
  • Reduces air density (moist air is less dense)
  • Decreases lift and engine performance
  • Increases power required
• Lower humidity:
  • Increases air density
  • Improves performance
   

Summary:
Higher humidity → reduced performance; lower humidity → improved performance.

14.100.4 Define pressure altitude.

Pressure altitude is the altitude indicated when the altimeter is set to the standard pressure setting of 1013.2 hPa (29.92 inHg).

It is the aircraft’s height above the standard datum plane, where atmospheric pressure corresponds to that found in the International Standard Atmosphere (ISA).