There are three main factors that affect aircraft performance: density altitude, weight, and wind. Density altitude refers to the air density corresponding to a pressure altitude adjusted for non-standard temperature. It is affected by atmospheric pressure, elevation, temperature, and moisture. Higher density altitude means lower air density, reducing an aircraft's performance. Weight also reduces performance, as more power is needed to lift heavier loads. Wind direction impacts takeoffs, landings, and hovering; headwinds are beneficial while tailwinds reduce performance.

1. HFH Ch. 7, PHAK Ch. 10, POH Sec. 5

2. Factors Affecting Performance
 There are three major factors that affect
performance
 Density Altitude
 Weight
 Wind

3. Density Altitude (DA)
 The altitude in the standard atmosphere
corresponding to a particular value of air
density
 High DA=low air density
 DA is the pressure altitude (PA) corrected
for non-standard temperature
 In order for us to understand DA we have
to understand PA

4. Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

5. Pressure Altitude
 Is the altitude, in feet,
above the standard datum
plane (29.92” hg)
 Datum (just like in W&B) is
an imaginary plane used for
measurement
 It makes allowances for
sea level pressure being
different than standard**

6. Atmospheric Pressure
 What is atmospheric pressure?
 Though air is very light, it has
mass and is affected by Earth’s
gravitational pull
 Therefore it has weight; because it
has weight, it has force
 Since it is a fluid substance, this
force is exerted equally in all
directions, and its effect on bodies
within the air is called pressure

7. Measuring AP
 To measure
atmospheric
pressure we use a
barometer
 Similar to a
thermometer is to
temperature
 Measures the weight
of the overlying air
column

8. Barometer

9. Atmospheric Pressure
 The pressure of the
atmosphere varies with time
and altitude
 Primarily caused by changes
in weather and uneven
heating of the Earth’s surface
 Due to the constantly
changing atmospheric
pressure
 A standard reference was
developed (ISA)
 Helped with consistent
performance planning

10. International Standard Atmosphere (ISA)
 Due to these changing atmospheric
pressures, a standard reference
was developed
 It was developed by International
Civil Aviation Organization (ICAO)
 It is often referred to as ISA or ICAO
standard atmosphere
 Any temperature or pressure that
differs from the standard lapse rate
is considered “non-standard”

11. ISA
 The standard atmosphere at sea level is a surface
temperature of 15 C and a surface pressure of 29.92
inches of mercury (hg)
 A standard temperature lapse rate is one in which
the temp decreases at the rate of approx. 2 C per
thousand feet up to 36,000 feet
 A standard pressure lapse rate is one in which the
pressure decreases at the rate of approx. 1” hg per
1,000’

12. Pressure Altitude Overview
 Is the altitude, in feet, above the
standard datum plane of 29.92” hg
 ISA
 It makes allowances for sea level
pressure being different than standard
 Let’s look at an example of this…

13. 1,000’
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sea Level Pressure: 29.92” hg
28.92” hg
Example #1
Standard day
SLP: 29.92” hg
Altimeter setting: 29.92
Indicated altitude: 0’
Pressure altitude: 0’
Altimeter setting: 29.92
Indicated altitude: 1,000’
Pressure altitude: 1,000’

14. 1,000’
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sea Level Pressure: 30.12” hg
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
29.92
Example #2
Non-standard day
SLP: 30.12
Altimeter setting: 30.12
Indicated altitude: 0’
Altimeter setting: 30.12
Indicated altitude: 1,000’
Pressure altitude: ?
Pressure altitude- the
altitude, in feet
above the standard
datum plane (29.92)
29.12” hg
1” hg=1,000’
29.92”-30.12”=-.20”
-.20”x1000=-200’
1000’-200’=800’ PA
800’
It makes allowances
for sea level pressure
being different than
standard

15. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

16. Elevation/Altitude
 Air becomes thinner or
less dense with
increases of altitude or
elevation
 Less of the overlying air
column weight is above
you
○ Loaf of bread

17. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

18. Temperature
 As warm air expands, air molecules move
farther apart creating less dense air

19. Four Factors Affecting DA
 Atmospheric pressure
 Elevation
 Temperature
 Moisture

20. Moisture Content
 Water vapor weighs less
than dry air
 Takes the place of air
molecules making the air less
dense
 Earth is composed of
Nitrogen 78%, oxygen 21%
and other 1%
 Nitrogen (1 gal) = 11.1
pounds
 Oxygen (1 gal) = 9.5 pounds
 H2O (1 gal) = 8 pounds

21. Calculating DA
 Tools:
 CX2
 E6B
 DA chart (Section 5
POH)
 Math
○ DA=PA+120(OAT-ISA)
 PA=Elev.+1,000(29.92-
current altimeter)

22. Weight
 Lift is the force that
opposes weight
 As weight is
increased, the
power required to
produce lift needed
to compensate for
the added weight
must also increase

23. Wind
 Translational lift occurs whenever there is
airflow across the rotor disc, wind or
forward movement
 Headwinds are best for performance
○ Takeoff
○ Landing
○ Hovering
 Crosswind
○ At low airspeeds requires more torque from the
engine
 Tail winds
○ Bad for takeoff landing and hovering
○ Takes away power from main rotor
○ Beneficial when we are flying cross country
 Faster ground speed