1. Principles of Flight

2. References Used Pilots Handbook of Aeronautical Knowledge (PHAK)
FAA-H A, Chapter 3
Available Online for Free at:

3. Atmosphere.... Brain teaser:
What percentage of oxygen is found at 0 ft?
What percentage of oxygen is found at 20,000 ft?

4. Atmospheric Pressure Air is very light...right?
But, it still has mass and is affected by gravity...
Because of its effects with gravity, it has weight
Because it has weight, it exerts a pressure
So where are we going with all this???

5. Atmospheric Pressure Cont’d.
On the ground air has an average pressure of 14.7 PSI or mb
Air has a limited thickness right?
So.... The higher the altitude, the less air there is above
Which means...that at 18,000 ft. the pressure is about half what it is at sea level. Cool huh?

6. Problems...and Solutions
Atmospheric pressure varies with time and location...
Well...we need a standard unit of measure so that all airplanes worldwide are “on the same page”
The standard atmospheric pressure was developed
This means...
Temperature of
59 ° F or 15 ° C
&
A surface pressure of
29.92 inHg or mb

7. Standard Lapse Rate -3.5 ° F or -2° C /thousand feet (up to 36,000ft)
From 36,000-80,000 the temperature is said to be constant at about -65 degrees F or -55 degrees C
Standard lapse rate is also when pressure decreases about 1 inHg/ 1000 ft (up to 10,000 ft.)

9. Pressure Altitude = 29.92
Height above a Standard Datum Plane (SDP), which is a theoretical level where the weight of the atmosphere is inHg or mb. as measured by a barometer (An instrument measuring atmospheric pressure)
Altimeter in essentially a sensitive barometer
If you dial in “29.92” on your altimeter
Then....
Your altimeter will indicate your PRESSURE ALTITUDE
Why is this important?
It will allow you to determine how the airplane will perform
And it will be used to use to assign altitudes above 18,000

10. How to find Pressure Altitude?
Dial in into your Kollsman Window on your altimeter
-or-
PA=(29.92-ambient pressure)x1000
Indicated altitude: What is indicated on your altimeter

11. Examples and Sample ?’s Find PA : Altimeter Field Elevation PA 30.16
1,000
760
29.91
1,850
1,860
28.57
963
2,313
31.10
256
-924

12. What is density? Density= Mass / Volume
What is Mass? The amount of material in an object
What is Volume? It is the amount of space enclosed by a shape or object, how much 3-dimensional space (length, width, and height) it occupies.

13. Density Altitude
SDP (Standard Datum Plane) is a theoretical pressure altitude (oft 15 degrees C)
But...aircraft operate in non-standard atmosphere
So, the term Density Altitude is used for correlating aerodynamic performance in the non-standard atmosphere
Density Altitude: Pressure Altitude corrected for Non-Standard Temp

14. Effect of Pressure on Density
Since air is a gas...it can be compressed or expanded
When air is compressed (increased pressure) you can have more air in a space-like inflating a tire
At a lower pressure, there is less mass of air
So we can say that density is directly related to pressure
If pressure is doubled then the density is doubled
If the pressure is lowered then the density is lowered
*All this is assuming a constant temperature

15. Effect of Temp. on Density
+ Temperature = - Density
- Temperature = + Density
Now, as you climb the pressure and temperature both generally decrease, thus having a conflicting effect on density. BUT... The deceasing pressure usually wins the battle and as you go higher, density decreases
*This is only true with constant pressure

16. Effect of Humidity (Moisture) on Density
Everything before assumed air was dry...but it’s not!
Water vapor is lighter than air....
So that means that air with high humidity is lighter (less dense)
As moisture increases, the air becomes less dense
As moisture decreases, the air becomes more dense

17. Brain Teaser: Can air have a RH of 100%?
Relative Humidity is the amount of water vapor contained in the atmosphere
Brain Teaser: Can air have a RH of 100%?
Warmer air can hold more water , colder air holds less
So that means...
Warm air with high humidity = less dense
Air that is cold with less humidity is more dense

18. Tying Everything Together
Pressure, Temperature and Humidity have a huge effect on aircraft performance because of their effect on density!!!
Why? Because the wing and engine all depend on density to operate... The less dense the air is then the less molecules of air there are for the wing and engines to “grab” onto

19. Why is it important to know density?
The HIGHER you go, the less dense the air is (less packed molecules) this is HIGH DENSITY ALTITUDE
The LOWER you go, the more dense the air is (tighter packed molecules) this is LOW DENSITY ALTITUDE
The less the density, the greater reduction in:
Power because engine takes in air
Thrust, because a propeller is less effective in thin air
Lift, because thin air exerts less force on airfoils

20. Finding Density Altitude
Write down current altimeter setting and indicated altitude
Find your Pressure Altitude (29.92 on altimeter)
Find your Temp (Celsius)
Using your E6B, line up your temp with your press alt. (in decimal form using inner-inner scale) and read density altitude off arrow
-OR-
DA=PA+/-(120Vt) (THIS EQUATION IS AN APPROXIMATION ONLY)
DA=PA +(120 *Temp Deviation from ISA)
Find the ISA standard temp for your altitude (Use the Graph on Slide 24 and interpolate the altitudes/temperatures to find ISA for your altitude)
Find the temp at the altitude in question
Find the difference between these two
Adjust the altitude by 120 ft for every 1 degree C deviation from ISA Standard temp
Add altitude if temp is more than ISA or subtract for a temp less than ISA
Use the graph

21. The Real DA Equation

23. Now you try...
Altimeter Field Elevation Temp ,

25. Website to help....

26. More about DA...
Regardless of you’re actual altitude, the airplane will fly as if it were at an altitude equal to the density altitude
Air density is affected by changes in temp, altitude and humidity
High density altitude refers to thin air.
High altitudes, temperatures, humidity
Low pressures
Low density altitude refers to dense air
Low altitudes, temperatures, humidity
High pressures

27. Just a note....
Pressure, temperature, and humidity have a great influence on aircraft performance because of their effect upon density.
There are no rules of thumb that can be easily conveyed but the affect of humidity can be determined using online formulas.