Principles of flight
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Principles of Flight. References Used. Pilots Handbook of Aeronautical Knowledge (PHAK) FAA-H-8083-25A, Chapter 3 Available Online for Free at: Atmosphere. Brain teaser: What percentage of oxygen is found at 0 ft ?

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References Used

  • Pilots Handbook of Aeronautical Knowledge (PHAK)

    • FAA-H-8083-25A, Chapter 3

      Available Online for Free at:


  • Brain teaser:

    • What percentage of oxygen is found at 0 ft?

    • What percentage of oxygen is found at 20,000 ft?

Atmospheric pressure
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???

Atmospheric pressure cont d
Atmospheric Pressure Cont’d.

  • On the ground air has an average pressure of 14.7 PSI or 1013.2mb

    • 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?

Problems and solutions
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 1013.2 mb

Standard lapse rate
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.)

Pressure altitude 29 92
Pressure Altitude = 29.92

  • Height above a Standard Datum Plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 inHg or 1013.2 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


    • 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

How to find pressure altitude
How to find Pressure Altitude?

  • Dial in 29.92 into your Kollsman Window on your altimeter


    • PA=(29.92-ambient pressure)x1000

  • Indicated altitude: What is indicated on your altimeter

Examples and sample s
Examples and Sample ?’s

  • Find PA :






    Field Elevation










What is density
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.

Density altitude
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

Effect of pressure on density
Effect of Pressure on Density

  • Since air is a 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

Effect of temp on density
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

Effect of humidity moisture on density
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

Tying everything together
Tying Everything Together the atmosphere

  • 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

Why is it important to know density
Why is it important to know density? the atmosphere

  • 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

Finding density altitude
Finding Density Altitude the atmosphere

  • 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



    • 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

The real da equation
The Real DA Equation the atmosphere

Now you try
Now you try... the atmosphere

Altimeter Field Elevation Temp

30.18 1069 25

28.56 10,003 -2

29.92 0 15

31.36 -279 12

30.00 551 36

Website to help
Website to help.... the atmosphere


More about da
More about DA... the atmosphere

  • 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

Just a note
Just a note.... the atmosphere

  • 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.