The Atmosphere

78% Nitrogen

21% Oxygen

1% other gases

Atmospheric Pressure

Standard atmosphere was developed as a baseline reference

59° F, 15° C

29.92 Hg, 1013.2 mb, 14.7 psi

All aircraft performance is referenced to this data

Winds of Change

Pressure decreases 1 inch per 1,000 feet

Temperature decreases 3.5° F or 2° C per 1,000 feet

It is therefor possible, based on standard rates, to determine temp and pressure for the altitude you wish to cruise

This is important to us because as pressure decreases and/or temperature increases performance decreases

ISA

International Standard Atmosphere

The black lines represent temps above and below standard

Often expressed as ISA +10 for example

Performance Charts

So aircraft manufactures, to make it easy, include columns with various temps above and below standard

Terms

Density altitude; measurement of the density of the air expressed in terms of altitude

High density altitude; a condition in which the air is less dense

Pressure effects on Density

Gases can compress or expand

Therefor a greater pressure indicates more molecules per volume (cubic foot or whatever)

This means we have a greater density

In fact density is directly proportional to pressure

If pressure doubles, density doubles (Boyle’s Law)

At a constant temp which goes without saying of course

Temp Effects on Density

Temperature as it turns out changes the density of a gas

Increasing temp decreases density

So density varies inversely with temp

Throw Altitude Into The Mix

So what happens as we go up in altitude?

The temp decreases increasing density

However the pressure decreases which decreases density

Pressure wins this one

The decrease in temp is not enough to offset the effects of a decrease in pressure as we increase altitude

High Density Altitude

Density altitude is pressure altitude corrected for non-standard temp

Density altitude then is the best means for correlating aerodynamic performance

Since high density altitude is where we can get into trouble, we should know a few markers so we know what to look for, in fact there are 4:

1. High field elevation

2. Atmospheric pressure

3. High temperature

4. High humidity

High Density Altitude

1. High field elevation

Density altitude is increased by an increase in field elevation i.e. Seattle to Denver

At 18,000 feet atmospheric pressure is roughly halved

Remember pressure wins over temp

High Density Altitude

2. Low pressure system

Density altitude is increased by a decrease in pressure

Low pressure weather system will lower pressure

High Density Altitude

3. High temperature

Density altitude is increased by an increase in temperature

Spreads out molecules makes air less dense

High Density Altitude

4. High humidity

Density altitude is increased by an increase in humidity

A given volume of dry air is more dense than the same volume of moist air

How can that be? Water is a lot heavier than air right?

High Density Altitude

Molecular weights:

Hydrogen weighs 1

Oxygen weighs 16

oxygen exists as O2 and weighs 32

Nitrogen weighs 14

exists as N2 and weighs 28

H20 weighs 18

The atmosphere is 70% nitrogen 28% oxygen 2% inert gases

Water vapor displaces the nitrogen

JD Garcia memorial slide

High Density Altitude

Hot, humid, high elevation and low pressure = high density altitude

Cold, dry, low elevation and high pressure = Low density altitude

High Density Altitude

Airplane effected several ways:

1. Wing is less effective

2. Propeller is less effective

3. Engine puts out less horsepower

Performance is effected several ways:

1. Rate of climb is lower

2. Time to climb is longer

3. Takeoff roll is longer

4. Acceleration is slower

5. Higher true airspeed means faster approach

6. Longer landing roll

Climb Performance

In order to climb, we need power above that required for level flight

This is referred to as Excess power

Climb Performance

So Vx is a function of excess thrust

Vy is a function of excess power

So how can the pilot minimize the effects of High density altitude?

1. Leave in the morning when its cooler

2. Off load some baggage and or passengers

3. Make two trips

4. Take a higher performing aircraft

So how can the pilot minimize the effects of High density altitude?

5. Lean the engine for more horsepower unless AFM says not to

6. Do your calculations ahead of time and make the go, no go decision early.

Do not let the people you travel with or Boss make you compromise safety

Density Altitude

The primary reason for computing density altitude is to determine aircraft performance

Remember Density altitude is Pressure altitude corrected for nonstandard temperature

Density Altitude

It can be found 2 ways

1. Use the chart

2. Use the computer

Example:

30.10

21° C

DA 2,000

Computing Density Altitude

1st step: convert elevation or altitude to pressure altitude.

This can be done 2 ways

By setting 29.92 into altimeter

By subtracting 29.92 from the current pressure then multiplying by 1000 then either adding or subtracting from the current pressure value

Computing Density Altitude

E.G. Field elevation 1185 pressure 30.55

29.92 (standard pressure)

– 30.55 (current pressure)

– .63 x 1000 = -630′

1185 + -630 = 555′ Pressure Altitude

Computing Density Altitude

So the airplane thinks its flying at 555 feet because of the extra pressure

555 is closer to sea level than 1185

2nd step: Take the pressure altitude and the temperature at your elevation or altitude and follow the lines on the density altitude chart until they intersect.

Temperature for this example is 32 C

PA=555

Temp=32 C

DA=2800′

Computing Density Altitude

E.G. Field elevation 1185 pressure 29.62

29.92

-29.62

.30 x 1000 = 300 1185 + 300 = 1485

Computing Density Altitude

So the airplane thinks its flying at 1485 feet because of the decrease in atmospheric pressure

2nd step: Take the pressure altitude and the temperature at your elevation or altitude and follow the lines on the density altitude chart until they intersect.

Temperature is 21 C

PA=1485

Temp=21 C

DA=2700′

Weight and Performance

More weight equals lower performance in almost every operation

Weight increases the angle of attack

This increases drag

More drag needs more thrust which means we have less reserve to climb with

So climb suffers

Takeoff distance increases

Landing distance increases

Fuel burn increases