Lesson 7 Aircraft Performance

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