Performance
Density Altitude
P1
A standard atmosphere has been established in which the temperature at sea level
is 59°F (15°C) and the barometric pressure is 29.92 in Hg.
P2
The atmosphere on a given day rarely matches standard conditions, therefore
corrections for nonstandard temperature must be made when considering aircraft
performance.
P3
Density altitude is pressure altitude corrected for nonstandard temperature.
P4
Air temperature, barometric pressure and humidity affect air density. When
temperature and pressure are not standard, density altitude will not equal true
altitude.
P5
As barometric pressure increases, the air becomes denser through compression.
As a result, density altitude decreases.
P6
As barometric pressure decreases, the air becomes less dense, and density altitude
increases.
P7
An increase in air temperature will cause the air to expand, which decreases air
density. This results in density altitude increasing.
P8
As air temperature decreases, the air contracts and becomes denser. Density
altitude is lowered.
P9
Air will become less dense as relative humidity increases since a volume of moist
air will weigh less than an equal volume of dry air. Density altitude will therefore
increase.
P10
As relative humidity decreases, density altitude decreases.
P11
Taken together, density altitude varies directly with temperature and humidity,
and varies inversely with pressure.
P12
High density altitude results in lower aircraft performance.
P13
Low density altitude results in better airplane performance.
Computing Density Altitude
P14
Density altitude is computed using pressure altitude and temperature.
P15
Pressure altitude is determined by adjusting the altimeter setting to 29.92 and
reading the indicated altitude.
P16
Once the pressure altitude and air temperature are known, a flight computer or a
chart may be used to determine density altitude.
P17
Consider the following conditions: altimeter setting 30.35, runway temperature
25°F, airport elevation 3,894 feet.
P18
Since pressure altitude is the altitude that results when the altimeter is set to
29.92, the altitude difference between 29.92 and 30.35 is -349 feet.
P19
Subtracting 349 feet from the airport elevation of 3,894 feet results in a pressure
altitude of 3,500 feet. The chart may now be used to find density altitude.
P20
At the 25°F mark, draw a line straight up to the 3,500 line then go straight across.
Density altitude is 2,000 feet.
P21
Since the actual air temperature is less than standard, density altitude is lower than
pressure altitude.
Takeoff Distance
P22
Takeoff distance includes ground roll and total distance over a 50-foot obstacle.
P23
Parameters such as altitude, temperature, airplane weight, wind, runway slope and
type of runway (pavement or grass) affect takeoff distance.
P24
Takeoff distances can be computed using a graph or a table. The FAA will test
your ability in using a graph like the one shown.
P25
Consider the following conditions: outside air temperature 90°F, pressure altitude
2,000 feet, takeoff weight 2,500 lb, headwind 20 knots. Find required ground roll.
P26
Enter the chart at 90°F and draw a line straight up to a pressure altitude of 2,000
feet.
P27
Move straight across to the first reference line, then parallel the closet guideline to
2,500 lb.
P28
Move straight across to the next reference line, then parallel the closet guideline
to 20 knots.
P29
Finally, move straight across, ignoring the 50-ft obstacle graph. The ground roll
turns out to be about 650 feet.
P30
If distance over a 50-ft obstacle is desired, stop at the reference line for 50-ft
obstacle graph and parallel the closest guideline to 50 feet.
P31
In this case, the distance to clear a 50-foot obstacle is about 1,100 feet.
Cruise Power Settings
P32
Cruise power settings are calculated using tables. The table in the center is
standard day values while the left and right tables are below and above standard
respectively.
P33
The set of tables shown corresponds to a power setting of 65%. If another power
setting is desired during cruise, such as 75%, another set of tables would be used.
P34
Interpolation between numbers may be required. Consider the following: pressure
altitude 9,500 feet, air temperature 36°F below standard. Find true airspeed in
MPH.
P35
The pressure altitude of 9,500 feet is between 8,000 feet and 10,000 feet in the
table.
P36
The true airspeed at a pressure altitude of 8,000 feet is 181 mph.
P37
The true airspeed at a pressure altitude of 9,500 feet is the unknown and we’ll call
it X.
P38
The true airspeed at a pressure altitude of 10,000 feet is 184 mph.
P39
X is found by calculating the ratios of the differences in altitude and true airspeed.
P40
Solving for X results in a true airspeed of just over 183 mph.
Crosswind Components
P41
Although not a limitation, the airplane’s demonstrated crosswind component
should not be exceeded when landing.
P42
The crosswind component graph provides headwind and crosswind information.
P43
For example, if landing on runway 18 with winds reported as 220° at 30 knots, the
wind will be 40° off the nose of the airplane.
P44
Find the 40° radial and move down until reaching the 30-knot wind speed arc.
P45
Move straight down and read the crosswind component, which is 19 knots.
P46
The headwind component is found by moving straight across from the point
where the 40° radial and the 30-knot wind speed arc intersect.
P47
In this case, headwind is 23 knots.
Landing Distance
P48
Landing distance is broken down into ground roll and total distance over a 50-ft
obstacle.
P49
The chart for calculating landing distance is similar to the takeoff distance chart.
P50
Enter the chart using the outside air temperature (OAT) and move upward until
reaching pressure altitude.
P51
Move straight across until reaching the reference line for airplane weight.
P52
Parallel the nearest guideline until reaching the airplane’s weight.
P53
Move straight across until reaching the wind component reference line.
P54
Parallel the nearest guideline until reaching the wind component.
P55
For ground roll move straight across while ignoring the obstacle height reference
line.
P56
For total distance over a 50-ft obstacle, stop at the obstacle height reference line,
then parallel the nearest guideline until reaching the 50-ft line.