Performance
Density Altitude
P1A 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.
P2The atmosphere on a given day rarely matches standard conditions, therefore corrections for nonstandard temperature must be made when considering aircraft performance.
P3Density altitude is pressure altitude corrected for nonstandard temperature.
P4Air temperature, barometric pressure and humidity affect air density. When temperature and pressure are not standard, density altitude will not equal true altitude.
P5As barometric pressure increases, the air becomes denser through compression. As a result, density altitude decreases.
P6As barometric pressure decreases, the air becomes less dense, and density altitude increases.
P7An increase in air temperature will cause the air to expand, which decreases air density. This results in density altitude increasing.
P8As air temperature decreases, the air contracts and becomes denser. Density altitude is lowered.
P9Air 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.
P10As relative humidity decreases, density altitude decreases.
P11Taken together, density altitude varies directly with temperature and humidity, and varies inversely with pressure.
P12High density altitude results in lower aircraft performance.
P13Low density altitude results in better airplane performance.
Computing Density Altitude
P14Density altitude is computed using pressure altitude and temperature.
P15Pressure altitude is determined by adjusting the altimeter setting to 29.92 and reading the indicated altitude.
P16Once the pressure altitude and air temperature are known, a flight computer or a chart may be used to determine density altitude.
P17Consider the following conditions: altimeter setting 30.35, runway temperature 25°F, airport elevation 3,894 feet.
P18Since 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.
P19Subtracting 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.
P20At the 25°F mark, draw a line straight up to the 3,500 line then go straight across. Density altitude is 2,000 feet.
P21Since the actual air temperature is less than standard, density altitude is lower than pressure altitude.
Takeoff Distance
P22Takeoff 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.
P24Takeoff distances can be computed using a graph or a table. The FAA will test your ability in using a graph like the one shown.
P25Consider 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.
P26Enter the chart at 90°F and draw a line straight up to a pressure altitude of 2,000 feet.
P27Move straight across to the first reference line, then parallel the closet guideline to 2,500 lb.
P28Move straight across to the next reference line, then parallel the closet guideline to 20 knots.
P29Finally, 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.
P31In this case, the distance to clear a 50-foot obstacle is about 1,100 feet.
Cruise Power Settings
P32Cruise 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.
P33The 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.
P34Interpolation between numbers may be required. Consider the following: pressure altitude 9,500 feet, air temperature 36°F below standard. Find true airspeed in MPH.
P35The pressure altitude of 9,500 feet is between 8,000 feet and 10,000 feet in the table.
P36The true airspeed at a pressure altitude of 8,000 feet is 181 mph.
P37The true airspeed at a pressure altitude of 9,500 feet is the unknown and we’ll call it X.
P38The 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.
P40Solving for X results in a true airspeed of just over 183 mph.
Crosswind Components
P41Although not a limitation, theairplane’s demonstrated crosswind component should not be exceeded when landing.
P42The crosswind component graph provides headwind and crosswind information.
P43For 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.
P44Find the 40° radial and move down until reaching the 30-knot wind speed arc.
P45Move straight down and read the crosswind component, which is 19 knots.
P46The headwind component is found by moving straight across from the point where the 40° radial and the 30-knot wind speed arc intersect.
P47In this case, headwind is 23 knots.
Landing Distance
P48Landing distance is broken down into ground roll and total distance over a 50-ft obstacle.
P49The chart for calculating landing distance is similar to the takeoff distance chart.
P50Enter the chart using the outside air temperature (OAT) and move upward until reaching pressure altitude.
P51Move straight across until reaching the reference line for airplane weight.
P52Parallel the nearest guideline until reaching the airplane’s weight.
P53Move straight across until reaching the wind component reference line.
P54Parallel the nearest guideline until reaching the wind component.
P55For ground roll move straight across while ignoring the obstacle height reference line.
P56For 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.