Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration)
Page 50
Another type of cruise chart is a best power mixture range graph. This graph gives the best range based on power setting and altitude. Using Figure 11-29, find the range at 65 percent power with and without a reserve based on the provided conditions.
Sample Problem 8
OAT Standard
Pressure Altitude 5,000 feet
First, move up the left side of the graph to 5,000 feet and standard temperature. Follow the line straight across the graph until it intersects the 65 percent line under both the reserve and no reserve categories. Draw a line straight down from both intersections to the bottom of the graph. At 65 percent power with a reserve, the range is approximately 522 miles. At 65 percent power with no reserve, the range should be 581 miles.
The last cruise chart referenced is a cruise performance graph. This graph is designed to tell the TAS performance of the airplane depending on the altitude, temperature, and power setting. Using Figure 11-30, find the TAS performance based on the given information.
Figure 11-28. Cruise power setting.
Sample Problem 9
OAT 16 °C
Pressure Altitude 6,000 feet
Power Setting 65 percent, best power
Wheel Fairings Not installed
Begin by finding the correct OAT on the bottom left side of the graph. Move up that line until it intersects the pressure altitude of 6,000 feet. Draw a line straight across to the 65 percent, best power line. This is the solid line, that represents best economy. Draw a line straight down from this intersection to the bottom of the graph. The TAS at 65 percent best power is 140 knots. However, it is necessary to subtract 8 knots from the speed since there are no wheel fairings. This note is listed under the title and conditions. The TAS is 132 knots.
Figure 11-29. Best power mixture range.
Crosswind and Headwind Component Chart
Every aircraft is tested according to Federal Aviation Administration (FAA) regulations prior to certification. The aircraft is tested by a pilot with average piloting skills in 90° crosswinds with a velocity up to 0.2 VS0 or two-tenths of the aircraft’s stalling speed with power off, gear down, and flaps down. This means that if the stalling speed of the aircraft is 45 knots, it must be capable of landing in a 9-knot, 90° crosswind. The maximum demonstrated crosswind component is published in the AFM/POH. The crosswind and headwind component chart allows for figuring the headwind and crosswind component for any given wind direction and velocity.
Sample Problem 10
Runway 17
Wind 140° at 25 knots
Refer to Figure 11-31 to solve this problem. First, determine how many degrees difference there is between the runway and the wind direction. It is known that runway 17 means a direction of 170°; from that subtract the wind direction of 140°. This gives a 30° angular difference or wind angle. Next, locate the 30° mark and draw a line from there until it intersects the correct wind velocity of 25 knots. From there, draw a line straight down and a line straight across. The headwind component is 22 knots and the crosswind component is 13 knots. This information is important when taking off and landing so that, first of all, the appropriate runway can be picked if more than one exists at a particular airport, but also so that the aircraft is not pushed beyond its tested limits.
Figure 11-30. Cruise performance graph.
Landing Charts
Landing performance is affected by variables similar to those affecting takeoff performance. It is necessary to compensate for differences in density altitude, weight of the airplane, and headwinds. Like takeoff performance charts, landing distance information is available as normal landing information, as well as landing distance over a 50 foot obstacle. As usual, read the associated conditions and notes in order to ascertain the basis of the chart information. Remember, when calculating landing distance that the landing weight is not the same as the takeoff weight. The weight must be recalculated to compensate for the fuel that was used during the flight.
Figure 10-31. Crosswind component chart.
Sample Problem 11
Pressure Altitude 1,250 feet
Temperature Standard
Refer to Figure 10-32. This example makes use of a landing distance table. Notice that the altitude of 1,250 feet is not on this table. It is, therefore, necessary to interpolate to find the correct landing distance. The pressure altitude of 1,250 is halfway between sea level and 2,500 feet. First, find the column for sea level and the column for 2,500 feet. Take the total distance of 1,075 for sea level and the total distance of 1,135 for 2,500 and add them together. Divide the total by two to obtain the distance for 1,250 feet. The distance is 1,105 feet total landing distance to clear a 50 foot obstacle. Repeat this process to obtain the ground roll distance for the pressure altitude. The ground roll should be 457.5 feet.
Sample Problem 12
OAT 57 °F
Pressure Altitude 4,000 feet
Landing Weight 2,400 pounds
Headwind 6 knots
Obstacle Height 50 feet
Using the given conditions and Figure 11-33, determine the landing distance for the aircraft. This graph is an example of a combined landing distance graph and allows compensation for temperature, weight, headwinds, tailwinds, and varying obstacle height. Begin by finding the correct OAT on the scale on the left side of the chart. Move up in a straight line to the correct pressure altitude of 4,000 feet. From this intersection, move straight across to the first dark reference line. Follow the lines in the same diagonal fashion until the correct landing weight is reached. At 2,400 pounds, continue in a straight line across to the second dark reference line. Once again, draw a line in a diagonal manner to the correct wind component and then straight across to the third dark reference line. From this point, draw a line in two separate directions: one straight across to figure the ground roll and one in a diagonal manner to the correct obstacle height. This should be 975 feet for the total ground roll and 1,500 feet for the total distance over a 50 foot obstacle.
Figure 11-32. Landing distance table.
Stall Speed Performance Charts
Stall speed performance charts are designed to give an understanding of the speed at which the aircraft stalls in a given configuration. This type of chart typically takes into account the angle of bank, the position of the gear and flaps, and the throttle position. Use Figure 11-34 and the accompanying conditions to find the speed at which the airplane stalls.
Figure 11-33. Landing distance graph.
Sample Problem 13
Power OFF
Flaps Down
Gear Down
Angle of Bank 45°
First, locate the correct flap and gear configuration. The bottom half of the chart should be used since the gear and flaps are down. Next, choose the row corresponding to a power-off situation. Now, find the correct angle of bank column, which is 45°. The stall speed is 78 mph, and the stall speed in knots would be 68 knots.
Figure 11-34. Stall speed table.
Performance charts provide valuable information to the pilot. By using these charts, a pilot can predict the performance of the aircraft under most flying conditions, providing a better plan for every flight. The Code of Federal Regulations (CFR) requires that a pilot be familiar with all information available prior to any flight. Pilots should use the information to their advantage as it can only contribute to safety in flight.
Transport Category Aircraft Performance
Transport category aircraft are certificated under Title 14 of the CFR (14 CFR) part 25. For additional information concerning transport category airplanes, consult the Airplane Flying Handbook, FAA-H-8083-3 (as revised).
Transport category helicopters are certificated under 14 CFR part 29.
Air Carrier Obstacle Clearance Requirements
For information on air carrier obstacle clearance requirements consult the Instrument Procedures Handbook, FAA-H-8083-16 (as revised).
Chapter Summary
Performance characteristics and capabil
ities vary greatly among aircraft. As transport aircraft become more capable and more complex, most operators find themselves having to rely increasingly on computerized flight mission planning systems. These systems may be on board or used during the planning phase of the flight. Moreover, aircraft weight, atmospheric conditions, and external environmental factors can significantly affect aircraft performance. It is essential that a pilot become intimately familiar with the mission planning programs, performance characteristics, and capabilities of the aircraft being flown, as well as all of the onboard computerized systems in today’s complex aircraft. The primary source of this information is the AFM/POH.
Chapter 12
Weather Theory
Introduction
Weather is an important factor that influences aircraft performance and flying safety. It is the state of the atmosphere at a given time and place with respect to variables, such as temperature (heat or cold), moisture (wetness or dryness), wind velocity (calm or storm), visibility (clearness or cloudiness), and barometric pressure (high or low). The term “weather” can also apply to adverse or destructive atmospheric conditions, such as high winds.
This chapter explains basic weather theory and offers pilots background knowledge of weather principles. It is designed to help them gain a good understanding of how weather affects daily flying activities. Understanding the theories behind weather helps a pilot make sound weather decisions based on the reports and forecasts obtained from a Flight Service Station (FSS) weather specialist and other aviation weather services.
Be it a local flight or a long cross-country flight, decisions based on weather can dramatically affect the safety of the flight.
Atmosphere
The atmosphere is a blanket of air made up of a mixture of gases that surrounds the Earth and reaches almost 350 miles from the surface of the Earth. This mixture is in constant motion. If the atmosphere were visible, it might look like an ocean with swirls and eddies, rising and falling air, and waves that travel for great distances.
Life on Earth is supported by the atmosphere, solar energy, and the planet’s magnetic fields. The atmosphere absorbs energy from the sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects life on Earth from high energy radiation and the frigid vacuum of space.
Composition of the Atmosphere
In any given volume of air, nitrogen accounts for 78 percent of the gases that comprise the atmosphere, while oxygen makes up 21 percent. Argon, carbon dioxide, and traces of other gases make up the remaining one percent. This volume of air also contains some water vapor, varying from zero to about five percent by volume. This small amount of water vapor is responsible for major changes in the weather. [Figure 12-1]
The envelope of gases surrounding the Earth changes from the ground up. Four distinct layers or spheres of the atmosphere have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. [Figure 12-2]
Figure 12-1. Composition of the atmosphere.
The first layer, known as the troposphere, extends from 6 to 20 kilometers (km) (4 to 12 miles) over the northern and southern poles and up to 48,000 feet (14.5 km) over the equatorial regions. The vast majority of weather, clouds, storms, and temperature variances occur within this first layer of the atmosphere. Inside the troposphere, the average temperature decreases at a rate of about 2 °Celsius (C) every 1,000 feet of altitude gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain.
Figure 12-2. Layers of the atmosphere.
At the top of the troposphere is a boundary known as the tropopause, which traps moisture and the associated weather in the troposphere. The altitude of the tropopause varies with latitude and with the season of the year; therefore, it takes on an elliptical shape as opposed to round. Location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence.
Above the tropopause are three more atmospheric levels. The first is the stratosphere, which extends from the tropopause to a height of about 160,000 feet (50 km). Little weather exists in this layer and the air remains stable, although certain types of clouds occasionally extend in it. Above the stratosphere are the mesosphere and thermosphere, which have little influence over weather.
Atmospheric Circulation
As noted earlier, the atmosphere is in constant motion. Certain factors combine to set the atmosphere in motion, but a major factor is the uneven heating of the Earth’s surface. This heating upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure. The movement of air around the surface of the Earth is called atmospheric circulation.
Heating of the Earth’s surface is accomplished by several processes, but in the simple convection-only model used for this discussion, the Earth is warmed by energy radiating from the sun. The process causes a circular motion that results when warm air rises and is replaced by cooler air.
Warm air rises because heat causes air molecules to spread apart. As the air expands, it becomes less dense and lighter than the surrounding air. As air cools, the molecules pack together more closely, becoming denser and heavier than warm air. As a result, cool, heavy air tends to sink and replace warmer, rising air.
Because the Earth has a curved surface that rotates on a tilted axis while orbiting the sun, the equatorial regions of the Earth receive a greater amount of heat from the sun than the polar regions. The amount of solar energy that heats the Earth depends on the time of year and the latitude of the specific region. All of these factors affect the length of time and the angle at which sunlight strikes the surface.
Solar heating causes higher temperatures in equatorial areas, which causes the air to be less dense and rise. As the warm air flows toward the poles, it cools, becoming denser and sinks back toward the surface. [Figure 12-3]
Figure 12-3. Circulation pattern in a static environment.
Atmospheric Pressure
The unequal heating of the Earth’s surface not only modifies air density and creates circulation patterns; it also causes changes in air pressure or the force exerted by the weight of air molecules. Although air molecules are invisible, they still have weight and take up space.
Imagine a sealed column of air that has a footprint of one square inch and is 350 miles high. It would take 14.7 pounds of effort to lift that column. This represents the air’s weight; if the column is shortened, the pressure exerted at the bottom (and its weight) would be less.
The weight of the shortened column of air at 18,000 feet is approximately 7.4 pounds; almost 50 percent that at sea level. For instance, if a bathroom scale (calibrated for sea level) were raised to 18,000 feet, the column of air weighing 14.7 pounds at sea level would be 18,000 feet shorter and would weigh approximately 7.3 pounds (50 percent) less than at sea level. [Figure 12-4]
The actual pressure at a given place and time differs with altitude, temperature, and density of the air. These conditions also affect aircraft performance, especially with regard to takeoff, rate of climb, and landings.
Coriolis Force
In general atmospheric circulation theory, areas of low pressure exist over the equatorial regions and areas of high pressure exist over the polar regions due to a difference in temperature. The resulting low pressure allows the high-pressure air at the poles to flow along the planet’s surface toward the equator. While this pattern of air circulation is correct in theory, the circulation of air is modified by several forces, the most important of which is the rotation of the Earth.
Figure 12-4. Atmosphere weights.
The force created by the rotation of the Earth is known as the Coriolis force. This force is not perceptible to humans as they walk around because humans move slowly and travel relatively short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly affects mot
ion over large distances, such as an air mass or body of water.
The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. The amount of deflection differs depending on the latitude. It is greatest at the poles and diminishes to zero at the equator. The magnitude of Coriolis force also differs with the speed of the moving body—the greater the speed, the greater the deviation. In the Northern Hemisphere, the rotation of the Earth deflects moving air to the right and changes the general circulation pattern of the air.
The Coriolis force causes the general flow to break up into three distinct cells in each hemisphere. [Figure 12-5] In the Northern Hemisphere, the warm air at the equator rises upward from the surface, travels northward, and is deflected eastward by the rotation of the Earth. By the time it has traveled one-third of the distance from the equator to the North Pole, it is no longer moving northward, but eastward. This air cools and sinks in a belt-like area at about 30° latitude, creating an area of high pressure as it sinks toward the surface. Then, it flows southward along the surface back toward the equator. Coriolis force bends the flow to the right, thus creating the northeasterly trade winds that prevail from 30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between 30° and 60° latitude and between 60° and the poles. This circulation pattern results in the prevailing upper level westerly winds in the conterminous United States.
Figure 12-5. Three-cell circulation pattern due to the rotation of the Earth.