Aircraft Performance

Transcription

1 Energy Management Aircraft Performance The energy state describes how much of each kind of energy the airplane has available at any given time. Pilots who understand energy management will know instantly what options they may have to maneuver their airplane. The three sources of energy available to the pilot are: Kinetic Energy (KE): The energy of speed; and Potential Energy (PE): The stored energy of position. Chemical Energy (CE): Energy created from chemical processes (e.g., the burning of fuel stored in the tanks). The objective is to manage energy so that kinetic energy stays between limits (stall and placards), the potential energy stays with limits (terrain to buffet altitude), and chemical energy stays above certain thresholds (not running out of fuel). During maneuvering, energy can be traded or exchanged: Airspeed (KE) can be traded for altitude (PE), and vice-versa. Stored energy (CE) can be traded for either altitude (PE) or airspeed (KE). Mechanical Energy Mechanical energy comes in two forms: kinetic energy and potential energy. Both KE and PE are directly proportional to the object s mass. KE is directly proportional to the square of the object s velocity (airspeed). That means for a twofold increase in speed, the kinetic energy will increase by a factor of four. PE is directly proportional to the object s height (altitude). The following formulas summarize these energy relationships. M = Mass V = Velocity M = Mass V = Gravity field strength H = Height Power Versus Thrust KE = ½M V² PE = M G H The terms power and thrust are sometimes used interchangeably, erroneously implying that they are synonymous. In a propeller-driven aircraft, the engine provides power to rotate the propeller which produces thrust. Thrust is a force or pressure exerted on an object. It is generated from the movement of air by propeller or jet engine. Thrust is measured in pounds (lb) or newtons (N). Power is a measurement of the rate of performing work or transferring energy (KE and PE). It can be thought of as the motion (KE and PE) a force (thrust) creates when exerted on an object over a period of time. Power is measured in horsepower (hp) or kilowatts (kw). Version: Page 1 of 12

2 Performance Charts Information in the AFM/POH is not standardized among manufacturers. Some provide the data in tabular form, while others use graphs. Also, the performance data may be presented on the basis of standard atmospheric conditions, pressure altitude, or density altitude. Some charts require interpolation for specific flight conditions. Interpolating means to find an intermediate value by calculating it from surrounding known values. For example, to get the winds aloft at 7,500, the pilot needs to average the wind speeds and directions reported at 6,000 and 9,000. Straight-and-Level Flight For the aircraft to remain in steady, level flight, equilibrium must be obtained by a lift equal to the aircraft weight and a powerplant thrust equal to the aircraft drag. Thus, the aircraft drag defines the thrust required to maintain steady, level flight. When an aircraft is in steady, level flight, the condition of equilibrium must prevail. The unaccelerated condition of flight is achieved with the aircraft trimmed for lift equal to weight and the powerplant set for a thrust to equal the aircraft drag. The maximum level flight speed for the aircraft will be obtained when the power or thrust required equals the maximum power or thrust available from the powerplant. The minimum level flight airspeed is not usually defined by thrust or power requirement since conditions of stall or stability and control problems generally predominate. Region of Reversed Command The power required to achieve equilibrium in constant-altitude flight at various airspeeds is depicted on a power required curve. The lowest point on the power required curve represents the speed at which the lowest brake horsepower sustains level flight. This is termed the best endurance airspeed. Flight at speeds below the best endurance airspeed is known as flying on the back side of the power curve or as flying in the region of reversed command. Speed instability occurs while operating in the region of reversed command: A higher airspeed requires a lower power setting to hold altitude; and A lower airspeed requires a higher power setting to hold altitude. Climb Performance An airplane can climb from one, or a combination of two factors: The excess power above that required for level flight. For example, an aircraft equipped with an engine capable of 200 horsepower, but using 130 horsepower to sustain level flight has 70 excess horsepower available for climbing. KE can be traded-off for PE by reducing airspeed. Factors that determine climb performance during a steady climb: Airspeed: Too much or too little will decrease climb performance. Drag: Configuration of gear, flaps, cowl flaps, and propellers must be made with consideration for the least possible drag. Power and Thrust: The rate of climb will depend on excess power, while the angle of climb is a function of excess thrust. Weight: Extra weight in the aircraft negatively affects performance. Page 2 of 12 My CFI Book

3 Climb Gradient Airplane climb performance in regard to certification requirements is calculated using a climb gradient. A climb gradient is a distance climbed per distance traveled across the ground, expressed in a percentage. For example, a 2% climb gradient would be 20 of altitude gained for every 1,000 traveled across the ground. The following formula can be used to convert a climb gradient as a percentage to a climb rate in hundreds of feet per minute. Climb Rate = (Ground Speed 60) Climb Gradient The following formula can be used to calculate a climb gradient as a percentage when given a required altitude to climb (rise) in a certain number of nautical miles (run). Climb Gradient = (Rise Run) 100 Note: The rise and the run should be in feet. There are approximately 6,076 feet in one nautical mile. Best Angle of Climb Maximum angle of climb, obtained at V X, provides the greatest altitude gain over a certain distance. V X is maintained when it is necessary for an airplane to clear obstacles after takeoff. For a given weight of an aircraft, the angle of climb depends on the difference between thrust and drag, or the excess thrust. The climb angle depends on the value of excess thrust. When the excess thrust is zero, the inclination of the flight path is zero, and the aircraft will be in steady, level flight. The maximum angle of climb occurs where there is the greatest difference between thrust available and thrust required. For a propeller-powered airplane, maximum excess thrust occurs typically at an airspeed below the maximum lift/drag ratio (L/D MAX ) and is frequently just above stall speed. Best Rate of Climb The maximum rate of climb, obtained at V Y, provides the greatest altitude gain over time. For a given weight of an aircraft, the rate of climb depends on the difference between the power available and the power required, or the excess power. When the excess power is zero, the rate of climb is zero and the aircraft is in steady, level flight. When power available is greater than the power required, the excess power will allow a rate of climb specific to the magnitude of excess power. The maximum rate of climb will occur where there exists the greatest difference between the power available and the power required. For a propeller-powered airplane, maximum excess power normally occurs at an airspeed and angle-of-attack combination close to L/D MAX. Effect of Weight If weight is added to an aircraft, it must fly at a higher AOA to maintain a given altitude and speed. This increases the induced drag of the wings, as well as the parasite drag of the aircraft. Increased drag means that additional thrust is needed to overcome it, which in turn means that less reserve thrust is available for climbing. Version: Page 3 of 12

4 An increase in an aircraft s weight produces a twofold effect on climb performance: Increased drag and the power required: Reserve power available is reduced, which in turn, affects both the climb angle and the climb rate. Reduced maximum rate of climb: The aircraft must be operated at a higher climb speed to achieve the smaller peak climb rate. Effect of Altitude An increase in altitude also will increase the power required and decrease the power available. Therefore, the climb performance of an aircraft diminishes with altitude. The speeds for maximum rate of climb, maximum angle of climb, and maximum and minimum level flight airspeeds vary with altitude. As altitude is increased, these various speeds finally converge at the absolute ceiling of the aircraft. At the absolute ceiling, there is no excess of power, and only one speed will allow steady, level flight. Consequently, the absolute ceiling of an aircraft produces a zero rate of climb. The service ceiling is the altitude at which the aircraft is unable to climb at a rate greater than 100 FPM. Cruise Performance In flying operations, the problem of efficient range operation of an aircraft appears in two general forms: To extract the maximum flying distance from a given fuel load; or To fly a specified distance with a minimum expenditure of fuel. Range Versus Endurance Range must be clearly distinguished from the item of endurance. Range involves consideration of flying distance, while endurance involves consideration of flying time. Maximum endurance is obtained at the point of minimum power required to keep the airplane in steady, level flight. When power is at a minimum, fuel burn is also at a minimum. Maximum range is obtained by conducting the cruise operation so that the aircraft obtains its maximum specific range throughout the flight. Since fuel is consumed during cruise, the gross weight of the aircraft varies and optimum airspeed, altitude, and power setting can also vary. The airspeed for maximum range corresponds with L/D MAX, the airspeed where total drag is at a minimum. Maximum specific range is a calculated flight condition that provides the maximum speed per fuel flow. It is expressed in nautical mile per pound of fuel. It is a useful calculation for comparing the efficiency and range of various aircraft. Specific Range = NM/Hour Pounds of Fuel/Hour Long-range cruise operations are normally conducted at the flight condition that provides 99% of the absolute maximum specific range. The advantage of such an operation is that 1% of range is traded for three to 5% higher cruise speed. Cruise Control Cruise control of an aircraft implies that the aircraft is operated to maintain the recommended long-range cruise condition throughout the flight. As fuel is consumed, the aircraft s gross weight decreases. The optimum airspeed and power setting decreases, or the optimum altitude increases. Effects of Wind Different theories exist on how to achieve max range when there is a headwind or tailwind present. Many say that speeding up in a headwind or slowing down in a tail wind helps to achieve max range. While this theory may be true in a lot of cases, it is not always true as there are different variables to every situation. Page 4 of 12 My CFI Book

5 Effects of Altitude A flight conducted at high altitude has a greater true airspeed (TAS), and the power required is proportionately greater than when conducted at sea level. The drag of the aircraft at altitude is the same as the drag at sea level, but the higher TAS causes a proportionately greater power required. If a change in altitude causes identical changes in speed and power required, the proportion of speed to power required would be unchanged. The fact implies that the specific range of a propeller-driven aircraft would be unaffected by altitude. An aircraft equipped with a reciprocating engine experiences very little, if any, variation of specific range up to its absolute altitude. An increase in altitude produces a decrease in specific range only when the increased power requirement exceeds the maximum cruise power rating of the engine. One advantage of supercharging is that the cruise power may be maintained at high altitude. The principal differences in the high altitude cruise and low altitude cruise are the TAS and climb fuel requirements. Declared Runway Distances Reference: AIM Declared distances for a runway represent the maximum distances available and suitable for meeting takeoff and landing distance performance requirements. All Title 14 CFR Part 139 airports report declared distances for each runway. For runways without published declared distances, the declared distances may be assumed to be equal to the physical length of the runway unless there is a displaced landing threshold. Definitions Takeoff Run Available (TORA): The runway length declared available and suitable for the ground run of an airplane taking off. The TORA may be shorter than the runway length if a portion of the runway must be used to satisfy runway protection zone requirements. Takeoff Distance Available (TODA): The takeoff run available plus the length of any remaining runway or clearway beyond the far end of the takeoff run available. Accelerate-Stop Distance Available (ASDA): The runway plus stopway length declared available and suitable for the acceleration and deceleration of an airplane aborting a takeoff. The ASDA may be longer than the physical length of the runway when a stopway has been designated available by the airport operator, or it may be shorter than the physical length of the runway if necessary to use a portion of the runway to satisfy runway design standards. Landing Distance Available (LDA): The runway length declared available and suitable for a landing airplane. This distance may be shorter than the full length of the runway due to a threshold displacement. Runway Conditions and Contaminants Reference: AC Runway Conditions Runways are classified as dry, wet, or contaminated. Version: Page 5 of 12

6 For purposes of runway condition reporting and airplane performance: A runway can be considered wet when more than 25% of its surface area is covered by any visible dampness or water that is 1/8 inch or less in depth; A runway is considered contaminated when more than 25% of its surface area is covered by frost, ice, and any depth of snow, slush, or water; and A runway is dry when it is neither wet nor contaminated. Note: Wet is a condition, water is a contaminant. Depending on its depth, water can cause a runway to be considered wet or contaminated. According to the AIM, a runway is considered contaminated whenever standing water, ice, snow, slush, frost in any form, heavy rubber, or other substances are present. Runway Contaminants Dry Snow: Snow that does not stick together. This generally occurs at temperatures well below freezing. If when making a snowball, it falls apart, the snow is considered dry. Wet Snow: Snow that has grains coated with liquid water, which bonds the snow together. A well-compacted snowball can be made, but water will not squeeze out. Slush: Snow that has water content such that it takes on fluid properties. Water will drain from slush when a handful is picked up. Compacted Snow: Snow that has been compressed into a solid form. An airplane will remain on its surface without displacing any of it. If a chunk is picked up by hand, it will hold together. Frost: Frost consists of ice crystals formed from airborne moisture that condenses on a surface whose temperature is below freezing. Frost has a more granular texture than ice. Water: Water in a liquid state greater than 1/8 inch in depth. Ice: The solid form of frozen water. Wet Ice: Ice that is melting or ice with any depth of water on top. Runway Conditions Reporting References: AIM 4-3-8, AIM 4-3-9, AC 91-79, AC 150/ Airport operators assess runway surfaces and report conditions using the Runway Condition Assessment Matrix (RCAM). Contaminants are reported by numerical Runway Condition Codes (RwyCC) defined in the RCAM. This allows ATC to communicate actual runway conditions to pilots in terms that relate to the way a particular aircraft is expected to perform. Each third of the runway, including the length of any displaced thresholds, is considered as a separate section. The RwyCCs may vary for each section if different contaminants are present. However, the same RwyCC may be applied when a uniform coverage of contaminants exists. Related: Appendix: Braking Action Codes and Definitions Matrix Braking Action NOTAMs A runway that is covered with two inches of dry snow would be reported in a field condition (FICON) NOTAM as follows: DEN RWY 17R FICON (5/5/3) 25 PRCT 1/8 IN DRY SN, 25 PRCT 1/8 IN DRY SN, 50 PRCT 2 IN DRY SN OBSERVED AT When the condition of a movement area cannot be monitored, a FICON NOTAM is issued, or the information is included in the Chart Supplements. If no condition reports will be taken for longer than a 24-hour period, an Aerodrome (AD) NOTAM will be issued stating SFC CONDITIONS NOT REPORTED. Braking Action Reports When available, ATC furnishes pilots the quality of braking action received from other pilots. Braking action quality is described by the terms good, good to medium, medium, medium to poor, poor, and nil. These terms are defined in the RCAM. When weather conditions are deteriorating or rapidly changing, the ATIS broadcast will include the statement, Braking action advisories are in effect. During this time, ATC will issue the most recent braking action report to each arriving and departing aircraft. Pilots should be prepared to provide a descriptive runway condition report to controllers after landing. Page 6 of 12 My CFI Book

7 Example pilot report: Braking action is poor the first half of the runway. Note: A NIL braking condition report requires the affected runway to be closed. Operations on Contaminated Runways Braking Effectiveness The amount of power that can applied to the brakes without skidding the tires is referred to as braking effectiveness. It occurs just prior to the point where the wheels begin to skid. Brake Application Brakes should be applied progressively throughout the deceleration process. The brakes should not be pumped. Should a skid occur, releasing brake pressure can stop skidding. Maximum braking can be then be reestablished. Directional Control Directional control problems on contaminated runways usually occur most frequently in the latter part of the landing rollout where the flight controls lose effectiveness due to the low airspeed. To help prevent directional control problems: Pick a nice, long runway oriented into the wind; Touch down at the slowest possible airspeed, on centerline with the longitudinal axis of the aircraft aligned with the runway; Use as much aerodynamic drag as possible; Do not lock the brakes; Do not try to turn onto a taxiway at too great of a speed; and Taxi slowly. Hydroplaning Types of Hydroplaning Dynamic hydroplaning is a condition where the tires ride on a layer of water on the runway. Tire friction is then lost resulting in the wheels failing to spin-up properly. This effect is the same as water skiing. The following formula can be used to determine the approximate speed at which dynamic hydroplaning will occur in knots. Minimum Dynamic Hydroplaning Speed = (Tire Pressure) 8.6 Viscous hydroplaning occurs when a thin film of dirt, oil, or rubber particles mixes with water and prevents tires from making contact with the surface. Viscous hydroplaning can occur at a much lower speed than dynamic hydroplaning but it requires a smooth surface. Reverted rubber hydroplaning occurs when the wheels slide across smooth wet or icy pavement. Entrapped water between the locked wheel and pavement is heated enough to form steam, which acts to lift the tire off the runway. The heat generated by steam reverts the rubber to a black gooey substance. This type of hydroplaning can occur at any speed over approximately 20 knots. Preventing Hydroplaning The chances of hydroplaning increase as the speed of the aircraft increases, when air pressure in the tires increases, and when the depth of water increases. Version: Page 7 of 12

8 To prevent hydroplaning, the pilot should: Touch down at the slowest speed that is safely possible; Use as much aerodynamic drag as possible; Apply moderate braking after the nosewheel is lowered; Maintain directional control with the rudder; Avoid a crosswind landing; and Land on a grooved runway (if available). The brakes should be applied firmly until reaching a point just short of a skid. At the first sign of a skid, the pilot should release brake pressure and allow the wheels to spin up. Runway Grooves: Runway grooving is the most effective means of preventing hydroplaning. One-quarter-inch grooves spaced approximately 1 1/4 inches apart are made on some runway surfaces to provide for better drainage and to provide an escape route for water under the tire. Density Altitude Aircraft performance is based on density altitude. High-density altitude refers to thin air, while low-density altitude refers to dense air. Regardless of the actual altitude of the aircraft, it will perform as though it were operating at an altitude equal to the existing density altitude. As density altitude increases: Power Decreases: The engine takes in less air; Thrust Decreases: A propeller is less efficient in thin air; and Lift Decreases: The thin air exerts less force on the airfoils. Factors that Increase Density Altitude Low Atmospheric Pressure: At a constant temperature, density decreases directly with pressure. High Temperature: Increasing the temperature of a substance decreases its density. High Humidity: Water vapor is lighter than air; consequently, air becomes less dense as its water content increases. Calculating Density Altitude Density altitude is determined by first finding pressure altitude, and then correcting this altitude for nonstandard temperature variations. At standard day conditions, density altitude can be read off the altimeter when it is set to Hg. Using a flight computer, density altitude can be computed by inputting the pressure altitude and outside air temperature at flight level. Density altitude can also be determined by referring to a table and chart. Related: Technical Subjects: Flight Instruments: Types of Altitude Preflight Preparation: Aviation Weather: Density Appendix: Density Altitude Chart Takeoff and Landing Performance Definitions Actual Landing Distance: The landing distance for the reported meteorological and runway surface conditions, runway slope, airplane weight, airplane configuration, approach speed, and use ground deceleration devices planned to be used for the landing. It does not include any safety margin and represents the best performance the airplane is capable of for the conditions. Adjusted Landing Distance: The actual landing distance adjusted for a landing safety margin. Clearway: A defined area connected to and extending beyond the runway end available for completion of the takeoff operation of turbine-powered airplanes. A clearway increases the allowable airplane operating takeoff weight without increasing runway length. It must be at least 500 wide. Page 8 of 12 My CFI Book

9 Factored Landing Distance: The factored landing distance is the certified landing distance multiplied by The resulting distance is required to be used in some types of operations during preflight planning. Runway Safety Area (RSA): The surface surrounding a runway, suitable for reducing the risk of aircraft damage during an undershoot, overshoot or off-side runway excursion. The RSA is free of all objects except those necessary to be placed due to their function. Stopway: An area beyond the runway, centered upon the extended runway centerline and no less than the runway width, able to support an airplane during a rejected takeoff without causing structural damage to the airplane. This area must be designated by the airport authority for use in decelerating an airplane during a rejected takeoff. Stopways often double as blast pads, or areas free of items that could be damaged by a departing aircraft s jet or propeller blast. Unfactored or Certified Landing Distance: The landing distance determined during certification as required by 14 CFR or Part The unfactored landing distance is not adjusted for any safety margin additives. The unfactored certified landing distance may be different from the actual landing distance because not all factors affecting landing distance are required to be accounted for by certification regulations. Touchdown Zone: The first 3,000 of the runway beginning at the threshold. The area is used for determination of Touchdown Zone Elevation (TDZE) in the development of straight-in landing minimums for instrument approaches. Safety Margins Once the actual takeoff and landing distances are determined, an additional safety margin of at least 15% should be added. The resulting distance should be within the runway length available and acceptable for obstacle clearance. Runway Surface Typically, performance chart information assumes paved, level, smooth, and dry runway surfaces. Any surface that is not hard and smooth will increase the ground roll during takeoff. Runway surfaces for specific airports are noted in the Chart Supplements. For small airplanes, the factors given below are often quoted in the flight manual as an alternative to data derived from testing or calculation. Surface Takeoff Landing Dry Grass Wet Grass Note: If the runway is not smooth, the grass is very long or very short, higher factors may be warranted. Runway Gradient The gradient (i.e., slope) of the runway is the amount of change in runway height over the length of the runway. Runway gradient information is contained in the Chart Supplements. It is expressed as a percentage. A positive gradient indicates the runway height increases, and a negative gradient indicates the runway decreases in height. For example, a 3% gradient means that for every 100 of runway length, the runway height changes by 3. An upsloping runway impedes acceleration and results in a longer ground run during takeoff. However, landing on an upsloping runway typically reduces the landing roll. A downsloping runway aids in acceleration on takeoff resulting in shorter takeoff distances. However, landing on a downsloping runway increases landing distances. Depending upon the airplane s manufacturer, runway slope may be accounted for in the AFM/POH performance data. Rules of thumb: An upslope increases takeoff distance by approximately 7% per degree; A downslope reduces takeoff distance by approximately 5% per degree; and A downslope increases landing distance by approximately 10% per degree. Version: Page 9 of 12

10 Takeoff Performance The most critical conditions of takeoff performance are the result of some combination of high gross weight, altitude, temperature, and unfavorable wind. Weight The effect of gross weight on takeoff distance is significant and proper consideration of this item must be made in predicting the aircraft s takeoff distance. Increased gross weight can be considered to produce a threefold effect on takeoff performance: Higher liftoff speed; Greater mass to accelerate; and Increased retarding force (drag and ground friction). If the gross weight increases, a greater speed is necessary to produce the greater lift necessary to get the aircraft airborne at the takeoff lift coefficient. A 10% increase in takeoff gross weight would cause: An estimated 5% increase in takeoff velocity; At least a 9% decrease in rate of acceleration; and At least a 21% increase in takeoff distance (high thrust-to-weight ratio aircraft); and At least a 25% increase in takeoff distance (low thrust-to-weight ratio aircraft). Wind A headwind that is 10% of the takeoff airspeed will reduce the takeoff distance approximately 19%. A headwind that is 50% of the takeoff airspeed will reduce the takeoff distance approximately 75%. A tailwind that is 10% of the takeoff airspeed will increase the takeoff distance approximately 21%. Density Altitude An increase in density altitude can produce a twofold effect on takeoff performance: Greater takeoff speed; and Decreased thrust and reduced net accelerating force. If an aircraft of given weight and configuration is operated at greater heights above standard sea level, the aircraft requires the same dynamic pressure to become airborne at the takeoff lift coefficient. Thus, the aircraft at altitude will takeoff at the same indicated airspeed (IAS) as at sea level, but because of the reduced air density, the TAS will be greater. Landing Performance References: 14 CFR , 14 CFR , 14 CFR , AC 91-79, SAFO The most critical conditions of landing performance are combinations of: High gross weight; High density altitude; Contaminated runways; Tailwind landings; Downhill slopes; Less than maximum landing flaps; and Short runways. Planning Requirements Note: Compliance with these takeoff planning requirements is encouraged but not required for operations under 14 CFR Part 91, excluding Subpart K. Operations conducted under 14 CFR Part 121, 135, or 91 Subpart K are required comply with certain landing distance requirements at the time of takeoff. Generally, the airplane must be found capable of landing and stopping within 60% of the most suitable runway when the runway is dry. When the runway is wet, an additional 15% to be added to the landing distance required. Page 10 of 12 My CFI Book

11 The following formula can be used to determine if the airplane is capable of stopping within 60% of the available landing distance. Actual Landing Distance = Required Runway Length to be Available The following formula can be used to include the additional 15% requirement for a wet runway. Actual Landing Distance 1.92 = Required Runway Length to be Available For example, if the calculated landing distance is 3,000, the landing distance available must be 5,001 (60%) when the runway is dry or 5,760 when the runway is wet. The regulations do not specify the type of landing distance assessment that must be performed at the actual time of arrival, but operators are required to restrict or suspend operations when conditions are hazardous. Minimum Landing Speed The minimum landing distance is obtained by landing at some minimum safe speed, which allows sufficient margin above stall and provides satisfactory control and capability for a go-around. Generally, the landing speed is some fixed percentage of the stall speed or minimum control speed for the aircraft in the landing configuration. As such, the landing will be accomplished at some particular value of lift coefficient and AOA. Minimum Landing Distances Versus Ordinary Landings A distinction should be made between the procedures for minimum landing distance and an ordinary landing roll with considerable excess runway available. Minimum landing distance will be obtained by creating a continuous peak deceleration of the aircraft; that is, extensive use of the brakes for maximum deceleration. On the other hand, an ordinary landing roll with considerable excess runway may allow extensive use of aerodynamic drag to minimize wear and tear on the tires and brakes. Aerodynamic Drag If aerodynamic drag is sufficient to cause deceleration, it can be used in deference to the brakes in the early stages of the landing roll; i.e., brakes and tires suffer from continuous hard use, but aircraft aerodynamic drag is free and does not wear out with use. The use of aerodynamic drag is applicable only for deceleration to 60% or 70% of the touchdown speed. At slower speeds, aerodynamic drag is of little use. Wheel braking must be utilized to produce continued deceleration. Height Above Touchdown Landing distances furnished in the AFM/POH are based on the landing gear being 50 above the runway threshold. For every 10 above the standard 50 threshold crossing height, landing distance will increase by approximately 200. Weight The minimum landing distance will vary in direct proportion to the gross weight. An increase in gross weight requires a faster approach speed and requires more effort to decelerate to a stop after landing. A 10% increase in gross weight would cause: An estimated 5% increase in landing velocity; and An estimated 10% increase in landing distance. Density Altitude An increase in density altitude increases the landing speed but does not alter the net retarding force. Thus, the aircraft at altitude lands at the same IAS as at sea level but, because of the reduced density, the TAS is greater. The minimum landing distance at 5,000 is 16% greater than the minimum landing distance at sea level. The approximate increase in landing distance with altitude is approximately 3.5% for each 1,000 of altitude. Version: Page 11 of 12

12 Excessive Airspeed and Wind The speed (acceleration and deceleration) experienced by any object varies directly with the imbalance of force and inversely with the mass of the object. For example, an airplane on the runway moving at 75 knots has four times the energy it has when moving at 37 knots. At the faster speed, the airplane requires four times as much distance to stop than it does when at half the speed. Rules of thumb: An increase of the approach speed by 10% increases the landing distance by 20%; and For each 10 knots of tailwind, increase the landing distance by at least 21%. Excessive speed upon touchdown places a greater working load on the brakes because of the additional kinetic energy to be dissipated. Also, excessive speed increases lift in the normal ground attitude after landing, which reduces braking effectiveness. Approach Speed Calculations Sometimes variations in the normal approach speed should be made to compensate for changes in weight and gusty wind conditions. Normal Approach Speed: In the absence of the manufacturer s recommended approach airspeed, a speed equal to 1.3 VSO should be used. Example: 58 V S0 1.3 = 75 KIAS Variations in Weight: The weight-variation rule of thumb is to decrease the AFM/POH approach speed by 1/2 of the percent under gross weight. Example: 2,000 Actual Weight 2,500 Gross Weight = 80% of gross (20% under gross) ½ of 20% = 10% reduction needed = 67.5 KIAS Wind Gust Factor: Slightly higher than normal airspeeds provide for more positive control during strong horizontal wind gusts. One procedure is to use the normal approach speed plus one-half of the wind gust factor. Example: Wind 180 at 10 knots gusting to 20 knots = 10 knot gust factor ½ of 10 = 5 KIAS = 72.5 KIAS Page 12 of 12 My CFI Book