Aircraft Performance

Energy Management

Energy management is the process of planning, monitoring, and controlling altitude and airspeed targets in relation to the airplane's energy state to:

• Attain and maintain desired vertical flightpath-airspeed profiles.
• Detect, correct, and prevent unintentional altitude-airspeed deviations.
• Prevent irreversible deceleration and/or sink rate that results in a crash.

The Energy System of an Airplane

The three sources of energy available to the pilot are:

• Kinetic Energy (KE) from airspeed.
• Potential Energy (PE) from altitude.
• Chemical Energy (CE) from burning of fuel stored in the tanks.

Airspeed (KE) and altitude (PE) make up an airplane's total mechanical energy.

The airplane's energy state describes how much of each kind of energy the airplane has available at any given time.

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).

A Framework for Managing the Energy State

A flying airplane is an "open" energy system, which means it can gain and lose energy. Energy can also be redistributed or exchanged without changing the airplane's total energy.

Examples of energy exchanges:

• Airspeed (KE) can be traded for altitude (PE), and vice-versa.
• Stored energy (CE) can be traded for altitude (PE) or airspeed (KE).

Total Energy Changes (Net Energy Change)

The airplane gains energy from engine thrust (T), and it loses energy from aerodynamic drag (D). When thrust exceeds drag, the pilot can store the surplus energy as altitude or airspeed.

Energy Exchanges (Change in Stored Energy)

Airspeed and altitude can be exchanged without contributing to energy gain or loss. When one increases, the other decreases, and vice-versa.

The Energy Balance Equation

The energy balance equation shows the relationship between energy flow and energy storage.

The left side of the energy balance equation represents the airplane's net energy change, while the right side reflects changes to the energy storage. A change in total energy (left side) always matches the change in total energy redistributed over altitude and airspeed (right side).

Roles of the Controls to Manage Energy State

Rather than asking what controls altitude and what controls airspeed, a pilot should ask what controls total energy and what controls its distribution over altitude and airspeed.

The central rule for managing the airplane's energy can be summed up as follows:

Coordinated throttle and elevator inputs control the airplane's energy state.

Primary Role of the Throttle and Elevator/Stabilator

The throttle regulates the rate of energy change. It is the total energy controller (left side of the energy balance equation).

The elevator or stabilator redistributes energy between altitude and speed. Thus, the elevator is the energy distribution controller (right side of the energy balance equation).

Additional Roles of the Elevator/Stabilator

The elevator can increase drag during a level turn. As the airplane banks, load factor (lift/weight) increases because total lift has to increase to pull the airplane into the turn.

On the backside of the power curve, an irreversible deceleration and/or sink rate can occur if a continuous loss of airspeed is coupled with insufficient power available. Pushing forward on the pitch control reduces drag; therefore, available energy increases. This role of the elevator or stabilator is critical in preventing excessive deceleration or sink rate.

Energy Management Errors

There are two types of energy errors:

• Total Energy Errors: The total amount of mechanical energy is not correct (e.g., low and slow).
• Energy Distribution Errors: The distribution between potential and kinetic energy is not correct (e.g., high and slow).

Correcting Energy Management Errors

• Total energy errors are corrected with the throttle.
• Energy distribution errors are corrected with the elevator or stabilator.
• A combination of both errors is corrected by using both controls simultaneously.

Energy Formulas

KE is directly proportional to the square of the object's velocity (airspeed). For a twofold increase in speed, KE increases by a factor of four.

KE = ½M × V²

PE is directly proportional to the object's height (altitude).

PE = M × G × H

• V = Velocity
• M = Mass
• G = Gravity field strength
• H = Height

Power Versus Thrust

The terms "power" and "thrust" are sometimes used interchangeably, erroneously implying that they are synonymous.

Thrust is a force or pressure exerted on an object. It is generated from the movement of air by a propeller or jet engine. Thrust is measured in pounds (lb) or newtons (N).

Jet engines produce thrust. High-energy air from the combustion chamber powers the turbine, which turns the compressor. When air leaves the engine, it is pushed backward at a high velocity. Thrust moves the airplane forward.

Power is a measurement of the rate of performing work or transferring energy. It can be thought of as the motion a force (thrust) creates when exerted on an object over a period of time. Power is measured in horsepower (hp) or kilowatts (kW).

In a propeller-driven aircraft, the engine provides power to rotate the propeller, which produces thrust.

Wing and Power Loading

Wing and power loading values provide insight into an airplane's handling and performance characteristics. They are typically published in the Introduction or General section of the AFM/POH.

Wing Loading

Wing loading indicates how hard each square-foot section of the wing is working to support the airplane's weight. An airplane's wing loading determines its takeoff and landing speed. A lightly loaded wing is ideal for short runways.

Wing Loading = Gross Weight ÷ Wing Area

Power Loading

Power loading indicates the power output in relation to the airplane's weight (thrust-to-weight ratio). It is a significant factor in takeoff and climb capabilities. A low power loading means better performance.

Power Loading = Gross Weight ÷ Rated Horsepower

Performance Regulations

Pilots must conduct all flight operations within the aircraft's limitations.

Aircraft limitations are published in:

• The Airplane Flight Manual (AFM).
• Placards or other markings.
• Type Certificate Data Sheet (TCDS).
• Supplements provided with aftermarket equipment.
• Limitations of Supplemental Type Certificates (STC).
• Maintenance manuals or instructions for continued airworthiness (ICA).

Performance Planning

14 CFR Parts 121 and 135 require aircraft to be operated within specified performance limits (parameters), such as planned runway lengths and climb performance. Operators must use data extracted from the performance data section of the AFM to show compliance.

Pilots operating under 14 CFR Part 91, excluding Subpart K, are not required to comply with performance planning limits; however, 14 CFR 91.103 requires the PIC to be familiar with the runway lengths of intended use and takeoff and landing distance information.

Advisory Information

Aircraft manufacturers may provide advisory information related to aircraft performance (e.g., wet runway landing distances). When advisory information is not placed in the limitations section, it is not a limitation. Pilots who do not observe such advice are not exhibiting good judgment.

Performance Charts

Information in the AFM/POH is not standardized among manufacturers. Some provide the data in tabular form, while others use graphs.

Performance data is usually based on:

• Standard atmospheric conditions (59°F or 15°C, and 29.92" Hg).
• Pressure altitude or density altitude.

Some charts require interpolation for specific flight conditions. Interpolating means finding an intermediate value by calculating it from surrounding known values.

Straight-and-Level Flight

In straight-and-level, unaccelerated flight, lift equals weight and thrust equals drag. Thus, drag determines the thrust required.

The maximum level flight speed for the aircraft is 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 curve represents the speed at which the lowest brake horsepower sustains level flight. This is the best endurance airspeed.

Flight at speeds below the best endurance airspeed is known as "flying on the backside of the power curve" or "flying in the region of reversed command."

In the region of reversed command:

• A higher airspeed requires a lower power setting to hold altitude.
• A lower airspeed requires a higher power setting to hold altitude.
• The aircraft likely exhibits no inherent tendency to maintain the trimmed speed.

Speed Instability

Speed instability is a condition in the region of reverse command that causes the aircraft to display negative longitudinal (pitch) stability because of the increased induced drag.

When a disturbance causes the airspeed to decrease, total drag increases, causing the airspeed to decrease further. Airspeed continues to decay without appropriate pilot action.

When an aircraft is established in steady, level flight in the region of reversed command:

• If airspeed is increased, an excess of power exists. The aircraft accelerates to an even higher speed.
• If airspeed is decreased, a deficiency of power exists. The aircraft continues to slow down.

Speed Stability During Visual Approaches

Speed instability may be encountered in slow-speed phases of flight, such as during takeoff and landing; however, this region is small and is below the normal approach speed (1.3 V_SO) of most general aviation aircraft.

Most landing approaches are flown in an area of neutral speed stability, where small speed changes result in little or no change in drag or power required.

Speed Stability During Instrument Approaches [IFR]

Instrument approaches are generally flown in an area of positive speed stability (approximately 1.8 V_SO) to reduce workload. Operating in this regime permits the pilot to make slight pitch changes without changing power settings. Minor speed changes are accepted, knowing that the speed returns to the original setting when the pitch is returned. The aircraft is slowed to a normal approach speed just before landing.

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 decreases 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 depends on excess power, while the angle of climb is a function of excess thrust.
• Weight: Extra weight in the aircraft negatively affects performance.

Climb Gradients

Airplane climb performance is calculated using a climb gradient to meet certification requirements. A climb gradient is a distance climbed per distance traveled across the ground, expressed in a percentage.

The following formula converts 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 calculates 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

Best Angle of Climb

The maximum angle of climb (AOC), 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 maximum angle of climb occurs where there is the greatest difference between the thrust available and the thrust required.

Maximum excess thrust occurs:

• For a jet-powered airplane, at approximately the maximum lift/drag ratio (L/D_MAX).
• For a propeller-powered airplane, at an airspeed just above stall speed and below L/D_MAX.

Best Rate of Climb

The maximum rate of climb (ROC), obtained at V_Y, provides the greatest altitude gain over time. V_Y is maintained when an airplane needs to reach the cruising altitude in the shortest time.

For a given weight of an aircraft, the climb rate depends on the difference between the power available and the power required, or the excess power. The maximum climb rate occurs where there is the greatest difference between the power available and the power required.

Maximum excess power occurs:

• For a jet-powered airplane, at an airspeed above L/D_MAX.
• For a propeller-powered airplane, at an airspeed 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.

An increase in an aircraft's weight produces a twofold effect on climb performance:

• Increased Drag and Power Required: Reserve power available is reduced, which in turn, affects both the climb angle and the climb rate.
• Reduced Rate of Climb: Less reserve thrust is available for climbing due to the increase in drag.

Effect of Altitude

As altitude increases, air density decreases, resulting in reduced available power. Airplanes with fixed-pitch propellers experience a reduction in RPM. Airplanes that are equipped with controllable propellers show a decrease in manifold pressure.

Speeds for the maximum rate of climb (V_Y) and maximum angle of climb (V_x) vary with altitude. As altitude increases, V_Y decreases and V_x increases until they converge at the aircraft's absolute ceiling.

At the absolute ceiling, there is no excess of power, and only one speed allows steady, level flight. Consequently, the aircraft produces a zero rate of climb. The service ceiling is the altitude at which the aircraft cannot 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.

Maximum Range

Maximum range (distance) occurs where the ratio of speed to power/thrust required is greatest. The maximum range speed is dependent on the type of powerplant.

The maximum range speed occurs:

• For a jet-powered airplane, above L/D_MAX (near the typical cruise speed).
• For a propeller-driven airplane, at L/D_MAX (minimum drag condition).

A variation in weight alters the values of airspeed and power required to obtain the L/D_MAX. Since fuel is consumed during cruise, the aircraft's gross weight varies, and optimum airspeed, altitude, and power setting can also vary.

The following formula determines the specific range for any given flight condition. It is a useful calculation for comparing the efficiency and range of various aircraft.

Specific Range = NM per Hour ÷ Pounds of Fuel per 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 the range is traded for 3 to 5% higher cruise speed.

Maximum Endurance

Maximum endurance (flying time) is obtained in a flight condition that requires the minimum amount of fuel flow to maintain steady, level flight.

The maximum endurance speed occurs:

• For a jet-powered airplane, at L/D_MAX (minimum drag and thrust condition).
• For a propeller-driven airplane, at approximately 75% of L/D_MAX (minimum power condition).

The following formula determines the specific endurance for any given flight condition.

Specific Endurance = Flight Hours per Hour ÷ Pounds of Fuel per Hour

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 decrease, or the optimum altitude increases.

Effects of Wind

Different theories exist on achieving maximum range when a headwind or tailwind is present. Many say that speeding up in a headwind or slowing down in a tailwind helps achieve the maximum range. While this theory may be true in many cases, there are variables in every situation.

Effects of Altitude

A flight conducted at a high altitude has a greater true airspeed (TAS) for the same indicated airspeed (IAS). Drag is the same, but the higher TAS causes a proportionately greater power required.

Range: An aircraft equipped with a reciprocating engine experiences very little, if any, variation of specific range up to its absolute altitude (not considering wind).

Endurance: Since the power required increases with altitude, the maximum endurance of a propeller-driven aircraft is achieved at sea level. If the airplane were over a flat surface, maintaining ground effect could reduce drag and extend the endurance.

Maximum Cruise Speed Performance

Airplanes are typically operated in cruise flight at their maximum cruise speed. As cruise altitude increases, very nearly the same TAS can be maintained with less power and less fuel burn per hour.

The airplane operates more efficiently at altitude because:

• The lower temperature increases engine performance.
• Drag depends mostly on IAS. The same IAS at altitude corresponds to a higher TAS.

Measuring Fuel Consumption

A measure of the economical use of fuel is called specific fuel consumption (SFC). The SFC can compare the engine's use of fuel at various power settings.

SFC = Pounds of Fuel Per Hour ÷ Horsepower

Declared Runway Distances

Declared distances for a runway represent the maximum distances available and suitable for meeting takeoff and landing distance performance requirements. For runways without published declared distances, the declared distances may be assumed to be equal to the runway's physical length 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 the stopway length declared available and suitable for the acceleration and deceleration of an airplane aborting a takeoff. The ASDA may be longer than the runway's physical length 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

Runway Conditions

Runways are classified as dry, wet, or contaminated. "Wet" is a condition, "water" is a contaminant. Depending on its depth, water can cause a runway to be considered wet or contaminated.

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 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.
• A runway is dry when it is neither wet nor contaminated.

The AIM defines a contaminated runway slightly differently. It states that a runway is 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 making a snowball and 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 does not squeeze out.

Slush: Snow that has water content such that it takes on fluid properties. Water drains from slush when a handful is picked up.

Compacted Snow: Snow that has been compressed into a solid form. An airplane remains on its surface without displacing any of it. If a chunk is picked up by hand, it holds 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

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 how a particular aircraft is expected to perform.

Each third of the runway, including the length of any displaced thresholds, is considered a separate section. The RwyCCs may vary for each section if different contaminants are present, or the same RwyCC may be applied when a uniform coverage of contaminants exists.

Braking Action NOTAMs

A runway covered with two inches of dry snow would be reported in a field condition (FICON) NOTAM as follows:

Example FICON NOTAM
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 1601010139. 1601010151-1601020145

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 is issued stating "SFC CONDITIONS NOT REPORTED."

Braking Action Reports

When available, ATC gives pilots the braking action reports 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 includes the statement, "braking action advisories are in effect." During this time, ATC issues 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.

Example Pilot Report
"Braking action is poor the first half of the runway."

Operations on Contaminated Runways

Maintaining Directional Control

Directional control problems on contaminated runways occur most frequently at slower speeds where the flight controls lose effectiveness.

To help prevent directional control problems, pilots should:

• Not lock the brakes.
• Slow down before turning onto a taxiway.
• Pick a nice, long runway oriented into the wind.
• Use as much aerodynamic drag as possible.
• Touch down at the slowest possible speed, on the centerline, and with no side drift.

Braking Techniques

On a contaminated runway, the brakes should be applied progressively throughout the deceleration process. They should not be pumped. Should a skid occur, releasing brake pressure can stop skidding. Maximum braking can then be reestablished.

The amount of power that can be applied to the brakes without skidding the tires is referred to as braking effectiveness. It occurs just before the point where the wheels begin to skid.

Landing Distance Calculations

When the landing runway is contaminated, pilots should consider the increase in landing distance required.

Pilots of reciprocating engine airplanes without published data can use the landing distance factors (LDF) from the following table. The LDF should be multiplied by the landing distance computed for a dry runway with no safety margin added (unfactored landing distance).

Example Landing Distance Calculation
Dry runway landing length: 500'
Braking action reported by ATIS: 4
Landing Distance Required = 1,400'

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 determines the approximate speed at which dynamic hydroplaning occurs 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 slower 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.

Hydroplaning Prevention

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.

To help prevent hydroplaning, pilots 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.
• 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 performs 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.
• 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 defined as "pressure altitude corrected for nonstandard temperature variations." Pressure altitude can be read off the altimeter when set to 29.92" Hg.

Density altitude can be found by:

• Using a flight computer.
• Referring to a table and chart.

Correcting for Humidity

Humidity is usually not considered an important factor in aircraft performance, but it is a contributing factor. Its effect can be determined using an online calculator.

Link: https://weather.gov/epz/wxcalc_densityaltitude

Example Calculation
Station Pressure: 22.22" Hg at 8,000'
Temperature: 80°F, Dew Point: 75°F
Density Altitude = 11,564'
With no humidity, the density altitude would be almost 500' lower.

When the temperature is greater than 5°C, a rule of thumb can be used: double the dew point in degrees Celsius and add a zero. Add the result to the density altitude.

Example Calculation
24°C + 24°C = 480' Correction

Performance on the Runway

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 of ground deceleration devices planned to be used for the landing. It does not include any safety margin and represents the airplane's best performance 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.

Factored Landing Distance: The factored landing distance is the certified landing distance multiplied by 1.67. 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 runway excursion (off the edge). 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. The airport authority must designate this area for use in decelerating an airplane during a rejected takeoff.

Unfactored or Certified Landing Distance: The landing distance determined during certification as required by 14 CFR 23.2130 or Part 25.125. The unfactored landing distance is not adjusted for any safety margin additives. The unfactored certified landing distance may differ from the actual landing distance because not all factors affecting landing distance must be accounted for by certification regulations.

Touchdown Zone: The first 3,000' of the runway beginning at the threshold. It is the area used for determining the Touchdown Zone Elevation (TDZE) during the development of straight-in landing minimums for instrument approaches.

Safety Margins

The FAA recommends adding a safety margin of at least 15% to the planned takeoff and landing distances. Some pilots add 50% to their takeoff and landing calculations. 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 increases 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.

Runway Gradient

The gradient (slope) of a runway is the amount of change in runway height over the length. It is expressed as a percentage. A positive gradient indicates the runway height increases, and a negative gradient indicates the runway decreases in height.

Runway gradient information is contained in the Chart Supplements. Depending upon the airplane's manufacturer, runway slope may be accounted for in the AFM/POH performance data.

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.

Rules of thumb:

• An upslope increases takeoff distance by approximately 7% per degree.
• A downslope reduces takeoff distance by approximately 5% per degree.
• A downslope increases landing distance by approximately 10% per degree.

Takeoff Performance

The most critical conditions of takeoff performance are combinations of:

• High gross weight
• High-density altitude
• Contaminated runways
• Tailwinds
• Uphill slopes
• Short runways

Rules of thumb:

• Abort the takeoff if no more than 70% of the takeoff speed is reached by 50% of the runway length (the "50/70" rule).
• Add 50% to the planned takeoff distance over a 50-foot obstacle as a safety margin (the "50/50" rule).

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.

An increase in gross weight:

• Requires a higher liftoff speed.
• Decreases acceleration.
• Increases the retarding force (drag and ground friction).

If the gross weight increases, more speed is required to get the aircraft airborne.

A 10% increase in takeoff gross weight causes:

• An estimated 5% increase in takeoff velocity.
• At least a 9% decrease in acceleration rate.
• At least a 21% increase in takeoff distance (high thrust-to-weight ratio aircraft).
• At least a 25% increase in takeoff distance (low thrust-to-weight ratio aircraft).

Wind

Rules of thumb:

• A headwind that is 10% of the takeoff airspeed reduces the takeoff distance by approximately 19%.
• A headwind that is 50% of the takeoff airspeed reduces the takeoff distance by approximately 75%.
• A tailwind that is 10% of the takeoff airspeed increases the takeoff distance by approximately 21%.

Density Altitude

An increase in density altitude:

• Requires a greater takeoff speed (true airspeed is higher than it would be at sea level).
• Decreases acceleration due to decreased thrust.

Planning for Intersection Departures

Pilots should assess the suitability of intersection departures during their preflight planning. Pilots may ask ATC for the distance between the intersection and the runway end. However, the distance may not be the same as any published declared distances.

Landing Performance

The most critical conditions of landing performance are combinations of:

• High gross weight
• High-density altitude
• Contaminated runways
• Tailwinds
• Downhill slopes
• Less than maximum landing flaps

• Short runways

Rules of thumb:

• Increase the landing distance by 50% for a wet runway.
• Increase the approach speed by 20% if ice is on the wings.
• For every knot above the recommended approach airspeed at the runway threshold, the touchdown point is 100' further down the runway.

Planning Requirements

Operations conducted under 14 CFR Part 121, 135, or 91 Subpart K must 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. For a wet runway, an additional 15% to be added to the landing distance required.

The following formula determines if the airplane can stop within 60% of the available landing distance.

Actual Landing Distance × 1.667 = Required Runway Length to be Available

The following formula calculates the additional 15% requirement for a wet runway.

Actual Landing Distance × 1.92 = Required Runway Length to be Available

• 5,001' (60%) when the runway is dry.
• 5,760' (60% + 15%) 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 must 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 near the stall speed or minimum control speed for the aircraft in the landing configuration.

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 is obtained by creating a continuous peak deceleration of the aircraft (maximum braking). 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

Aerodynamic drag can be used in the early stages of the landing roll. Unlike wheel brakes and tires, which suffer from continuous hard use, aerodynamic drag is free and does not wear out with use.

The use of aerodynamic drag is most beneficial for decelerating to 60% to 70% of the touchdown speed. At slower speeds, aerodynamic drag becomes less effective. Wheel braking must be increased 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, the landing distance increases by approximately 200'.

Weight

The minimum landing distance varies 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 causes:

• An estimated 5% increase in landing velocity.
• An estimated 10% increase in landing distance.

Density Altitude

An increase in density altitude increases the landing speed. The aircraft at altitude lands at the same indicated airspeed (IAS) as at sea level, but the true airspeed (TAS) is greater because of the reduced density.

Because a given IAS corresponds to a higher TAS at higher density altitudes, pilots are sometimes "tricked" by visual cues and fly slower than they should.

The approximate increase in landing distance with altitude is approximately 3.5% for each 1,000' of altitude. At 5,000', the required landing distance is 16% greater than at sea level.

Excessive Airspeed and Wind

The speed (acceleration and deceleration) experienced by any object varies directly with the imbalance of force and inversely with the object's mass.

Rules of thumb:

• An increase in the approach speed by 10% increases the landing distance by 20%.
• For every 10 knots of tailwind, increase the landing distance by at least 21%.

Excessive speed upon touchdown places a greater load on the brakes because of the additional kinetic energy. 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 V_SO should be used.

Example Calculation
58 V_S0 × 1.3 = 75 KIAS

Variations in Weight: Manufacturers typically publish the approach speed at the maximum gross weight of the airplane. The approach speed for a lower operating weight can be approximated with the following formula, however, the manufacturer's published speeds and recommendations should be followed.

V_REF1 = V_REF2 × √(Current Weight ÷ Maximum Gross Weight)

• V_REF1 is the calculated approach speed for the current weight.
• V_REF2 is the AFM/POH approach speed at the maximum gross weight.

Example Calculation
2,000 Actual Weight ÷ 2,500 Gross Weight = .80
√.80 = .894
75 KIAS X .894 = 67 KIAS

Wind Gust Factor: Slightly higher than normal airspeeds provide 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 Calculation
"Wind 180° at 10 knots gusting to 20 knots."
½ of the 10 knot gust factor = 5 KIAS
67 + 5 = 72 KIAS