Pilot's Handbook of Aeronautical Knowledge
Chapter 11: Aircraft Performance
The performance or operational information section of the Aircraft Flight Manual/Pilot's Operating Handbook (AFM/ POH) contains the operating data pertaining to takeoff, climb, range, endurance, descent, and landing. The use of this data in flying operations is mandatory for safe and efficient operation.
Manufacturers' information and data is not standardized, and performance data may be presented on the basis of standard atmospheric conditions. Pressure and temperature have a major effect on aircraft performance, which pilots should take into account when predicting performance.
Pressure and Density Altitude
Pilots are mainly concerned with atmospheric pressure. Air a fluid substance that has mass, weight, and a force that is exerted equally in all directions. The effect of air on bodies within the air is called pressure. The pressure of the atmosphere varies with altitude and temperature. As air becomes less dense, it reduces power, thrust, and lift.
A standard temperature lapse rate observes the temperature decreasing at the rate of approximately 3.5 degrees F (2 degrees C) per thousand feet, up to 36,000 feet. Above this, the temperature is constant up to 80,000 feet.
A standard pressure lapse rate observes pressure decreasing at a rate of approximately 1" Hg per 1,000 feet of altitude gain, up to 10,000 feet. This often is referred to as International Standard Atmosphere (ISA) or ICAO Standard Atmosphere.
Any temperature or pressure that differs from the standard lapse rates is considered nonstandard temperature and pressure.
All aircraft instruments are calibrated for the standard atmosphere. Thus, corrections must be applied to the instrumentation and predicted performance if actual operating conditions vary from the standard atmosphere.
The Standard Datum Plane (SDP) is a theoretical level at which atmospheric pressure is 29.92" Hg and the weight of air is 14.7 psi. SDP may be below, at, or above sea level as atmospheric pressure changes.
Pressure altitude is the height above the SDP. Pressure altitude is a basis for determining aircraft performance. It also is used for assigning flight levels to aircraft operating at above 18,000 feet (FL 180).
If the altimeter is set to 29.92", the indicated altitude is pressure altitude. Pressure altitude also can be determined by applying a correction factor to the reported altimeter setting, or by using a flight computer.
Density altitude is pressure altitude corrected for nonstandard temperature. Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is to be found.
As the density of the air increases (low density altitude), aircraft performance increases. As air density decreases (higher density altitude), aircraft performance decreases
High density altitude refers to thin air while low density altitude refers to dense air. An increase in the air density results in "lower" density altitude, not "higher" density altitude, even though the increased weight of the air has more (a higher amount) of pressure. "High density altitude" refers to a higher altitude with air that is less dense.
Density altitude is determined by first finding pressure altitude and then correcting this altitude for nonstandard temperature variations. A known density occurs for any one temperature and pressure altitude. Regardless of the actual altitude at which the aircraft is operating, it will perform as though it were operating at an altitude equal to the existing density altitude.
Air density is affected by changes in altitude, temperature, and humidity. Contributors to high density altitude are high elevations, low atmospheric pressures, high temperatures, and high humidity.
Density altitude can be computed with a flight computer or a tabular chart.
Air density is directly proportional to pressure. If the pressure is doubled, the density is doubled, and if the pressure is lowered, so is the density. (This statement is true only at a constant temperature.)
Increasing the temperature of a substance decreases its density. (This statement is true only at a constant pressure.)
In the atmosphere, both temperature and pressure decrease with altitude and have conflicting effects upon density. With less pressure, the air becomes less dense, while lower temperature will make the air more dense. However, the fairly rapid drop in pressure as altitude is increased usually has the dominant effect. Pilots can expect the density to decrease with altitude.
Air Pressure, Air Density, and Pressure Altitude
When thinking about air pressure, it's best to use a stack of blankets as a metaphor. Imagine 300 blankets, piled high in a column.
Now get under them. You'll feel squished.
The atmosphere is similar to a stack of blankets. Air is matter — it has molecules, and these molecules become compressed if there's a lot of air overhead, because that overhead air is pressing down.
The atmosphere's downward force is easy to measure with a mercury barometer (although these aren't widely used anymore). At sea level, we typically measure about 30 inches of mercury within a sealed tube. An open dish at the base of the tube also contains mercury, and the mercury in this dish is resisting the air that is pressing down on it.
If we take this barometer to a location other than sea level, we'll see that the mercury in the measuring tube will fall, because the mercury in the dish is better able to resist the atmosphere's reduced downward force at higher elevations. The mercury in the dish rises, and the mercury in the measuring tube falls. If we drive up a mountain road, we'll see that (generally speaking) the atmosphere loses one inch of downward force for every 1,000 feet of elevation. As we climb, the mercury in the open dish continues to rise while the mercury in the measuring tube continues to fall.
So if we're looking at a barometer, we might say "Air pressure is 30 inches of mercury." We might even say "Current pressure is 30 inches." But we also could say "three hundred blankets" or "three thousand layers," because what we really care about are the measured layers of air molecules that are piled thousands of feet high.
People at or near sea level exist under about 3,000 layers of air molecules, most of the time. You can think of these as thin layers. In most of the earth's inhabited areas, each measurable layer of atmosphere — 0.01 inch of mercury — is about 10 vertical feet.
However, people who live at higher elevations, such as Denver, have adjusted to life under 2,500 layers of air molecules. Compared to a coastal atmosphere, the atmosphere in the Rocky Mountains is receiving less downward force, and this somewhat expanded air has fewer available oxygen molecules. That said, almost everyone adjusts to it.
Tibetans in the high Himalayas, above 14,000 feet, live under a mere 1,600 layers of air molecules. That's why visitors to camps at the base of Mount Everest feel a bit light-headed when they first arrive. There isn't as much air above them to press down on the air at the surface. Therefore, the air molecules aren't as squished together compared to a sea-level atmosphere, and this includes oxygen molecules. Up high, the result is "thin air."
Oxygen diminishes with altitude, but how?
Does the atmosphere at 14,000 feet have less oxygen than a sea-level atmosphere? Yes… and no. Because the air isn't being squished down to the same degree as at lower elevations, there aren't as many molecules roaming around in a given parcel of air. The reduction in air pressure results in a reduction in air density. Without question, oxygen becomes scarce at higher elevations.
However, it would be inaccurate to say "The atmosphere has a reduced amount oxygen at higher elevations" — at least, if we are comparing oxygen to other atmospheric matter. About 99% of the tropopause is made up of nitrogen and oxygen. As air density reduces with increased elevation, these molecules become more disperse, which means that there is less oxygen available for organisms that require it. But since both the nitrogen and oxygen molecules become more disperse, it's incorrect to say that the relative amount of oxygen in the atmosphere decreases with altitude. Instead, all atmospheric matter becomes more scarce. (You may come across knowledge test questions on this, which is why it's worth noting.)
Density vs. Pressure
It's also important to note that air overhead pressing down on the air below — air pressure — is only one contributing factor when it comes to air density.
Air density also is affected by changes in temperature, and this force is somewhat counter-intuitive. Heat causes the molecules in a parcel of air to expand, reducing density. This suggests that the air pressure, i.e. the air's weight, would reduce as well.
However, warmer air actually causes air pressure to rise — something we are familiar with during summer weather, when domes of high pressure can trap us in heat, haze, and smoke.
Why is this? When air molecules expand, they speed up, and they start to collide. This increases the air pressure, even though the parcel of air is less dense. As we fly from one location to another, we can expect to find higher pressures at warmer destinations.
As air cools, the air becomes more dense. However, the velocity of air molecules reduces. This creates a reduction in air pressure.
Humidity
Air density also increases as humidity increases. Water attaches to oxygen molecules in a parcel of air, provided that it's expansive enough to accept the moisture content. Thus, humid air often feels "wet" or "thick."
Important note: Some aviation sources teach that air pressure decreases as it is expanded by heat, and that air pressure increases as it is compressed by cooling.
However, there are FAA test questions that expect test-takers to disregard this model and instead accept that air density decreases and air pressure increases in warm conditions, while cooling has the opposite effects.
There are several online discussions about why a warm atmosphere becomes both less dense with an increase in pressure/weight, with varying explanations. This probably is one of the more substantial deep-dives in the aviation curriculum — interesting, but not required.
The Vertical and Horizontal Pressure Gradients
The vertical pressure gradient is easy to define, because we are provided with a standard pressure lapse rate: The atmosphere loses one inch of barometric pressure for every 1,000-foot increase in altitude/elevation. When the lapse rate exceeds this standard, then we have an unstable lapse rate, which is one of the key ingredients for thunderstorms.
The horizontal pressure gradient is more dynamic than the vertical component, because there isn't a consistent barometric pressure at the earth's surface. Variable heating of the surface contributes to pressure differentials, which creates wind.
Both air pressure and air density reduce as we ascend the vertical pressure gradient — but don't forget that density and pressure can work at cross-purposes along the horizontal pressure gradient. When air is heated it becomes less dense, reducing its pressure. However, the active molecules in warmer air create more energy, which increases the air pressure. As air is cooled, it becomes more dense, but its sedate molecules decrease air pressure.
The activity of the molecules within the parcel of air is the dominant effect, overriding its density characteristics. Thus, warmer air weighs more, while cooler air weighs less. You'll need to remember this rule for the knowledge test.
Redacted Field Elevations
Reported barometric pressures for purposes of aviation redact field elevation.
Because of this, the reported barometric pressure always will be subterranean when provided for any airport that's above sea level — which is to say, you couldn't actually get that reading with a mercury barometer.
At airports that are above sea level, the barometric pressure at the airport's surface always will be lower than the reported pressure, due to the surface's higher location in the vertical presssure gradient.
For example, an airport with a 250-foot field elevation that's reporting 30.25 inches of barometric pressure has 30.00 inches of surface pressure. The weather-reporting station knows this, but it redacts its own 250-foot elevation.
You can enter surface pressure in your altimeter — just set the indicated value to zero feet when at the surface. However, by doing so you will then track an indicated absolute altitude. (There are multiple reasons why this is impractical in the flight environment.)
When sea-level pressure is entered in the altimeter, only airports at sea level will have indicated altitudes of zero feet at the surface.
At airports where the field elevation is below sea level — such as Furnace Creek, Calif. (L06) where the field elevation is −206 MSL — the reported barometric pressure would be higher than field elevation. However, we can't fly to L06 and test this, since the airport doesn't have a weather station.
The Standard Datum Plane (SDP)
The Standard Datum Plane is a location in the horizontal pressure gradient. It exists where the air pressure is 29.92 inches of mercury. Or, in our analogy, it's always buried under 2,992 layers of air molecules.
The SDP is at sea level in a standard atmosphere. However, variable surface pressure means that the SDP typically is somewhere above or below sea level.
For example, if sea-level barometric air pressure at your location is reported to be 29.75", then the ocean at your location is buried under 2,975 layers of air. If you're on the ground at a sea-level airport, then that describes the atmosphere you currently occupy.
This is fewer than the 2,992 layers that press down on the SDP. Therefore, the airport is above the SDP.
By how much? The standard pressure lapse rate says that we lose one inch of mercury for every 1,000 feet that we ascend. Therefore, we must descend 10 feet to add 1/100th-inch of mercury.
At your sea-level airport, the atmosphere has enough downward force to elevate 29.75 inches of mercury.
At the location of the SDP, the atmosphere has enough downward force to elevate 29.92 inches of mercury.
.92 − .75 = 17 one hundredths of an inch.
Your atmosphere is 170 feet above the SDP, since each .01 inch of mercury is about 10 feet tall.
In this example, if you're sitting on the ground, the standard datum plane is somewhere underground.
What's happening in the Flight Levels?
By regulation, every aircraft flying at or below 17,999 MSL must set its altimeter to a reporting station within 100 miles of its location. When this is done, the altimeter is continuously reset along the route of flight to account for variations in the horizontal pressure gradient. The altimeter thus measures the aircraft's true altitude — the height above sea level — and the aircraft, at constant indicated altitude, will fly a horizontal path above sea level.
Every station's elevation is redacted from the barometric pressure reading so that all pressure values are reported as if the station were at sea level. This permits aircraft to remain at a consistent height above sea-level. Only sea-level airports report the actual barometric pressure at field elevation.
"Pressure altitude" is the altitude that appears on a barometric altimeter when the pressure is set to 29.92 inches of mercury. This is done above 17,999 MSL, starting at Flight Level 180 — and "The Levels" replace MSL because, when an altimeter is set to 29.92 and disregards reported pressures from ground stations, the aircraft no longer tracks a height above sea level, but instead a height above the Standard Datum Plane. This permits high-speed, high-altitude aircraft in the Flight Levels adequate vertical separation, while freeing each from the task of continuously resetting altimeters to ground stations.
Because of this, aircraft in the Flight Levels don't fly a perfect horizontal path. The true altitude drifts up and down, since the indicated altitude is tracking the horizontal pressure gradient, maintaining a determined height above where 29.92 inches of mercury registers directly below.
Uncorrected Altimeter Problems
Student pilots learn the phrase "High to low, look out below," but many find the meaning behind it difficult to visualize.
Let's start with a basic statement: The altimeter targets a baseline of air pressure.
Imagine that your altimeter has a sensor that sends a laser beam toward the earth, based on where you set the pressure in the Kollsman window. If you set your laser beam's depth to 30.30", then the altimeter will make that barometric pressure its baseline.
This a powerful laser. It doesn't matter if this weight of atmosphere exists at the surface, in the air, within the ocean, or underground. The laser beam will find the requested barometric pressure and track it.
Your selected 30.30" baseline is a datum plane. And when the altimeter indicates zero (0) feet, it is at this datum plane.
You can then use the altimeter to fly any height above your selected datum plane of 30.30". By maintaining a level altitude, such as 4,000 feet, you will remain 4,000 above the datum plane.
If you fly to 4,500 feet above this 30.30" layer, the altimeter will indicate an increase in altitude — to 4,500 feet. If you descend to 3,500 feet above the 30.30" layer, the altimeter will indicate your descent and new altitude.
But you're a skilled pilot who can hold altitude, so you will maintain your selected altitude as read on the instrument. And wherever you go, the altimeter will target a pressure reading somewhere below you that weighs 30.30 inches of mercury. This is how you know that you're in level flight.
However…
The horizontal pressure gradient isn't consistent. It's likely to change if you fly cross country, and even if you remain in the same location for a while.
Let's say you depart from a location where the sea-level barometric pressure is 30.30", maintaining an indicated altitude of 4,000 feet MSL. If you fly into an area where the pressure is 29.75" and you maintain indicated altitude, your altimeter will guide you closer to sea level. You have flown from high pressure to low pressure: "High to low, look out below."
When operating below 18,000 feet, aircraft should maintain a consistent height above sea level. This only can be done when the altimeter is continuously reset during a cross-country flight.
High to Low
Altimeter questions on pilot knowledge tests are some of the most difficult, because they must be visualized. Drawing a diagram of the horizontal and vertical pressure gradients can help students arrive at correct answers.
Let's use our established example, from above: You departed an airport where the sea-level barometric pressure was reported to be 30.30". You have arrived at an airport where the sea-level pressure is reported to be 29.75". You haven't listened to the reporting station, so your altimeter is uncorrected and in error.
But by how much?
And if you enter the correct barometric pressure, how will the altimeter respond?
The Differential
The difference between 30.30" and 29.75" is .55", or 55/100 inch of mercury. The standard vertical pressure lapse rate is 1" of mercury for every 1,000 feet. Therefore, your altimeter is off by 550 feet. This step gives us the vertical value of the error.
Following the Horizontal Gradient
When flying "high to low," the horizontal pressure gradient is descending. If you maintain a consistent indicated altitude, your true altitude will decrease along your route of flight.
Because you've been holding 4,000 feet indicated altitude, you may think that you're still 4,000 feet above sea level. In reality, you're flying closer to the ground than you may think. You're still 4,000 feet above 30.30" but the datum plane your altimeter has been tracking is lower than at your departure airport.
The Reset
Because you are "high to low" and flying closer to the ground, what can you expect if you enter the correct altimeter setting?
Your contract with the altimeter is simple: "I will enter a barometric pressure; you will tell me how high I am above that datum plane."
You have flow "high to low," which means that the sea-level pressure at your point of departure is now beneath sea level at this new location.
If you provide the altimeter with the correct pressure — in this case, a reduced pressure — then you will get a new indicated altitude that is lower than your current indicated altitude.
Why? Because your true altitude has been decreasing. You are now closer to the current sea-level datum plane, which is higher in the vertical pressure gradient.
In this case, re-setting your altimeter from 30.30" to 29.75" will cause the indicated altitude to decrease by 550 feet. Your indicated altitude will be 3,450 feet. You will have to climb to return to your cruising altitude of 4,000 feet.
"How high am I above _____?"
This is one of the most difficult concepts in the pilot curriculum. Do not beat yourself up if you don't understand it right away.
While all pilots should have a solid understanding of the vertical and horizontal pressure gradients, you might need to fall back on the fundamentals when taking a knowledge test. Put simply, lowering the pressure in the Kollsman window always lowers the indicated altitude, and increasing the pressure always increases indicated altitude.
The explanation is simple, even if it doesn't seem that way at first.
When you interact with an altimeter's pressure setting, you are asking it to tell you how high you are above a given datum plane.
In level flight:
- You are vertically closer to smaller numbers, such as 29.75".
- Therefore, if you set the altimeter to a datum plane that is vertically closer to you, the indicated altitude will decrease.
- You are vertically more distant from larger numbers, such as 30.30".
- Therefore, if you set the altimeter to a datum plane that is vertically farther from you, the indicated altitude will increase.
You can model this by holding one hand level (like an airplane) and using your other hand to visualize levels within the vertical pressure gradient.
You also can search for an interactive altimeter online to experiment. Start at a cruising altitude with a pressure of 29.92". Then come up with any new pressure, higher or lower, and ask yourself if the altimeter will increase or decrease, and by how many feet. Enter the new pressure to see if you're right.
Humidity
Humidity can be an important factor in aircraft performance. A small amount of water vapor suspended in the atmosphere may be negligible under certain conditions, but the air is never completely dry.
Relative humidity refers to the amount of water vapor contained in the atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can hold. Warm air can hold more water vapor, while colder air can hold less.
Water vapor is lighter than air. As the water content of the air increases, the air becomes less dense, causing an increase in density altitude, which reduces performance. Air is at its lightest and least dense when it contains the maximum amount of water vapor.
Humidity alone is usually not considered an essential factor in calculating density altitude and aircraft performance, but it does contribute. There is no rule-of-thumb or chart used to compute the effects of humidity on density altitude, but pilots should expect a decrease in overall performance in high humidity conditions.
Performance
Performance describes an aircraft's useful abilities. This can include takeoff, landing, rate of climb, payload capacity, altitudes, airspeeds, range, maneuverability, stability, fuel burn, and any other metrics deemed useful. All result from the combination of aircraft and powerplant.
When an aircraft is in straight and level flight, the condition of equilibrium must prevail.
Thrust is a force or pressure exerted on an object, and is measured in pounds or Newtons.
Power is a measurement of the rate of performing work or transferring energy, and is measured in horsepower (hp) or kilowatts (kw).
Mechanical energy can be kinetic or potential. Kinetic energy (KE) comes from speed, while potential energy (PE) is stored.
A climb can be achieved by applying excess horsepower. A climb also can be achieved by an exchange of energy, if the pilot converts the kinetic energy of airspeed (KE) to a gain in altitude (PE). As the altitude increases, airspeed is reduced.
Angle of Climb (AOC) is a comparison of altitude gained relative to distance traveled, i.e. the inclination (angle) of the flight path. Maximum AOC is used to clear obstacles, represented by the performance speed Vx. If excess thrust is available, the greater force can result in a steeper climb.
Rate of Climb (ROC) is a comparison of altitude gained relative to the time needed to reach that altitude, represented by the performance speed Vy, and resulting a maximum gain in altitude over a given period of time. Maximum ROC occurs at an airspeed and AOA combination that produces the maximum excess power.
Climb performance is directly dependent upon the ability to produce either excess thrust or excess power.
A change in weight changes the drag and the power required, affecting both the climb angle and the climb rate. An increase in weight also reduces the maximum ROC.
An increase in altitude also increases the power required and decreases the power available. Therefore, the climb performance of an aircraft diminishes with altitude.
The service ceiling is the altitude at which the aircraft is unable to climb at a rate greater than 100 feet per minute (fpm). At the absolute ceiling, there is no excess of power and only one speed allows steady, level flight. Consequently, the absolute ceiling of an aircraft produces zero ROC.
Power loading is expressed in pounds per horsepower and is obtained by dividing the total weight of the aircraft by the rated horsepower of the engine..
Wing loading is obtained by dividing the total weight of an airplane in pounds by the wing area. Wing loading determines the airplane's landing speed.
Endurance involves consideration of flying time, rather than distance traveled. Range is the ability of an aircraft to convert fuel energy into flying distance.
Maximum endurance requires a flight condition with minimum fuel flow.
Maximum specific range requires a flight condition with a maximum of speed per fuel flow.
Maximum endurance occurs at the point of minimum power required, since this requires the lowest fuel flow to keep the airplane in steady, level flight. Maximum range condition occurs where the ratio of speed to power required is greatest. The maximum range condition is obtained at maximum lift/ drag ratio (L/D max).
Long-range aircraft have a fuel weight that is a considerable part of the gross weight. Cruise control procedures employ scheduled airspeed and power changes to maintain optimum range conditions.
A flight conducted in a propeller-driven aircraft at high altitude has a greater true airspeed (TAS), and the power required is proportionately greater than when conducted at sea level because of the higher TAS. The drag of the aircraft at altitude is the same as the drag at sea level.
An aircraft equipped with a reciprocating engine experiences very little, if any, variation of specific range up to its absolute altitude.
Region of Reversed Command
The aerodynamic properties of an aircraft determine the power requirements at various conditions of flight. Powerplant capabilities determine the power available at various conditions of flight.
The power required to achieve equilibrium in constant-altitude flight at various airspeeds is depicted on a power required curve.
At low airspeeds, the power setting required for steady, level flight is high.
In the region of normal command, a higher airspeed requires a higher power setting and a lower airspeed requires a lower power setting to hold altitude. Most flight is conducted in this region.
In the region of reversed command while holding a constant altitude, a higher airspeed requires a lower power setting and a lower airspeed requires a higher power setting to hold altitude. This exists for flight speeds below the speed for maximum endurance, i.e. the lowest point on the power curve. This is often referred to as "flight behind the power curve" or "the backside of the curve." [11-14]
If an unacceptably high sink rate should develop on the backside of the power curve, it may be possible for the pilot to reduce or stop the descent by applying power. If additional power is not available, the airplane may stall or be incapable of flaring for landing. The only recourse is to lower the pitch attitude in order to increase airspeed, which inevitably results in a loss of altitude. If sufficient altitude does not exist, impact is possible.
Takeoff and Landing Performance
Takeoff
The most critical conditions of takeoff performance are the result of some combination of high gross weight, altitude, temperature, and unfavorable wind. An accurate prediction of takeoff distance can be determined from the performance data of the AFM/POH.
An airplane on the runway moving at 80 knots has four times the energy it has traveling at 40 knots. Thus, an airplane requires four times as much distance to stop as required at half the speed.
Any runway surface that is not hard and smooth increases the ground roll during takeoff. Runways can be concrete, asphalt, gravel, dirt, or grass.
The amount of power that is applied to the brakes without skidding the tires is referred to as braking effectiveness. Water on the runway can reduce braking effectiveness.
The gradient or slope of the runway is the amount of change in runway height over the length of the runway. The gradient is expressed as a percentage, such as a 3 percent gradient. This means that for every 100 feet of runway length, the runway height changes by 3 feet. A positive gradient indicates the runway height increases, and a negative gradient indicates the runway decreases in height. Up-sloping and down-sloping runways affect takeoff performance.
Dynamic hydroplaning is a condition in which the aircraft tires ride on a thin sheet of water rather than on the runway's surface. The minimum hydroplaning speed (in knots) is determined by multiplying the square root of the main gear tire pressure in psi by nine. (If the tire pressure is 36 psi, the square root is 6, and thus the hydroplaning speed is 54 knots.) Once hydroplaning starts, it can continue well below the minimum initial hydroplaning speed.
The lift-off speed is a fixed percentage of the stall speed or minimum control speed for the aircraft in the takeoff configuration. It can be anywhere from 1.05% to 1.25% the stall speed or minimum control speed.
Any item that alters the takeoff speed or acceleration rate during the takeoff roll affects the takeoff distance. Increased gross weight creates a higher lift-off speed, because there is a greater mass to accelerate and an in increase in drag and ground friction. For example, a 21% increase in takeoff weight requires a 10% increase in lift-off speed to support the greater weight.
The effect of a headwind is to allow the aircraft to reach the lift-off speed at a lower groundspeed, while the effect of a tailwind is to require the aircraft to achieve a greater groundspeed to attain the lift-off speed. The effect of wind on landing distance is identical to its effect on takeoff distance.
An increase in density altitude can produce a greater takeoff speed, as well as decreased thrust and reduced net accelerating force.
Proper accounting of pressure altitude and temperature is mandatory for accurate prediction of takeoff roll distance.
If an aircraft of given weight and configuration is operated at a pressure altitude above standard sea level, the aircraft requires the same dynamic pressure to become airborne at the takeoff lift coefficient. Thus, the aircraft at altitude takes off at the same indicated airspeed (IAS) as at sea level, but because of the reduced air density, the TAS is greater.
An increase in altitude above standard sea level brings an immediate decrease in power output for the unsupercharged reciprocating engine.
Landing
The most critical conditions of landing performance are combinations of high gross weight, high density altitude, and unfavorable wind.
The landing speed is some fixed percentage of the stall speed or minimum control speed for the aircraft in the landing configuration. Runway slope and condition are additional factors.
Minimum landing distance is obtained with extensive use of the brakes for maximum deceleration. An ordinary landing roll may allow use of aerodynamic drag to minimize wear on the tires and brakes. Aerodynamic drag is applicable only for deceleration to 60 or 70 percent of the touchdown speed.
Wind alters the groundspeed at which the aircraft touches down.
An increase in density altitude increases the landing speed. An aircraft at altitude lands at the same IAS as at sea level but, because of the reduced density, the TAS is greater. Deceleration is the same as with the landing at sea level, but the TAS will affect the landing distance.
The approximate increase in landing distance with altitude is approximately three and one-half percent for each 1,000 feet. At 5,000 feet, the minimum landing distance is 16 percent greater than the minimum landing distance at sea level.
A ten percent excess landing speed causes at least a 21 percent increase in landing distance. A tail wind of ten knots also increases the landing distance by about 21 percent.
Performance Speeds
- True airspeed (TAS): The speed of the aircraft in relation to the air mass in which it is flying.
- Indicated airspeed (IAS): The speed of the aircraft as observed on the ASI. It is the airspeed without correction for indicator, position (or installation), or compressibility errors.
- Calibrated airspeed (CAS): The ASI reading corrected for position (or installation) and instrument errors. (CAS is equal to TAS at sea level in standard atmosphere.) The color coding for various design speeds marked on ASIs may be IAS or CAS.
- Equivalent airspeed (EAS): The ASI reading corrected for position (or installation), for instrument error, and for adiabatic compressible flow for the particular altitude. (EAS is equal to CAS at sea level in standard atmosphere.)
- VS0: The calibrated power-off stalling speed or the minimum steady flight speed at which the aircraft is controllable in the landing configuration.
- VS1: The calibrated power-off stalling speed or the minimum steady flight speed at which the aircraft is controllable in a specified configuration.
- VY: The speed at which the aircraft obtains the maximum increase in altitude per unit of time. This best ROC speed normally decreases slightly with altitude.
- VX: The speed at which the aircraft obtains the highest altitude in a given horizontal distance. This best AOC speed normally increases slightly with altitude.
- VLE: The maximum speed at which the aircraft can be safely flown with the landing gear extended. This is a problem involving stability and controllability.
- VLO: The maximum speed at which the landing gear can be safely extended or retracted. This is a problem involving the air loads imposed on the operating mechanism during extension or retraction of the gear.
- VFE: The highest speed permissible with the wing flaps in a prescribed extended position. This is because of the air loads imposed on the structure of the flaps.
- VA: The calibrated design maneuvering airspeed. This is the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. Operating at or below maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time.
- VN0: The maximum speed for normal operation or the maximum structural cruising speed. This is the speed at which exceeding the limit load factor may cause permanent deformation of the aircraft structure.
- VNE: The speed that should never be exceeded. If flight is attempted above this speed, structural damage or structural failure may result.
Performance Charts
Performance charts are found in the AFM/POH, as provided by the manufacturer. They allow a pilot to predict the takeoff, climb, cruise, and landing performance of an aircraft. Data from the charts will not be accurate if the aircraft is not in good working order or when operating under adverse conditions.
Charts are not standardized and may appear in a variety of formats. Pilots should be familiar with the conventions of performance charts in the AFM/POH of their airplanes.
See the Pilot's Handbook of Aeronautical Knowledge for examples of various performance charts.
The Crosswind Component Graph
Each airplane has an upper limit to the amount of direct crosswind in which it can land. The crosswind component graph is used to determine if your airplane will be able to safely operate in observed or forecast wind conditions when the wind is not aligned with the runway at your destination.
Crosswind component graphs have two variables:
- The angle between wind direction and runway
- Wind velocity (in knots)
While the graph is fairly simple, it's easy to forget how it should be used to determine headwind and crosswind component values, since it's a series of numbers repeated along four metrics. (Fortunately, the graph included in the test booklet includes a sample problem.)
Think of the crosswind/headwind diagram as a depiction of the airplane coming in for a landing, with the '0' as the touchdown zone.
What is the angle between the wind and the runway? This the final approach course.
What is the wind velocity? Stop your approach here, on the airspeed arc. Look directly out your left and right windows at the horizontal and vertical metrics. You are looking at the crosswind and headwind components.
The most common reason why test-takers will select an incorrect answer is because they don't compute the initial variable correctly and select the wrong differential between the runway heading and the crosswind angle. Always double-check that you are using the correct angle. Often, the test will offer correct answers for wrong crosswind angles.
Note: As a rule of thumb, most airplanes have a maximum crosswind capability equal to 0.2 of VS0. Therefore, an airplane with a VS0 of 40 knots would have a maximum crosswind component of 8 knots.
Knowledge Test Tip: While student pilots may need to demonstrate mastery of the crosswind component graph during an oral exam, Sportys' electronic E6B has a "Wind > X/H-Wind" function. Students should use the calculator during the knowledge test because of its increased precision.
Commercial Pilot & Flight Instructor Test Questions
If an aircraft is flown from a region of high pressure to a region of low pressure without any altimeter reset, the altimeter will indicate an altitude higher than the true altitude.
If an altimeter's pressure is reset from 29.35 to 30.10, how does the indicated altitude change? It increases by 850 feet.
How will variations in temperature change your indicated altitude? Air pressure increases on warm days; true altitude is higher than indicated altitude.
If an airplane flying at a constant power setting, indicated altitude, and indicated airspeed travels from an area of colder temperature to an area of warmer temperature, how will the true airspeed and true altitude be affected? True airspeed will increase; true altitude will increase.
What effect does high density altitude, as compared to low density altitude, have on propeller efficiency? Efficiency is reduced because the propeller exerts less force at high density altitudes than at low density altitudes. As density altitude increases, which will occur if a constant indicated airspeed is maintained in a no-wind condition? True airspeed increases; groundspeed increases.
Which statement is true regarding takeoff performance with high density altitude conditions? The acceleration rate is slower because the engine and propeller efficiency is reduced. If atmospheric pressure and temperature remain the same, how would an increase in humidity affect takeoff performance? Longer takeoff distance; the air is less dense.
What effect does an uphill runway slope have upon takeoff performance? Increases takeoff distance. In a propeller-driven aircraft, maximum range occurs at maximum lift/drag ratio.
What can a pilot expect when landing at an airport in the mountains? Higher true airspeed and longer landing distance. What can a pilot expect when landing at an airport located at a much higher elevation? Higher true airspeed and longer landing distance. GIVEN:
From the conditions given, the approximate density altitude is 14,130 feet.
— The question describes "high to low, look out below," so it's easy to select "higher than true altitude" as the correct answer since it conveys risk. You can sketch this to reference a mental model.
— The height-differential between the two altimeter settings can be calculated using the standard vertical-pressure lapse rate of 1" per 1,000 feet. The difference here is .85 inches, or 850 feet. Because the pilot is resetting the altimeter to a baseline datum plane that is lower in the vertical pressure gradient, the distance between the altimeter's baseline and the aircraft will increase. Thus, the indicated altitude increases.
— This one is "cold to hot, look to the sky," the opposite of "hot to cold, look out below." Remember that heat increases air pressure, even as the expanded air is less dense. Thus, if you travel to a warmer region (or if you park your aircraft on a cold night and return on a hot day), you are "low to high." In this scenario, you have followed an ascending horizontal pressure gradient and your true altitude is higher than your indicated altitude.
— This question has a two-part answer. Air density decreases as it warms, which means that an aircraft traveling in a consistent manner from more-dense to less-dense air will increase its true airspeed as the atmosphere provides less resistance. However, warmer air increases air pressure, which will elevate the vertical pressure gradient in the area of warmer temperature. The aircraft will climb as the altimeter follows the horizontal pressure gradient to a higher true altitude.
— An increase in density altitude will indicate that the air has become less dense.
— Water vapor is less dense than dry air.
— "Maximum endurance" occurs at minimum power required.
Pressure altitude 12,000 ft
True air temperature +50 °F