Understanding airspeed is essential for safe flying, accurate navigation, and aircraft performance.
In this lesson, you will discover how pilots measure, interpret, and use different airspeed values. We will move from flight instruments to practical calculations and real flying situations.
How it works, how to read its colored markings, and what V-speeds mean.
Common indication errors and the effects of pitot-static system failures.
IAS, CAS, EAS, TAS, groundspeed, and Mach number.
Airspeed versus groundspeed, the wind triangle, and using an E6-B flight computer.
Using airspeed during each phase of flight and understanding vertical speed.
Review the essential concepts and test your knowledge before your exam.
By the end of this lesson, you should be able to interpret airspeed indications, distinguish the different speed measurements, and use them in basic flight planning and flying decisions.
How Does an Airspeed Indicator Work?
The airspeed indicator (ASI) is a flight instrument that shows an airplane's indicated airspeed (IAS), usually in knots. It works by comparing two air pressures rather than directly measuring how fast the airplane moves over the ground.
These pressures come from the pitot-static system, which uses a forward-facing pitot tube and one or more static ports. The difference between their pressures allows the instrument to indicate airspeed.
The Two Pressure Sources
To understand the airspeed indicator, you first need to know what each part of the pitot-static system measures.
The pitot tube faces the relative airflow. It measures total pressure, which includes static pressure and the pressure caused by the airplane's movement through the air.
The static port measures the surrounding atmospheric pressure without intentionally capturing the oncoming airflow.
How the Needle Responds to Airspeed Changes
In a traditional mechanical ASI, the flexible diaphragm reacts whenever the difference between pitot pressure and static pressure changes.
Total pressure rises relative to static pressure. The diaphragm expands, moving the mechanical linkage and turning the needle toward a higher indicated airspeed.
The pressure difference becomes smaller. The diaphragm contracts, allowing the needle to move toward a lower indicated airspeed.
When the airplane is stationary in calm air, pitot pressure and static pressure are equal, so the indicator reads zero. In flight, the instrument converts the pressure difference into a speed reading.
Modern aircraft with electronic flight displays use pressure sensors and an air data computer instead of a mechanical needle mechanism. The basic principle remains the same: indicated airspeed comes from the pitot-static pressure measurements.
The airspeed indicator displays indicated airspeed (IAS), not groundspeed. A steady headwind or tailwind does not directly change IAS if the airplane maintains the same speed relative to the air.
Airspeed Indicator Markings and V-Speeds
The colored markings on an airspeed indicator help pilots recognize important operating ranges and airspeed limitations at a glance. Each arc has a specific meaning, while the boundaries between the colors correspond to important V-speeds.
V-speeds are reference speeds used for aircraft operation and performance. Some appear as markings on the airspeed indicator, while others must be obtained from the aircraft's Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM).
Understanding the Colored Arcs
Extends from VSO, the stall speed in the landing configuration, to VFE, the maximum flap-extended speed.
Extends from VS1, the stall speed in a specified configuration, to VNO, the maximum structural cruising speed.
Extends from VNO toward VNE. Flight in this range is permitted only in smooth air and with caution.
Marks VNE, the maximum permitted airspeed. Exceeding this limit can cause structural damage or failure.
Other Important V-Speeds
Not all V-speeds are shown by colored arcs. Student pilots must also know the speeds used for takeoff, climbing, maneuvering, and gliding. Their actual values depend on the airplane.
Produces the greatest altitude gain over a given horizontal distance. Used when clearing obstacles after takeoff.
Produces the greatest altitude gain in a given amount of time.
An important structural reference speed for maneuvering. It decreases as aircraft weight decreases and does not make abrupt or combined control inputs harmless.
The speed at which the pilot begins raising the airplane's nose during takeoff, when specified.
Provides maximum gliding distance for a given loss of altitude in still air with the specified aircraft configuration.
VLE is the maximum speed with landing gear extended. VLO limits gear extension or retraction, as applicable.
Using the Airspeed Markings in Flight
- Before extending flaps: check the applicable flap speed limit. Some airplanes have different limits for different flap settings.
- During normal flight: recognize the green arc as the normal operating range and avoid the yellow arc unless the air is smooth.
- During approach: use the recommended approach speed from the aircraft's POH/AFM. The white arc alone does not tell you the correct approach speed.
The colored arcs provide important limits, but they do not replace the aircraft's POH/AFM. Always check the published V-speeds for your specific airplane, including any changes required for weight or configuration.
Airspeed Indicator Errors and Pitot-Static Failures
The airspeed indicator (ASI) is an essential flight instrument, but its indications are not always perfectly accurate. Some differences are caused by the way airspeed is measured, while others result from instrument problems or blocked pitot-static openings.
Understanding these errors helps pilots recognize unusual airspeed indications, identify possible causes, and avoid making incorrect control inputs.
Position Error
Position error occurs when the pressure measured by the pitot tube or static ports differs slightly from the pressure that should be measured in undisturbed airflow.
Air flowing around the aircraft changes direction and pressure near the fuselage and wings. Because the pitot tube and static ports are installed on the aircraft, their position can influence the pressure reaching the ASI.
What causes position error?
- Pitot tube location: Airflow around the aircraft can affect the pressure entering the pitot tube.
- Static port location: Local airflow can make the pressure at the static port slightly different from the surrounding atmospheric pressure.
- Aircraft configuration: Changes in angle of attack, flap position, and flight attitude can change the airflow around these openings.
Position error is one reason indicated airspeed (IAS) may differ from calibrated airspeed (CAS). The correction is aircraft-specific and can be found in the aircraft's operating handbook when applicable.
Density Error: Indicated Airspeed Versus True Airspeed
The ASI measures a difference in air pressure, not the airplane's actual speed through the surrounding air directly. Because dynamic pressure depends on air density, the same IAS can correspond to different true airspeeds (TAS).
Higher altitude
Air density generally decreases as altitude increases. At the same indicated airspeed, the airplane must travel faster through the less dense air to produce approximately the same dynamic pressure.
Warmer or colder air
At the same pressure, warmer air is less dense than colder air. Lower density increases TAS for a given IAS; higher density reduces that difference.
Important distinction: The difference between IAS and TAS caused by air density does not mean the airspeed indicator is malfunctioning. IAS remains an important reference for controlling the airplane. Later in this lesson, you will learn how to calculate TAS using an E6B flight computer.
Lag and Instrument Errors
An airspeed indicator contains a flexible diaphragm and a mechanical linkage that moves the pointer as pressure changes. Errors can occur if the mechanism does not respond correctly or if pressure reaches the instrument too slowly.
A properly functioning ASI should respond normally to changes in speed. A pointer that becomes unusually slow, sticks, jumps, or behaves inconsistently requires investigation. Delayed indications are particularly concerning during takeoff, approach, and slow flight.
Icing Errors
Ice can obstruct the pitot tube, its drain hole, or the static ports. When pressure cannot enter or leave the system correctly, the ASI may display misleading information.
Ice in the pitot system
Ice at the pitot opening prevents ram air from entering. The result depends on whether the drain hole also remains open or becomes blocked.
Ice in the static system
Ice blocking the static ports prevents outside atmospheric pressure from reaching the instruments. This can affect the ASI, altimeter, and vertical speed indicator (VSI).
Pitot heat helps prevent ice from forming in the pitot tube when the aircraft is equipped with that system. Some aircraft also have heated static ports. Pilots must follow the aircraft's operating procedures for their use.
Water and Other Obstructions
Ice is not the only possible cause of unreliable airspeed indications. Water, insects, dirt, and other debris can also restrict the pitot-static system.
How water affects the ASI
Water entering a pressure line can partially obstruct the passage of air or interfere with pressure changes. Depending on where the water collects, the ASI may respond slowly, fluctuate, or provide incorrect readings.
The pitot drain hole helps moisture escape. If this hole becomes blocked, water may remain in the system. Pilots should check pitot and static openings during the preflight inspection without attempting to clear them using methods not approved for the aircraft.
How Pitot-Static Blockages Affect the ASI
A blocked pitot opening does not always produce the same indication. The result depends on which openings are blocked and whether pressure remains trapped.
Pitot Opening Blocked — Drain Hole Open
Ram air cannot enter the pitot tube, but the trapped pressure can escape through the drain hole. The pressure inside the pitot line approaches outside atmospheric pressure.
The ASI loses the pressure difference needed to measure airspeed.
Pitot Opening and Drain Hole Both Blocked
Pressure becomes trapped inside the pitot line. The instrument no longer receives changes in ram air pressure.
If the static system remains open, the ASI responds to altitude changes instead of accurately responding to airspeed changes.
Static Ports Blocked — Pitot System Open
The static pressure becomes trapped at the pressure that existed when the blockage occurred. The pitot tube still receives ram air.
As the airplane changes altitude, the ASI compares pitot pressure with an incorrect static pressure. The following applies when comparing indications at the same actual airspeed.
Pitot and Static Systems Both Completely Blocked
If the pitot opening, pitot drain hole, and static system are all blocked, pressure becomes trapped on both sides of the ASI diaphragm.
The pressure difference can no longer change, even if the airplane accelerates, slows down, climbs, or descends.
Remember: With a blocked pitot opening and drain hole, the ASI tends to read higher during a climb and lower during a descent. With a blocked static system, the altitude-related error works in the opposite direction. These comparisons assume the actual airspeed remains unchanged.
What Happens to the Altimeter and VSI?
A pitot blockage mainly affects the airspeed indicator because the altimeter and VSI do not use pitot pressure. A blocked static system is different: all three pitot-static instruments may be affected.
Altimeter
With a completely blocked static system, the altimeter remains at approximately the altitude where the blockage occurred, even if the aircraft climbs or descends.
Vertical Speed Indicator
With a completely blocked static system, the VSI eventually indicates approximately zero vertical speed, even when the aircraft is climbing or descending.
A partially restricted static line can produce delayed or misleading indications instead of completely frozen instruments. Comparing the behavior of the ASI, altimeter, and VSI can therefore help identify a static system problem.
Recognizing Unreliable Airspeed Indications
Pilots should check the ASI before takeoff and remain alert for indications that do not match the airplane's expected performance.
Remove the pitot cover. Inspect the pitot opening, drain hole, and static ports for obstructions. Check instrument indications using the aircraft's approved procedures.
Confirm that the airspeed increases normally as the airplane accelerates. An abnormal indication must be handled according to the aircraft's takeoff procedures.
Cross-check attitude, power, altitude, and other available instruments. If airspeed becomes unreliable, use the appropriate aircraft procedures and known pitch-and-power references.
When available, an alternate static source may restore a usable pressure reference during a static system failure. Its use and any resulting indication corrections depend on the aircraft's operating handbook.
Key Point: Not every difference between indicated and true airspeed is an instrument failure. Position, instrument, and density effects must be distinguished from genuine pitot-static problems. A blocked pitot tube, drain hole, or static port can produce very different indications, so pilots must understand how each failure affects the instruments.
Types of Airspeed Explained: IAS, CAS, EAS, TAS, GS and Mach Number
An airplane does not have just one useful speed. Pilots use different airspeed measurements to control the aircraft, understand its aerodynamic performance, and calculate navigation and flight time.
In this chapter, you will learn the meaning of the six main speed references, how they differ, and which ones you actually use when preparing a navigation log.
Indicated Airspeed
Indicated Airspeed (IAS) is the speed displayed directly on the airspeed indicator. It is the reading the pilot sees without applying corrections for measurement errors, air density, or compressibility.
IAS is closely related to the aerodynamic pressure acting on the aircraft. This makes it the principal airspeed reference for controlling an airplane during normal flight.
For example, when your instructor asks you to maintain 80 knots indicated, you adjust the airplane until the ASI reads 80 knots.
Calibrated Airspeed
Calibrated Airspeed (CAS) is IAS corrected for instrument error and position error.
As explained in the previous chapter, airflow around the airplane can affect the pressure measured by the pitot tube and static ports.
The correction may change with airspeed, angle of attack, or flap configuration. Aircraft manufacturers provide calibration data when needed.
CAS is used when interpreting aircraft performance data. Always check whether the POH presents a particular airspeed in IAS or CAS.
Equivalent Airspeed
Equivalent Airspeed (EAS) is calibrated airspeed corrected for the effects of air compressibility.
It represents the speed that would produce the same dynamic pressure in standard sea-level air as the aircraft experiences in its actual flight conditions.
EAS is especially useful when evaluating aerodynamic loads and aircraft performance at higher speeds and altitudes.
This approximation is generally useful for low-speed flight, not for every aircraft or flight condition.
True Airspeed
True Airspeed (TAS) is the airplane's actual speed relative to the surrounding air mass.
TAS differs from IAS because air density changes with pressure altitude and temperature. At the same indicated airspeed, the airplane generally has a higher TAS in less dense air.
TAS is particularly important in navigation. It tells you how quickly the airplane moves through the air before wind is taken into account.
To determine TAS, pilots use aircraft performance data or a flight computer with the appropriate altitude and temperature information.
Groundspeed
Groundspeed (GS) is the speed of the airplane relative to the Earth's surface.
Unlike TAS, groundspeed includes the effect of wind. An airplane can maintain exactly the same TAS while its groundspeed changes considerably.
A headwind reduces groundspeed, while a tailwind increases it. With a crosswind, wind direction must also be considered.
These examples use an airplane flying at 120 KTAS with the wind directly along its flight path.
Groundspeed is the value used to calculate flight time, estimated arrival times, and fuel requirements for each flight leg.
Mach Number
Mach number compares the airplane's TAS with the local speed of sound.
The speed of sound depends mainly on air temperature. Therefore, the same TAS can correspond to different Mach numbers under different conditions.
Mach number becomes especially important during high-speed and high-altitude flight. As speed increases toward the transonic range, compressibility effects and shock waves can affect the airplane's aerodynamic behavior.
The critical Mach number is the lowest aircraft Mach number at which airflow somewhere over the airplane first reaches the speed of sound.
Mach number is a ratio. It is not expressed in knots.
The Order of Airspeed Conversions
Each airspeed represents a different stage in understanding the airplane's movement. Starting with the ASI reading, corrections lead to TAS. Wind is then applied to determine groundspeed.
Important: This is the conceptual conversion sequence, not a list of five speeds that must appear in every navigation log. In a typical light-airplane flight, compressibility corrections are often negligible. Mach number is a separate speed ratio rather than another step toward groundspeed.
Which Airspeed Goes into a Navigation Log?
A navigation log helps pilots plan each leg of a flight. The two most important speeds for these calculations are true airspeed (TAS) and groundspeed (GS).
Determine TAS
Use aircraft cruise performance data or the appropriate airspeed conversion with pressure altitude and temperature.
Calculate GS
Apply forecast wind direction and speed to TAS. With crosswinds, calculate wind correction angle as well.
Calculate ETE
Use planned groundspeed and leg distance to determine estimated time en route and fuel requirements.
Assume the airplane has a planned TAS of 120 knots, a direct 20-knot headwind, and a leg distance of 50 nautical miles.
Which values are actually entered? A typical student-pilot VFR navigation log commonly includes planned TAS, forecast wind, calculated GS, leg distance, and estimated flight time. Some log formats also provide an IAS field.
CAS and EAS are normally intermediate calculation values, not separate entries in a basic navigation log.
Key Point: IAS is the speed you read and commonly fly. CAS corrects measurement errors. EAS corrects compressibility effects. TAS is your speed through the air. GS is your speed over the ground. Mach compares TAS with the local speed of sound.
For navigation planning, remember the practical sequence: determine TAS, apply wind, calculate GS, then calculate flight time.
How to Calculate True Airspeed (TAS) with an E6B — Step by Step
A flight computer helps pilots convert calibrated airspeed (CAS) into true airspeed (TAS) using pressure altitude and outside air temperature. TAS is then used with forecast winds to plan groundspeed and flight time.
In this chapter, you will learn how to calculate TAS using a traditional manual E6B and an electronic flight computer such as the ASA CX-3. Both methods use the same flight information, even though the calculations are performed differently.
What Information Do You Need?
Before using an E6B, collect three values for the altitude at which you plan to fly.
Calibrated Airspeed
Obtain CAS from aircraft performance data or correct IAS using the aircraft's airspeed calibration chart when necessary.
Pressure Altitude
Use pressure altitude for the planned cruising level. It is not always equal to the indicated altitude shown with the current altimeter setting.
Outside Air Temperature
Use the forecast or measured outside air temperature at flight altitude. A manual E6B normally requires the temperature in degrees Celsius.
Pressure altitude: It is the altitude indicated when an altimeter is set to the standard pressure of 29.92 inHg. It can also be calculated from altitude and the current altimeter setting. Always restore the required altimeter setting after any check, following aircraft procedures.
Worked Example: Calculating TAS
We will use the same example with both flight computers so you can compare their results.
An outside air temperature of −5°C is equivalent to 23°F. This example produces a TAS of approximately 141 knots.
Manual E6B Flight Computer — Step by Step
A traditional E6B has a circular calculation side and a wind side. For this TAS calculation, use the circular side with its airspeed correction window.
Prepare the three values Use 125 knots CAS, 8,500 feet pressure altitude, and −5°C outside air temperature.
Find the airspeed correction window On the circular side of the E6B, locate the window used for altitude and temperature corrections.
Set altitude and temperature Rotate the disk until 8,500 feet pressure altitude is aligned with −5°C outside air temperature in the correction window.
Find calibrated airspeed Without moving the disk, locate 125 knots on the middle scale marked CAS.
Read true airspeed Read the value opposite 125 CAS on the outer scale marked TAS. The result is approximately 141 knots true airspeed.
Why is TAS higher than CAS?
At this altitude and temperature, the air is less dense than standard sea-level air. The airplane must move faster through the air to produce the same pressure difference.
Reading the E6B correctly: On the ASA manual E6B, CAS is read on the middle scale and TAS on the outer scale. Do not move the disk after aligning pressure altitude and temperature. Other E6B models may arrange their windows differently.
Electronic E6B: Using the ASA CX-3
An electronic flight computer performs the same conversion after you enter the required values. The ASA CX-3 also provides other useful flight-planning calculations.
Open the flight functions Select the FLT menu on the CX-3.
Select Airspeed Choose the Airspeed function for a planned true airspeed calculation.
Enter CAS Input 125 knots as the planned calibrated airspeed.
Enter OAT and pressure altitude Input 23°F outside air temperature and 8,500 feet pressure altitude. Check that the selected units are correct.
Read TAS The CX-3 calculates approximately 141.03 knots true airspeed.
Manual versus electronic E6B: Both use the same basic airspeed principles. The manual computer requires you to align and read the scales, while the electronic computer performs the calculation from the values you enter.
Estimating TAS Without an E6B
Pilots sometimes use a simple rule of thumb to make a quick estimate of TAS. It is useful as a rough mental check, but it does not replace an accurate calculation or aircraft performance data.
The 2% Rule of Thumb
About 2% per 1,000 ftAs a rough estimate, increase calibrated airspeed by approximately 2% for each 1,000 feet of altitude.
For example, at 5,000 feet, 100 knots CAS gives a rough TAS estimate of 110 knots.
Why It Is Only an Estimate
The relationship depends on pressure and temperature. An unusually warm or cold atmosphere can produce a noticeable difference from this simple approximation.
For navigation planning, use the aircraft performance information and an appropriate flight computer.
Using Your TAS in a Navigation Log
Once TAS has been calculated, the next step is to use forecast wind information to determine groundspeed.
A navigation log commonly includes planned TAS and groundspeed for each leg. The calculated TAS is also used to determine wind correction angle and the heading required to maintain the intended ground track.
These wind calculations will be explained in the next chapter on the wind triangle.
Manual and Electronic E6B Flight Computers
An E6B is a useful training tool for practicing airspeed conversions, wind calculations, flight time, and fuel planning.
You can compare manual and electronic E6B flight computers through PilotMall.
Common E6B Mistakes to Avoid
Check whether a correction is required before starting the TAS calculation.
The calculation requires pressure altitude, which may differ from indicated altitude.
Confirm whether the computer expects Celsius or Fahrenheit before entering outside air temperature.
The E6B TAS conversion does not include wind. Groundspeed must be calculated separately.
Key Point: To calculate TAS with an E6B, start with calibrated airspeed, pressure altitude, and outside air temperature. A manual E6B uses its correction window and circular scales; an electronic E6B calculates the result from entered values.
The resulting TAS becomes a starting value for wind and groundspeed calculations in your navigation log.
The Triangle of Velocities: Wind, Heading and Groundspeed
An airplane moves through the air, but the air itself may also be moving. Because of the wind, an airplane does not always travel over the ground in the same direction its nose is pointing.
The triangle of velocities, also called the wind triangle, combines true airspeed and wind velocity to determine the heading needed to maintain a planned course and the resulting groundspeed.
True Airspeed (TAS)
The airplane's speed through the surrounding air. The TAS vector points in the direction of the true heading (TH).
Wind Velocity
The movement of the air, described by wind speed and direction. Its vector points toward the direction the wind is blowing.
Groundspeed (GS)
The airplane's speed over the ground. The GS vector points along its actual ground track.
Understanding the Wind Triangle
Consider an airplane planning to fly east while a wind blows from the north. Without a correction, the wind pushes the airplane south of its intended course.
To stay on course, the pilot must point the airplane slightly north of east. The wind triangle shows the relationship between this heading, the wind and the resulting groundspeed.
Example: Flying East with a Northerly Wind
True Heading: about 080°
The air moves south.
Groundspeed: about 113 kt
True Heading (TH)
The direction the airplane's nose points, measured clockwise from true north.
A crosswind usually requires the pilot to point the airplane into the wind to maintain the desired course.
True Course (TC)
The planned direction of travel over the ground, measured clockwise from true north.
When the wind correction is correct, the airplane's ground track follows the planned true course.
How to Construct the Wind Triangle Manually — Step by Step
You can solve the wind triangle yourself using paper, a ruler, a compass and a protractor. This graphical method shows exactly how the wind changes an airplane's heading and groundspeed.
The important rule is to use the same scale for every vector.
TAS of 115 kt = 11.5 cm
Wind of 20 kt = 2 cm
Draw the True Course
Mark a starting point A. Using your protractor, draw a straight line from A toward 090° (east).
This line represents the planned ground track. Its final length is not known yet.
Draw the Wind Vector
The wind is reported FROM 360°, so it blows toward 180° (south).
Starting at A, draw a line 2 cm straight south. Mark its endpoint B.
Set the TAS Distance
Open your drawing compass to 11.5 cm, representing the airplane's TAS of 115 kt.
Place the compass point at B, the endpoint of the wind vector.
Locate the Third Point
Draw an arc with the compass until it intersects the true course line east of A.
Mark the intersection C. Draw a line from B to C. This is the TAS vector.
Measure Groundspeed
Use your ruler to measure the distance from A to C.
It measures about 11.3 cm. With our scale, this corresponds to a groundspeed of about 113 kt.
Measure Wind Correction
Use your protractor to measure the acute angle between the TAS line (B–C) and the course line (A–C).
The angle is about 10°. The correction is to the left because the wind comes from the left.
Length: 2 cm
Speed: 20 kt
TAS: 115 kt
Heading: about 080°
GS: about 113 kt
Course: 090°
Calculating the True Heading from the Wind Triangle
Once the triangle is complete, measure the angle between the airspeed vector and the true course. This is the Wind Correction Angle (WCA).
In our example, the wind comes from the north, pushing the airplane to the right of its easterly course. The pilot must therefore correct to the left.
Wind Correction Angle
Measure the angle between the TAS vector and the true course.
True Heading
Subtract the left wind correction from the planned course.
The planned true heading is 080°.
Wind Drift: With and Without Correction
The wind triangle explains why the airplane must point into the wind to maintain its planned course.
Without Wind Correction
The airplane points east, but the north wind pushes it south. Its ground track drifts to the right of the planned course.
With Wind Correction
The airplane points slightly north of east. The correction keeps its ground track on the planned easterly course.
How Wind Affects Groundspeed
The effect of wind on groundspeed depends on the direction of the wind relative to the airplane's course.
Headwind
Air moves against the airplane's direction of travel.
Groundspeed decreases.
Tailwind
Air moves in the same direction as the airplane's travel.
Groundspeed increases.
Crosswind
Air moves across the planned course. The pilot corrects the heading to oppose drift.
Groundspeed is determined from the wind triangle.
Our example: The airplane has a TAS of 115 kt and maintains an easterly course despite a 20 kt crosswind. The resulting groundspeed is approximately 113 kt.
Solving the Wind Triangle with an E6B Flight Computer
The wind side of a traditional E6B performs the same graphical calculation without requiring a separate ruler, compass and protractor.
Enter the true course, true airspeed, wind direction and wind speed to obtain the wind correction angle and groundspeed.
Set wind direction.
Rotate the E6B until 360°
is beneath the true index.
Mark wind speed.
Place a pencil mark 20 kt above
the center grommet.
Set true course.
Rotate the disk until 090°
is beneath the true index.
Set true airspeed.
Slide the grid until the wind mark
lies on the 115 kt speed arc.
Read groundspeed.
Read beneath the center grommet:
approximately 113 kt.
Read wind correction.
The wind mark indicates approximately
10° left.
Both methods give the same practical result: a wind correction of about 10° left, a true heading of 080° and a groundspeed of approximately 113 kt.
Entering the Wind Triangle Results in a Navigation Log
Once the triangle has been solved, the pilot enters the planned heading and expected groundspeed into the navigation log.
| TC | Wind | TAS | WCA | TH | GS | Distance | ETE |
|---|---|---|---|---|---|---|---|
| 090° | 360° / 20 kt | 115 kt | 10° L | 080° | 113 kt | 113 NM | 60 min |
Heading for the Flight
The wind triangle gives a true heading of approximately 080°.
If magnetic heading is required, apply magnetic variation and any applicable compass deviation.
Estimated Time En Route
Groundspeed is used to estimate the time needed to fly each leg.
Using a planned groundspeed of 113 kt, a 113 NM leg has an estimated time en route of 60 minutes.
Common Wind Triangle Mistakes
Important Points to Remember
- Wind direction means where the wind comes from. A wind reported from 360° blows toward 180°.
- True heading and true course are not always equal. With crosswind correction, the airplane points into the wind while maintaining its planned track.
- Use the same scale for every vector. Changing the scale during manual construction produces incorrect results.
- Correct into the wind. Wind pushing the airplane right of course requires a heading correction to the left.
- Do not automatically add or subtract the full wind speed. The wind's direction relative to the course determines its effect on groundspeed.
- Do not mix true and magnetic directions. Solve the triangle using true course and true wind direction before applying magnetic corrections.
Wind Triangle — Key Takeaways
The triangle of velocities shows how an airplane's movement through the air combines with the wind to produce its movement over the ground.
True Airspeed
Describes the airplane's speed through the air.
Wind Velocity
Changes the airplane's movement relative to the ground.
Groundspeed
Describes how fast the airplane travels over the ground.
By constructing the triangle manually or using an E6B, pilots can determine the wind correction angle, required heading and expected groundspeed for each leg of a flight.
Vertical Speed: Understanding Rate of Climb and Descent
Vertical speed describes how quickly an airplane gains or loses altitude. Unlike airspeed, which measures movement through the air, vertical speed measures the rate of altitude change.
In U.S. aviation, vertical speed is usually expressed in feet per minute (fpm). Pilots monitor it during climbs, descents and approaches to help maintain the desired flight profile.
Positive Vertical Speed
+500 fpmThe airplane gains 500 feet of altitude each minute at a constant climb rate.
Zero Vertical Speed
0 fpmThe airplane maintains a constant altitude, with no sustained climb or descent.
Negative Vertical Speed
−500 fpmThe airplane loses 500 feet of altitude each minute at a constant descent rate.
Vertical Speed Indicator (VSI)
The Vertical Speed Indicator (VSI), also called a rate-of-climb indicator, shows whether the airplane is climbing, descending or maintaining altitude.
A conventional VSI uses the aircraft's static pressure system to detect how quickly atmospheric pressure changes as altitude changes.
The needle moves above zero during a climb and below zero during a descent. At a constant altitude, it should indicate zero.
A conventional VSI has a small delay. The needle begins showing a trend when vertical speed changes, but its indication may take several seconds to stabilize.
VSI scale: On many traditional indicators, a marking of 0.5 represents 500 fpm, and a marking of 1 represents 1,000 fpm. Always read the markings on the specific instrument installed in the aircraft.
How to Calculate Vertical Speed
Vertical speed is calculated by dividing the altitude change by the time taken.
Use feet for altitude and minutes for time. A climb gives a positive result, while a descent gives a negative result.
Example 1 — Climbing
The airplane climbs from 3,000 ft to 4,000 ft in two minutes.
Example 2 — Descending
The airplane descends from 4,500 ft to 3,000 ft in three minutes.
Vertical Speed in Flight Planning
Pilots use vertical speed to plan climbs and descents, estimate the time needed to change altitude and monitor the aircraft's performance.
Climb Performance
An airplane's rate of climb depends on factors such as weight, engine power and atmospheric conditions.
Pilots use the aircraft's performance charts to determine the climb capability for the conditions.
Descent Planning
Pilots plan when to begin descending and select an appropriate descent rate.
They must consider altitude restrictions, aircraft speed, configuration and the distance remaining.
ATC Instructions
Air traffic control may assign a climb or descent rate, or require an altitude to be reached by a specified point.
The pilot monitors the VSI while ensuring the aircraft remains within its capabilities.
Practical Example: Planning a Climb
Suppose an airplane is at 4,000 ft and needs to climb to 6,000 ft. The pilot plans a constant climb rate of 500 fpm, provided the aircraft can maintain that rate under the conditions.
Climb Planning Example
Altitude to Gain
The airplane must gain 2,000 feet.
Time Required
At a constant 500 fpm, the climb takes four minutes.
In flight: The pilot uses the appropriate pitch attitude and power setting, monitors airspeed and altitude, and checks the VSI to confirm the climb rate. Aircraft performance and operating limitations always take priority over a planned rate.
Groundspeed and Rate of Descent
Groundspeed also matters when planning a descent. Two airplanes following the same descent path may need different vertical speeds if their groundspeeds are different.
Lower Groundspeed
The airplane covers less ground each minute. To follow the same descent angle, it needs a lower rate of descent.
Higher Groundspeed
The airplane covers more ground each minute. To follow the same descent angle, it needs a higher rate of descent.
For a descent path close to 3°, a commonly used planning shortcut is:
Example: Groundspeed of 120 kt
Remember: This is a quick estimate, not an exact calculation for a 3° path. For an instrument approach, use the published procedure and the appropriate glidepath or descent guidance.
Monitoring Vertical Speed During Flight
The VSI is useful, but pilots should never rely on it alone to control a climb or descent.
Altimeter
Shows the aircraft's altitude and confirms whether altitude is increasing or decreasing.
Airspeed Indicator
Helps the pilot maintain the required airspeed during the climb or descent.
Vertical Speed Indicator
Shows the trend and rate of altitude change as the aircraft climbs or descends.
Avoid Chasing the VSI Needle
- A conventional VSI has a delay. The indication may take several seconds to stabilize after a change in vertical speed.
- Turbulence can cause rapid needle movement. Avoid making large pitch corrections in response to every fluctuation.
- Cross-check other instruments. Use the altimeter, airspeed indicator, attitude information and the outside view when available.
- Vertical speed does not guarantee terrain clearance. Follow the required altitudes, obstacle-clearance procedures and aircraft limitations.
Vertical Speed — Key Takeaways
Positive VSI
The airplane is climbing.
Zero VSI
The airplane is maintaining altitude.
Negative VSI
The airplane is descending.
Vertical speed is not a type of airspeed, but it provides essential information about the airplane's climb and descent. By understanding how to read the VSI and calculate rates of altitude change, pilots can plan and monitor these phases of flight more effectively.
How Pilots Use Airspeed in Real Flight
Knowing the different types of airspeed is useful only when you understand which speed matters at each stage of a flight. Pilots use indicated airspeed to control the airplane, true airspeed to plan its performance, and groundspeed to track progress toward the destination.
Let's follow a typical flight and see how these speeds influence real cockpit decisions.
Takeoff
Accelerating along the runwayDuring the takeoff roll, the pilot monitors the airspeed indicator and checks that the airplane is accelerating normally.
The airplane must reach the recommended rotation or lift-off speed specified in its Airplane Flight Manual or Pilot's Operating Handbook (AFM/POH).
A headwind can reduce the groundspeed needed to reach the required takeoff airspeed. The pilot still follows the airplane's published airspeed targets, not a target groundspeed.
Climb and Maneuvering
Controlling performance after takeoffAfter takeoff, pilots use the recommended indicated airspeed for the desired climb.
Vx provides the greatest altitude gain over a given horizontal distance. It is useful when obstacle clearance is the priority.
Vy provides the greatest altitude gain in a given amount of time.
The appropriate climb speed depends on the airplane, altitude, conditions, and the procedure being flown. Pilots also monitor airspeed while turning and maneuvering to maintain safe operating margins.
Cruise and Navigation
Planning and monitoring the routeDuring cruise, true airspeed (TAS) helps pilots evaluate aircraft performance at the selected power setting and altitude.
They combine TAS with the forecast wind to estimate groundspeed (GS). GS is then used to calculate the expected time between waypoints and the flight time remaining.
If actual GS is lower than planned, the airplane will take longer to reach its destination. The pilot updates the estimated arrival time and checks whether the remaining fuel is sufficient.
Descent, Approach and Landing
Managing speed and the flight pathDuring the approach, pilots maintain the recommended indicated airspeed for the aircraft's weight, configuration, and operating conditions.
Groundspeed helps with approach timing and descent planning. The vertical speed indicator helps the pilot monitor the rate of descent.
A strong headwind may produce a low groundspeed on final approach. The pilot must still maintain the appropriate indicated approach speed, including any adjustment required by the aircraft's procedures.
Other Situations Where Airspeed Matters
Airspeed information also helps pilots respond to changing conditions, aircraft limits, and air traffic control instructions.
Wind Changes the Flight Time
A stronger headwind reduces groundspeed and increases the time needed to travel a given distance.
A tailwind increases groundspeed and can shorten the flight.
During flight planning, pilots consider these effects when estimating fuel requirements and deciding whether they can safely continue to their destination.
Important: Sudden wind changes or wind shear can also cause rapid changes in indicated airspeed. This requires immediate attention to aircraft control and performance.
Respecting Airspeed Limits
Every airplane has published operating speeds and limitations.
These may include Vne (never-exceed speed), Vfe (maximum flap-extended speed), and Va (design maneuvering speed).
Limits and their applicable airspeed references must be checked in the AFM/POH. Maneuvering speed can also change with aircraft weight.
In practice: Before extending flaps or performing a maneuver, the pilot verifies that the airplane is operating within its applicable speed limits.
Following ATC Speed Instructions
When ATC assigns an airspeed in knots, the instruction normally refers to indicated airspeed (IAS).
For suitably equipped high-altitude aircraft, ATC may instead assign a Mach number.
For example, an instruction to maintain 120 knots means the pilot should use the indicated airspeed, not groundspeed.
Important: If an assigned speed conflicts with the aircraft's limitations or safe operation, the pilot must advise ATC rather than attempt to fly an unsafe speed.
High-Altitude and High-Speed Flight
At high altitude and high speed, Mach number becomes an important operating reference because the effects of compressibility become significant.
Some airplanes have a maximum operating Mach number (Mmo) as well as a maximum operating airspeed (Vmo).
Equivalent airspeed (EAS) is useful when evaluating aerodynamic loads and high-speed performance, although it is not normally a speed that pilots of basic training airplanes select on their instruments.
Which Airspeed Should You Use?
The key is to select the speed that answers the question you are trying to solve.
Conclusion and Key Takeaways
Airspeed is one of the most important references in the cockpit. However, not all airspeeds tell the pilot the same thing. Understanding their differences helps pilots control the airplane safely, respect its limitations, and make accurate navigation decisions.
Fly at the Correct Speed
Indicated airspeed (IAS) is the pilot's primary speed reference for takeoff, climb, maneuvering, and landing. It is also used with the aircraft's published operating speeds and limitations.
Understand Airspeed Accuracy
Calibrated airspeed (CAS) corrects indicated airspeed for instrument and installation errors. Equivalent airspeed (EAS) also accounts for compressibility effects and is useful when evaluating aerodynamic performance and loads.
Plan and Monitor the Flight
True airspeed (TAS) describes the airplane's speed through the surrounding air. Groundspeed (GS) describes its speed over the ground and is essential for estimating flight time, arrival times, and fuel requirements.
Recognize High-Speed Limits
For high-speed aircraft, Mach number compares true airspeed with the local speed of sound. It becomes an important reference when compressibility effects influence aircraft performance and operating limits.
Wind Changes Your Groundspeed
Headwinds, tailwinds, and crosswinds affect the airplane's movement over the ground. The wind triangle and E6B flight computer help pilots determine heading corrections and expected groundspeed.
Vertical Speed Completes the Picture
Vertical speed is not a type of airspeed. It measures the rate of climb or descent, normally in feet per minute. The VSI helps pilots monitor altitude changes alongside the airspeed indicator and altimeter.
Three Essential Habits for Every Pilot
A good pilot does more than read an airspeed indicator. A good pilot understands what each speed means and knows when to use it.
Typical Exam Questions
Test your understanding of the airspeed indicator, V-speeds, pitot-static failures, the E6B, wind correction and vertical speed. Choose one answer for each question. You will receive an explanation after every answer.
