In this lesson, you will learn how to understand and interpret aircraft flight instruments. You will explore how they work, what their indications mean, and how to recognize possible instrument problems.
Understanding What Your Airplane Is Telling You
Flight instruments provide pilots with important information about how an airplane is flying. They are useful during every flight and become especially important when outside visual references are limited.
In traditional aircraft, flight instruments fall into two main groups: pitot-static instruments, which provide information about airspeed, altitude and vertical speed; and gyroscopic instruments, which help pilots understand the airplane's attitude, heading and turning motion.
Understanding these instruments helps pilots make better decisions and recognize when an indication may be unreliable.
Pitot-Static System: Pressure Sources and Connected Flight Instruments
The pitot-static system uses air pressure to provide information about an airplane's airspeed, altitude, and rate of climb or descent. It includes two pressure sources and three main flight instruments.
Static Port vs Pitot Tube: What Is the Difference?
Static Port
The static port is a small opening on the side of the airplane.
It measures static pressure: the pressure of the air surrounding the airplane.
Static pressure normally decreases as altitude increases and increases as altitude decreases.
Pitot Tube
The pitot tube has an opening that faces forward into the oncoming airflow. On many training airplanes, it is mounted under a wing.
It measures total pressure: the surrounding static pressure plus the pressure caused by movement through the air.
The extra pressure caused by airflow is called dynamic pressure.
Remember the difference: The static port measures surrounding air pressure. The pitot tube faces the oncoming airflow and measures total pressure.
Which Instruments Use Each Pressure Source?
The pressure sources are connected to the flight instruments by small pressure lines. The diagram below shows which instruments receive each pressure.
Static pressure is supplied to all three instruments. Total pressure from the pitot tube is supplied only to the ASI.
How Does the Airspeed Indicator Work?
The airspeed indicator (ASI) shows indicated airspeed (IAS), usually in knots.
Inside a traditional ASI, total pressure acts inside a flexible metal chamber called a diaphragm. Static pressure acts outside it. The pressure difference makes the diaphragm move, which moves the instrument needle.
The ASI therefore responds to dynamic pressure. As this pressure increases, the indicated airspeed rises.
Reading the ASI
A reading of 80 kt IAS means the instrument indicates 80 knots. It is not necessarily the airplane's true airspeed (TAS) or groundspeed.
Understanding ASI Color Markings
Many traditional airspeed indicators use colored markings to show important operating ranges.
From VS0 to VFE. This identifies the normal flap operating range.
From VS1 to VNO on a typical light-airplane ASI.
Above VNO and below VNE. In typical light airplanes, this range is used only in smooth air and with caution.
Marks VNE, the speed that must not be exceeded.
Aircraft-specific limits: Speed markings and operating limits vary by aircraft. Always use the approved AFM/POH. Some important speeds are not marked on the ASI.
How Does the Altimeter Work?
The altimeter indicates altitude by measuring atmospheric pressure.
A traditional mechanical altimeter contains sealed metal chambers called aneroid capsules. When outside pressure decreases, these capsules expand. When outside pressure increases, they contract.
Their movement is transferred to the altimeter pointers.
Altimeter Setting and the Kollsman Window
Atmospheric pressure changes with the weather. To obtain a useful altitude indication, the pilot must enter the appropriate altimeter setting.
On many traditional altimeters, the setting appears in a small window called the Kollsman window. In the United States, it is commonly expressed in inches of mercury (inHg).
Reading the Altimeter
With the appropriate local setting, an indication of 3,500 feet represents an altitude of approximately 3,500 feet above mean sea level (MSL), subject to instrument and atmospheric errors.
MSL is not AGL. Mean sea level (MSL) is the altitude reference. Above ground level (AGL) is the airplane's height above the terrain directly below.
How Does the Vertical Speed Indicator Work?
The vertical speed indicator (VSI) shows the airplane's rate of climb or descent, usually in feet per minute.
A traditional VSI has a flexible diaphragm and a small restricted opening called a calibrated leak. These allow the instrument to detect how quickly static pressure is changing.
The pressure difference moves the needle, which indicates the rate of climb or descent.
Climbing at approximately 500 feet per minute.
No significant change in altitude.
Descending at approximately 500 feet per minute.
Why Does the VSI Have a Delay?
Because air passes slowly through the calibrated leak, a traditional VSI usually takes a few seconds to settle after the airplane changes its vertical speed.
This delay is called VSI lag. Some aircraft have an instantaneous VSI (IVSI) or an electronic system that responds more quickly.
Example: If the VSI indicates +500 ft/min during a steady climb, the airplane gains approximately 500 feet in one minute.
The pitot-static system connects two pressure sources to three flight instruments. The ASI shows indicated airspeed, the altimeter shows indicated altitude, and the VSI shows the rate of climb or descent.
Pitot-Static Blockages and Alternate Static Source
Ice, water, dirt, or insects can block parts of the pitot-static system. When this happens, one or more flight instruments may show incorrect information.
The effect depends on which part of the system is blocked. We will look at the static system first, then the pitot tube.
What Happens When the Static System Is Blocked?
Imagine that the airplane is flying at 3,000 feet when its normal static pressure supply becomes completely blocked.
The air pressure inside the static lines is now trapped. Even if the airplane climbs or descends, this pressure cannot change normally.
Effect on the Three Instruments
The indication stays near the altitude where the blockage occurred. The airplane may be climbing or descending, but the altimeter does not follow.
The VSI settles at zero because it no longer detects the normal changes in static pressure.
The ASI can still move, but its reading becomes inaccurate when the airplane changes altitude.
How Does a Blocked Static System Affect Airspeed?
The direction of the error depends on whether the airplane is above or below the altitude where the pressure became trapped.
The airplane climbs above the altitude where the blockage occurred.
ASI reads too LOWIt indicates less airspeed than it should for the actual airspeed.
The airplane descends below the altitude where the blockage occurred.
ASI reads too HIGHIt indicates more airspeed than it should for the actual airspeed.
Example: If the static pressure becomes trapped at 3,000 feet, the altimeter stays near 3,000 feet even when the airplane climbs to 4,000 feet. The VSI settles at zero, and the ASI reads lower than it should.
What Is an Alternate Static Source?
Some airplanes have an alternate static source. If the normal static pressure supply is blocked, the pilot may be able to select another source.
This source usually takes air from inside the cockpit. The instruments can then receive pressure information again.
What Changes When Cockpit Air Is Used?
Air pressure inside the cockpit is often slightly lower than outside. Because of this, the instruments may show small differences.
Important: Not every airplane has an alternate static source. Its location, use, and possible instrument errors are aircraft-specific. Always follow the AFM/POH.
What Happens When the Pitot Tube Is Blocked?
The pitot tube has a forward opening where air enters. It also normally has a small drain hole that allows water to escape.
When the forward opening is blocked, the behavior of the ASI depends on whether the drain hole is still open.
Forward Opening Blocked, Drain Hole Open
Air cannot enter through the front opening, but pressure can escape through the drain hole.
The pressure difference measured by the ASI disappears, so the needle moves toward zero.
Forward Opening and Drain Hole Both Blocked
The pressure inside the pitot system is now trapped.
With a working static system, the ASI may increase during a climb and decrease during a descent, even when actual airspeed stays the same.
One important difference: In these two pitot-blockage cases, if the static system still works, the altimeter and VSI continue to operate normally. The pitot blockage affects the ASI.
How Can Pilots Reduce the Risk of Blockage?
During preflight inspection, make sure the pitot cover has been removed and check the pitot and static openings for visible blockage or damage.
Some airplanes have pitot heat to help prevent ice from blocking the tube. Use it according to the aircraft's procedures. Pitot heat does not, by itself, make an airplane approved for flight in icing conditions.
Recognizing a Possible Instrument Problem
If the altimeter stops responding and the VSI remains at zero during a real change in altitude, suspect a static system problem. If those instruments appear normal but the ASI behaves strangely, suspect a pitot system problem.
Check other available flight information. Follow the aircraft's emergency or abnormal procedures, and do not rely on a single unreliable instrument.
Gyroscopic Instruments: Attitude, Heading and Turn Indicators
A gyroscope, or gyro, contains a wheel that spins very fast. This wheel is called a rotor.
Traditional gyroscopic instruments use this spinning wheel to help pilots understand how the airplane is moving. They work using two important ideas: rigidity in space and precession.
Which Instruments Use a Gyroscope?
Attitude Indicator
Shows whether the airplane's nose is pointing up or down (pitch).
It also shows whether the wings are level or one wing is lower (bank).
Heading Indicator
Shows the airplane's heading.
Heading means the direction the airplane's nose is pointing.
Turn Indicator or Turn Coordinator
Shows whether the airplane is turning left or right.
It also shows how quickly the airplane is turning.
How are they different from pitot-static instruments? Traditional gyroscopic instruments use a spinning wheel. They do not measure pitot or static pressure.
Rigidity in Space: Keeping the Same Direction
When a gyroscope spins very fast, it tends to keep pointing in the same direction. This is called rigidity in space.
Imagine a gyroscope inside an airplane. The airplane turns right, but the spinning gyro tends to keep its original direction.
Before the Turn
After a Right Turn
Why is this useful? The attitude indicator and heading indicator use this property to provide a stable reference while the airplane moves.
Precession: How a Spinning Gyro Reacts to a Force
A spinning gyroscope tries to keep its direction. But what happens if we push it and try to change that direction?
The gyro does not react in the way we might expect. It responds in another direction. This is called gyroscopic precession.
Understanding Precession
In the example below, a force acts on a spinning wheel. The effect appears about 90 degrees farther around the wheel, in the direction it spins.
What does 90 degrees mean? It describes where the effect appears around the spinning wheel in this simple example. It does not mean the airplane turns 90 degrees.
How Does This Help a Turn Indicator?
When an airplane starts turning, its movement acts on the spinning gyro inside the instrument.
Because of precession, the gyro reacts by moving to one side. The instrument uses this movement to show the direction and rate of turn.
Example: An Airplane Turns Right
The airplane begins a right turn. The gyro reacts to this movement. The turn indicator then shows that the airplane is turning right.
The faster the airplane turns, the greater the response of the traditional turn indicator.
Rigidity in space: A spinning gyro tends to keep the same direction.
Precession: A spinning gyro reacts in another direction when a force acts on it.
Attitude Indicator and Heading Indicator
The attitude indicator shows pitch and bank relative to the horizon, while the heading indicator shows the direction the airplane is pointing. Both are especially useful when outside visual references are limited.
Understanding the Two Instruments
Attitude Indicator (AI)
The attitude indicator, also called the artificial horizon, shows the airplane's position relative to the horizon.
On this traditional instrument, the miniature airplane remains fixed while the artificial horizon moves as the aircraft changes attitude.
- Pitch: When the horizon moves below the miniature airplane, the indication is nose-up. When it moves above, the indication is nose-down.
- Bank: A tilted horizon shows that the wings are banked.
- Bank angle: The markings near the top of the instrument help the pilot estimate the angle of bank.
Heading Indicator (HI / DG)
The heading indicator, also called a directional gyro (DG), displays the airplane's heading on a rotating compass card.
The heading indicator makes it easier to maintain a constant direction and to follow heading changes during turns.
- Heading: Read the compass card at the fixed index at the top.
- Numbers: The numbers commonly represent tens of degrees. For example, 24 means 240°.
- Turns: The heading indication changes as the airplane turns.
A nose-up attitude does not necessarily mean that the airplane is climbing. An airplane may have its nose above the horizon while maintaining altitude or even descending.
The attitude indicator shows how the airplane is positioned. The altimeter and vertical speed indicator help confirm its actual vertical performance.
Using Both Instruments During a Turn
Imagine you are maintaining a heading of 090° and need to turn right to 120°. Each instrument provides different information during the maneuver.
As the airplane enters the right turn, the attitude indicator shows the bank angle. At the same time, the heading indicator shows the heading changing toward 120°.
Begin rolling out before the desired heading as appropriate for the bank angle and turn rate. When the turn is complete, the attitude indicator should show wings level and the heading indicator should stabilize near 120°.
Why Does a Traditional Heading Indicator Need Resetting?
A traditional, unslaved heading indicator does not automatically identify magnetic north. It must be aligned with the magnetic compass.
Over time, gyroscopic drift can cause the indication to become inaccurate. This means the heading indicator may no longer show the airplane's correct magnetic heading.
Fly straight and level at a constant speed. This helps avoid magnetic compass errors caused by turning or acceleration.
Compare the heading shown by the traditional, unslaved heading indicator with the magnetic compass.
If the indications differ, reset the heading indicator to the magnetic compass heading using the aircraft's approved procedure.
The FAA recommends checking a traditional heading indicator frequently, approximately every 15 minutes, and resetting it when needed.
Some modern heading systems use automatic magnetic referencing. Their operation and checking procedures may be different.
An attitude indicator or heading indicator can give misleading information if its gyroscope or power source is not operating correctly.
Compare the indications with other flight instruments. Unexpected differences may indicate an instrument problem. Gyroscopic instrument failures are covered later in this lesson.
The attitude indicator shows pitch and bank relative to the horizon. The heading indicator shows the direction the airplane is pointing.
A traditional, unslaved heading indicator needs periodic comparison with the magnetic compass. Always cross-check instrument indications rather than relying on a single instrument.
Turn Coordinator vs Turn-and-Slip Indicator
The turn coordinator and the turn-and-slip indicator show the direction and rate of turn. Both also include an inclinometer ball to indicate flight coordination.
The main difference is how they respond to aircraft motion.
How Are the Two Instruments Different?
Turn Coordinator (TC)
The turn coordinator uses a miniature airplane symbol. Its gyroscope responds to both rolling and turning motion.
The instrument can therefore show the beginning of a roll before the airplane has established a steady rate of turn.
Turn-and-Slip Indicator
The traditional turn-and-slip indicator, also called a turn-and-bank indicator, uses a pointer or needle.
This instrument senses rotation about the airplane's vertical axis (yaw) rather than roll rate.
| Feature | Turn Coordinator | Turn-and-Slip |
|---|---|---|
| Roll rate | Responds to roll | No direct indication |
| Turn rate | Yes, in a stable turn | Yes |
| Turn direction | Yes | Yes |
| Slip / skid | Inclinometer ball | Inclinometer ball |
| Typical display | Miniature airplane | Needle or pointer |
| Pitch information | No | No |
Although the turn coordinator displays a miniature airplane, it does not measure the airplane's actual bank angle or pitch attitude. Use the attitude indicator for that information.
Understanding a Standard-Rate Turn
A standard-rate turn changes heading at 3° per second. On a typical instrument marked 2 MIN TURN, the appropriate reference mark indicates this rate during a stabilized turn.
For example, a standard-rate right turn from heading 090° reaches heading 180° after approximately 30 seconds.
Reading the Inclinometer: The Ball
The small ball at the bottom of either instrument indicates whether a turn is coordinated, slipping, or skidding. Here is how to read it during a right turn.
To restore coordination, the familiar reminder is "step on the ball": apply appropriate rudder pressure toward the displaced ball to bring it back to the center.
For more about coordinated turns, slips and skids, see Aircraft Turns .
Use these instruments to monitor turn rate and coordination, not actual pitch or bank angle. Cross-check their indications with the other flight instruments.
Vacuum and Electrical System Failures
Traditional gyroscopic instruments need power to operate. In many training airplanes, some instruments use vacuum power and others use electrical power.
If one power source fails, the instruments connected to it may become unreliable or stop working.
Loss of Suction
In many traditional airplanes, the attitude indicator and heading indicator are vacuum-driven.
If suction is lost, these instruments may gradually become unreliable.
Loss of Electrical Power
Electrical power may operate the turn coordinator, radios, and other avionics.
If electrical power is lost, these systems may stop working.
During engine run-up, check the vacuum and electrical indications required by your aircraft checklist.
Confirm that the suction indication is within the normal range for your airplane.
Check the ammeter, loadmeter, voltage indication, or warning lights installed in the aircraft.
Know which instruments use vacuum power and which use electrical power. If indications become unreliable, cross-check the remaining instruments and follow the AFM/POH.
Instrument Flying Review Questions
Test your understanding of pitot-static instruments, gyroscopic principles, instrument indications, system blockages, and common instrument failures.
Try to answer each question before opening the explanation. Focus on understanding what each instrument indicates and how to recognize unreliable information.
Part 1 — Pitot-Static Instruments
Review the instruments that use air pressure and the effects of blocked pitot or static sources.
Which flight instruments are part of the pitot-static system?
The three traditional pitot-static instruments are: the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI).
All three use static pressure, but only the ASI also receives pitot pressure.
The pitot system provides impact pressure for which instrument?
The airspeed indicator. It compares pitot pressure with static pressure to determine indicated airspeed.
The altimeter and VSI use static pressure only.
What do the altimeter and vertical speed indicator show?
The altimeter indicates altitude relative to the selected pressure setting. With the correct local altimeter setting, it normally indicates altitude above mean sea level.
The VSI indicates the rate of climb or descent, usually in feet per minute. A conventional VSI has a slight indication lag.
What happens if the static port becomes completely blocked?
The altimeter freezes at the altitude where the blockage occurred, while the VSI indicates zero.
The ASI becomes inaccurate. At a higher altitude than where the blockage occurred, it tends to read too low; at a lower altitude, it tends to read too high.
What happens if the pitot tube becomes blocked?
The airspeed indicator is affected.
If the pitot inlet is blocked but the drain remains open, the ASI reading drops toward zero.
If both the inlet and drain are blocked while static pressure remains available, the ASI behaves like an altimeter: its indication increases during a climb and decreases during a descent.
What is the purpose of an alternate static source?
An alternate static source, when installed, provides another source of static pressure if the primary source becomes blocked.
It can restore usable indications, but small errors may occur. Follow the aircraft's AFM/POH for operation and corrections.
Part 2 — Gyroscopic Instruments
Review gyroscopic principles, attitude and heading indications, turn rate, and coordination.
How do gyroscopic instruments work?
Traditional gyroscopic instruments use a spinning gyroscope. Two important principles are:
Rigidity in space: A spinning gyroscope tends to maintain its orientation.
Precession: An applied force produces a gyroscopic reaction displaced approximately 90° in the direction of rotation.
What is the difference between the attitude indicator and the heading indicator?
The attitude indicator shows pitch and bank relative to the horizon.
The heading indicator shows the direction the airplane is pointing.
The attitude indicator does not directly measure climb rate, and the heading indicator does not measure bank angle.
Why must a traditional heading indicator be checked periodically?
A traditional, unslaved heading indicator can drift because of gyroscopic errors.
It should be compared with the magnetic compass approximately every 15 minutes and realigned when necessary, preferably during straight-and-level, unaccelerated flight.
What is the main difference between a turn coordinator and a turn-and-slip indicator?
The turn coordinator responds to both rolling and turning motion.
The traditional turn-and-slip indicator responds to yawing motion and indicates turn rate.
Both contain an inclinometer ball for checking coordination. Neither directly measures the airplane's bank angle.
What does the inclinometer ball tell the pilot?
The ball indicates whether a turn is coordinated.
Centered: Coordinated turn.
Inside the turn: Slipping turn.
Outside the turn: Skidding turn.
The training reminder "step on the ball" means applying appropriate rudder pressure toward the displaced ball to restore coordination.
What is a standard-rate turn, and how long does a full 360° turn take?
A standard-rate turn changes heading at 3° per second.
At that rate, a 90° turn takes 30 seconds, and a complete 360° turn takes 2 minutes.
Part 3 — Instrument Reliability and Failures
Check your understanding of instrument power sources and how to recognize unreliable indications.
Which instruments are commonly vacuum-driven, and which are electrically powered?
In many traditional training airplanes, the attitude indicator and heading indicator are vacuum-driven.
The turn coordinator is commonly electrically powered.
Power arrangements vary by aircraft. Always check which source powers each instrument in your airplane.
What should you check before flight, and what should you do if instrument indications disagree?
Before flight, inspect the pitot tube and static openings. During preflight checks and engine run-up, verify instrument, vacuum, and electrical indications as required by the aircraft checklist.
In flight, maintain aircraft control, cross-check other reliable instruments, and follow the appropriate AFM/POH procedures.
Pitot-static instruments use air pressure to indicate airspeed, altitude, and vertical speed. Gyroscopic instruments provide attitude, heading, turn-rate, and coordination information.
Understanding each instrument's power source, limitations, and possible failure indications helps pilots recognize unreliable information and maintain a safe instrument cross-check.
