Leroy R. Grumman Cadet Squadron

Civil Air Patrol - The official auxiliary of the United States Air Force

Aircraft Instrumentation - The "Steam Gauge" Cockpit


In comparison to the electronic flight instrumentation (i.e., "Glass Cockpit" instrumentation) that has become widely available over the past few years, many older aircraft use vacuum-driven pumps and mechanical gyroscopes that provide critical situational and directional information to the pilot through the use of analogue gauges.  This type of flight instrumentation has become known as “steam gauge” instrumentation, as they replicate the round dial-type gauges that originated with steam-engine powered trains many years ago.


Most aircraft built since about 1953 have four of the flight instruments located in a standardized pattern called the T arrangement.  The Attitude Indicator is in the top center, Airspeed Indicator to the left, Altimeter to the right and Heading Indicator under the Attitude Indicator.  The other two, Turn Coordinator and Vertical Speed Indicator, are usually found under the Airspeed Indicator and Altimeter, but are given more latitude in placement.  In newer aircraft with glass cockpit instruments the layout of the displays conform to the basic T arrangement.


Let’s take a closer look at each of these six flight instruments and how they work.


Pitot Static Instruments
The Pitot Static System relies on a Pitot Tube to measure the dynamic pressure due to the forward motion of the airplane through the air, and Static Vents to measure the static, outside barometric pressure as the airplane gains or loses altitude. The three flight instruments connected to the Pitot Static System include the Airspeed Indicator, Altimeter, and Vertical Speed Indicator.


Gyroscopic Instruments
A Gyroscope is a rotor or spinning wheel, rotating at a high speed. Usually, this is powered by the Vacuum System Pump. Gyroscopic Inertia is the tendency of a rotating body to maintain its plane of rotation, known as Rigidity in Space. Gyroscopic Precision is the tendency of a rotating body to consistently react to a force being applied by turning in the direction of its rotation exactly 90 degrees to its axis. These principles of physics are used to make very precise Flight Instruments including the Attitude Indicator, Heading Indicator, and Turn Coordinator.




T Arrangement


Six basic instruments in a light twin-engine airplane arranged in a "Basic-T". From top left, clockwise: Airspeed Indicator, Attitude Indicator, Altimeter, Vertical Speed Indicator, Heading Indicator, and Turn Coordinator

 #1 - Airspeed Indicator



The Airspeed Indicator shows the aircraft's speed (usually in knots) relative to the surrounding air. It works by measuring the ram-air pressure in the aircraft's pitot tube. The indicated airspeed must be corrected for air density (which varies with altitude, temperature and humidity) in order to obtain the true airspeed, and for wind conditions in order to obtain the speed over the ground.  The Airspeed Indicator measures the speed of the aircraft through the air, but really this is the speed at which the air is flowing over the airplane. And remember, this is not a measurement of ground speed. The dial is usually calibrated in nautical miles known as knots.  The airspeed indicator is connected to the Pitot Static System. To give a reading of speed through the air, the flight instrument measures the difference between the dynamic pressure in the pitot tube and the atmospheric pressure from the static vent. When the airplane is standing still on the ground, the pressure in the two systems will be the same resulting in a reading of zero. However, when the airplane is traveling through the air, the dynamic pressure in the Pitot System will increase and a reading is registered.

Knots vs. Miles

Knots are a measure of speed based on nautical, or sea miles. Aviation uses both nautical and statute miles for measuring distance and speed, but the Airspeed Indicator typically shows knots.


Nautical Mile = 6,076 feet
Statute Mile = 5,280 feet

Therefore, 1 Nautical Mile distance = 1.15 Statute Mile distance


Indicated Airspeed

The Indicated airspeed (IAS) is the reading displayed on the face of the instrument. The small windows at the top and bottom of the Airspeed Indicator are used for determining True Airspeed (TAS). Remember, the Airspeed Indicator displays the Indicated Air Speed (IAS), and adjustments are needed to calculate the Calibrated Airspeed (CAS) and True Airspeed (TAS).

Speed Ranges


Speed ranges and limitations are marked on the Airspeed Indicator and are specific to the make and model of the aircraft. Different makes and models of airplanes will have the markings at different speeds based on limitations of each aircraft. Typically Green markings on instruments reflect normal operations, and Red markings reflect abnormal operations or limitations.


The Red Line
The speed marked by the Red Line is the Never Exceed Speed (Vne). This speed should never be exceeded in the Aircraft or structural damage may occur.


The Yellow Arc
The speed range marked by the Yellow Arc is the Caution Speed Range. Speed range indicated by the Yellow Arc is for Smooth Air Only.


The Green Arc
The Green Arc denotes the Normal Operating Airspeed Range.


The White Arc
The Flaps Operating Range is denoted by the White Arc. Flaps may only be used within this range of speeds.

#2 - Attitude Indicator



The Attitude Indicator (also known as an Artificial Horizon) shows the aircraft's attitude relative to the horizon. From this the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument flight and is also useful in conditions of poor visibility. Pilots are trained to use other instruments in combination should this instrument or its power fail.


A pair of wings represents the attitude of the aircraft. Behind the aircraft is a ball. The top is blue, representing the sky, and the bottom half is usually brown, representing the ground. As the airplane manoeuvres in the air, the pair of wings will show the degree of bank and pitch attitude.


The Attitude Indicator is a Gyroscopic Instrument, and it uses a Gyroscope to stabilize a horizon bar that stays parallel to the natural horizon.

#3 - Altimeter

Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an altitude of 10,180 feet.


The Altimeter shows the aircraft's height (usually in feet or meters) above some reference level (usually sea-level) by measuring the local air pressure. It is adjustable for local barometric pressure (referred to sea level) which must be set correctly to obtain accurate altitude readings.  This requires the altimeter to be set prior to every flight, and during flight as barometric pressure in your flying area changes.  The Altimeter measures the Altitude or height of the aircraft above Sea Level. Remember, ground elevation varies widely, so the Altimeter reading does not measure height about the Ground, but instead above Sea Level.


Similar to a clock, an Altimeter has three hands. The fastest moving hand reads in Hundreds of Feet. The shortest hand reads in Thousands of Feet. The longest hand, which moves the slowest, reads in Tens of Thousands of Feet (on some altimeters, the Tens of Thousands of Feet is represented with the shortest hand, instead of the longest hand).



The Altimeter pictured here has a reading of 1,410 feet above sea level. The fastest moving hand (Hundreds) is between the 4 and 5, and the small hash marks represent 20 feet each. Therefore, this hand has a reading of 410 feet. The shortest hand (Thousands) is between the 1 and 2. Therefore, the current altitude would be 1,410.

 #4 - Vertical Speed Indicator

The Vertical Speed Indicator (VSI, also sometimes called a Variometer), senses changing air pressure, and displays that information to the pilot as a rate of climb or descent in feet per minute (displayed above as Hundreds of FPM), meters per second or knots.


The VSI flight instrument measures the vertical speed (vertical velocity, or rate of climb). This instrument is connected to the static air pressure system. There is a standard barometric pressure change with altitude changes, and this standard rate of change is calibrated to measure the aircraft’s change in altitude and rate of change.


The pilot relies on both the Altimeter and the Vertical Speed Indicator to monitor altitude and altitude changes. At a glance, the VSI shows the pilot if they are flying at a steady altitude, or if they are ascending or descending, and the rate at which their altitude is changing in feet per minute.

#5 - Heading Indicator


The Heading Indicator (also known as the Directional Gyro, or DG; sometimes also called the gyrocompass, though usually not in aviation applications) displays the aircraft's heading with respect to geographical north. Principle of operation is a spinning gyroscope, and is therefore subject to drift errors (called precession) which must be periodically corrected by calibrating the instrument to the magnetic compass. In many advanced aircraft (including almost all jet aircraft), the heading indicator is replaced by a Horizontal Situation Indicator (HSI) which provides the same heading information, but also assists with navigation.  Unlike the magnetic compass, the Directional Gyro is not as affected by banks, turns, and speed changes. However, the Heading Indicator is NOT a magnetic compass.


The Heading Indicator must be set according to the Magnetic Compass indication before takeoff, and occasionally adjusted to the Magnetic compass while the aircraft is in steady, level flight. Precision error must be corrected for at regular intervals of about 15 minutes by re-calibrating the Heading Indicator (HI) to the Magnetic Compass.


The outline of an aircraft is positioned over a 360 degree scale with markings for North, East, South and West. The larger markings indicate 10 degrees each, and the smaller markings denote 5 degree variations.

#6 - Turn Coordinator



The Turn Coordinator displays direction of turn and rate of turn. Internally mounted inclinometer displays 'quality' of turn, i.e. whether the turn is correctly coordinated, as opposed to an uncoordinated turn, wherein the aircraft would be in either a slip or a skid. The original Turn and Bank Indicator was replaced in the late 1960s and early '70s by the newer Turn Coordinator, which is responsive to roll as well as rate of turn, the turn and bank is typically only seen in aircraft manufactured prior to that time, or in gliders manufactured in Europe.


If the aircraft is slipping or skidding during a turn, the ball (or inclinometer) in the bottom portion of the Turn Coordinator will not be centered. During a coordinated turn, the ball will remain centered. If the ball is not centered, the pilot must adjust the turn by using more or less rudder to correct for adverse yaw.


Standard Rate Turn

The white lines indicate the bank amount for a Standard Rate Turn. The turn indicator indicates the rate of turn, and not the amount of turn. A Standard Rate Turn, or Rate One Turn, will give a standard rate of turn of 3 degrees per second. Therefore, a 360 degree turn will be exactly 2 minutes. This allows the pilot to determine by time, the degrees of turn. For instance, a pilot could use a standard rate of turn for 60 seconds, and confidently know they have changed their course by 180 degrees based on 3 degrees per second. This becomes particularly important when pilots begin Instrument flying.

Additional Panel Instrumentation:

The Magnetic Compass shows the aircraft's heading relative to magnetic north. While reliable in steady level flight it can give confusing indications when turning, climbing, descending, or accelerating due to the inclination of the Earth's magnetic field. For this reason, the heading indicator is also used for aircraft operation. For purposes of navigation it may be necessary to correct the direction indicated (which points to a magnetic pole) in order to obtain direction of true north or south (which points to the Earth's axis of rotation).  The Magnetic Compass will be above the instrument panel, often on the windscreen centerpost.

The Course Deviation Indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a track, which can be provided by a VOR (short for VHF Omnidirectional Radio Range) or an Instrument Landing System (ILS).  This instrument can also be integrated with the heading indicator in a Horizontal Situation Indicator (HSI).


A Radio Magnetic Indicator (RMI) is generally coupled to an Automatic Direction Finder (ADF), which provides bearing for a tuned Non-Directional Beacon (NDB). While simple ADF displays may have only one needle, a typical RMI has two, coupled to different ADF receivers, allowing for position fixing using one instrument.

Aircraft Instrumentation - The "Glass" Cockpit

The term “Glass Cockpit" is defined as a system of cathode ray tubes or LCD flat-panels that display key critical information about an aircraft’s flight, situation, position, and progress.


From a performance standpoint, whether it is a turbine (jet) or reciprocal (piston) power plant, information such as power output and all the various attributes associated with it are displayed. Glass cockpits have been available in commercial large-scale passenger and cargo aircraft since the early 80s and the space shuttle since its inception. The Airbus A320 commercial airliner is known for its advanced glass cockpit.


Glass Cockpit - Airbus A-320


Glass Cockpit - Airbus A-320 in flight

How and why did glass cockpits become popular?

Prior to the 1970s, the operation of an aircraft was not considered sufficiently demanding to require advanced equipment like electronic flight displays. The increasing complexity of transport aircraft, the advent of digital systems and the growing air traffic congestion issue began to change that notion.


The average transport aircraft in the mid-1970s had more than 100 cockpit analogue instruments and controls, and the primary flight instruments were already crowded with indicators, crossbars, and symbols. The issue approaching aviation was that a growing number of cockpit elements were competing for cockpit space and pilot attention.


Analogue Cockpit - Boeing B727 Flight Deck


Analogue Cockpit - Boeing B727 Flight Engineer's Station


As a result, NASA performed the initial research on displays that could process the raw aircraft system and flight data into an integrated, easily understood picture of the aircraft flight situation, culminating in a series of demonstration flights to demonstrate a full glass cockpit system.

 As seen in the example graphic above, a single electronic display screen can provide airspeed, attitude, altitude, heading, turn coordination and magnetic heading in a relatively small space.



Glass Cockpit - Dynon FlightDEK D180 showing Engine Management Readings


The success of the NASA-led glass cockpit work is reflected in the total acceptance of electronic flight displays beginning with the introduction of the Boeing 767 in 1982.  Safety and efficiency of flight have been increased with improved pilot understanding of the airplane's situation relative to its environment.


Glass Cockpit - Boeing B767-300R

Trickle-Down Technology


Significant decreases in costs due to economies of scale and overall decreases in the costs associated with flat-panel display and electronics production has allowed glass cockpit technology to reach the “grass roots” level of general aviation.


Glass Cockpit - Cessna C172 SP

(Note that some "Steam Gauges" [Air Speed, Attitude and Altitude] have been retained)



Glass Cockpit - Garmin G600


Glass Cockpit - Aspen EFD 1000


All the displays/ LCDs used in the glass cockpit environment utilize an architecture that applies data bus technology which implements line-replaceable units (LRUs) that are integrated with sensors throughout the aircraft.


The sensors transmit data seamlessly to the Primary Flight Display/Multi-function Display (PFD/MFD) units to deliver real-time information to the pilot regarding the aircraft and its environment. Several leading systems use the air data/attitude and heading reference system (ADAHRS). The compact, lightweight, ADAHRS system uses a 3-axis solid state gyro and accelerometer system combined with a flux-gate compass to replace the traditional mechanical vertical and directional gyros thus avoiding wear and tear with age – the reliability far exceeds the legacy system that it replaces. The separate air data computers are integrated to the aircraft’s pitot-static system and provide altitude, vertical speed, and outside air temperature (OAT). The system continually updates the winds aloft and true airspeed (TAS) indications on the PFD.


The key success factors in general aviation are affordability, applicability and increases in safety for weather reporting, traffic avoidance and situational awareness.  The successful application in commercial aviation has started to “trickled down” to general aviation – smaller, more affordable aircraft that operate out of your typical hometown airport.  The same issues that caused the migration from older, analog “steam gauge” instrumentation in commercial aircraft have resulted in an emerging market for the General Aviation industry.


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