Jul 20, 2024

As part of their flight training, all pilots must learn how to understand and interpret aircraft instruments in order to safely operate their aeroplanes. Aircraft instruments are vital to the safe operation of any aircraft, ultimately helping the pilot maintain control while identifying any potential issues at a glance.

Understanding the way these instruments work means a pilot is able to recognise when the equipment is malfunctioning, avoiding unnecessary mistakes during flight and on take-off and landing. Read on for more information on aircraft flight instruments and how they work.

Types Of Aircraft Instruments

Aircraft InstrumentsPilots rely on aircraft instruments to gain a better understanding of where the plane is, how fast it is travelling and what it is doing in relation to the ground, as well as a large amount of other information.

There are four basic kinds of aircraft instruments grouped according to the job they perform. These are flight instruments, engine instruments, navigation instruments and miscellaneous position/condition instruments.

Flight Instruments

The instruments that give information on the aircraft’s in flight performance. Examples are the Altimeter, the Airspeed Indicator, the Heading Indicator, the Attitude Indicator (artificial horizon), the Turn Coordinator, and the Vertical Speed Indicator.

Engine Instruments

These are instruments designed to constantly measure operating parameters relating to the aircraft’s engine(s). Examples are tachometers, temperature gauges, fuel and oil quantity displays, and engine pressure gauges.

Navigation Instruments

These instruments provide guidance information to enable the aircraft to follow its intended path. Examples include various kinds of navigational devices ranging from the simple compass and radiolocation to GPS location devices.

Miscellaneous Position/Condition Instruments

This category covers a range of miscellaneous gauges and indicators not included in the first three groups that provide data on the positions of moveable components on the aircraft and the condition of various aircraft components or systems. Examples include cabin environment (pressure, temperatures etc.), flight control position, and auxiliary power units etc.

Flight Instruments Explained

For the purposes of this post, we will take a more in-depth look at the basic flight instruments, starting with a further classification into two groups.

  • Pitot-Static Systems. Using differences in air pressure, namely ambient air pressure affected (pitot pressure) and unaffected (static pressure), to determine flight parameters such as the speed and altitude of the aircraft.
  • Gyroscopic Instruments. Using gyroscopic principles to provide information on the aircraft’s attitude during flight (the aircraft’s orientation in relation to its surroundings).

The Original Aviation 6 Pack

Sometimes referred to as the “aviation six pack”, these are the 6 basic flight instruments that are found in almost every aircraft in some way, shape or form – whether as individual instruments or merged together as part of the newer glass cockpit technology. For more in-depth information on the 6 pack see more below.

  1. Altimeter
  2. Airspeed Indicator
  3. Vertical Speed Indicator
  4. Attitude Indicator
  5. Heading Indicator
  6. Turn Coordinator

1. The Altimeter

AltimeterAn Altimeter displays the aircraft’s current height above sea level (not ground level). A traditional Altimeter has three hands measuring hundreds, thousands, and tens of thousands of feet. These three hands move at different speeds, and when the readings are added together, they give an indication of the aircraft’s current altitude.

5 Types Of Altitudes

  1. Indicated Altitude. The altitude indicated on the altimeter when the correct barometric pressure is set.
  2. True Altitude. Height above sea level (MSL).
  3. Absolute Altitude. Height above ground level (AGL).
  4. Pressure Altitude. The altitude indicated on the altimeter based on a ‘standard atmospheric level’, this is sometimes used in flight planning calculations.
  5. Density Altitude. This is the Pressure Altitude adjusted for temperature variations (density altitude affects aircraft performance).

How Does An Altimeter Work?

The Altimeter’s readings are based on barometric pressure, however, due to the constantly changing nature of barometric pressure the Altimeter needs to be pre-set prior to, and also during, every flight as the barometric pressure changes.

As a very basic description, the Altimeter works by utilising a static port on the outside of the aircraft. Increases and decreases in altitude cause the device to expand and contract altering the reading on the gauge. This information is used in conjunction with the pre-set barometric pressure to provide a more accurate altitude reading.

Common Errors Associated With Altimeters

  • Inconsistent Airflow. Interrupted airflow to the external static port during flight can cause the altimeter to give inaccurate readings. This is commonly associated with gusty wind conditions, or during certain manoeuvres.
  • Elasticity. The continual expansion and contraction of the altimeter’s operating parts during normal use can result in the parts losing some of their rigidity, becoming naturally more flexible resulting in inaccurate readings.
  • Pilot Error. The correct barometric pressure must be entered into the altimeter in order for it to give accurate results. Pilot error is one of the most common reasons altimeters fail to give accurate readings; a difference of 1″ Hg can cause an altitude deviation of 1,000 feet.
  • Air Density. The density of air alters from one area to the next, just as it does on the ground. Errors in altimeter readings over long flights are commonly associated with changes in air density.
  • Static Port Blockages. Something blocking the external static port would obviously prevent the altimeter from detecting and changes to altitude.

2. Airspeed Indicator (ASI)

Airspeed IndicatorThe Airspeed Indicator (ASI measures the speed of the aircraft as it moves through the air using air pressure differences from both a static port and a pitot tube. A traditional ASI has graduated numbers over a round dial with a single clock-like hand indicating the aircraft’s current speed. This measurement is usually given in knots (Nautical Miles per Hour) but sometimes in other forms such as kilometres per hour.

4 Types Of Airspeeds

  1. Indicated Airspeed (IAS). The Airspeed Indicator reading without any consideration for atmospheric conditions or potential installation and instrument errors. The Indicated Airspeed is used to give the manufacturers recommendations for aircraft performance indications relating to take off, landing, and stall speeds.
  2. Calibrated Airspeed (CAS). The Indicated Airspeed corrected for installation error and instrument error. Under certain operating conditions installation and instrument errors may total several knots.
  3. True Airspeed (TAS). The Calibrated Airspeed corrected for altitude related atmospheric conditions such as temperature variations and air density. The True Airspeed is used for flight planning calculations.
  4. Groundspeed (GS). The aircraft’s actual speed over the ground, or the True Airspeed adjusted for wind resistance factors (headwind, tailwind etc.).

How Does An Airspeed Indicator Work?

Utilising both an external static port and a pitot tube system on the aircraft, the ASI takes into account the airflow and equalising pressure differences to provide speed indications during flight.

While on the ground, the Airspeed Indicator will show a reading of zero as the pressures are equal, when airborne, air entering the external pilot tube places pressure on an internal diaphragm causing the Airspeed Indicator to move upwards.

Common Errors Associated With Airspeed Indicators

  • Static Port Blockages. Debris, insects, water or ice blocking the external static port prevents the Airspeed Indicator from giving a correct reading as air is unable to enter the port. If the static system is blocked but the pitot tube remains clear, it is important to note that the Airspeed Indicator will continue to operate but will give inaccurate readings.
  • Pitot Tube Blockages. As with above, any debris or blockages to the external pitot tube will result in incorrect readings.

3. Vertical Speed Indicator (VSI)

Sometimes referred to as a rate of climb indicator, the Vertical Speed Indicator senses changes in air pressure, displaying this information as a rate of climb or descent (generally in feet per minute). The VSI is used to monitor the rate of climb/descent and is useful to confirm the aircraft/pilot is maintaining level flight and isn’t unintentionally pitching up or down.

Types Of Vertical Speed Indicators

  1. Vertical Speed Indicator (VSI). Standard variety using air leak diaphragm system consisting of a bypass restriction, a dashpot piston, the diaphragm, a restricted passage, and a static port connection.
  2. Instantaneous Vertical Speed Indicators (IVSIs). Uses accelerator-actuated air pumps in conjunction with the traditional VSI components, which enables much faster response times, effectively reducing lag times in response to differential pressure variations.

How Does A Vertical Speed Indicator Work?

The Vertical Speed Indicator works by collecting and comparing static pressure inside of a calibrated air leak diaphragm system, effectively measuring the rate of pressure change (which changes as the aircraft climbs or descends). The changes in pressure difference are displayed using a pointer/needle on the face of the indicator, which moves up when climbing and down when descending. Under a constant pitch the needle will stabilise.

Put simply, as air enters through an external static port it enters a diaphragm system where it expands or retracts depending on the aircraft’s vertical movements. The VSI diaphragm/aneroid provides a calibrated exit point where the air can exit creating a higher pressure in the casing than the diaphragm. This pressure differential is used to indicate the air pressure changes. As pressure inside the VSI drops the aneroid compresses, indicating a climb, and as the pressure increases the aneroid expands, indicating a descent. When maintaining level flight or when the aircraft is on the ground the pressure doesn’t change so the needle/pointer will return to its zero position.

Common Errors Associated With Vertical Speed Indicators

  • Lag. Because the calibrated leak may take a while to equalise the reading may take a while to stabilise (approx. around 6-9 seconds). This is especially prevalent during abrupt aircraft movements such as turbulence, or during rapid prolonged climbs and descents.
  • Blocked Static Port. A blocked static port will disrupt the flow of air, resulting in inaccurate readings (it is likely the VSI will give a zero indication or read incorrectly).

4. Attitude Indicator (AI)

Also known as an artificial horizon or gyro horizon or artificial horizon, the Attitude Indicator (AI) depicts the aircraft’s position in relation to the earth’s horizon. An Attitude Indicator enables the pilot to instantly see whether the plane is in level flight, climbing, turning or descending.

Types Of Attitude Indicators

  1. Traditional Attitude Indicators. Standard vacuum or air-powered gyroscopic attitude indicator.
  2. Electric Attitude Indicators. Use an electrically driven gyroscope instead of a vacuum.
  3. Solid State Digital Attitude Indicators. Commonly part of a glass cockpit installation, digital indicators rely on solid state electronics to display attitude and don’t have pitch or bank restrictions, providing more accurate results.
  4. All-In-One Attitude Indicators. Modern aircraft displays will incorporate multiple instruments into the primary flight display unit including the Attitude Indicator as well as aircraft heading, altitude, airspeed and vertical speed indicators.

How Does An Attitude Indicator Work?

Traditionally speaking, the Attitude Indicator is based around the principle of gyroscopic rigidity, where an internal gyroscope (built to pivot on two axes using a gimbal) maintains its orientation in relation to its space. More specifically, as the vacuum is pulled or air blown into the instrument case, it causes the gyroscope to spin depicting changes in pitch and bank attitudes.

Most Attitude Indicator gauges will depict an artificial sky, earth and horizon, with bank index increments of 10, 20, 30, 45, and 60 degrees, along with a central depiction of the aircraft indicating pitch accordingly.

Common Errors Associated With Attitude Indicators

  • Leaks. Vacuum gyros are susceptible to contamination and leaks in the system that can result in inaccurate indications.
  • Pitch/Bank Restrictions. Depending on the age of the instrument the display capabilities may have limitations on the degrees of bank and pitch that are able to be displayed without tumbling (related to the maximum physical rotation capabilities of the gimbals/gyro).
  • Tumbling. If excessive bank or pitch angles are encountered this can cause the gyroscope to tumble or spin, rendering the instrument unusable.
  • Rapid Change. Sudden accelerations and decelerations can temporarily affect the pitch indication.
  • Turn Errors. Following a 180-degree turn, it is possible to encounter small bank and pitch errors.

5. Heading Indicator (HI)

The Heading Indicator uses a rotating gyro to show the compass rose direction in which the aircraft is currently flying (with respect to magnetic north when set with a compass). Pilots use heading indicators to monitor the direction the aircraft is pointing.

Types Of Heading Indicators

  1. Directional Indicator (Directional Gyro). Standard heading indicator type that indicates the aircraft’s heading (direction) when aligned with a magnetic compass.
  2. Horizontal Situation Indicator (HIS). Most modern aircraft have replaced the traditional Heading Indicator with a horizontal situation indicator that provides the same information with the addition of navigational assistance.

How Does A Heading Indicator Work?

The heading indicator uses a rotating gyro to depict the direction the aircraft is flying in based on a 360-degree azimuth similar to a magnetic compass. The instrument features an aircraft image and 360-degree compass like points that turn with the motion of the plane driven by suction from a vacuum pump (or electrical system).

The Heading Indicator contains a gyro wheel spinning on a horizontal axis, pivoting to match the aircraft’s centre line with a gimbal that only allows movement on the vertical axis. Friction and the earth’s rotation causes drift errors which need to be periodically corrected manually by calibrating the instrument to the magnetic compass.

Common Errors Associated With Heading Indicators

  • Human Error. The pilot may forget to reset the heading indicator before take-off or during flight resulting in inaccurate headings.
  • Mechanical Failure. Vacuum pump failure (the pump that provides suction for the heading indicator’s gyro), or there is an electrical failure in an electrically powered gyro the HI will also stop working.
  • Drift Failures. Friction within the heading indicator’s gimbal components can build up over time causing accumulated heading errors if not corrected. Due to the rotating nature of the earth, the AI’s gyroscope will ‘drift’ by an average of 4° every fifteen minutes (Called Apparent Drift).
  • Lag. Sometimes the gimbal can’t react as fast as you can turn the aeroplane which may cause tumbling/lag.

6. Turn Coordinator (TC)

The Turn Coordinator is used to provide a visual depiction of the direction and rate of heading change plus any slipping/skidding during a turn, effectively monitoring the aircraft’s roll and yaw. Turn Coordinators are important because during a turn aircraft roll and yaw at the same time, which can cause in slippage/skidding resulting in unintentional loss of altitude.

How Does A Turn Coordinator Work?

The Turn Coordinator instrument is made up of two parts; the top part is the turn indicator (looks like a little plane) which when entering a turn will move correspondingly to show the direction and rate of turn. The second part is a small ball-and-fluid inclinometer which is used to measure yaw in turns. For a coordinated turn, the black ball should remain in the centre/bottom of the gauge, if it drifts left or right, the aircraft is skidding or slipping (the tail of the plane isn’t following the intended flight path).

The turn coordinator (TC) works using gyroscope precession and is generally powered by the aircraft’s electrical system. The gyro is mounted with its front at a 30-to-45-degree angle higher than its rear and spins at about 10,000-15,000 rpms. The angled gyro allows the gyro gauge rotation of the vertical and longitudinal axis, effectively showing both the rate of heading change and the rate of roll.

Common Errors Associated With Turn Coordinators

  • Electrical Failure. If there is no power being supplied to the Turn Coordinator, it will not function (a warning flag should be displayed).

Thinking about learning to fly in NZ or want more information on how to become a pilot? See the specialists in flight training at Southern Wings Invercargill or Auckland today.

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