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Flight instruments
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Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank (artificial horizon), direction (directional gyro) and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.[1]: 3–1
The term is sometimes used loosely as a synonym for cockpit instruments as a whole, in which context it can include engine instruments, navigational and communication equipment. Many modern aircraft have electronic flight instrument systems.
Most regulated aircraft have these flight instruments as dictated by the US Code of Federal Regulations, Title 14, Part 91. They are grouped according to pitot-static system, compass systems, and gyroscopic instruments.[1]: 3–1
Pitot-static systems
[edit]Instruments which are pitot-static systems use air pressure differences to determine speed and altitude.
Altimeter
[edit]
The altimeter shows the aircraft's altitude above sea-level by measuring the difference between the pressure in a stack of aneroid capsules inside the altimeter and the atmospheric pressure obtained through the static system. The most common unit for altimeter calibration worldwide is hectopascals (hPa), except for North America and Japan where inches of mercury (inHg) are used.[2] The altimeter is adjustable for local barometric pressure which must be set correctly to obtain accurate altitude readings, usually in either feet or meters. As the aircraft ascends, the capsules expand and the static pressure drops, causing the altimeter to indicate a higher altitude. The opposite effect occurs when descending. With the advancement in aviation and increased altitude ceiling, the altimeter dial had to be altered for use both at higher and lower altitudes. Hence when the needles were indicating lower altitudes i.e. the first 360-degree operation of the pointers was delineated by the appearance of a small window with oblique lines warning the pilot that he or she is nearer to the ground. This modification was introduced in the early sixties after the recurrence of air accidents caused by the confusion in the pilot's mind. At higher altitudes, the window will disappear.[1]: 3–3
Airspeed indicator
[edit]
The airspeed indicator shows the aircraft's speed relative to the surrounding air. Knots is the currently most used unit, but kilometers per hour is sometimes used instead. The airspeed indicator works by measuring the ram-air pressure in the aircraft's pitot tube relative to the ambient static pressure. The indicated airspeed (IAS) must be corrected for nonstandard pressure and temperature in order to obtain the true airspeed (TAS). The instrument is color coded to indicate important airspeeds such as the stall speed, never-exceed airspeed, or safe flap operation speeds.[1]: 3-7 to 3-8
Vertical speed indicator
[edit]
The VSI (also sometimes called a variometer, or rate of climb indicator) senses changing air pressure, and displays that information to the pilot as a rate of climb or descent in feet per minute, meters per second or knots.[1]: 3-8 to 3-9
Compass systems
[edit]Magnetic compass
[edit]
The compass shows the aircraft's heading relative to magnetic north. Errors include Variation, or the difference between magnetic and true direction, and Deviation, caused by the electrical wiring in the aircraft, which requires a Compass Correction Card. Additionally, the compass is subject to Dip Errors. 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, but periodically calibrated against the compass.[1]: 3-9 to 3-13, 3–19
Gyroscopic systems
[edit]Attitude indicator
[edit]
The attitude indicator (also known as an artificial horizon) shows the aircraft's relation to the horizon. From this the pilot can tell whether the wings are level (roll) and if the aircraft nose is pointing above or below the horizon (pitch).[1]: 3-18 to 3-19 Attitude is always presented to users in the unit degrees (°).[citation needed] The attitude indicator 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.
Heading indicator
[edit]
The heading indicator (also known as the directional gyro, or DG) displays the aircraft's heading in compass points, and with respect to magnetic north when set with a compass. Bearing friction causes drift errors from precession, which must be periodically corrected by calibrating the instrument to the magnetic compass.[1]: 3-19 to 3-20 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.
Turn and slip indicator
[edit]
These include the turn and slip indicator and the turn coordinator, which indicate rotation about the longitudinal axis. They include an inclinometer to indicate whether the aircraft is in coordinated flight, or in a slip or skid. Additional marks indicate a standard rate turn.[1]: 3-20 to 3-22 The turn rate is most commonly expressed in either degrees per second (deg/s) or minutes per turn (min/tr).[citation needed]
Flight director systems
[edit]These include the Horizontal Situation Indicator (HSI) and Attitude Director Indicator (ADI). The HSI combines the magnetic compass with navigation signals and a Glide slope. The navigation information comes from a VOR/Localizer, or GNSS. The ADI is an Attitude Indicator with computer-driven steering bars, a task reliever during instrument flight.[1]: 3-22 to 3-23, 7–10
Navigational systems
[edit]Very-high frequency omnidirectional range (VOR)
[edit]
The VOR indicator instrument includes a course deviation indicator (CDI), omnibearing selector (OBS), TO/FROM indicator, and flags. The CDI shows an aircraft's lateral position in relation to a selected radial track. It is used for orientation, tracking to or from a station, and course interception.[1]: 7-8 to 7-11 On the instrument, the vertical needle indicates the lateral position of the selected track. A horizontal needle allows the pilot to follow a glide slope when the instrument is used with an ILS.
Non-directional radio beacon (NDB)
[edit]
The automatic direction finder (ADF) indicator instrument can be a fixed-card, movable card, or a radio magnetic indicator (RMI). An RMI is remotely coupled to a gyrocompass so that it automatically rotates the azimuth card to represent aircraft heading.[1]: 7-3 to 7-4 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.
Layout
[edit]
Most aircraft are equipped with a standard set of flight instruments which give the pilot information about the aircraft's attitude, airspeed, and altitude.
T arrangement
[edit]Most US aircraft built since the 1940s have flight instruments arranged in a standardized pattern called the T arrangement.[3] The attitude indicator is in the top center, airspeed to the left, altimeter to the right and heading indicator under the attitude indicator. The other two, turn-coordinator and vertical-speed, are usually found under the airspeed and altimeter, but are given more latitude in placement. The magnetic compass will be above the instrument panel, often on the windscreen centerpost. In newer aircraft with glass cockpit instruments, the layout of the displays conform to the basic T arrangement.
Early history
[edit]In 1929, Jimmy Doolittle became the first pilot to take off, fly and land an airplane using instruments alone, without a view outside the cockpit. In 1937, the British Royal Air Force (RAF) chose a set of six essential flight instruments[4] which would remain the standard panel used for flying in instrument meteorological conditions (IMC) for the next 20 years. They were:
- altimeter (feet)
- airspeed indicator (knots)
- turn and bank indicator (turn direction and coordination)
- vertical speed indicator (feet per minute)
- artificial horizon (attitude indication)
- directional gyro / heading indicator (degrees)
This panel arrangement was incorporated into all RAF aircraft built to official specification from 1938, such as the Miles Master, Hawker Hurricane, Supermarine Spitfire, and 4-engined Avro Lancaster and Handley Page Halifax heavy bombers, but not the earlier light single-engined Tiger Moth trainer, and minimized the type-conversion difficulties associated with blind flying, since a pilot trained on one aircraft could quickly become accustomed to any other if the instruments were identical.
This basic six set, also known as a "six pack",[5] was also adopted by commercial aviation. After the Second World War the arrangement was changed to: (top row) airspeed, artificial horizon, altimeter, (bottom row) turn and bank indicator, heading indicator, vertical speed.
Further development
[edit]
In glass cockpits, the flight instruments are shown on monitors. Primary flight display, is given a central place on the panel, superseding the artificial horizon, often, with a horizontal situation indicator next to it or integrated with the PFD. The indicated airspeed, altimeter, and vertical speed indicator are displayed as moving "tapes" with the indicated airspeed to the left of the horizon and the altimeter and the vertical speed to the right in the same layout as in most older style "clock cockpits".
See also
[edit]References
[edit]- ^ a b c d e f g h i j k l Instrument Flying Handbook, 2001, FAA-H-8083-15, US Dept. of Transportation, Federal Aviation Administration, Flight Standards Service
- ^ Aviation's Crazy, Mixed Up Units of Measure - AeroSavvy
- ^ Mark Natola, ed. (2002). Boeing B-47 Stratojet. Schiffer Publishing Ltd. p. 46. ISBN 0764316702.
- ^ Williamson, G. W. (19 August 1937). "Instrument Planning: The New Service Blind-Flying Panel Described". Flight. p. 193. Archived from the original on 27 July 2014. Retrieved 3 May 2024.
- ^ "Six Pack - The Primary Flight Instruments". LearnToFly.ca. 13 March 2010. Retrieved 31 January 2011.
External links
[edit]- Instrument Flying Handbook 2012
- Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A) 2008
- The Gyro Horizon Enables Instrument Flying A history of how aircraft instrumentation was developed with an emphasis on the gyro horizon. 2007
- "How Aircraft Instruments Work." Popular Science, March 1944, pp. 116–123/192.
- Current Practice in Instrument Panel Layout – Aero Digest
Flight instruments
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Pitot-Static Instruments
Altimeter
The altimeter is a critical flight instrument that measures an aircraft's altitude above a reference level, primarily by detecting changes in atmospheric pressure via the static pressure port of the pitot-static system. It operates on the principle that atmospheric pressure decreases with increasing altitude in a predictable manner according to the standard atmosphere model, allowing the instrument to infer height from pressure readings. The core mechanism is an aneroid barometer consisting of sealed, flexible metal capsules (aneros) that expand or contract with pressure variations, mechanically linked to pointers on a dial to display altitude.[1] The primary type is the pressure altimeter, which provides altitude relative to sea level or a standard pressure datum. Pressure altimeters are calibrated to the International Standard Atmosphere (ISA), where sea-level pressure is defined as 1013.25 hPa (29.92 inHg) and temperature is 15°C, with a lapse rate of 6.5°C per km up to 11 km. Altitude is calculated using the hypsometric equation derived from the ISA model for the troposphere: where is geopotential altitude in meters, K (sea-level temperature), K/m (lapse rate), is ambient pressure in Pa, Pa (sea-level pressure), J/(mol·K) (universal gas constant), m/s² (standard gravity), and kg/mol (molar mass of air). This equation assumes hydrostatic equilibrium and ideal gas behavior but has limitations above the tropopause or in non-standard conditions, where more complex models are needed. A simplified approximation for pressure altitude in feet is , often used in aviation computations.[3] Calibration involves setting the altimeter to local conditions using the Kollsman window, a subscale for adjusting the reference pressure in inches of mercury (inHg) or hectopascals (hPa). For operations near the surface, it is set to QNH (altimeter setting reduced to sea level using local station pressure), yielding altitude above mean sea level; for high-altitude or standard pressure regions, it is set to QNE (29.92 inHg), providing pressure altitude above the standard datum plane. The Kollsman window is named after inventor Paul Kollsman, who patented the first sensitive barometric altimeter in 1928 (U.S. Patent No. 2,036,581, issued 1936 based on 1930 application), revolutionizing instrument flight. Altimeters display in feet (common in U.S. aviation) or meters internationally, with multi-pointer dials showing tens of thousands, thousands, and hundreds of feet.[1][4][5] Errors arise from deviations in the actual atmosphere from ISA assumptions, notably temperature and pressure variations. In cold temperatures, air density increases, causing the aircraft to be lower than indicated (e.g., at -15°C and 4,000 ft indicated, true altitude may be 3,600 ft, requiring a 4% correction per 10°C below standard); conversely, hot temperatures yield higher true altitudes. Non-standard pressure also introduces errors: flying from high to low pressure or temperature decreases true altitude by about 1,000 ft per inHg (or 30 ft per hPa) difference. These necessitate corrections using flight computers or charts for precise operations. Under FAA regulations (14 CFR § 91.205), a sensitive altimeter adjustable for barometric pressure is required for instrument flight rules (IFR) operations, with preflight accuracy checks ensuring deviation no more than 75 ft from known elevation.[1]Airspeed Indicator
The airspeed indicator (ASI) is a critical flight instrument that measures and displays an aircraft's speed relative to the surrounding air mass by sensing the dynamic pressure generated by the aircraft's motion. It operates using the pitot-static system, where the pitot tube captures total pressure (a combination of static and dynamic pressure), and the static port measures ambient static pressure; the difference between these, known as dynamic pressure , drives a diaphragm or aneroid capsule within the instrument to indicate speed.[6][7] This differential pressure is calibrated to provide an uncorrected reading under standard sea-level conditions. The ASI displays several types of airspeed, each serving distinct operational purposes. Indicated airspeed (IAS) is the direct, uncorrected reading from the instrument, while calibrated airspeed (CAS) adjusts IAS for instrument and installation errors, such as those from the pitot-static system's positioning on the aircraft. True airspeed (TAS) further corrects CAS for air density variations due to altitude and temperature, becoming essential for navigation and performance calculations; for low speeds, TAS is approximated as , where is the density ratio relative to sea-level density . At higher speeds approaching Mach 0.3 or above, compressibility effects require additional corrections using isentropic flow relations to account for air compressibility, ensuring accurate TAS derivation from the dynamic pressure equation .[6][7] The instrument face features color-coded arcs and specific markings to guide safe operation: the green arc represents the normal operating range, the yellow arc indicates caution speeds to be avoided in turbulence, and the red radial line marks the never-exceed speed (V_NE). Key V-speeds, such as V1 (decision speed), Vr (rotation speed), and V2 (takeoff safety speed), are often marked or referenced on the ASI for critical phases like takeoff. To prevent icing-related blockages that can cause erroneous readings—such as a blocked pitot tube leading to zero or fluctuating indications—modern ASIs incorporate heated pitot probes, activated in visible moisture to maintain clear airflow. A notable incident illustrating this vulnerability occurred on February 6, 1996, when Birgenair Flight 301, a Boeing 757, crashed into the Atlantic Ocean shortly after takeoff from Puerto Plata, Dominican Republic, due to the captain's pitot tube blockage by insect debris, resulting in conflicting airspeed data, crew confusion, and loss of control that killed all 189 occupants.[6][8]Vertical Speed Indicator
The Vertical Speed Indicator (VSI), also known as a rate-of-climb and descent indicator, measures the aircraft's vertical speed by detecting the rate of change in atmospheric static pressure, displaying it as the rate of ascent or descent.[6] It operates using only the static pressure source from the pitot-static system and is calibrated in feet per minute (fpm) in imperial units or meters per second in metric systems, with a typical range of -6,000 to +6,000 fpm to cover most operational climb and descent rates in general aviation and commercial aircraft.[6][9] The core mechanism consists of an aneroid diaphragm (or capsule) housed within an airtight instrument case, both connected to the aircraft's static pressure line.[6] The diaphragm receives direct static pressure, allowing it to expand or contract immediately with pressure changes, while the case interior equalizes to the same pressure through a calibrated restrictor or leak—a small orifice designed to delay pressure equalization by 6 to 9 seconds.[10][6] This creates a temporary pressure differential across the diaphragm proportional to the rate of pressure change (dP_s/dt), which is mechanically linked via gears and a pointer to indicate vertical speed on a circular scale; in level flight, pressures equalize, and the indicator reads zero.[11] The VSI provides two types of readings: an instantaneous "trend" indication, which shows the initial direction of climb or descent almost immediately as the diaphragm responds first, and a steady-state "rate" indication, which stabilizes after the lag period to reflect the constant vertical speed once pressures equilibrate across the restrictor.[6] This dual output helps pilots anticipate changes, but the inherent lag means the needle may initially deflect by 1 to 2 scale widths before settling, particularly during abrupt maneuvers.[12] Lag errors arise from the restrictor's time constant, modeled as a first-order system where the case pressure follows the external static pressure with a delay τ (typically 6-9 seconds), causing the indicated rate to approach the true rate exponentially: the error decreases as e^{-t/τ}.[6] To mitigate this, two main types exist: standard (unbalanced) VSIs, which rely solely on the restrictor and exhibit full lag, and instantaneous VSIs (IVSIs or balanced designs), which incorporate accelerometer-driven air pumps or vanes to accelerate pressure equalization and provide near-immediate rate readings with minimal delay.[6][13] Calibration ensures the instrument reads zero during unaccelerated level flight and is sensitive to pressure changes corresponding to altitude variations, but errors occur during rapid maneuvers or turbulence, where rough air can prolong the lag or cause erratic readings.[6][11] The VSI is essential for instrument flight rules (IFR) operations, particularly in non-precision approaches, where it helps maintain the required glide slope by cross-checking with the altimeter to control descent rates precisely.[6] The indicated vertical speed (VSI) is derived from the rate of static pressure change scaled to altitude:where is the altitude sensitivity factor from the altimeter scale, approximately 27 feet per millibar near sea level under standard atmospheric conditions (derived from the hydrostatic equation , integrated for the lapse rate).[6] Full sensitivity calibration adjusts the restrictor size and linkage gearing so that the pressure differential produces a deflection proportional to this rate, with lag modeling incorporated via the time constant τ to predict settling time during certification.[10][11]
Heading Reference Instruments
Magnetic Compass
The magnetic compass, also known as the whiskey compass, is a fundamental flight instrument that provides aircraft heading relative to magnetic north by utilizing the Earth's magnetic field. It consists of a magnetized needle or card attached to a float within a sealed, liquid-filled bowl, typically containing compass fluid similar to kerosene, which damps oscillations and supports the assembly's weight to prevent excessive pivoting. The float pivots on a low-friction jewel-and-pivot mount, allowing the card—marked with cardinal and intermediate headings—to align freely with the horizontal component of the Earth's magnetic field lines, visible through a transparent dome and referenced against a fixed lubber line. This design ensures readability and stability during flight, though it is most accurate in level, unaccelerated flight up to an 18-degree bank angle.[6] Several inherent errors affect the magnetic compass's accuracy. Magnetic variation, or declination, is the angular difference between true north and magnetic north, caused by the Earth's geographic and magnetic poles not coinciding; for example, it measures about 11 degrees west in Washington, D.C., and changes annually by approximately 0.02–0.03 degrees due to shifts in the magnetic field. Deviation arises from the aircraft's own magnetic fields, such as those from electrical systems, metal structures, or engines, which distort the compass reading depending on heading; this is minimized through compensation but not eliminated. Northerly turning error, also called acceleration and deceleration error, occurs during changes in speed or turns: in the Northern Hemisphere, the compass indicates a turn toward north (UNOS: Undershoot North, Overshoot South) when accelerating on east or west headings, and the opposite when decelerating, due to the dip of the magnetic field tilting the card. Additionally, the compass becomes unreliable in polar regions where the horizontal magnetic component weakens, causing erratic indications near the magnetic poles.[6] To mitigate deviation, a pre-flight compass swing is performed by an aviation maintenance technician (AMT) at a certified compass rose or equivalent site, aligning the aircraft to multiple headings (e.g., every 30 degrees) with engines and electrical systems operating normally, then adjusting onboard compensators to reduce errors. Remaining deviations are recorded on a compass correction card, placarded near the instrument, which pilots consult to apply corrections; for instance, if the card shows a 5-degree easterly deviation on a 090-degree heading, the pilot adds 5 degrees to the compass reading for magnetic heading. Separate cards may be needed if deviations exceed 10 degrees with radios or lights on versus off. In the United States, a magnetic direction indicator is required by regulation for all powered civil aircraft conducting visual flight rules (VFR) day operations, making the whiskey compass standard in light aircraft as a reliable, non-powered backup to gyroscopic heading systems for basic navigation.[14][15][6]Example Deviation Card
| Magnetic Heading | Deviation (Degrees) | Corrected Magnetic Heading |
|---|---|---|
| 000° | 0° E | 000° |
| 030° | 2° W | 028° |
| 060° | 3° E | 063° |
| 090° | 5° E | 095° |
| 120° | 2° W | 118° |
| 150° | 1° E | 151° |
| 180° | 0° | 180° |
| 210° | 2° E | 212° |
| 240° | 4° W | 236° |
| 270° | 3° W | 267° |
| 300° | 1° E | 301° |
| 330° | 0° | 330° |