Flight instruments

The cockpit of a Slingsby T-67 Firefly two-seat light airplane. The flight instruments are visible on the left of the instrument panel

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as altitude, speed and direction. The flight instruments are of particular use in conditions of poor visibility, such as in clouds, when such information is not available from visual reference outside the aircraft.

The term is sometimes used loosely as a synonym for cockpit instruments as a whole, in which context it can include engine instrument, navigational and communication equipment.

Flight instruments

Most regulated aircraft have these flight instruments:

Altimeter

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. It is adjustable for local barometric pressure which must be set correctly to obtain accurate altitude readings. As the aircraft ascends, the capsules expand as the static pressure drops therefore causing the altimeter to indicate a higher altitude. The opposite occurs when descending.

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.

Schempp-Hirth Janus-C glider Instrument panel equipped for "cloud flying". The turn and bank indicator is top center. The heading indicator is replaced by a GPS-driven computer with wind and glide data, driving two electronic variometer displays to the right.

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.

Magnetic compass

The 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).

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 magnetic 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

Turn indicator

The turn indicator (also known as turn and slip) 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, where in 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 indicator is seen typically in aircraft manufactured only prior to that time, or in gliders manufactured in Europe.

Vertical speed indicator

The 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, meters per second or knots.

Additional panel instruments that may not be found in smaller aircraft

Course deviation indicator

The 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 or an Instrument Landing System.

This instrument can also be integrated with the heading indicator in a horizontal situation indicator.

Radio Magnetic Indicator

An 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.

Layout

Six basic instruments in a light twin-engine airplane arranged in a "basic-T". From top left: airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator, and vertical speed indicator

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

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 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.

Basic Six

In 1937 the British Royal Air Force (RAF) chose a set of six essential flight instruments 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 every RAF aircraft, from the light single engined Tiger Moth trainer, to the 4-engined Avro Lancaster heavy bomber, 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", was also adopted by commercial aviation. After the Second World War the arrangement was changed to: (top row) airspeed, artificial horizon, altimeter, (bottom row) radio compass, direction indicator, vertical speed.

Further development

Of the old basic six instruments, the turn and bank indicator is now obsolete. The instument was included, but of little use in the first generation of jet airliners. It was removed on many aircraft prior to the glass-cockpits became available. With improved artificial horizon, including gyros and flight directors, the turn and bank indicator became needless. But the other five flight instruments, somtemes known as "the big five", are still included in all cockpits. The way of displaying them has changed over time though. In glass cockpits the flight instruments are shown on monitors. But the display is not showed by numbers, but as images of analoge instruments. The artificial horizon is given a central place in the monitor, with a heading indicator just below (usually this is displayed only as a part of the compass). The indicated airspeed, altimeter, and vertical speed indicator are displayed as columns. The two latters to the right of the horizon and the first to the left. In the same pattern as in most older style "clock cockpits".

Different sigificance and some other instumentation

However all five instuments are not eaqually importaint. Only the artificial horizon (or gyrohorizon) is imperative. In good weather conditions small aircraft can be landed without the use of any instrument, called visual landing. But on large jet airliners, landing without any functional artificial horizon, then the landing will most presumably not end well. It's impossible to a pilot (of a large aircraft) to estimate if the aircraft flies without a banking of f.i 3-4 degrees at landing setting. And if only one side of the central gear wheels touches ground (in a speed of 140-150 knots), a disaster is the most likely result. Due to this all passenger airliners are equipped with a third gyrohorizon (appart from those on the captain and flight officer instrumentation) even in the modern glass-cockpits. On the other hand the vertical speed indicator, or VSI, is more of "a good help" then absolutely essential. On jet aircraft it displays the vertical speed in thousands of feet per minute, usually in the range -6 to +6. The heading indicator (or compass) can be used for navigation, but it is indeed a flight instrument aswell. It's needed to control the adjustment of the heading, to be the same as the heading of the landing runway. Indicated airspeed, or IAS, is the second most importaint instrument and indicates the airspeed very accurate in the range of 45 to 250 knots. At higher altitude a MACH-meter is used instead, to prevent aircraft from overspeed. An instrument called true airspeed, or TAS, exists on some aircraft. TAS shows airspeed in knots in the range from 200 knots and higher. (It's like the MACH-meter not really a flight instument) The altimeter display the altitude in feet, but must be correct to local air preassure at the landing airport. The altimeter may be adjusted to an altitude of show zero feet on any runway, but far more common is to adjust the altimeter to show the actual altitude when aircraft has landed. In the latter case pilots must keep the runway hight in mind. However a radio altimeter (displaying the altitude to the ground if lower than around 2000-2500 feet) has been standard for decades. This instrument is however not among the "big five", but must still be concidered as a flight instrument.

See also

• Acronyms and abbreviations in avionics
• Cockpit
• Glass cockpit

External links

• Instrument Flying Handbook (FAA-H-8083-15A) 2007
• The Gyro Horizon Enables Instrument Flying A history of how aircraft instrumentation was developed with an emphasis on the gyro horizon. (c) 2007

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