Six Pack

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.

Early history[edit]

Typical instrument configuration of a Cessna 172

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[2] 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) turn and bank indicator, heading indicator, vertical speed.

Further development

Of the old basic six instruments, the turn and bank indicator is now obsolete. The instrument was included, but it was of little use in the first generation of jet airliners. It was removed from many aircraft prior to glass cockpits becoming available. With an improved artificial horizon, including gyros and flight directors, the turn and bank indicator became needless except when performing certain types of aerobatics (which would not be intentionally performed in IMC to begin with). But the other five flight instruments, sometimes 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 shown by numbers, but as images of analog 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 with the indicated airspeed and altitude to the right of the horizon and the vertical speed to the left in the same pattern as in most older style "clock cockpits".

Different significance and some other instrumentation

However, all five instruments are not equally important (in good weather conditions small aircraft can be landed without the use of any instrument, called visual landing.) Only the artificial horizon, altimeter, airspeed indicator and compass are imperative. Due to this all passenger airliners are equipped with a battery-powered integrated standby instrument system along with a magnetic compass.

The vertical speed indicator, or VSI, is more of "a good help" than absolutely essential. On jet aircraft it displays the vertical speed in thousands of feet per minute, usually in the range -6 to +6. The gyrocompass can be used for navigation, but it is indeed a flight instrument as well. 

It is 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 important instrument and indicates the airspeed very accurately in the range of 45 to 250 knots. At higher altitude a MACH-meter is used instead, to prevent the 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 is like the Mach-meter: not really a flight instrument). The altimeter displays the altitude in feet, but must be corrected to local air pressure at the landing airport. The altimeter may be adjusted to show an altitude of zero feet on the runway, but far more common is to adjust the altimeter to show the actual altitude when the aircraft has landed. 

In the latter case pilots must keep the runway elevation in mind. However a radio altimeter (displaying the height above 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 considered as a flight instrument.