Flight instruments

thumb|right|400px|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 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.

Pitot-static systems

Instruments which are pitot-static systems use air pressure differences to determine speed and altitude.

Altimeter

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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. 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. The attitude indicator is in the top center, airspeed to the left, altimeter to the right and heading indicator under the attitude indicator. The magnetic compass will be directly above the attitude indicator, and above the instrument panel to limit electromagnetic interference. The placement of the various instruments were a recommendation of Howard Stark, who taught blind flying following his 1-2-3 method in the 1930s.

The other two, turn-coordinator and vertical-speed, are usually found under the airspeed 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 Tarrangement.

Early history

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 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", 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

thumb|400px|[[Primary flight display|Primary Flight Display (PFD)]]

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".

References

External links

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