Flight Instruments - Layout - Different Significance and Some Other Instrumentation

Different Significance and Some Other Instrumentation

However, all five instruments are not equally important. Only the artificial horizon (or gyro-horizon) is imperative. In good weather conditions small aircraft can be landed without the use of any instrument, called visual landing. Due to this all passenger airliners are equipped with a third gyro-horizon (apart from those on the captain's and first officer's instrumentation) even with modern glass cockpits. On the other hand, 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 heading indicator (or compass) 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 height 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.