|Instrument flying is a common occurrence today but it wasn’t always so. In 1929 James Doolittle, most famous for the WWII “Doolittle Raid”, became the first pilot to takeoff, fly, and land an aircraft based solely by reference to flight instruments with no outside visual views. He also helped develop the artificial horizon and directional gyroscope. These accomplishments made all-weather airline and general aviation operations practical.
Typically any time the daytime weather that doesn’t meet VFR (visual flying rules) minimums of 3 statute miles flight visibility, cloud clearances of 500′ below, 1,000′ above, and 2,000′ feet horizontally requires a pilot to follow instrument flight rules (IFR). Many general aviation aircraft have all of the flight instruments required to fly in less than perfect weather but what instruments do they use?
|The illustration above shows the basic cockpit instruments used for instrument flying. Since 1953 they have been grouped in this “basic T” or “six pack” arrangement and are as follows:
(Starting with the top left and going from left to right)
Airpseed indicator- The airspeed indicator shows the aircraft’s speed. It measures ram air collected at the pitot tube compared to static pressure from a static port to give an airspeed indication.
Attitude Indicator- The attitude indicator (also known as an artificial horizon) shows the aircraft’s attitude in pitch and bank relative to the horizon. It is usually powered by vacuum from an engine driven vacuum pump but some can be electrically powered as well.
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.
Turn/ Slip Indicator- The turn indicator 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, wherein 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. Usually these are electrically powered.
Directional Gyro- The heading indicator (also known as the directional gyro, or DG; sometimes also called the gyrocompass, ) displays the aircraft’s heading with respect to geographical north. These are usually vacuum powered.
Vertical Speed Indicator- The VSI senses changing air pressure, and displays that information to the pilot as a rate of climb or descent in feet per minute.