Six primary flight instruments have become standard in any instrument panel and, since the early 1970's, these have been arranged in a certain, standard layout commonly referred to as the 'six pack'. They are laid out in two rows of three instruments each. The top row, left to right, contains the Airspeed Indicator (ASI), the Attitude Indicator (AI) and the Altimeter (Alt). The bottom row contains the Turn Coordinator (TC) the Directional Gyro (DG) and the Vertical Speed Indicator (VSI). A quick summary of these instruments follows:
The Airspeed Indicator (ASI)Edit
The ASI indicates airspeed, or the speed at which the aircraft is traveling through the air. It is a differential pressure gauge that measures the dynamic pressure of the air through which the aircraft is flying. Dynamic pressure is the difference in the ambient static air pressure and the total, or ram, pressure caused by the motion of the aircraft through the air. These two pressures are taken from the pitot-static system. Beginning pilots need to remember that airspeed will include local wind and does not necessarily equate to ground speed. Throttle back a Cessna 150 in slow flight with a stiff head wind and you could be flying in one place or even backwards.
The Attitude Indicator (AI)Edit
As discussed earlier, the AI informs the pilot of his or her attitude in space. This is accomplished by fixing the case of the instrument to the aircraft and measuring the displacement of the case with reference to a fixed gyroscope inside.
The Altimeter (ALT)Edit
The altimeter looks somewhat like the face of a clock and relays to the pilot her altitude, measured by the expansion or contraction of a fixed amount of air, acting on some springs. As the airplane climbs or descends the relative air pressure outside the aircraft changes and the altimeter reports the difference between the outside air pressure and a reference, contained in a set of airtight bellows.
The Turn Coordinator (TC)Edit
The TC relays the rate of turn for the aircraft. The instrument is only accurate when the turn is coordinated, that is that the airplane is not skidding or slipping through the turn. For a skid, imagine a car that is understeering, where the front wheels do not have enough traction to overcome the car's momentum and the front of the car is thus plowing through the turn, resulting in a turn radius that is larger than that commanded by the driver). A slip is a bit more difficult for you to imagine unless you're a pilot already. This results from an aircraft that is banked too steeply for the rate of turn selected. To correct the slip all the pilot has to do is increase back pressure on the yoke to pull the airplane 'up' into a tighter turn, such that the turn rate is in equilibrium with the bank angle.
The major limitation of the older turn-and-slip indicator is that it senses rotation only about the vertical axis of the aircraft. It tells nothing of the rotation around the longitudinal axis, which in normal flight occurs before the aircraft begins to turn. A turn coordinator operates on precession, the same as the turn indicator, but its gimbals frame is angled upward about 30° from the longitudinal axis of the aircraft. This allows it to sense both roll and yaw. Therefore during a turn, the indicator first shows the rate of banking and once stabilized, the turn rate. Some turn coordinator gyros are dual-powered and can be driven by either air or electricity. Rather than using a needle as an indicator, the gimbal moves a dial that is the rear view of a symbolic aircraft. The bezel of the instrument is marked to show wings-level flight and bank angles for a standard rate turn.
The Directional Gyro (DG)Edit
A magnetic compass is a dependable instrument used as a backup instrument. Although very reliable, it has so many inherent errors that it has been supplemented with gyroscopic heading indicators.
The gyro in a heading indicator is mounted in a double gimbal, as in an attitude indicator, but its spin axis is horizontal permitting sensing of rotation about the vertical axis of the aircraft. Gyro heading indicators, with the exception of slaved gyro indicators, are not north seeking, therefore they must be manually set to the appropriate heading by referring to a magnetic compass. Rigidity causes them to maintain this heading indication, without the oscillation and other errors inherent in a magnetic compass.
The heading of the aircraft is shown against the nose of the symbolic aircraft on the instrument glass, which serves as the lubber line. A knob in the front of the instrument may be pushed in and turned to rotate the gyro and dial. The knob is spring loaded so it disengages from the gimbals as soon as it is released. This instrument should be checked about every 15 minutes to see if it agrees with the magnetic compass.
The Vertical Speed Indicator (VSI)Edit
The VSI reports your climb or descent rate in feet per minute. Typically, non-pressurized airplanes will climb comfortably at about 700 fpm (if they can) and descend at about 500 fpm. Descent rates faster than this cause discomfort on the occupants which can be felt in your ears. Pressurized airplanes can climb and descend much more rapidly and still maintain the cabin rate of change at about these levels, since the cabin altitude is not related to the ambient altitude unless the pressurization system fails.