Using Flight Instruments for Pilot

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Figure shows a basic panel layout. Let’s begin with the center of the panel. At the top center is the artificial horizon, or attitude indicator. This is a gyroscopic instrument. A vacuum pump attached to the engine causes a flow of air through the instrument, which spins its gyroscope up to speed. When powered, the gyroscope will attempt to maintain a constant orientation as the plane moves, giving it the ability to mimic the horizon’s constant position.

Figure shows the artificial horizon when the plane is in level flight. The small airplane in the instrument’s foreground represents he plane you are flying, while the background represents the horizon, earth, and sky, outside the airplane. As the plane moves, the background of the artificial horizon moves. When viewed in relation to the small airplane in the instrument, you can maintain a reference to the horizon, even if the outside horizon is obscured.

The instrument located directly below the artificial horizon is the directional gyro, or DG. Like the artificial horizon, it is a gyroscopic instrument powered by suction on most single-engine aircraft. Figure shows the directional gyro. The instrument acts much like a compass but does not use the magnetic field of the earth for directionfinding purposes. Instead, as part of the run-up prior to takeoff, you set the DG to the same heading indicated on the compass. The gyro in the instrument then maintains this alignment and indicates the plane’s heading with more stability than the floating compass.

A word of warning concerning the DG, though. The directional gyro in most airplanes has some tendency to drift off the correct heading as time passes. This precession is easily corrected by periodic comparisons with the DG and magnetic compass while in steady flight. For planes with older instruments, or those with failing suction pumps, you may find that the DG will not hold the correct heading for any period of time. When it comes to low vacuum pressure, the same is true of the artificial horizon. I have had flights where low vacuum pressure from the pump caused the artificial horizon to give erroneous readings, indicating a bank when the plane was actually flying straight and level. In heavy instrument conditions this can cause the pilot some distress, but with a proper instrument technique you should be able to quickly determine that an instrument is failing, which instrument it is, and then continue to fly using alternative instruments.

Figure shows the airspeed indicator. The needle moves clockwise to indicate higher airspeed. The airspeed indicator measures pressure differentials between air entering a probe called a pitot tube and static air pressure from the static vent. There are typically several color bands on the airspeed indicator: white, green, yellow, and red. The white band denotes the recommended speeds for flap operation. The high end is the maximum airspeed the flaps may be extended, while the lower end of it is the stall speed of the plane with flaps fully extended. If flaps are kept out above the airspeed indicated at the top of the white arc, damage to the flaps and other structures of the wing could potentially result.

The bottom of the green arc is the stalling speed of the plane with flaps retracted, or in the clean configuration. Please note the stalling speed is higher when flaps are not used, which fits with our discussion of flaps earlier. At the top of the green arc is the airspeed that should be used for normal cruise, although some planes do not have sufficient power at altitude to actually reach this indicated airspeed.

A yellow arc begins at the end of the green arc and depicts the range of airspeeds wherein the plane should be flown only with caution. At these airspeeds it is possible to damage the airplane if severe turbulence is encountered or rapid control inputs are made. 

At the top of the yellow arc is a red line, the maximum airspeed. Above this airspeed the airplane can be structurally damaged due to the dynamic load.

Figure shows an altimeter, which operates by measuring changes in barometric pressure. Altimeters normally have three needles: one showing feet, one showing hundreds of feet, and finally one showing thousands of feet. Most altimeters also have a small window that is used to set the reference barometric pressure for your location.

Like the altimeter, the vertical speed indicator is a pressure-sensing device that is used to indicate the rate of change in your airplane’s altitude. This is usually expressed in hundreds of feet per minute.

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