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Some helicopters are equipped with an alternate static source, which may be selected in the event that the main static system becomes blocked. The alternate source generally vents into the cabin, where air pressures are slightly different than outside pressures, so the airspeed and altimeter usually read higher than normal. Correction charts may be supplied in the flight manual.
GYROSCOPIC INSTRUMENTS
The three gyroscopic instruments that are required for instrument flight are the attitude indicator, heading indicator, and turn indicator. When installed in helicopters, these instruments are usually electrically powered.
Gyros are affected by two principles—rigidity in space and precession. Rigidity in space means that once a gyro is spinning, it tends to remain in a fixed position and resists external forces applied to it. This principle allows a gyro to be used to measure changes in attitude or direction.
Precession is the tilting or turning of a gyro in response to pressure. The reaction to this pressure does not occur at the point where it was applied; rather, it occurs at a point that is 90° later in the direction of rotation from where the pressure was applied. This principle allows the gyro to determine a rate of turn by sensing the amount of pressure created by a change in direction. Precession can also create some minor errors in some instruments.
ATTITUDE INDICATOR
The attitude indicator provides a substitute for the natural horizon. It is the only instrument that provides an immediate and direct indication of the helicopter’s pitch and bank attitude. Since most attitude indicators installed in helicopters are electrically powered, there may be a separate power switch, as well as a warning flag within the instrument, that indicates a loss of power. A caging or “quick erect” knob may be included, so you can stabilize the spin axis if the gyro has tumbled. [Figure 12-5]
HEADING INDICATOR
The heading indicator, which is sometimes referred to as a directional gyro (DG), senses movement around the vertical axis and provides a more accurate heading reference compared to a magnetic compass, which has a number of turning errors. [Figure 12-6].
Due to internal friction within the gyroscope, precession is common in heading indicators. Precession causes the selected heading to drift from the set value. Some heading indicators receive a magnetic north reference from a remote source and generally need no adjustment. Heading indicators that do not have this automatic north-seeking capability are often called “free” gyros, and require that you periodically adjust them. You should align the heading indicator with the magnetic compass before flight and check it at 15minute intervals during flight. When you do an in-flight alignment, be certain you are in straight-and-level, unaccelerated flight, with the magnetic compass showing a steady indication.
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