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. Older directional gyros use a drum-like card marked in the  same way as the magnetic compass card. The gyro and the  card remain rigid inside the case with the pilot viewing the  card from the back. This creates the possibility the pilot might  start a turn in the wrong direction similar to using a magnetic  compass. A knob on the front of the instrument, below the  dial, can be pushed in to engage the gimbals. This locks the  gimbals allowing the pilot to rotate the gyro and card until  the number opposite the lubber line agrees with the magnetic  compass. When the knob is pulled out, the gyro remains rigid  and the aircraft is free to turn around the card. Directional gyros are almost all air-driven by evacuating the  case and allowing filtered air to flow into the case and out  through a nozzle, blowing against buckets cut in the periphery  of the wheel. The Earth constantly rotates at 15° per hour  while the gyro is maintaining a position relative to space, thus  causing an apparent drift in the displayed heading of 15° per  hour. When using these instruments, it is standard practice to  compare the heading indicated on the directional gyro with  the magnetic compass at least every 15 minutes and to reset  the heading as necessary to agree with the magnetic compass.  Heading indicators like the one in  Figure 5-31  work on the  same principle as the older horizontal card indicators, except  that the gyro drives a vertical dial that looks much like the  dial of a vertical card 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. Attitude and heading indicators function on the principle  of rigidity, but rate instruments such as the turn-and- slip indicator operate on precession. Precession is the  characteristic of a gyroscope that causes an applied force to  produce a movement, not at the point of application, but at  a point 90° from the point of application in the direction of  rotation.  [Figure 5-32] The first gyroscopic aircraft instrument was the turn indicator  in the needle and ball, or turn-and-bank indicator, which  has more recently been called a turn-and-slip indicator.  [Figure 5-33] The inclinometer in the instrument is a black glass ball sealed  inside a curved glass tube that is partially filled with a liquid  for damping. This ball measures the relative strength of the  force of gravity and the force of inertia caused by a turn.  When the aircraft is flying straight-and-level, there is no  inertia acting on the ball, and it remains in the center of the  tube between two wires. In a turn made with a bank angle  that is too steep, the force of gravity is greater than the inertia  and the ball rolls down to the inside of the turn. If the turn is  Turn Indicators Figure 3-32. Precession causes a force applied to a spinning Figure 5-32. Precession causes a force applied to a spinning  wheel to be felt 90° from the point of application in the direction  of rotation. Figure 3-31. The heading indicator is not north-seeking, but must be set to agree with the magnetic compass. Figure 5-31. The heading indicator is not north seeking, but must  be set periodically (about every 15 minutes) to agree with the  magnetic compass. Plane of Precession  Plane of Force  Plane of Rotation  FORCE  33  30  24  2I  I5  I2  6  3  5-20 Turn-and-Slip Indicator