Vacuum manifold Pilot’s turn and bank Copilot’s turn and bank Pilot’s artificial horizon Copilot’s artificial horizon Pilot’s directional gyro Needle valve Needle valve Check valve Check valve Vacuum gauge Relief valve Relief valve Left engine vacuum pump Right engine vacuum pump Vacuum 4-way selector valve Turn-and-bank selector valve Right turn and bank Left turn and bank Main Figure 10-92.  An example of a twin-engine instrument vacuum system.  form of a red flag or mark of some sort often with the word  “off” written on it.  Three of the most common flight instruments, the attitude  indicator, heading indicator, and turn needle of the turn-and- bank indicator, are controlled by gyroscopes. To understand  how these instruments operate, knowledge of gyroscopic  principles and instrument power systems is required.  A mechanical gyroscope, or gyro, is comprised of a wheel or  rotor with its mass concentrated around its perimeter. The rotor  has bearings to enable it to spin at high speeds.  [Figure 10-93A]  Different mounting configurations are available for the rotor  and axle, which allow the rotor assembly to rotate about one  The supporting ring can also be mounted inside an outer  ring. The bearing points are the same as the bracket just  described, 90° around the supporting ring from where the  spin axle attached. Attachment of a bracket to this outer ring  allows the rotor to rotate in two planes while spinning. Both  of these are perpendicular to the spin axis of the rotor. The  plane that the rotor spins in due to its rotation about its axle  is not counted as a plane of rotation. or two axes perpendicular to its axis of spin. To suspend the  rotor for rotation, the axle is first mounted in a supporting  ring.  [Figure 10-93B]  If brackets are attached 90° around  the supporting ring from where the spin axle attached, the  supporting ring and rotor can both move freely 360°. When  in this configuration, the gyro is said to be a captive gyro.  It can rotate about only one axis that is perpendicular to the  axis of spin.  [Figure 10-93C] Principles of Gyroscopic Instruments Mechanical Gyros 10-53