PUSH TO CAGE Erection mechanism Magnet 44–48 rpm Gyro universally  mounted 21,000 rpm Reaction to precession forces Caging mechanism OFF 30 60 90 10-59 Turn-and-Slip Indicator  Turn Coordinators Figure 10-103.  A typical vacuum-powered gyroscopic direction  indicator, also known as a directional gyro.  Figure 10-102.  Erecting and caging mechanisms of an electric attitude indicator.  A vacuum-powered DG is common on many light aircraft. Its  basis for operation is the gyro’s rigidity in space. The gyro  rotor spins in the vertical plane and stays aligned with the  direction to which it is set. The aircraft and instrument case  moves around the rigid gyro. This causes a vertical compass  card that is geared to the rotor gimbal to move. It is calibrated  in degrees, usually with every 30 degrees labeled. The nose  of a small, fixed airplane on the instrument glass indicates  the aircraft’s heading.  [Figure 10-103] Vacuum-driven direction indicators have many of the same  basic gyroscopic instrument issues as attitude indicators.  Built-in compensation for precession varies and a caging  device is usually found. Periodic manual realignment with  the magnetic compass by the pilot is required during flight. Many aircraft make use of a turn coordinator. The rotor of  the gyro in a turn coordinator is canted upwards 30°. As  such, it responds not only to movement about the vertical  axis, but also to roll movements about the longitudinal axis.  This is useful because it is necessary to roll an aircraft to  turn it about the vertical axis. Instrument indication of roll,  therefore, is the earliest possible warning of a departure from  straight-and-level flight.  the aircraft (yaw). Most turn-and-slip indicators are located  below the airspeed indicator of the instrument panel basic  T, just to the left of the direction indicator. The turn-and-slip indicator is actually two separate devices  built into the same instrument housing: a turn indicator  pointer and slip indicator ball. The turn pointer is operated  by a gyro that can be driven by a vacuum, air pressure, or  by electricity. The ball is a completely independent device.  It is a round agate, or steel ball, in a glass tube filled with  dampening fluid. It moves in response to gravity and  centrifugal force experienced in a turn. Typically, the face of the turn coordinator has a small airplane  symbol. The wing tips of the airplane provide the indication  of level flight and the rate at which the aircraft is turning.  [Figure 10-104] The turn-and-slip indicator may also be referred to as the turn-  and-bank indicator, or needle-and-ball indicator. Regardless,  it shows the correct execution of a turn while banking the  aircraft and indicates movement about the vertical axis of  Turn indicators vary. They all indicate the rate at which the  aircraft is turning. Three degrees of turn per second cause  an aircraft to turn 360° in 2 minutes. This is considered a 