Figure 8-20.  Typical vacuum system. Figure 8-21.  Turn indicators rely on controlled precession for their operation. The gimbal in the turn coordinator is canted; therefore, its  gyro can sense both rate of roll and rate of turn. Since turn  coordinators are more prevalent in training aircraft, this  discussion concentrates on that instrument. When rolling into  or out of a turn, the miniature aircraft banks in the direction  the aircraft is rolled. A rapid roll rate causes the miniature  aircraft to bank more steeply than a slow roll rate. The turn coordinator can be used to establish and maintain  a standard-rate turn by aligning the wing of the miniature  aircraft with the turn index.  Figure 8-22  shows a picture of a  turn coordinator. There are two marks on each side (left and  right) of the face of the instrument. The first mark is used to  reference a wings level zero rate of turn. The second mark  on the left and right side of the instrument serve to indicate  a standard rate of turn. A standard-rate turn is defined as a  turn rate of 3° per second. The turn coordinator indicates only  the rate and direction of turn; it does not display a specific  angle of bank. Turn Coordinator 8 6 4 2 0 I0 SUCTION INCHES MERCURT 33 30 24 2I I5 I2 6 3 20 20 I0 I0 I0 I0 20 20 TEST STBY PWR Vacuum relief valve Vacuum air filter Attitude indicator Vacuum pump Heading indicator Overboard vent line Suction gauge Turn coordinator Turn-and-slip indicator Gyro rotation Standard rate turn index Horizontal gyro Inclinometer Inclinometer Gimbal rotation Standard rate turn index Gyro rotation Gimbal rotation Canted gyro Gimbal 8-17