of lead varies since the angle of bank for a standard rate turn  varies with the true airspeed. Practice with a lead of one-half the angle of bank until  the precise lead a given technique requires is determined.  If rates of roll-in and roll-out are consistent, the precise  amount of lead suitable to a particular roll-out technique  can be determined.  A timed turn is a turn in which the clock and the turn  coordinator are used to change heading by a specific number  of degrees in a given time. For example, in a standard rate turn  (3 degrees per second), an airplane turns 45° in 15 seconds; in  a half standard rate turn, the airplane turns 45° in 30 seconds.  Prior to performing timed turns, the turn coordinator should  be calibrated to determine the accuracy of its indications.  [Figure 7-34]  Establish a standard rate turn as indicated by  the turn coordinator, and as the sweep-second hand of the  clock passes a cardinal point (12, 3, 6, 9), check the heading  on the heading indicator. While holding the indicated rate  of turn constant, note the indicated heading changes at 10  second intervals. If the airplane turns more than or less than  30° in that interval, a respectively larger or smaller deflection  of the miniature aircraft of the turn coordinator is necessary  to produce a standard rate turn. After calibrating the turn  coordinator during turns in each direction, note the corrected  deflections, if any, and apply them during all timed turns.  In most small airplanes, the magnetic compass is the only  direction-indicating instrument independent of other airplane  instruments and power sources. Because of its operating  characteristics, called compass errors, pilots are prone to  use it only as a reference for setting the heading indicator,  but knowledge of magnetic compass characteristics permits  full use of the instrument to turn the airplane to correct and  maintain headings. Practice timed turns with a full instrument panel and check  the heading indicator for the accuracy of turns. If the turns are  executed without the gyro heading indicator, use the magnetic  compass at the completion of the turn to check turn accuracy,  taking compass deviation errors into consideration.  The same cross-check and control technique is used in making  a timed turn that is used to execute turns to predetermined  headings, except the clock is substituted for the heading  indicator. The miniature aircraft of the turn coordinator is  primary for bank control, the altimeter is primary for pitch  control, and the ASI is primary for power control. Start the  roll-in when the clock’s second hand passes a cardinal point,  hold the turn at the calibrated standard rate indication (or  half-standard rate for small heading changes), and begin the  roll-out when the computed number of seconds has elapsed.  If the rates of roll-in and roll-out are the same, the time taken  during entry and recovery does not need to be considered in  the time computation.  Timed Turns Compass Turns 30.0  29.9  29.8  Supporting pitch and bank Supporting pitch Primary power Primary pitch Primary bank Figure 7-34.  Turn coordinator calibration. 7-21