power changes to accelerate the rate of airspeed change. (For  small speed changes, or in airplanes that decelerate or accelerate  rapidly, overpowering or underpowering is not necessary.)  Consider the example of an airplane that requires 23 inches  of mercury ("Hg) to maintain a normal cruising airspeed of  120 knots, and 18 "Hg to maintain an airspeed of 100 knots.  The reduction in airspeed from 120 knots to 100 knots while  maintaining straight-and-level flight is discussed below and  illustrated in  Figures 7-57, 7-58,  and  7-59 .  Instrument indications, prior to the power reduction, are  shown in  Figure 7-57 . The basic attitude is established and  maintained on the attitude indicator. The specific pitch,  bank, and power control requirements are detected on these  primary instruments:  Altimeter—Primary Pitch  Heading Indicator—Primary Bank  Airspeed Indicator—Primary Power  Supporting pitch and bank instruments are shown in  Figure 7-57 . Note that the supporting power instrument is  the manifold pressure gauge (or tachometer if the propeller  is fixed pitch). However, when a smooth power reduction to  approximately 15 "Hg (underpower) is made, the manifold  pressure gauge becomes the primary power instrument.  [Figure 7-58]  With practice, power setting can be changed  with only a brief glance at the power instrument, by sensing  the movement of the throttle, the change in sound, and the  changes in the feel of control pressures.  As the thrust decreases, increase the speed of the cross-check  and be ready to apply left rudder, back-elevator, and aileron  control pressure the instant the pitch and bank instruments  show a deviation from altitude and heading. As proficiency  is obtained, a pilot will learn to cross-check, interpret, and  control the changes with no deviation of heading and altitude.  Assuming smooth air and ideal control technique, as airspeed  decreases, a proportionate increase in airplane pitch attitude  is required to maintain altitude. Similarly, effective torque  control means counteracting yaw with rudder pressure.  As the power is reduced, the altimeter is primary for  pitch, the heading indicator is primary for bank, and the  manifold pressure gauge is momentarily primary for power  (at 15 "Hg in  Figure 7-58 ). Control pressures should be  trimmed off as the airplane decelerates. As the airspeed  approaches the desired airspeed of 100 knots, the manifold  pressure is adjusted to approximately 18 "Hg and becomes  the supporting power instrument. The ASI again becomes  primary for power.  [Figure 7-59] Practice of airspeed changes in straight-and-level flight  provides an excellent means of developing increased  proficiency in all three basic instrument skills and brings  out some common errors to be expected during training  in straight-and-level flight. Having learned to control the  airplane in a clean configuration (minimum drag conditions),  increase proficiency in cross-check and control by practicing  speed changes while extending or retracting the flaps and  landing gear. While practicing, be sure to comply with the  Airspeed Changes in Straight-and-Level Flight INSET  PFD  CDI  XPDR  IDENT  TMR/REF  NRST  ALERTS  5 338  $_{1}$1652  200  46  13.7  2300  23.0  VOR 1  270 °  2  1  1  2  4 300  4 200  4 100  4 000  3 900  3 800  4300  3600  3500  3400  3300  3200  3100  60 20 000 000  150  140 130 110 100 90  1 120 9 TAS 106KT  OAT 7°C  134.000 118.000 COM1 123.800 118.000 COM2 $_{WPT }$_ _ _ _ _ _$_{ DIS }$_ _ ._ $_{NM}$ $_{DTK }$_ _ _$_{°}$ TRK 360° Supporting pitch and bank Primary power Supporting pitch Supporting power Primary bank Primary pitch Supporting bank Figure 7-57.  Straight-and-level flight (normal cruising speed). 7-40