p ressure. As the needle returns to zero, relax the corrective  p ressure. If control pressures have been smooth and light, the  n eedle reacts immediately and slowly, and the altimeter shows  little or no change of altitude. As a rate instrument, the VSI  requires consideration of lag characteristics.  Lag refers to the delay involved before the needle attains a  stable indication following a pitch change. Lag is directly  proportional to the speed and magnitude of a pitch change.  If a slow, smooth pitch change is initiated, the needle moves  w ith minimum lag to a point of deflection corresponding  to the extent of the pitch change, and then stabilizes as the  aerodynamic forces are balanced in the climb or descent.  A large and abrupt pitch change produces erratic needle  movement, a reverse indication, and introduces greater time  delay (lag) before the needle stabilizes. Pilots are cautioned  not to chase the needle when flight through turbulent  conditions produces erratic needle movements. The apparent  lag in airspeed indications with pitch changes varies greatly  among different airplanes and is due to the time required for  the airplane to accelerate or decelerate when the pitch attitude  is changed. There is no appreciable lag due to the construction  or operation of the instrument. Small pitch changes, smoothly  executed, result in an immediate change of airspeed. When using the VSI as a rate instrument and combining it  with the altimeter and attitude indicator to maintain level  flight, a pilot should know that the amount the altimeter  needle moves from the desired altitude governs the rate that  should be used to return to that altitude. A rule of thumb is to  make an attitude change that results in a vertical-speed rate  approximately double the error in altitude. For example, if  altitude is off by 100 feet, the rate of return to the desired  altitude should be approximately 200 feet per minute (fpm).  If it is off by more than 100 feet, the correction should  be correspondingly greater, but should never exceed the  optimum rate of climb or descent for the airplane at a given  airspeed and configuration.  A deviation of more than 200 fpm from the desired rate  of return is considered overcontrolling. For example, if  attempting to change altitude by 200 feet, a rate in excess of  400 fpm indicates overcontrolling.  When returning to an altitude, the VSI is the primary pitch  instrument. Occasionally, the VSI is slightly out of calibration  and may indicate a climb or descent when the airplane is in  level flight. If the instrument cannot be adjusted, take the  error into consideration when using it for pitch control. For  30.0  29.9  29.8  30.0  29.9  29.8  30.0  29.9  29.8  30.0  29.9  29.8  Figure 7-9.  Altitude error, less than 100 feet. Figure 5-9. Altitude error, less than 100 feet.  Figure 7-10. Pitch correction, less than 100 feet—one-half bar low  to correct altitude error. Figure 7-12. Pitch correction, greater than 100 feet—one bar  correction initially. 7-5 Figure 7-11.  Altitude error, greater than 100 feet. Figure 5-11. Altitude error, greater than 100 feet. 