At constant power, any deviation from level flight (except  in turbulent air) must be the result of a pitch change. If the  power is constant, the altimeter gives an indirect indication  of the pitch attitude in level flight. Since the altitude should  remain constant when the airplane is in level flight, any  deviation from the desired altitude signals the need for a  pitch change. For example, if the aircraft is gaining altitude,  the nose must be lowered. In the PFD, as the pitch starts to change, the altitude trend  indicator on the altitude tape begins to show a change in  the direction of displacement. The rate at which the trend  indicator grows and the altimeter numbers change aids the  pilot in determining how much of a pitch change is necessary  to stop the trend.  As a pilot becomes familiar with a specific aircraft’s  instruments, he or she learns to correlate pitch changes,  altimeter tapes, and altitude trend indicators. By adding the  altitude tape display and the altitude trend indicator into the  scan along with the attitude indicator, a pilot starts to develop  the instrument cross-check. One important skill to practice is partial panel flight by  referencing the altimeter as the primary pitch indicator.  Practice controlling the pitch by referencing the altitude  tape and trend indicator alone without the use of the attitude  indicator. Pilots need to learn to make corrections to altitude  deviations by referencing the rate of change of the altitude  tape and trend indicator. When operating in IMC and in a  partial panel configuration, the pilot should avoid abrupt  changes to the control yoke. Reacting abruptly to altitude  changes can lead to large pitch changes and thus a larger  divergence from the initial altitude.  When a pilot is controlling pitch by the altitude tape and  altitude trend indicators alone, it is possible to overcontrol  the aircraft by making a larger than necessary pitch  correction. Overcontrolling causes the pilot to move from  a nose-high attitude to a nose-low attitude and vice versa.  Small changes to pitch are required to insure prompt  corrective actions are taken to return the aircraft to its  original altitude with less confusion. When an altitude deviation occurs, two actions need to be  accomplished. First, make a smooth control input to stop  the needle movement. Once the altitude tape has stopped  moving, make a change to the pitch attitude to start back to  the entry altitude. The VSI tape should be used to assist in determining what  pitch changes are necessary to return to the desired altitude.  A good rule of thumb is to use a vertical speed rate of change  that is double the altitude deviation. However, at no time  should the rate of change be more than the optimum rate of  climb or descent for the specific aircraft being flown. For  example, if the altitude is off by 200 feet from the desired  altitude, then a 400 feet per minute (fpm) rate of change  would be sufficient to get the aircraft back to the original  Unlike older analog VSIs, new glass panel displays have  instantaneous VSIs. Older units had a lag designed into the  system that was utilized to indicate rate information. The  new glass panel displays utilize a digital air data computer  that does not indicate a lag. Altitude changes are shown  immediately and can be corrected for quickly. The VSI tape provides for an indirect indication of pitch  attitude and gives the pilot a more immediate indication of a  pending altitude deviation. In addition to trend information,  the vertical speed also gives a rate indication. By using the  VSI tape in conjunction with the altitude trend tape, a pilot has  a better understanding of how much of a correction needs to  be made. With practice, the pilot will learn the performance  of a particular aircraft and know how much pitch change  is required in order to correct for a specific rate indication. Instrumentation needs to be utilized collectively, but failures  will occur that leave the pilot with only limited instrumentation.  That is why partial panel flying training is important. If the  pilot understands how to utilize each instrument independently,  no significant change is encountered in carrying out the flight  when other instruments fail. As a general rule of thumb, for altitude deviations less than  100 feet, utilize a pitch change of 1 degree, which equates to  1 ⁄5 of the thickness of the chevron. Small incremental pitch  changes allow the performance to be evaluated and eliminate  overcontrolling of the aircraft. During instrument flight with limited instrumentation, it is  imperative that only small and precise control inputs are  made. Once a needle movement is indicated denoting a  deviation in altitude, the pilot needs to make small control  inputs to stop the deviation. Rapid control movements only  compound the deviation by causing an oscillation effect.  This type of oscillation can quickly cause the pilot to become  disoriented and begin to fixate on the altitude. Fixation on  the altimeter can lead to a loss of directional control as well  as airspeed control. Altimeter Partial Panel Flight VSI Tape 7-36