To maintain a straight flightpath, the pilot must keep the airplane's wings level with the horizon. Any degree of bank (in coordinated flight) will result in a deviation from straight flight, and a change in the airplane's heading. If an attitude indicator is available, straight flight is simplified by merely keeping the wings of the representative airplane level with the representative or artificial horizon (Fig. 13-1). This is accomplished by applying the necessary coordinated aileron and rudder pressures. The needle of a turn indicator or the representative wings on a turn coordinator will deflect whenever the airplane is turned and will be centered or level when the airplane is in straight flight. Thus, they also can be used to maintain straight flight by applying coordinated aileron and rudder pressures as needed to keep the needle centered or the turn coordinator's airplane wings level.

   Regardless of which of these instruments is being used, the heading indicator should be checked frequently to determine whether a straight flightpath is actually being maintained. This is particularly true when flying in turbulent air since every little gust may bank the airplane and make it turn.

   Using only the magnetic compass as a reference for maintaining a straight course is difficult but is possible. Therefore, before using the compass, the pilot must learn its characteristics.

   When turning from a northerly heading, the compass will lag behind the actual heading of the airplane and in fact, indicate a turn in the opposite direction. The only general compass heading that accurately indicates when and in which direction a turn is started, is southerly. While the magnetic compass may be used to maintain a direction and detect the start of a turn on any heading from approximately 120 degrees to 240 degrees, the pilot will find it easier to detect a turn on a heading of south. On this heading when a turn is inadvertently started, the magnetic compass exaggerates it and correctly shows in which direction the airplane is turning. With practice, even in turbulence, the pilot can hold a reasonably accurate heading in the southern portion of the compass card. Consequently, relatively large movements of the compass should not be regarded with alarm. The compass should be allowed to oscillate if turbulence makes it unavoidable. Smooth and alert control action will result in a surprisingly small actual deviation from desired heading.

   At the same time that straight flight is being maintained, the pilot must also control the pitch attitude to keep the airplane level - that is, no gain or loss of altitude. This can be accomplished by reference to several instruments.

   The first is the altimeter, which tells when a constant altitude is being maintained or if the flight altitude is changing. A vertical speed indicator, when available, will indicate the rate at which altitude is changing. Either of these instruments, therefore, shows the pilot whether a change in pitch attitude is needed and approximately how much (Fig. 13-1).

   Just as when flying by reference to outside visual references, a change in altitude when level flight is desired, requires that the airplane's nose be raised or lowered in relation to the horizon. This can be done most easily with reference to the attitude indicator, by applying elevator pressure to adjust the representative airplane in relation to the horizon bar. The application of elevator pressure should be very slight to prevent overcontrolling. It must be emphasized that turn coordinators provide NO PITCH INFORMATION even though they have an appearance similar to attitude indicators.

   In lieu of an attitude indicator, the vertical speed indicator may be used. However, if the instrument shows a climb or descent, the pilot should apply only sufficient elevator pressure to start the pointer moving toward the zero indication, since there is a certain amount of lag in the indication. Trying to obtain an immediate zero indication usually results in overcontrolling. When the pointer stabilizes again, additional pressure, if needed, can then be added in increments to get a zero indication and gradually stop the climb or descent. Only after the vertical speed is zero and the altimeter remains constant should an attempt be made to return to the original altitude.

   In the case of an airplane having neither attitude indictor nor vertical speed indicator, the airspeed indicator can be used much like the vertical speed indicator to maintain level flight. Remember though that it, too, lags somewhat as a result of the time required for the airplane to accelerate and decelerate after a pitch change is made.

   Pilots must be cautioned not to "chase" the pointers on the instruments when flight through turbulent air produces erratic movements.