It is impossible to emphasize too strongly the necessity for forming
correct habits in flying straight and level. All other flight
maneuvers are in essence a deviation from this fundamental flight
maneuver.

http://w3n-a.blogspot.com/2008/11/st...el-flight.html

Many flight instructors and students are prone to believe that
perfection in straight-and-level flight will come of itself, but such
is not the case. It is not uncommon to find a pilot whose basic flying
ability consistently falls just short of minimum expected standards,
and upon analyzing the reasons for the shortcomings to discover that
the cause is the inability to fly straight and level properly.

Straight-and-level flight is flight in which a constant heading and
altitude are maintained. It is accomplished by making immediate and
measured corrections for deviations in direction and altitude from
unintentional slight turns, descents, and climbs. Level flight, at
first, is a matter of consciously fixing the relationship of the
position of some portion of the airplane, used as a reference point,
with the horizon. In establishing the reference points, the instructor
should place the airplane in the desired position and aid the student
in selecting reference points. The instructor should be aware that no
two pilots see this relationship exactly the same. The references will
depend on where the pilot is sitting, the pilot's height (whether
short or tall), and the pilot's manner of sitting. It is, therefore,
important that during the fixing of this relationship, the pilot sit
in a normal manner; otherwise the points will not be the same when the
normal position is resumed.

In learning to control the airplane in level flight, it is important
that the student be taught to maintain a light grip on the flight
controls, and that the control forces desired be exerted lightly and
just enough to produce the desired result. The student should learn to
associate the apparent movement of the references with the forces
which produce it. In this way, the student can develop the ability to
regulate the change desired in the airplane's attitude by the amount
and direction of forces applied to the controls without the necessity
of referring to instrument or outside references for each minor
correction.

The pitch attitude for level flight (constant altitude) is usually
obtained by selecting some portion of the airplane's nose as a
reference point, and then keeping that point in a fixed position
relative to the horizon. Using the principles of attitude flying,
that position should be cross-checked occasionally against the
altimeter to determine whether or not the pitch attitude is correct.
If altitude is being gained or lost, the pitch attitude should be
readjusted in relation to the horizon and then the altimeter rechecked
to determine if altitude is now being maintained. The application of
forward or back-elevator pressure is used to control this attitude.

The pitch information obtained from the attitude indicator also will
show the position of the nose relative to the horizon and will
indicate whether elevator pressure is necessary to change the pitch
attitude to return to level flight. However, the primary reference
source is the natural horizon.

In all normal maneuvers, the term "increase the pitch attitude"
implies raising the nose in relation to the horizon; the term
"decreasing the pitch attitude" means lowering the nose.

Straight flight (laterally level flight) is accomplished by visually
checking the relationship of the airplane's wingtips with the horizon.
Both wingtips should be equidistant above or below the horizon
(depending on whether the airplane is a high-wing or low-wing type),
and any necessary adjustments should be made with the ailerons, noting
the relationship of control pressure and the airplane's attitude. The
student should understand that anytime the wings are banked, even
though very slightly, the airplane will turn. The objective of
straight-and-level flight is to detect small deviations from laterally
level flight as soon as they occur, necessitating only small
corrections. Reference to the heading indicator should be made to note
any change in direction.

Continually observing the wingtips has advantages other than being the
only positive check for leveling the wings. It also helps divert the
pilot's attention from the airplane's nose, prevents a fixed stare,
and automatically expands the pilot's area of vision by increasing the
range necessary for the pilot's vision to cover. In practicing
straight-and-level-flight, the wingtips can be used not only for
establishing the airplane's laterally level attitude or bank, but to a
lesser degree, its pitch attitude. This is noted only for assistance
in learning straight-andlevel flight, and is not a recommended
practice in normal operations.

The scope of a student's vision is also very important, for if it is
obscured the student will tend to look out to one side continually
(usually the left) and consequently lean that way. This not only gives
the student a biased angle from which to judge, but also causes the
student to exert unconscious pressure on the controls in that
direction, which results in dragging a wing.

With the wings approximately level, it is possible to maintain
straight flight by simply exerting the necessary forces on the rudder
in the desired direction. However, the instructor should point out
that the practice of using rudder alone is not correct and may make
precise control of the airplane difficult. Straight–and-level flight
requires almost no application of control pressures if the airplane is
properly trimmed and the air is smooth. For that reason, the student
must not form the habit of constantly moving the controls
unnecessarily. The student must learn to recognize when corrections
are necessary, and then to make a measured response easily and
naturally.

To obtain the proper conception of the forces required on the rudder
during straight-and-levelflight, the airplane must be held level. One
of the most common faults of beginning students is the tendency to
concentrate on the nose of the airplane and attempting to hold the
wings level by observing the curvature of the nose cowling. With this
method, the reference line is very short and the deviation,
particularly if very slight, can go unnoticed. Also, a very small
deviation from level, by this short reference line, becomes
considerable at the wingtips and results in an appreciable dragging of
one wing. This attitude requires the use of additional rudder to
maintain straight flight, giving a false conception of neutral control
forces. The habit of dragging one wing, and compensating with rudder
pressure, if allowed to develop is particularly hard to break, and if
not corrected will result in considerable difficulty in mastering
other flight maneuvers.

For all practical purposes, the airspeed will remain constant in
straight-and-level flight with a constant power setting. Practice of
intentional airspeed changes, by increasing or decreasing the power,
will provide an excellent means of developing proficiency in
maintaining straight-and-level flight at various speeds. Significant
changes in airspeed will, of course, require considerable changes in
pitch attitude and pitch trim to maintain altitude. Pronounced changes
in pitch attitude and trim will also be necessary as the flaps and
landing gear are operated.

Common errors in the performance of straight-andlevel flight a

Attempting to use improper reference points on the airplane to
establish attitude.
Forgetting the location of preselected reference points on subsequent
flights.
Attempting to establish or correct airplane attitude using flight
instruments rather than outside visual reference.
Attempting to maintain direction using only rudder control.
Habitually flying with one wing low.
"Chasing" the flight instruments rather than adhering to the
principles of attitude flying.
Too tight a grip on the flight controls resulting in overcontrol and
lack of feel.
Pushing or pulling on the flight controls rather than exerting
pressure against the airstream.
Improper scanning and/or devoting insufficient time to outside visual
reference. (Head in the cockpit.)
Fixation on the nose (pitch attitude) reference point.
Unnecessary or inappropriate control inputs.
Failure to make timely and measured control inputs when deviations
from straight-and-level flight are detected.
Inadequate attention to sensory inputs in developing feel for the
airplane.