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Flying Handbook Menu > Transition to Complex Airplanes > Wing Flaps > Operational Procedures
It would be impossible to discuss all the many
airplane design and flap combinations. This emphasizes the importance
of the FAA-approved Airplane Flight Manual and/or Pilot’s
Operating Handbook (AFM/POH) for a given airplane. However,
while some AFM/POHs are specific as to operational use of flaps,
many are lacking. Hence, flap operation makes pilot judgment
of critical importance. In addition, flap operation is used
for landings and takeoffs, during which the airplane is in close
proximity to the ground where the margin for error is small.
Since the recommendations given in the AFM/POH
are based on the airplane and the flap design combination,the
pilot must relate the manufacturer’s recommendation to
aerodynamic effects of flaps. This requires that the pilot have
a basic background knowledge of flap aerodynamics and geometry.
With this information, the pilot must make a decision as to
the degree of flap deflection and time of deflection based on
runway and approach conditions relative to the wind conditions.
The time of flap extension and degree of deflection
are related. Large flap deflections at one single point in the
landing pattern produce large lift changes that require significant
pitch and power changes in order to maintain airspeed and glide
slope. Incremental deflection of flaps on downwind, base, and
final approach allow smaller adjustment of pitch and power compared
to extension of full flaps all at one time. This procedure facilitates
a more stabilized approach.
Asoft- or short-field landing requires minimal
speed at touchdown. The flap deflection that results in minimal
groundspeed, therefore, should be used. If obstacle clearance
is a factor, the flap deflection that results in the steepest
angle of approach should be used. It should be noted, however,
that the flap setting that gives the minimal speed at touchdown
does not necessarily give the steepest angle of approach; however,
maximum flap extension gives the steepest angle of approach
and minimum speed at touchdown. Maximum flap extension, particularly
beyond 30 to 35°, results in a large amount of drag. This
requires higher power settings than used with partial flaps.
Because of the steep approach angle combined with power to offset
drag, the flare with full flaps becomes critical. The drag produces
a high sink rate that must be controlled with power, yet failure
to reduce power at a rate so that the power is idle at touchdown
allows the airplane to float down the runway. A reduction in
power too early results in a hard landing.
Crosswind component is another factor to be
considered in the degree of flap extension. The deflected flap
presents a surface area for the wind to act on. In a crosswind,
the “flapped” wing on the upwind side is more affected
than the downwind wing. This is, however, eliminated to a slight
extent in the crabbed approach since the airplane is morenearly
aligned with the wind. When using a wing low approach, however,
the lowered wing partially blankets the upwind flap, but the
dihedral of the wing combined with the flap and wind make lateral
control more difficult. Lateral control becomes more difficult
as flap extension reaches maximum and the crosswind becomes
perpendicular to the runway.
Crosswind effects on the “flapped”
wing become more pronounced as the airplane comes closer to
the ground. The wing, flap, and ground form a “container”
that is filled with air by the crosswind. With the wind striking
the deflected flap and fuselage side and with the flap located
behind the main gear, the upwind wing will tend to rise and
the airplane will tend to turn into the wind. Proper control
position, therefore, is essential for maintaining runway alignment.
Also, it may be necessary to retract the flaps upon positive
ground contact.
The go-around is another factor to consider
when making a decision about degree of flap deflection and about
where in the landing pattern to extend flaps. Because of the
nosedown pitching moment produced with flap extension, trim
is used to offset this pitching moment. Application of full
power in the go-around increases the airflow over the “flapped”
wing. This produces additional lift causing the nose to pitch
up. The pitch-up tendency does not diminish completely with
flap retraction because of the trim setting. Expedient retraction
of flaps is desirable to eliminate drag, thereby allowing rapid
increase in airspeed; however, flap retraction also decreases
lift so that the airplane sinks rapidly.
The degree of flap deflection combined with
design configuration of the horizontal tail relative to the
wing requires that the pilot carefully monitor pitch and airspeed,
carefully control flap retraction to minimize altitude loss,
and properly use the rudder for coordination. Considering these
factors, the pilot should extend the same degree of deflection
at the same point in the landing pattern. This requires that
a consistent traffic pattern be used. Therefore, the pilot can
have a preplanned go-around sequence based on the airplane’s
position in the landing pattern.
There is no single formula to determine the
degree of flap deflection to be used on landing, because a landing
involves variables that are dependent on each other. The AFM/POH
for the particular airplane will contain the manufacturer’s
recommendations for some landing situations. On the other hand,
AFM/POH information on flap usage for takeoff ismore precise.
The manufacturer’s requirements are based on the climb
performance produced by a given flap design. Under no circumstances
should a flap setting given in the AFM/POH be exceeded for takeoff.
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