It would be impossible to discuss all the many airplane design and flap combinations. This emphasizes the importance of the Federal Aviation Administration (FAA) approved Airplane Flight Manual and/or Pilot’s Operating Handbook (AFM/ POH) for a given airplane. While some AFM/POHs are specific as to operational use of flaps, others leave the use of flaps to pilot discretion. Hence, flap operation makes pilot judgment of critical importance. Since 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, the pilot must relate the manufacturer’s recommendation to aerodynamic effects of flaps. This requires basic background knowledge of flap aerodynamics and geometry. With this information, a decision as to the degree of flap deflection and time of deflection based on runway and approach conditions relative to the wind conditions can be made.
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.
While all landings should be accomplished at the slowest speed possible for a given situation, a soft or short-field landing requires minimal speed at touchdown while a short field obstacle approach requires minimum speed and a steep approach angle. Flap extension, particularly beyond 30°, results in significant levels of drag. As such, large angles of flap deployment require higher power settings than used with partial flaps. When steep approach angles and short fields combine with power to offset the drag produced by the flaps, the landing flare 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 can result in a hard landing and damage or loss of control.
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. With flaps extended in a crosswind, the wing on the upwind side is more affected than the downwind wing. The effect is reduced to a slight extent in the crabbed approach since the airplane is more nearly aligned with the wind. When using a wing-low approach, the lowered wing partially blocks the upwind flap. 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.
With flaps extended, the crosswind effects on the 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. Since the flap is located behind the main landing gear when the wind strikes the deflected flap and fuselage side, the upwind wing tends to rise and the airplane tends to turn into the wind. Proper control position is essential for maintaining runway alignment. Depending on the amount of crosswind, it may be necessary to retract the flaps soon after touchdown in order to maintain control of the airplane.
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 nose down 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 wing. This produces additional lift causing significant changes in pitch. 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; however, the pilot must be prepared for rapid changes in pitch forces as the result of trim and the increase in airflow over the control surfaces. [Figure 11-5]
The degree of flap deflection combined with design configuration of the horizontal tail relative to the wing require carefully monitoring of pitch and airspeed, carefully control flap retraction to minimize altitude loss, and properly use the rudder for coordination. Considering these factors, it is good practice to extend the same degree of flap deflection at the same point in the landing pattern for each landing. This requires that a consistent traffic pattern be used. This allows for 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 contains the manufacturer’s recommendations for some landing situations. On the other hand, AFM/POH information on flap usage for takeoff is more 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.