As discussed in chapter 2, the maintenance of lift and control of an aircraft in slow flight requires a certain minimum airspeed and angle of attack. This critical airspeed depends on certain factors, such as gross weight, load factors, and density altitude. The minimum speed below which further controlled flight is impossible is called the stalling speed. An important feature of pilot training is the development of the ability to estimate and “feel” the margin of speed above the stalling speed. Also, the ability to determine the characteristic responses of the aircraft at different airspeeds is of great importance to the pilot. The student pilot, therefore, must develop this awareness in order to safely avoid stalls and to operate an aircraft correctly and safely at slow airspeeds.
As discussed in chapter 2, the nose stalls while the tips keep flying. Therefore, the definition of stall speed of the WSC aircraft is the speed at which the nose starts stalling. The control bar is pushed forward and buffeting is felt on the control bar as the root reaches the critical angle of attack. Separation of the laminar airflow occurs, creating turbulence that can be felt in the control bar. There is a loss of positive roll control as the nose buffets and lowers as it loses lift.
The objective of maneuvering during slow flight is to develop the pilot’s sense of feel and ability to use the controls correctly and to improve proficiency in performing maneuvers that require slow airspeeds.
Slow flight is broken down into two distinct speeds:
- VX and the short field descent speed that was discussed earlier, and,
- Minimum controlled airspeed, the slowest airspeed at which the aircraft is capable of maintaining controlled flight without indications of a stall—usually 2 to 3 knots above stalling speed as discussed below.
The minimum controlled airspeed maneuver demonstrates the flight characteristics and degree of controllability of the aircraft at its minimum flying speed. By definition, the term “flight at minimum controllable airspeed” means a speed at which any further increase in angle of attack or load factor causes an immediate stall. Instruction in flight at minimum controllable airspeed should be introduced at reduced power settings with the airspeed sufficiently above the stall to permit maneuvering, but close enough to the stall to sense the characteristics of flight at very low airspeed—sloppy control, ragged response to control inputs, difficulty maintaining altitude, etc. Maneuvering at minimum controllable airspeed should be performed using both instrument indications and outside visual reference. It is important that pilots form the habit of frequent reference to the flight instruments, especially the airspeed indicator, while flying at very low airspeeds. However, the goal is to develop a “feel” for the aircraft at very low airspeeds to avoid inadvertent stalls and to operate the aircraft with precision.
The objective of performing the minimum controlled airspeed is to fly straight and level and make shallow level turns at minimum controlled airspeed. To begin a minimum controlled airspeed maneuver, the WSC is flown at trim speed straight and level to maintain a constant altitude. The nose is then raised as the throttle is reduced to maintain a constant altitude.
As the speed decreases further, the pilot should note the feel of the flight controls, pitch pressure, and difficulty of maintaining a straight heading with the increased side-to-side pilot input forces required to keep the wings level. At some point the throttle must be increased to remain level after the WSC has slowed below it’s maximum LD speed. The pilot should also note the sound of the airflow as it falls off in tone. There is a large difference by manufacturer and model, but the bar generally should not be touching the forward tube at minimum controlled airspeed. For example, the control bar would be 1 to 3 inches from the front tube at minimum controlled airspeed. [Figure 6-21]
The pilot should understand that when flying below the minimum drag speed (L/DMAX), the aircraft exhibits a characteristic known as “speed instability.” If the aircraft is disturbed by even the slightest turbulence, the airspeed decreases. As airspeed decreases, the total drag increases resulting in a further loss in airspeed. Unless more power is applied and/or the nose is lowered, the speed continues to decay to a stall. This is an extremely important factor in the performance of slow flight. The pilot must understand that, at speeds less than minimum drag speed, the airspeed is unstable and will continue to decay if allowed to do so.
It should also be noted that the amount of power to remain level at minimum controlled airspeed is greater than that required at the minimum drag speed which is also the best glide ratio speed and the best rate of climb speed.
When the attitude, airspeed, and power have been stabilized in straight-and-level flight, turns should be practiced to determine the aircraft’s controllability characteristics at this minimum speed. During the turns, power and pitch attitude may need to be increased to maintain the airspeed and altitude. The objective is to acquaint the pilot with the lack of maneuverability at minimum controlled airspeed, the danger of incipient stalls, and the tendency of the aircraft to stall as the bank is increased. A stall may also occur as a result of turbulence, or abrupt or rough control movements when flying at this critical airspeed.
Once flight at minimum controllable airspeed is set up properly for level flight, a descent or climb at minimum controllable airspeed can be established by adjusting the power as necessary to establish the desired rate of descent or climb.
Common errors in the performance of slow flight are:
- Failure to adequately clear the area.
- Inadequate forward pressure as power is reduced, resulting in altitude loss.
- Excessive forward pressure as power is reduced, resulting in a climb, followed by a rapid reduction in airspeed and “mushing.”
- Inadequate compensation for unanticipated roll during turns.
- Fixation on the airspeed indicator.
- Inadequate power management.
- Inability to adequately divide attention between aircraft control and orientation.