AFH 5-10

When flying above the minimum drag speed (L/DMAX), more power is required to fly even faster. When flying at speeds below L/DMAX, more power is required to fly even slower. Since slow flight will be performed well below L/DMAX, the pilot should be aware that large power inputs or a reduction in AOA will be required to prevent the aircraft from decelerating. It is important to note that when flying below L/DMAX or on the backside of the power curve, as the AOA increases toward the critical AOA and the airplane’s speed continues to decrease, small changes in the pitch control result in disproportionally large changes in induced drag and therefore changes in airspeed. As a result, pitch becomes a more effective control of airspeed when flying below L/DMAX and power is an effective control of the path. It is also important to note that an airplane flying below L/DMAX, exhibits a characteristic known as “speed instability” and the airspeed will continue to decay without appropriate pilot action. For example, if the airplane is disturbed by turbulence and the airspeed decreases, the airspeed may continue to decrease without the appropriate pilot action of reducing the AOA or adding power. [Figure 5-6] Figure 5-6. Angle-of-attack in degrees. Performing the Slow Flight Maneuver Slow flight training includes: ⦁ Slowing the airplane smoothly and promptly from cruising to approach speeds without changes in altitude or heading, while increasing the angle of attack and setting the required power and trim. ⦁ Configuration changes, such as extending the landing gear and adding flaps, while maintaining heading and altitude. ⦁ Turning while maintaining altitude. ⦁ Straight-ahead climbs and climbing medium-banked (approximately 20 degrees) turns, and straight-ahead power-off gliding descents and descending turns, which represent the takeoff and landing phases of flight. Slow flight in a single-engine airplane should be conducted so the maneuver can be completed no lower than 1,500 feet AGL (3,000 for multiengine airplanes), or higher, if recommended by the manufacturer. In all cases, practicing slow flight should be conducted at an adequate height above the ground for recovery should the airplane inadvertently stall.