The placement of the base leg is one of the more important judgments to be made by the pilot in any landing approach. The pilot must accurately judge the altitude and distance from which a gradual descent will result in landing at the desired spot. The distance will depend on the altitude of the base leg, the effect of wind and amount of wing flaps used. When there is a strong wind on final approach or the flaps will be used to produce a steep angle of descent, the base leg must be positioned closer to the approach end of the runway than would be required with a light wind or no flaps. Normally, the landing gear should be extended and the before landing check completed prior to reaching the base leg.
After turning onto the base leg, the pilot should start the descent with reduced power and an airspeed of approximately 1.4 Vs0. (Vs0 - the stalling speed with power off, landing gear and flaps down.) For example, if Vs0 is 60 knots, the speed should be 1.4 times 60, or 84 knots. Landing flaps may be partially lowered if desired at this time. Full flaps are not recommended until the final approach is established and the landing assured. Drift correction should be established and maintained to follow a ground track perpendicular to the extension of the centerline of the runway on which the landing is to be made. Since the final approach and landing will normally be made into the wind, there will be somewhat of a crosswind during the base leg. This requires that the airplane be angled (crabbed) sufficiently into the wind to prevent drifting farther away from the intended landing spot.
The base leg should be continued to the point where a medium to shallow banked turn will align the airplane's path directly with the centerline of the landing runway. This descending turn should be completed at a safe altitude which will be dependent upon the height of the terrain and any obstructions along the ground track. The turn to the final approach should also be sufficiently above the airport elevation to permit a final approach long enough for the pilot to accurately estimate the resultant point of touchdown, while maintaining the proper approach airspeed. This will require careful planning as to the starting point and the radius of the turn. Normally, it is recommended that the angle of bank not exceed a medium bank because the steeper the angle of bank, the higher the airspeed at which the airplane stalls. Since the base to final turn is made at a relatively low altitude, it is vitally important that a stall not occur at this point. If an extremely steep bank is needed to prevent overshooting the proper final approach path, it is advisable to discontinue the approach, go around, and plan to start the turn earlier on the next approach rather than risk a hazardous situation.