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FLIGHT INSTRUCTION MANUAL 151 An altitude of 2,000 feet has been arbitrarily set as the altitude for spirals during flight tests for pilot certificates of competency, but the student should be required to practice them from various altitudes in order to obtain the maximum of benefit to his judgement and accuracy. FORWARD SLIPS The forward slip is one of three types of slips normally used by pilots but in its true from is probably the least used of the three, as the average pilot prefers a shallow side slip or a slipping turn. All slips have definite uses and many circumstances requiring their use. However, their use at any other time is a confession of error in technique or judgement. They are indicated where it is necessary to land short over obstructions, or in a forced landing, in which it is wise to allow an extra margin of altitude for safety in making the original estimate of the approach. When the accuracy of this original estimate has been confirmed by the arrival at the boundary of the field selected with some excess altitude, this is then dissipated by slipping. The primary object of all three types is the dissipation of altitude without increase in speed. The forward slip is usually the least violent of the three, although the loss of altitude during a slipping turn may be made very slight. To properly execute a "forward slip," the wing on the side to which the slip is to be made is lowered 20° to 30° and the fuselage swung, by the use of opposite rudder, to an angle of approximately 45° to the flight path. The flight path is maintained in the same straight line over the ground, the same forward speed is maintained, and the nose is raised very slightly. This results in increased loss of altitude per unit of forward distance covered, and is accomplished mainly through the braking action of the side of the fuselage. The nose must be held down against the action of the rudder tending to raise it, but it is allowed to stay above the position for the normal glide just enough to keep the speed down to that of the normal glide. Any increase in speed will nullify the effects of the slip. It is a common tendency of all students to try to slip with the nose low, resulting in a great increase in speed. Constant speed must be maintained both during the slip and the recovery. This requires carefully coordinated control action during recovery. If the nose is allowed to swing free during recovery by sudden release of the rudder, the result will be a very abrupt and sharp increase in speed, almost as much as would have been gained by holding the nose too low during the slip. The bank must be recovered and the rudder released while the drop of the nose is controlled by the elevators to the extent that the speed remains constant. After proper explanation and demonstration, this technique will be easily grasped by the student. He should be shown the effects of improper recovery, and the effects of the resulting increase of speed on any attempts at accuracy.