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150 U.S. CIVIL AERONAUTICS AUTHORITY execution of a 180º, and the same procedure followed in arriving at the key position as is done during the 180º. The student estimates his heading according to the speed of the airplane and the velocity of the wind, so that his actual track over the ground more or less follows the pattern shown in figure 23. The same factors govern the conservation or dissipation of altitude as in the 180º. the 360º is merely another extension of the familiar 90º turn for a landing, with the addition of a different altitude, a new position, and a new heading from which to maneuver to the key position. Varying wind conditions and altitudes will vary the location and the necessary altitude at this key position and further develop judgement in arriving at the proper place to execute the final approach. This final approach should clear any obstacle at the boundary of the field by at least 50 feet. During the final approach the ship should be in a straight glide with the wings level. The 360º will also serve to fix in the student's mind another altitude and the amount of maneuvering practical and possible from it to safely effect a landing at the desired spot. Both 360º's and 180º's should be practiced constantly throughout the remainder of the pilot's training, until technique and judgement are as near perfection as it is possible to attain. However, the instructor must repeat and stress the point that the maneuvers in themselves are not as important as the principles they teach and the technique and judgement they perfect, which may be used in many other practical applications. SPIRALS FOR A LANDING Spirals for a landing are the next extension after the 360º's. In executing this maneuver the throttle is closed at an altitude of 2,000 feet or more, either immediately above the spot or in its vicinity. Spiral gliding turns are then made, and the spot is kept in the center of the turn by shallowing the bank on the upwind side and steepening it on the downwind side. The spiral is continued until by widening the last turn the familiar "key position" can be reached and the regular 90º turn for a landing executed. As usual, constant, normal, gliding speed must be maintained throughout the maneuver and slipping or skidding prohibited. The number of turns to be made, or that can be made, with a given airplane, will depend on the steepness of the initial bank or the radius of the turn around the spot, the wind velocity, and the initial altitude. This maneuver requires considerable judgement of the true glide path through an accurate extension of it if the arrival at the proper location for the key position is to be made at the proper altitude. Failure in this causes more trouble than any other part of the maneuver. This makes the spiral an excellent maneuver for further developing such ability, and in addition adds still a different altitude and position from which to maneuver to the same familiar position and execute the 90º turn for a landing. It also brings about a further development of orientation over the 180º's and 360º's.