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FLIGHT INSTRUCTORS MANUAL 143 field and make a 90° turn for the landing lane and enter the landing area. This position is a general location in the air, and in relation to the field, from which he has made so many landings that once he reaches it, or is able to maneuver through it, he feels reasonably certain that he knows where the airplane will land form a normal glide. This position is called the "key positions." (See figs. 18 to 24.) power-off accuracy work except low-altitude forced landings, and a position through which the student attempts to maneuver in all accuracy work. This so-called "key position" must not be over emphasized or it may become the primary objective rather than the landing. The position is merely an aid, and an intermediate point, which will vary with different conditions, but which if constantly varied as the exact location and altitude, will quickly give the student a basis upon which to build an accurate conception of the results of maneuvering to correct for errors in judgment or estimate of wind. and make his final approach to the spot. The development of this concept should be begun shortly after landing practice is started before solo, although the explanation of the principles may be deferred until a broader experience is gained. When started on 90° turns for the landing area, the student should be required to close the throttle at an altitude of 400 to 500 feet in the general position shown in figure 18, and to reach the landing area as a result of a 90° turn in the glide. During the landing practice which follow, including the 180° turn in a glide, he will spend so much time gliding from and through this one general position that he will have a very good conception of this position and altitude relative to the landing area and some idea of the limits and results of the maneuvering that can be accomplished from it. The student will have found that it is somewhat difficult to judge accurately the relative loss of altitude from forward speed in a lond, straight glide from any considerable altitude and that it is much easier, once the principles are learned, to judge the flight path through a 90° turn. The straight glide approach offers the additional difficulty of poor visibility and the possibility of hitting some unseen obstruction, hidden by the winds in the straight approach, but easily seen from a side of 90° approach. However, if he lands from a turn in the glide, visibility is increased and he is better able to note the rapidity of his approach to the landing area or spot and the loss of altitude relative to the speed of the approach, and can make a more accurate estimation of the termination of his true flight path. With this information, he may increase of decrease the radius of his turn in the glide, slightly vary the angle of the glide path with relation of the ack of the field, or delay or advance the start of his final turn to the spot, so as to reach this objective with the least difficulty and at the proper time. This practice is sometimes called "playing the turn." Knowledge of this procedure, plus ability to estimate and project the true flight path through the results of anticipated change, insures accuracy in power-off work without the necessity for the use of slips or constant practice. Students should not be permitted to use slips in power-off accuracy work.