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 138    U. S. CIVIL AERONAUTICS AUTHORITY

result of the increasing speed. The reaction of the ship is similar to that during a snap roll. (See p. 194.)
Since this is not necessarily a maximum performance maneuver, a speed of from 10 to 15 miles per hour above the normal glide may be used, but it should not be more. 
Particular attention must be given to the recoveries made by the student. Smoothness must be attained and the controls must be so coordinated that no increase or decrease of speed results when the straight glide is resumed. Considerable practice will be required by most students before this can be accomplished consistently from a 70* spiral held through six or more complete turns.
Toward the end of the practice, the student should be required to make precision recoveries toward an object or point, then he should be given the point before starting the maneuver and instructed to execute a specified number of turns and come out on the point. The greater the number of turns the more difficult it will be for the average student to retain his orientation. 
Such practice will be of great value as a preliminary to the perfection of accuracy landings and advanced maneuvers requiring a high degree of orientation, such as lazy eights and chandelles.
ACCURACY LANDINGS
The following discussion of power-off accuracy work in general is given to assist the instructor in better understanding the difficulties faced by the student and to bring to his mind some of the features that may have been overlooked or forgotten, particularly those factors involved in the estimation of altitude, the distance the airplane will glide under varying wind conditions, and the effects of maneuvering on the gliding distance.
Concepts and perceptions are based on experience, but this experience must be directed and its highlights and important features called to the student's attention and emphasized if the maximum of progress is to be made with the minimum of experience.
The technique of the student is a matter of instruction and practice over which the instructor exercises full control. The estimation of altitude and gliding distance is a matter of accurate estimation and association of the factors effecting them. The ability to develop proficiency in correlating these is more of a personal attribute of the student, dependent on his powers of observations and the ability to make practical use of such observations. The instructor can only call the student's attention to the various factors and explain the methods of association that may be used to arrive at accurate results. The ability to make use of them depends on the aptitude of the student.
  Few persons realize the extent to which the apparent size of an object, the actual size of which is known, controls estimation of distance. This is the result of comparing its apparent size with a mental picture of its known actual size and obtaining a concept of distance as a result, The accuracy with which this is done depends on the experience and training of the individual. Comparison with objects in between, whose distance is known or can be more accurately estimated, often plays an important part. In such cases,