Advanced Pilot Training - US AAF TM 1-205: Air Navigation - Section I General

1 Purpose and Scope.-The purpose of this manual is to describe and illustrate the various methods of air navigation used by pilots of the Army Air Corps. Pilotage, elementary dead reckoning, and the use of the Federal aids to air navigation will be covered in detail in this chapter, so that this manual may be used as a text for students undergoing flying instruction.

2. Definition and development of air navigation.-

a. Air navigation is the art of determining the geographical position and maintaining the desired direction of an aircraft, relative to the earth's surface, by means of pilotage, dead reckoning, radio aids, or celestial observations. It is not an exact science.

b. The basic principles of air navigation are very similar to those used centuries ago by mariners. Present day navigators depend upon the magnetic compass for direction as did Columbus when he sailed the Atlantic. Gradually, through the centuries, better instruments have been developed, and methods have been refined. When airplanes were developed to the point where they could fly for short distances on cross-country flights, there was an immediate need for navigation methods suitable for use in the air. Following the path made by rivers, railroads, and highways was one of the earliest methods, but with improvement of airplanes and engines, overwater flights were made and a better method of navigation was required. It became necessary to borrow methods from the marine navigator. The air surrounding the earth is simply an ocean through which aircraft may cruise, guided by navigational methods very similar to those used to guide boats over the surface of the sea. With long flights, many of them over water now a routine thing, it is apparent that there is need for a scientific method of air navigation.

This must include one or more ways of flying accurate courses when

(1) Over poorly mapped country.

(2) Over water.

(3) Over fog and in any condition of poor visibility.

(4) Making flights at night.

3. Methods used in air navigation. -There are four general methods used in air navigation: pilotage, dead reckoning, radio navigation, and celestial navigation. Two or more of these methods are usually combined in actual practise, but for purposes of instruction they are treated separately. An explanation of their combined use during flight conditions will be made in section X.

4. Pilotage.-Pilotage is the method of conducting an aircraft from one point to another by observation of landmarks either previously known or recognized from a map. This method is, therefore, very limited when navigating over poorly mapped country, over bodies of water, at night, off regularly established airways where no beacons exist, or during weather when visibility is poor. However, pilotage is always used, whenever possible, in conjunction with the other methods of navigation. The use of ranges and cross bearings is included in this method and can often be used to good advantage. This method is analogous to that used by the motorist, where the highway is equivalent to the compass course and towns passed through are the check points.

5. Dead reckoning.-Dead reckoning (D. R.) is the method of determining the geographical position and maintaining the required course by applying the ground speed and track, as estimated or calculated, over a certain period of time from the point of departure or from the last known position. It is deduced reckoning and is the basic method of navigation, being used at all times, by itself, and in conjunction with the other methods of navigation. The use of this  method allows flights to be made with very little or no reference to outside objects. It is used extensively on over-water flights where pilotage methods fail because of the lack of check points.

6. Radio navigation.-Radio navigation is the method of conducting an aircraft from one point to another by radio aids, such as the radio beacon, radio direction finder, or radio bearings. Development of this method of navigation is receiving considerable attention. Its possibilities are great due to the fact that no view of the ground is necessary and that instrument landings are made possible.

The reliability, accuracy, and power of radio equipment in the hands of a skilled operator constitute the only limiting factors of this method. Should this equipment fail or the radio signals be interfered with, some other method of navigation must be relied upon. It is likewise used in conjunction with other methods. This method has a doubtful value in time of war, due to radio interference or to restrictions imposed by "radio silence."

7. Celestial navigation.-Celestial navigation is the method of determining the geographical position of an aircraft by observation of celestial bodies. The use of celestial bodies for determining position is as old as history, and although the basic principles have been unchanged for some time, constant improvements in methods and procedure have made celestial observations a fairly simple and yet accurate means of navigation.

8. Marine and air navigation compared.-The ocean currents are plotted on charts so that the navigator can refer to them and can accurately gage their effect on the vessel. In the air this is not the case; there are variable currents moving in many directions, so that -it is impossible to plot them on a map. It is necessary to resort to some other method to determine the effect of air currents. There is a great difference in the relative speeds of the currents; in the ocean they seldom exceed 4 knots, while in the air they may move more than 90 knots. On board a ship the visibility is usually about 8 or 10 miles, while in the air at 5,000 feet the calculated visibility is 81 miles, although under normal conditions, depending upon the atmosphere, it is usually about 20 miles. The heading of an airplane cannot be as accurately maintained due to the smaller compass that is used. The patent log of the mariner is much more accurate for determining distance traveled on a course than is the determination of ground speed from the air. The mariner can fix his position by celestial bodies to within I mile, while in the air 5 to 10 miles is average under normal air conditions. The modern airplane travels at speeds of more than 200 miles per hour as compared to the 30 knots of the modern ocean liner. The ocean vessel may be stopped completely when danger threatens in conditions of poor visibility, while the airplane must continue to travel at relatively high speeds in order to maintain its altitude.