Advanced Pilot Training: TM 1-205 Air Navigation - Section VII Elementary Dead Reckoning

50. Advantages of dead reckoning (D. R.).-a. Definition.-Dead reckoning is the method of determining the geographical position of an aircraft by applying the track and ground speed, as estimated or calculated, over a certain period of time from the point of departure or from the last known position.

b. Use.-(1) Cross-country flying by elementary methods of piloting is simple under conditions of good visibility. Piloting a plane by reference to visible landmarks is fundamental and must be combined with any other form of navigation that may be used; however, when a pilot is limited to flying by landmarks alone, he loses the saving in distance of the direct air route. Furthermore, if the weather should close in unexpectedly during the flight and the familiar landmarks could not be found, the results might be extremely serious not only to the pilot but to the life and property of others as well.

(2) By means of dead reckoning between check points, a pilot can fly directly to or fairly close to the landmarks for which he is looking. Because he knows when and where to look for them, he will often succeed in finding them when a pilot without such training would miss them altogether. If he has an accurate knowledge of his own course and speed, and of wind direction and force, he may proceed even under adverse weather conditions with more certainty than an untrained pilot might in clear weather. In any event, the ability to navigate by more advanced methods results in increased safety and greater operating efficiency, gives considerable confidence and mental satisfaction to the pilot, and is essential for the missions required of military personnel.

(3) Dead reckoning, or deduced reckoning as it was originally called, consists of keeping the position of the aircraft known by means of estimating the path and distance traveled since a last known check point. This method is really the basis of all navigation. All other methods may be said to supplement dead reckoning.

(4) A pilot can fly cross country with no instruments and equipment except a map, and even without that if he has sufficient knowledge of the terrain to be covered ' but only very foolish and inexperienced pilots attempt such flights. Elementary dead reckoning methods require the use of several instruments.

51. Air speed meter-a. Definitions.- (1) Air speed is the true speed of an aircraft relative to the air. It is the true air speed unless otherwise stated. Air speed is obtained by correcting the calibrated air speed for density, using temperature and pressure altitude corrections.

(2) Indicated air speed is the reading of the air speed indicator.

(3) Calibrated air speed is the reading of the air speed indicator, corrected for instrumental and installation errors.

b. Relationship.-(1) The air speed indicator indicates the calibrated air speed of the airplane traveling through the air only when the instrumental and installation errors are so small that they may be ignored. Instrumental errors are usually so small that they may be neglected, but the installation error is so large that the air speed installation is always calibrated when precision results are desired.

In elementary dead reckoning the difference between indicated air speed and calibrated air speed is ordinarily ignored or estimated. In nearly all air speed indicator installations, it will be found that the indicated air speed is less than the calibrated air speed by an amount varying considerably between different airplanes and also at differer,t speeds in the same airplane. The calibration of air speed meters is described in paragraph 154.

(2) Except in still air at normal sea level temperature and atmospheric pressure, the calibrated air speed is different from the true air speed. However, the pilot may calculate the true air speed from the calibrated air speed if he knows the pressure altitude at which he is flying and the free air temperature. The conversion of calibrated air speed to true air speed may be accomplished by formula. However, the conversion is usually made by using the type D-3, E-613, or E-1 dead reckoning computers. The procedure when using the D-3 is described in paragraph 55. The procedure when using the other two computers is described in paragraph 145. If a computer is not available, the air speed may be obtained from calibrated air speed by the following rule: For every 1,000 feet of altitude above sea level, increase the calibrated air speed by 2 percent. For example, at 10,000 feet above sea level, the true air speed is 20 percent greater than the calibrated air speed.

c. Use.-Specific uses of the air speed indicator are as follows:

(1) To aid in estimating the actual ground speed of the airplane. This is necessary in cross-country flying when the time required to reach a landing field must be determined, during bomb sighting and gunnery missions, and in aerial camera work.

(2) To aid in determining the best throttle setting for the most efficient flying speed.

(3) To aid in determining the best climbing and gliding angles.

(4) To determine whether the speed attained in a dive is within the limits of safety for the structure of the airplane.

(5) To indicate to the pilot when the airplane has attained flying speed during the take-off and when the stalling speed is being approached when landing. This is especially true when flying closed cabin airplanes. Pilots are trained to judge these speeds without the aid of instruments, and in open cockpit airplanes this is easily done. In large, enclosed types, the air speed indicator is a very essential aid.

d. Principle of operation.-The air speed meter is operated by changesin air pressure, introduced into the instrument through tubing. If leaks or stoppages develop in the tubing the result will be a lower instrument reading. The indicator is a sealed unit with a pressure sensitive cell inside. The cell is a hollow circular box made of thin sheet bronze. It is capable of expanding and contracting as the pressure is varied on its surfaces. The movement of the cell is transmitted to a pointer through a system of levers and gears, and this pointer registers the rate of travel on the scale on the instrument face.

52. Altimeter-a. Purpose and use.-Altimeters are used for two distinct purposes in an aircraft:

(1) To measure the elevation of the aircraft above some point on the ground (regardless of its elevation above sea level). This method of altitude measurement is used in instrument landing procedure and will give a zero altimeter reading upon landing. It is called the "zero setting" system.

(2) To measure the elevation of the aircraft above sea level. This method of altitude measurement is used for cross-country and airways flights and is called the "altimeter setting" system. Specific uses of the altimeter setting system are as follows:

(a) To show at all times elevation of the airplane above sea level so that the indication can be compared with maps for the purpose of clearing critical points and mountain peaks safely.

(b) To use advantageously meteorological data which is supplied by weather stations, such as wind velocities and directions, and cloud and storm formations which are to be avoided in flight when possible.

(c) To observe and follow correctly airways traffic regulations.

b. Principle of operation.-Altimeters and barometers operate on the same principle. The mechanism is actuated by changes in atmospheric pressure. Atmospheric pressure varies with altitude, decreasing as altitude increases. Air is compressible so the atmospheric pressure does not decrease uniformly with a uniform increase of altitude. In the standard atmosphere at sea level the atmospheric pressure is 14.7 pounds per square inch; at 10,000 feet it is 10.8 pounds per square inch; at 20,000 feet it is 7.06 pounds per square inch; and at 30,000 feet the pressure is only 4.9 pounds per square inch. Atmospheric pressure is usually determined by measuring the height of the column of mercury it will support. This height at sea level in the standard atmosphere is 29.92 inches; at 10,000 feet it is 20.73 inches; at 20,000 feet it is 14.38 inches; and at 30,000 feet it is 9.97 inches. Any instrument which will indicate variations in pressure can be calibrated to indicate the approximate altitude. It cannot, however, always indicate the exact altitude, because the temperatures and the pressures, and therefore the pressure altitude relation, change with time and place. Consequently the altitude pressure relation in the standard atmosphere is assumed, and the altimeter is adjusted to conform to it.

c. Description. - (1) The altimeter in most common use is a modification of the aneroid barometer. The dial, instead of being graduated in units of pressure, is graduated in units of height. All of the altimeters used on military aircraft are the sensitive type. Standard models for tactical operation have a calibrated range of from - 1,000 feet below sea level to + 35,000 feet above. By use of a multiple pointer system, the instrument can be accurately read to at least one-half the smallest unit graduation on the scale which is 20 feet. Late types of altimeters have one altitude scale, one barometric scale and index marker, two reference markers, and three pointers. A setting knob located at the bottom front of the instrument case drives two pinions in opposite directions. One of these pinions rotates the barometric scale and reference markers and the other Pinion rotates the aneroid mechanism assembly and the pointers. The altitude scale is graduated from 0 to 10. This scale is fixed, and all the pointers, the reference markers, and the barometric scale rotate and indicate with reference to it.

(2) The aneroid mechanism is exceptionally well built. It is very

sensitive and well balanced. A temperature compensator is included in the mechanism to correct for any mechanical error from this source that results when changing from one altitude to another.

The minute hand makes one revolution for a change of 1,000 feet, each numeral being 100 feet, and the small graduations correspond to 20 feet. Due to the wide spacing between the 20-foot graduations a change of 5 feet is readily apparent. The hour hand makes one revolution for a change in altitude of 10,000 feet, each numeral being 1,000 feet. The second hand indicates the 10,000 feet, each numeral being 10,000 feet. To cover the full range of the instrument the long hand makes a total of 36 revolutions, the intermediate hand 3.6 revolutions, and the small hand 0.36 revolution. The standard range for the barometric scale is from 28.1 to 31.0 inches Hg, with unit graduations of 0.02 inch Hg. When the limit of the range of the barometric scale is reached at either extreme, a shutter blanks out the indication of the barometric dial, and the barometric pressure is read from the position of the reference markers. Thus, by introducing a limited range barometric scale, the actual unlimited possibilities of setting barometric pressure by means of the reference markers are not in any way affected.

d. Operation. - (1) General. - Since the altimeter mechanism consists of an aneroid which is designed to measure absolute pressure, its operation is entirely automatic. Extreme sensitivity is obtained by use of a high ratio multiplying mechanism in the linkage and the use of the multiple pointer system. When installed on an airplane it is essential that the aneroid be subjected to undisturbed (static) air. The air in the cabin or cockpit of an airplane under flight conditions is highly disturbed and if allowed to enter the case of the altimeter would cause serious errors in the instrument's indication. The extent of these errors varies; on high-speed airplanes they may be as much as 500 feet. Consequently, for correct operation and indication, the altimeter must be vented to the static line, and the instrument case and entire static system must not have any leaks.

(2) Definitions- (a) Altimeter setting is a pressure in inches of mercury, and is the existing station pressure reduced to sea level in accordance with the United States standard atmosphere. Altimeter setting is also the standard atmosphere pressure corresponding to pressure altitude variation.

(b) Station pressure is the existing atmospheric pressure at the elevation of the mercurial barometer located in the weather station.

(c) Field elevation pressure is the existing atmospheric pressure at a point 10 feet above the mean elevation of the runway and is obtained by applying a suitable correction to the station pressure. It is assumed that the altimeter in an airplane is 10 feet higher than the landing surface.

(d) Pressure altitude is the altitude in the standard atmosphere corresponding to the existing barometric pressure.

(e) Pressure altitude variation is the algebraic difference between the existing pressure altitude and the surveyed elevation of the field. The pressure altitude variation is also the equivalent in feet of the altimeter setting in accordance with the standard atmosphere.

(f) Meteorological sea-level pressure is the station pressure reduced to sea level in a manner dependent upon the prevailing conditions of station temperature. Meteorological sea level pressure should not be confused with altimeter setting, should never be broadcast to aircraft, and should never be used in connection with aircraft altimeters. It is designed to give smooth, consistent isobars on the sea level plane for the purpose of drawing weather maps.

(3) Zero setting system.- (a) When it is desired to set the altimeter so that the pointers indicate the height of the airplane above the ground or runway at some specific point, regardless of its elevation above sea level, the pilot in the airplane will contact the ground station at that point by radio and ask for the pressure altitude at that station.

(b) On late type altimeters which have the barometric scale, the pilot would call for the field elevation pressure.

(c) After the pilot sets these on his altimeter, his altimeter pointers would then indicate the elevation of the airplane above the runway and upon landing would read zero within the tolerance of the instruments. (See fig. 34.)

(d) The accuracy of this system on properly maintained instruments will be very close, normally, within + 30 feet; however, it is not recommended to depend on the altimeter for an indication accuracy closer than + 75 feet.

(e) For stations below an elevation of approximately 1,200 feet, either field elevation pressure or pressure altitude setting of the zero setting system will give a zero altimeter reading upon landing, and one may be used as a check against the other within the operating limits of the instrument. For elevations above approximately 1,200 feet, only pressure altitude can be set on the zero setting system due to the limited range of the zero setting scale graduated in pressure.

(4) Altimeter setting system.-(a) Upon receipt of altimeter setting by radio, the zero setting scale graduated in inches of mercury pressure is set to correspond. For example, if the altimeter setting received from a nearby ground station is 30.03 inches of mercury' ~ this should be set on the zero setting scale by turning the knob. The altimeter will then read altitude above sea level (uncorrected for atmospheric temperature) within the operating limits of the instrument and the accuracy of measurement of the altimeter setting. If the aircraft is landed, the instrument will read the surveyed elevation of the field within the operating limits of the instruments.

(b) The older types of Air Corps altimeters (type C-7 and earlier) are not equipped with a zero setting scale graduated in inches of mercury pressure but are provided only with reference marks reading on the altitude dial. These instruments may readily be used with the altimeter setting received by radio by referring to a table (par. 148) to obtain the pressure altitude variation, which is the altitude in feet corresponding to the altimeter setting, and by setting the reference marks on the altimeter to correspond. The pressure altitude variation may be plus or minus, depending upon existing conditions. Care should be taken in setting negative pressure altitude variations because the numerals on the dial do not apply in this case.

(c) For example, if the altimeter setting received is 30.03, then from the table the pressure altitude variation is found to be -100 feet. The hundred-foot zero reference mark is rotated counterclockwise from zero until it reads 9, and the thousand-foot zero reference mark is slightly to the left of zero.

(d) Altimeter settings are continuously changing and as a rule are never the same at any two stations. It is therefore imperative that the pilot obtain the altimeter setting by radio at every scheduled broadcast from the ground station nearest his location at the time in order to permit the safe execution of traffic control.

(e) For accuracy in clearing mountains, elevations, and critical Points along a route it is necessary to correct the altimeter reading for atmospheric temperature. This may readily be done by using the type C-1 true altitude computer or the E-6B dead reckoning computer. Caution: It should be noted that for obtaining the prescribed flight level for the purpose of traffic control, all aircraft operators must use the altimeter reading uncorrected for atmospheric temperature or all operators must correct their altimeter readings for atmospheric temperature. It is considered impracticable to require altimeter readings to be corrected by all operators at the present time for traffic control purposes. Therefore, prescribed flight levels will be altitudes uncorrected for atmospheric temperatures. Flight and traffic control personnel must realize that adequate margin of clearance must be allowed over mountains, etc., in order to use indicated altitudes uncorrected for atmospheric temperature. In preparing and maintaining flight plans, this should be carefully considered.

1. The latter case is one which may bring real danger to a pilot in case of bad weather. When the barometer reading is low the visibilty is often poor. Under these conditions the altimeter, if not properly set, will indicate an altitude above the actual height of the airplane. A pilot, depending on the altimeter reading in such a case, might think his altitude is 1,000 feet above sea level when it is actually several hundred feet (or more) below the 1,000 foot level he desires to use.

2. Many accidents have been caused by the failure to allow properly for the above adjustment of the altimeter. Safety demands that under conditions of poor visibility the pilot will allow a big margin of clearance when judging what altitude he should use. Without knowledge of the barometric pressure, increase this allowance to make up for a possible low pressure.

53. Turn indicator~a. Purpose and use.-The purposes and uses of the turn indicator are

(1) To determine accurately the magnitude of any turn made by the airplane.

(2) To relieve the pilot of the strain and mental fatigue resulting in attempting to maintain a directional bearing with a magnetic compass.

(3) To provide a positive azimuth indication at all times.

(4) To help locate radio beacons.

(5) For straight course keeping and for exact course changes when necessary.

(6) To show bank, by showing turn, and by movement of the inclinometer.

(7) To keep the airplane out of acrobatics under instrument flight conditions.

(8) To direct the pilot in bombing exercises.

(9) To keep a straight course on photographic mapping missions.

b. Description.-The type A-2 turn indicator is a two in one istrument. It contains a gyroscopic rotor fcr showing turn, and a ball, in a glass inclinometer, for showing bank. The gyro turn mechanism consists of a small rotor, spun by means of a moving column of air striking small cups on the rotor wheel. A circular card graduated in degrees is attached to the vertical ring in which the rotor and its gimbal ring are mounted. The vertical ring and card are free to turn in vertical bearings, and a rectangular opening in the front of the instrument case permits a view of an ample section of the card. The rotor is horizontal in normal operation. This entire assembly is placed inside of a sealed case. A lubber line painted on the window is used as a fixed reference mark with which to aline and read the instrument. The gimbal ring which carries the gyro and card is mounted so that it can be set to any degree in azimuth. This is accomplished by means of a caging knob which meshes with the azimuth gear. In the bottom at the rear of the case two vents are provided for connection of the instrument to the vacuum system of the airplane. An atmospheric vent located in the bottom of the instrument case is covered with a screen. The air passing through this vent is directed onto the gyro which causes its spin. At the top and in front of the window in the rectangular opening a small 12-volt light bulb is inserted into a recess, providing light for use at night.

c. Operation- (1) To use the turn indicator the pilot determines the course which he desires to follow. He gets on this course by means of the magnetic compass. He sets the turn indicator to the same reading as the compass, or to zero if he desires. Now every turn, however great or small, will be indicated instantly by this instrument. Each 10 or 15 minutes it should be checked with the magnetic compass and reset if necessary. Any time that a change of course is desired it can be accomplished accurately with this instrument. It neither lags, swings, nor oscillates, and it is therefore an accurate and safe indicator of directions and turns. The gyro will maintain its fixity in banks, climbs, and dives up to 551. Any maneuver in excess of this amount is beyond the operating limits of the instrument. Such turns may cause the gyro to upset, with the result that its indications cannot be depended upon until level flight is resumed, the gyro caged, and the instrument reset by compass.

(2) The following Points must be understood and observed to use the instrument properly:

(a) It is not a direction seeking instrument.

(b) It does not contain any magnets and can only be used to maintain a direction after the course has been found by means of the magnetic compass.

(c) It will not perform satisfactorily when maneuvers of the airplane exceed its operating limits.

(d) Due to the torque of the rotor and slight friction in the bearing, it will drift slightly from a set plane. This drift will not exceed 5° in 15 minutes on any properly operative indicator.

54. Clock. - An accurate timepiece is essential to navigation. Time is the basis for all computations in determining ground speed and subsequently estimated time of arrival, or in determining position from terrestrial bearings or dead reckoning methods. Standard equipment in Air Corps airplanes is the 8-day, sweeping secondhand clock. It is mounted with the other instruments on the instrument board. There are many types of navigation watches issued by the Air Corps. One that incorporates a "stop watch" feature has some advantages over the usual watch.

55. D-3 computer.-a. Description and use.-The D-3 computer is for the use of the pilot in solving speed, time, distance problems and altitude, temperature, air speed problems. It is of convenient pocket size and consists of a circular slide rule about 31/2 inches in diameter with a suitable scale on each side.

b. Front side.- (1) Using the front face of the computer, if any two of the three factors of speed, time, and distance are known, the third factor can be obtained with one setting. The following are examples of typical problems of this type:

(2) Given: Time 12 minutes.

Distance 35 miles.

Required: Speed.

Solution: Set the time (12 minutes) on the inner scale (smaller disk) opposite the distance (35 miles) on the outer scale. The speed will then be shown on the outer scale opposite the large arrow (1 hour or 60 minutes) which is printed on the inner disk. Answer: 175 m. p. h.

(3) Given: Distance 360 miles.

        Speed 148 m. p. h.

Required:       Time.

Solution:       Set the large arrow (l.hour) of the inner disk opposite the speed 148 on the outer scale. Then find the distance 360 on the outer scale and read the time opposite this figure on the inner scale. Answer: 2 hours 26 minutes.

(4) Any such problems may be quickly worked by using the inner disk for time and the outer scale for distance and speed.

c. Back side.- (1) On the reverse side of the computer are scales for applying the pressure altitude and temperature correction to the calibrated air speed in order to obtain the true air speed. To use, rotate the transparent disk until the calibrated air speed on the disk is opposite the free air temperature scale, which is printed on the background so as to be read through the disk. Then read the true air speed opposite the mark which agrees with the altitude of the airplane. Note that the altitude used is the pressure altitude and not the altitude above the terrain. Also that the air speed used is the calibrated and not the indicated air speed. The temperature used is the temperature of the free air outside the airplane and not the ground temperature.

(2) The transparent disk is constructed with a roughened surf ace so that the corresponding indicated air speeds may be penciled on it; the data being transcribed from the air speed meter calibration card, or the estimated calibrated air speed if no calibration card is available.

56. Thermometer.-The air thermometer shows the temperature of the air at the altitude being maintained by the aircraft. Groundtemperature is the temperature of the air at a ground station. All Air Corps thermometers installed in airplanes read degrees Centigrade. Free air temperature is used to correct air speed readings as well as altimeter readings, and also to determine if icing conditions are present.

57. Elementary methods,-a. General.-Very accurate results f rom dead reckoning methods may be obtained when a well-trained navigator with a complete set of instruments and equipment is present in the airplane. The pilot in the smaller airplanes does not have the drift meter, the aperiodic compass, the pelorus, and the other aids to precise navigation. He usually does not have the range for long overwater flights, or for long periods of flight during instrument flying conditions.

b. Desired accuracy- (1) The pilot navigator usually combines elementary dead reckoning with pilotage methods, together with aids from radio. He uses dead reckoning for short periods of time when he maintains a certain heading for a definite period and then expects to be within sight of the next check point.

(2) The following example illustrates how elementary dead reckoning is used. In figure 36 a pilot is flying along an easily followed codrse A, B, to C. At C the combination of highway and railroad, which has provided his check points, changes direction toward the south. The pilot wishes to go on to D, but the terrain between C and D is such that no check points are available, so he uses simple dead reckoning methods to proceed from C to D. He checks his exact compass heading along the course from A, through B, to C, and also determines his ground speed on this part of his course. When he i)asses C, he merely continues the same compass heading and figures his estimated time of arrival (E. T. A.) at D, using the figure for his known speed.

(3) Using the above method, the pilot may arrive at his destination a minute or two early or late. Also, he may be a mile or two to the right or left of D when he comes within sight of it. For this method, such accuracy is sufficient, and in actual practice it frequently gives very precise results.

58. Line of Position and fix.-a. The "line of position" is a line on which the aircraft is observed to be. The intersection of two lines of Position determines a "fix." The following example shows clearly a line of position. In Figure 37 a pilot proceeds from A to B, flying over terrain which offers little chance for him to identify his position. positively. He knows that his course is approximately clue east but cannot check his ground speed. When he comes to the C railroad it gives him a chance to identify his position as being soineivItere -ilong the railroad, as shown on his map. He cannot be sure whether he is at D, E or F, but by knowing that he is on the line of position (the railroad) he has an excellent check on his ground speed.

b. If, in the above example, the pilot had been able as he crossed the railroad to see that he was in line with the tops of the two hills G and H, he would have had a second line of position F, G, H. The intersection of the two, or in this case the point F, would then be a fix. Then he would know his position as well as his speed.

59. Steering a range.-a. In order to keep on a desired course it is good practice, when convenient, to select two landmarks ahead, which are known to be on the course, and to steer the plane so as to keep the two objects in line. This is known as steering a range. " Before the first of the two landmarks is reached, another more distant object in line with them may be selected and a second range steered.

b. Sometimes the selection of a range is very easy, as when a road or railroad parallels the route; at other times, the selection of a continuous series of ranges may prove difficult. For this reason, and also as an added factor of safety, it is desirable to use the magnetic compass for keeping a constant direction. For this purpose we need not be concerned with magnetic variation, compass deviation, or wind drift. It is only necessary, while steering a range that is definitely known to lie along the route, to note the compass heading. This heading is the correct course to steer, and it should be maintained until another range is available. Then, if the compass heading is compared again, any change in magnetic variation or wind conditions will be taken care of in the new compass heading noted.

c. As an example, the route from Springfield airport to Marion airport, in Ohio, lies between and roughly parallel to the Erie Railroad and the Springfield-Marion highway. Flying this route, when a plane was about opposite Marysville a compass heading of 35° was noted. Shortly afterward an area of poor visibility was encountered unexpectedly, but the compass heading of 35° was maintained. Flying into better conditions again, -near Claiborne, it was found that the plane was still on the intended track, and its position could easily be identified from the highway pattern east of Richwood.