Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 22. Aircraft Compasses (Magnetic)

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXII - AIRCRAFT COMPASSES (MAGNETIC)

101. Purpose and use.-a. In general, aircraft compasses are used for the following purposes :

(1) To indicate the heading of the aircraft.

(2) For locating the position of the aircraft by cross bearings.

b. The compass is a necessary navigational instrument for cross-country flying, patrol work, bombing expeditions, aerial photography, and flight through clouds. Its principal function is to indicate the direction in which the airplane is headed when flying on a straight course. However, it does not indicate correctly on turns made at velocities which require an appreciable banking of the airplane. The errors increase rapidly with the increase in the banking angle, a bank of about 20° being sufficient to destroy completely the function of the compass. This condition is inherent in any compass depending upon the earth's magnetic field for its directive effect and upon gravity for its stabilization.

102. Description.-a. The magnetic compass for use on aircraft consists essentially of a metal bowl filled with liquid and containing a card element carrying a system of magnetized needles so suspended on a pivot that it is free to aline itself with the meridian of the earth's magnetic field. The indications of the card and the reference marker or lubber's line are visible through a glass window of the bowl. An expansion chamber is built into the compass to provide for expansion and contraction of the liquid resulting from altitude and temperature changes. The liquid dampens the oscillations of the card. An individual lighting system is generally provided in the compass for illumination of the card when required for use during darkness.

b. Magnetic compasses for use on aircraft may be divided into two general types, type B for instrument board mounting to be used by the pilot, and type D for floor or table mounting to be used by the navigator.

(1) The type B compasses are available with cards graduated in either 1° or 5° increments. The models having the 1° graduations are intended for use on larger airplanes or when the 1° graduations are required by the pilot for navigation purposes. A front view of the type B compass having 5° increments is shown in figure 57(1). The various models of this type are interchangeable insofar as mounting dimensions on the instrument panel are concerned but are not interchangeable as to dimensions back of the panel. The construction features of this type of compass are shown in figure 57(2).

(2) The main characteristic of the type D (aperiodic) compass (fig. 58(1)) is the tendency of the card to return after displacement by one direct movement to its equilibrium position without executing a series of oscillations. This feature is obtained by the use of radial arms and damping vanes on the card which serve to increase the damping of the card movement in the liquid. These compasses are available with verge rings graduated in either 1° or 2° increments. Because of their different mounting dimensions, the two models are not interchangeable. Figure 58(2) shows the construction features of this type of compass.

103. Operation.-a. The earth acts as a huge magnet, with one pole near the north geographic pole and the other end near the south geographic pole. If a bar magnet is so suspended as to turn in any direction about its center of gravity, it will take a position with one end pointing toward the north magnetic pole and the other pointing toward the south magnetic pole. For this reason, the ends of magnets are known as the north-seeking or N-end and the south-seeking or S-end, respectively. Since the magnetic force acting on the N-end is equal and opposite to the force acting on the S-end, the effect on the N-end only is considered. The position taken by a freely suspended bar magnet gives the direction of the magnetic force or magnetic north. Thus the compass is a direction-indicating instrument. However, when installed in an airplane it is subject to additional forces.

b. There are four main causes for inaccuracy in aircraft compasses, incorrect installation, vibration, magnetism, and northerly turning error. Aircraft and instrument designers reduce or eliminate compass inaccuracies due to faulty installation and vibration by careful selection of compass location, use of vibration absorption mounts, provisions for level mounting, etc. During the construction of air-craft, the vibration and jarring of steel parts while being forged, machined, or fitted in place impart a certain amount of permanent magnetism which is induced by the earth's magnetic field. When the aircraft is placed in service, this permanent magnetism will vary due to vibrations of the engine, shocks from gunfire, landings, etc. This changing field of permanent magnetism correspondingly affects the action of the earth's magnetic field on the compass and thus causes the compass card to deviate from the magnetic north. Further deviations of the compass result from electric currents flowing in the electrical system of the aircraft, radio equipment, electrical instruments, and from varying positions of metallic masses such as bomb loads, re-tractable landing gears, etc. If not excessive, corrections of compass errors resulting from the permanent magnetic influences referred to may be accomplished within close limits by the proper application of compensating magnets.

(1) Front view.

(2) Cross Section
FIGURE 57.-Type B pilot's compass.

c. The error of any compass is the angular difference between true north and compass north, or the angle between the true meridian and a vertical plane passing through the length of the compass needle. This angle is the algebraic sum of the variation and the deviation. Variation is caused by terrestrial magnetic influences and is the angular difference between true north and magnetic north measured from the true meridian. It is termed "west" when the terrestrial magnetism draws the compass needle to the left or west; and "east" when the needle is drawn to the right or east. Deviation is caused by the local magnetic influence of the aircraft in which the compass is mounted. It is the angular difference between magnetic north and compass north. As in variation, it is termed "west" when the influence acting upon the needle draws it to the left or west, and "east." when the needle is drawn to the right or east. Ordinarily the vertical component of the aircraft magnetic field has no effect on the compass readings, being at right angles to the compass card. However, when the aircraft is not in level flying position, this vertical component affects the compass card and produces a deviation known as "heeling" error. Correction for this error is not necessary as all compass readings, either when compensating or during flight, are made only with the aircraft in level flying position and engines running.

d. The type B compass for pilot's use indicates magnetic directions continuously and does not require a setting by the pilot to obtain the heading of the airplane. The heading of the airplane may be read by viewing the indications on the compass card in reference to the lubber's line through a glass window in the front of the compass bowl.

(1) Top view

(2) Cross Section
FIGURE 58.-Type D navigator compass.

e. The type D compass is for use by the navigator, and although this compass indicates magnetic directions continuously, it requires setting of the reference lines to obtain the heading of the airplane. This type compass is provided with a verge ring, which may be rotated to bring the grid wires or the lubber's line into alinement with the card pointer, after which the heading of the airplane may be read directly by means of the graduated scale on the verge ring. It is viewed through a glass window located in the top of the compass bowl.

104. Installation.-a. Installation of compasses by service activities should always be made in the location provided in the air-plane unless relocation is found necessary and is authorized. Compasses are installed by depots and airplane manufacturers so that a vertical plane passing through the card pivot and lubber's line will be parallel to the longitudinal axis of the airplane, and the card pivot supporting post substantially perpendicular to the horizontal plane when the airplane is in flying posit ion. The type B compass is so constructed that these two conditions are fulfilled when the instrument board upon which it is mounted is perpendicular to the longitudinal axis of the airplane. The type D compass is so constructed that these two conditions are fulfilled when the surface of the base upon which the compass is mounted is parallel with the airplane leveling studs and a vertical plane through the card pivot and lubber's line of the compass is parallel to the longitudinal axis of the airplane. This can be accomplished by using a plumb line to establish the fore and aft axis or by marking a reference center line on the floor by measurements across the fuselage. The compass compensating chamber and adjusting screws must be easily accessible. The brackets required for mounting compasses are made of brass, duralumin, or other nonmagnetic materials and the mounting screws for compasses are of brass.

b. Special attention is given to the prevention of disturbing magnetic fields in the vicinity of the compass, either of a permanent nature such as may result from the proximity of electrical equipment, radio, armament, or structural members, or particularly of a varying nature such as may result from variations in current flow in electrical wiring or the position of retractable landing gears or kindred equipment. While a reasonable amount of permanent magnetism in the vicinity of the compass can be compensated for, the effect of variable magnetic fields cannot. The maximum deviation of the compass before compensation resulting from the effects of permanent magnetic fields must not be more than 25° for type B compasses. and not more than 15° for type D compasses. Deviations of the former after compensation must not exceed 10° and for the latter not more than 5°.

105. Maintenance.-a. The general points on maintenance given in section II are applicable to compasses. The normal service or line maintenance consists of the replacement of defective lamps, tightening of screws to eliminate leakage of liquid, checking lighting system for defective electrical connections, compensation, and replacement of defective compasses.

b. Compasses are removed and replaced by serviceable instruments if any of the following conditions exist:

(1) Clouded or discolored liquid which impairs visibility.

(2) Card markings are illegible due to discoloration, fading, or loss of luminous paint.

(3) Card does not rotate freely in a horizontal plane when airplane is in normal flying position. This may be checked by deflecting the card with a small permanent magnet.

(4) Bowl is cracked or mounting frame or lugs broken.

(5) Compass is not responsive or is erratic in action after proper efforts to compensate.

(6) Lubber's line is loose or misalined.

(7) Compass requires bench test, disassembly operations, additional liquid, or has any other major defects which might render it inoperative.

c. All compasses installed on aircraft, except primary training airplanes when used solely for airdrome flying, will be compensated and the readings recorded at each engine change period, at each change of guns or electrical equipment likely to affect the compass, or at least once during each 3-month period. However, if at any time the compass is suspected of being in error, it should be checked and compensated. The process of compensating for errors in compasses after installation in aircraft, that is, correcting within close limits the errors caused by magnetic influences and obtaining and recording the final deviations at the various points on the compass, is termed "swinging the compass." The compensation of compasses installed in aircraft cannot be expected to remain accurate for very long periods of time as it is known that under service conditions the magnetism of the aircraft structure is constantly changing both in intensity and direction. The following instructions should be observed during the compensation of all magnetic aircraft compasses:

(1) See that the airplane is at least 100 yards from any steel structure, underground cables, or metal drainage pipes, concrete containing steel reinforcements, or other aircraft.

(2) Insofar as possible place all controls, guns, etc., containing ferrous material in flying position, except when engines are running the elevators should be used to assist in keeping the tail of the air-plane down.

(3) If the compass employs loose magnet compensators remove all magnets from the chambers prior to compensation and keep all unused magnets away from the compass at least 2 feet.

(4) Cause the compass card to deflect through a small angle using a small permanent magnet for this purpose (compensating magnet).

Note whether the card rotates freely on its pivot and its path of rotation is in a horizontal plane. Read the compass only from a position directly in front of (pilot's type) or above (navigator's type) the lubber's line to avoid erroneous readings.

FIGURE 59. -Diagram showing procedure used when swinging aircraft compasses.

(5) To swing an airplane on the magnetic bearing swinging base, first place the airplane in level flying position on the base with the longitudinal;axis parallel with the magnetic meridian ; that is, parallel to the N—S line as shown in figure 59. Then if the compass does not indicate north (0°), insert sufficient compensating magnets in the athwartship chamber if compensator is of the loose magnet type, or adjust the compensating screw marked N-S if it is of the perma. Dent magnet type, until the compass indicates north (0°). Then, with the engine or engines running and speeded up sufficiently so that the maximum charge is shown on the ammeter, note whether the compass still indicates north (0°). If the compass heading is affected by the electrical current flow, it will be necessary to make further magnet corrections in order that the compass will indicate north (0°) under flight conditions. The airplane should then be headed east (90°) and the above process repeated, bearing in mind that in this position compensating magnets should be inserted in the fore and aft chamber, or the compensating screw marked E–W adjusted to obtain the desired compass heading of east (90°).

(6) If during the compensation of the compass on the north and east headings there is no apparent change noted in the indications of the compass as a result of running the engines, there will be no further occasion to keep them running during the remaining period of compensation. However, with the engines stopped, the compass should be tapped lightly by hand before each reading is taken to eliminate errors due to friction.

(7) The airplane should next be headed south (180°) and if the compass does not indicate south, change the compensating magnets in the athwartship chamber or adjust the N–S compensating screw to eliminate one-half the error. By so doing, the error is divided between the north and south headings. The airplane should then be headed west (270°) and if the compass does not indicate west, change the compensating magnets in the fore and aft chamber or adjust the E–W compensating screw to eliminate one-half the error on this heading, thus dividing the error between the east and west headings. This procedure results in a more even distribution of the compass errors at all points.

(8) The actual compensation of the compass is now completed. The airplane should next be swung on each successive 15° heading, starting at any convenient point, and the compass readings recorded in the "radio off" space provided on the compass correction card. Then, with the radio receiver in the "on" position, the airplane should again be swung on each successive 15° heading and the compass readings recorded in the "radio on" space provided on the compass correction card. The engines will be operated as specified in (6) above if the N and E headings were affected when compensating.

(9) All entries on the compass correction card must be the actual indicated readings on all headings. After all entries have been properly made and the card dated and signed. it is placed in the holder provided for ready reference during flight.

(10) To swing an airplane by using a master compass, the same procedure as outlined in (5) above is followed, except that instead of the magnetic bearing swinging base, any serviceable magnetic compass may be used as a master compass, provided the compensating magnets have been removed before being so used. A compass having compensating magnets installed, either of the loose type or permanent type, cannot be used as a master compass. When swinging an airplane with a master compass, the compass should be attached to the wing as near the tip as possible by using a compass swinging frame. These swinging frames have been supplied to service activities.

d. With the advent of aircraft of such size as to be unwieldy on the ground, the problem of swinging such airplanes by ground methods is extremely difficult. An increased amount of electrical and radio equipment and the incorporation of retractable landing gears and similar equipment are reasons for a more vigilant attitude with respect to compass errors, since the increased range of present-day aircraft demands a more precise method of navigation. It is definitely known that the compass errors on the larger aircraft as determined by ground swinging methods and air swinging methods are far from identical although every reasonable precaution is taken on the ground to duplicate air conditions. Further information on both ground and air swinging of aircraft compasses may be found in Air Corps Technical Orders.