CAA UK Civil Aircraft Airworthiness Information and Procedures

PART 8 AIRCRAFT INSTRUMENTS

LEAFLET 81 COMPASS BASE SURVEYING INTRODUCTION

In carrying out a swinging procedure for direct-reading and remote-reading compasses, the primary object is to determine the deviations caused by the magnetic field components of an aircraft. It is, therefore, necessary for swinging to be undertaken at a location where only these aircraft field components and the earth's magnetic field, can affect the readings of compasses. The location must be carefully chosen and surveyed to prove that it is free from any interfering local magnetic fields and also to establish it as the base on which all aspects of swinging procedures are to be carried out. The effect of interfering fields is to cause distortion of the direction and intensity of the earth's field. The effects on direction are the most critical and therefore, it is necessary for these to be determined during a survey. Any significant effects on the horizontal intensity will be detected as a change in direction, if suitable procedures are employed. The purpose of this Leaflet is to outline the basic requirements of a compass base, to define the accepted base classifications and also to outline the procedures which may be adopted for surveying selected locations.

1.1

The CAA does not carry out surveys or approve compass bases, but its interest in surveying procedures lies in the fact that the accuracy of a base is a significant factor in meeting British Civil Airworthiness Requirements relevant to the overall accuracy of compasses installed in aircraft. Surveys may be carried out by an operator or by an airport authority and in this connection, the standards set by the Admiralty Compass Observatory (ACO) are recognised by the CAA. The ACO is a Ministry of Defence department and although it has the responsibility for surveying compass bases for military aircraft, its services in an advisory capacity, or for carrying out an entire survey, can be obtained. It should, however, be noted that the latter service is confined to airline operators and airport authorities.

2. BASE REQUIREMENTS

A compass base must meet the following minimum requirements:

(a) It must be accessible, reasonably level in all directions and its use should not interfere with normal aircraft movements on the airport.

(b) It must be free from magnetic fields, other than that of the earth, which might cause aircraft compass errors. Most surface causes of errors are obvious, i.e. buildings and installations containing ferromagnetic components such as wire fences, drain and duct covers, picket points and lighting installations. The most likely underground causes of magnetic interference are:‑

  1. Buried scrap metal and old brickwork.
  2. Reinforced concrete.
  3. Pipelines including drainage systems.
  4. Magnetic soil and rocks.
  5. Electrical cables, conduits and airfield lighting transformers.
  6. Ferromagnetic pipes.

If such items are found at the selected location they should be removed if possible. Even though the area may be within permitted maximum limits of deviation (see paragraph 3) it is recommended that any ferromagnetic material present should still be removed as its magnetic effect may change with time and thereby down‑grade the accuracy of the base. Where electrical cables cannot be avoided, their effects, with and without current flowing, must be checked at intervals along their length, especially around known joints. If a new base is being constructed great care must be taken to ensure that the area is not magnetically contaminated after survey and during construction. Steel reinforcing must obviously be avoided and any aggregate or hardcore used in the foundations must not be magnetic or contain magnetic items such as steel wire or drums, bricks, boiler clinker, blast furnace slag or magnetic rock. All steel shuttering and associated pins used when laying concrete must be removed. On completion of all work a full survey must be repeated.

(c) A base should he sited so that its datum circle (see paragraph 2 (f) (iii)) is at least 46 metres (50 yards) away from hangars and other steel‑framed buildings and at least 91 metres (100 yards) away from buildings containing electrical power generation and distribution equipment and also from overhead or underground power cables.

NOTE: Proposed building programmes should be examined to ensure that the site is not scheduled for other work.

(d) A base must be large enough and of such load-bearing strength as to take all types of aircraft for which it is likely to be used. In this connection, some important factors to be considered are:

  1. Whether an aircraft will be towed or taxied during the swing.
  2. The radii of the turning circles of the aircraft.
  3. The position of sighting rods and target fixtures on the aircraft and their likely path during the swing.
  4. The likely positions of flux detector units of remote-reading compass systems.

(e) The surface of the base should not preclude its use in wet weather.

(f) The base should be clearly and permanently marked to show:

(i) The base centre.

(ii) The central area in which a direct-reading compass, or flux detector unit of a remote-reading compass system, should remain during the swing.

(iii) The datum compass circle, i.e. the circle around the central area showing where the datum compass should be placed.

(iv) Areas of magnetic anomalies which cannot be removed.

(v) Nose wheel turning circles.

(vi) If the base is to be used for carrying out 'electrical' swings a North-South line should be painted on the base, together with markings to indicate the locations of the compass calibrator monitor and turntable and the bearing of the reference target used when sighting the monitor (see also paragraph 5.6).

NOTE: Paint is the best medium for marking concrete. The datum compass circle, which may be on grass, must be marked permanently with a narrow continuous path of non-magnetic material such as tarmac or gravel.

3. BASE CLASSIFICATIONS

Compass bases may be established as either Class 1 or Class 2, the difference between them being only in the limits of permitted maximum deviation to be found anywhere within the base area as follows:

(a) Class 1. The maximum permissible deviation is ±0.10. Bases of this accuracy are required for carrying out refined swings, e.g. swinging of aircraft in which remote‑reading compasses are used as magnetic heading reference systems, in conjunction with such equipment as Doppler Systems.

(b) Class 2. The maximum permissible deviation is ±0.25'. Bases of this accuracy are suitable. for carrying out standard swings, e.g. swinging of aircraft in which the primary heading reference is provided by a remote-reading compass system, with a direct-reading compass serving as a standby.

NOTE: A location, the permissible deviation of which is greater than ±0.25' may be used where a direct-reading compass is used as the primary heading reference (see paragraph 5.5).

4. TYPES OF SURVEY

The following types of survey are normally carried out to assess the suitability of a location at which a compass base is to be finally established:‑

(a) Initial Survey. This is the first assessment survey of a location to determine gross errors and should be carried out by the aircraft operator or airport authority. If the deviations obtained appear to be within the permissible limits laid down for Class 1 and Class 2 bases there is justification for carrying out an establishment survey.

(b) Establishment Survey. This survey is of a more detailed nature in that measurements are taken at a greater number of more closely spaced points. The survey may also be carried out by the aircraft operator or airport authority, but, where appropriate, it is recommended that the services of the ACO be obtained (see paragraph 1.1).

(c) Periodic Re-survey. After a base has been established a detailed re-survey must be carried out at the following intervals:

(i) Class 1, every 5 years.

(ii) Class 2, every 2 years. In addition, bases of this accuracy should where possible, be surveyed by the ACO every 6 years.

(d)Annual Check. All bases should be checked annually to ensure that markings and boundaries are clearly defined and that no work has been done which might affect their magnetic properties and also to take into account changes in magnetic variation. If any doubt exists, the suspect area should be given a detailed magnetic survey.

(e)Area Survey. An area survey (see paragraph 5.6) is normally confined to the selection of a location which is to be used for carrying out a more specialised form of compass calibration procedure known as an 'electrical' swing.

5. SURVEY METHODS

There are two principal methods which may be adopted for the surveying of a compass base: (a) the reciprocal bearing method (see paragraph 5.3) and (b) the distant bearing method (see paragraph 5.4). In both methods, the use of accurate magnetic bearing compasses of either the medium landing type or the high‑precision datum type will be required to determine the effects of interference from local magnetic fields. There is also a third surveying method (see paragraph 5.5), but being of a lower order of accuracy its adoption should be strictly limited. For the area survey referred to in paragraph 4(e) units of a specially designed compass calibrator set are used (see paragraph 5.6).

5.1

Checking and Correction of Survey Instruments. Before carrying out a survey the appropriate survey instrument(s) should be given the full serviceability checks prescribed in the relevant operating manual, paying particular attention to checks which affect the repeatability of readings, e.g. pivot friction. Reference should also be made to any associated instrument test certificates to ascertain any instrument errors requiring correction. When the reciprocal bearing method (see paragraph 5.3) is used, bearings should be taken on a distant object with both compasses to establish a correction which can be applied to every reading taken from one of the compasses.

5.2

Positioning of Survey Instruments. In order that the deviation limits of a chosen location may be accurately assessed for base classification purposes from an establishment survey, the survey instrument(s) should be set up at close regular intervals. e.g. every 6 metres (20 feet), to cover the area quadrant by quadrant. The instrument(s) should be at the maximum height of its tripod which is approximately 1.5 metres (5 feet). In most types of aircraft, direct‑reading compasses and flux detector units of remote‑reading compasses are above this height. If a base is also to be used for an aircraft the compasses and detector units of which are below 1.5 metres (5 feet), assessment should then be made closer to the ground.

NOTE: At certain stages of an area survey procedure, assessments are made with the tripod set at both minimum and maximum heights (see paragraph 5.6).

5.3

Reciprocal Bearing Method. This method is the most accurate and may be adopted for an initial assessment survey, a detailed establishment survey and a periodic re‑survey. It requires the use of two precision datum compasses, one being designated the master compass and the other the mobile compass. The procedure is as follows:

(a) Following the checks for serviceability, both compasses should be aligned to a common magnetic datum. This is done by setting them up, in turn, on a tripod positioned as near to the anticipated centre of the base as is practical and sighting the compass on a distant object and noting, for each compass, the average of several determinations of the magnetic bearing. This produces the correction referred to in paragraph 5.1 which is subsequently applied to the readings of the mobile compass. Frequent cheeks on the accuracy of this correction should be made throughout the subsequent stages of the survey procedure.

(b) The master compass and tripod should be kept in its original position and the mobile compass and its tripod should be positioned at various points around the area to give good coverage (see paragraph 5.2). At each point, the two compasses are aligned on each other's sighting telescope object lenses.

(c) When the compasses are aligned, bearings should be taken from the bearing plates of the compasses and the magnetic deviation between the two compass positions should be obtained by taking the difference between bearing plate readings and subtracting 180'. The sign convention used for the deviation is that, if the reading of the mobile compass is greater than that of the master compass, the deviation is negative. Conversely, the deviation is positive if the master compass reading is greater than that of the mobile compass.

(d) The deviations should be recorded on an observation log, which should take the form of a scaled diagram of the area. The positions of any objects in the area such as drains, cable duct covers, lights, picketing points, etc., should also be indicated on the log. Areas in which deviations are in excess of the limits permitted by the appropriate base classification, should be investigated and, where possible, the source of magnetic interference should be eliminated. Where magnetic interference cannot be eliminated the area should also be indicated on the observation log as a prohibited area, i.e. an area which must be avoided when positioning an aircraft and datum compass for the purpose of swinging.

(e) Care should be taken to ensure that there are no obvious magnetic objects near the chosen centre of the base. If the deviations are such that their mean is more than half the deviation limits for the class of base being surveyed, it should be assumed that there is a buried object near the base centre.

5.4 Distant Bearing Method. This method should be used only for initial surveys and for gross error checks of Class 2 bases carried out because of doubts raised during annual checks. The procedure to be adopted is as follows:-

(a) Select a distant object at least 2 nautical miles away and accurately locate its position and die position of the compass base on a larger scale map and measure the distance between them. Mark the line of sight from the centre of the compass base to the distant object.

(b) Calculate the angular correction to be applied to bearings taken away from the base centre using the formula:–

(c) After ensuring that there are no objects in the area of the base centre likely to have a magnetic influence, the bearing compass, which may be of the medium landing type or precision datum type, should be set up and a datum bearing obtained by measuring the bearing of the distant object.

(d) Take bearings of the distant object from selected points around the base area and after applying the calculated corrections, compare the bearings with the datum bearing. Any difference obtained will be due to deviations present, assuming that the base centre is free from deviation. The deviations should be recorded on an observation log (see paragraph 5.3(d)).

5.5

Surveying Pole Method. This method is simpler than those already described and requires the use of two poles similar to those used by a land surveyor and in addition, a medium landing compass. Its survey accuracy is, however, of a lower order, principally because it does not utilise magnetic bearings of distant objects as a datum. The use of this method should, therefore, be restricted to the surveying of locations at which deviation limits outside those permitted under Class 1 or Class 2 (see paragraph 3) are acceptable; for example, a location for swinging aircraft using direct‑reading compasses as the primary heading reference. It may also be used in such cases as the initial assessment of gross errors prior to a detailed establishment survey of a base and where, in the absence of an established Class 1 or Class 2 base or more accurate surveying equipment, a swing is necessary to enable an aircraft to undertake a positioning flight. The procedure for carrying out this method is as follows:~

(a) One surveying pole should be placed in the centre of the area chosen and the medium landing compass should be positioned and levelled 9.1 metres (30 feet) to the south of the pole. A plumb bob should be suspended from the centre of the compass to the ground and the sighting device should be set to read due North. The second pole should be positioned 9.1 metres (30 feet) to the North of the centre pole, so that, when viewed with the compass sights as set. the two poles are in alignment.

(b) The plumb bob position should be marked with a peg or a painted mark and the position of the second pole and the compass should be interchanged. The compass reading with the poles as now positioned should be checked and should be within ±1° of the reciprocal of the initial reading.

(c) A further check should be made by moving and sighting the compass along a line between the North and South points already obtained, taking at least four readings at approximately equidistant intervals. The compass should not deviate by more than 1° from the original reading at any position.

(d) The same procedure outlined in paragraphs (a), (b) and (c) should be followed for determining the East and West positions.

(e) The geometric location of the cardinal points should be proved by checking the chord distances between the points, as indicated by pegs or painted marks. If they are equal then the North‑South and East‑West lines are at right angles. In any case, the measurements should agree within ±76 millimetres (3 inches).

(f) The whole of the foregoing procedure should be followed for determining deviations in the inter-cardinal areas and in the area around which the compass is to be positioned during the swinging procedure.

5.6

Area Survey. This survey is carried out using the monitor and console control units of a compass calibrator set which is.also designed for the 'electrical' swinging of remote‑reading compass systems. In this type of survey the principal objectives are (a) to determine the direction and strength of the earth's magnetic field at the locations of aircraft flux detector units (detector unit location check); (b) to select the point at which the compass calibrator monitor unit should be located in order to carry out an 'electrical' swing (monitor location check) and (c) to mark out the monitor and turntable location points and also a North‑South line over which an aircraft must be positioned during a swing. The full setting‑up procedures and operating instructions are detailed in the calibrator operating manual, and reference to this document should, therefore, always be made. The information given in the following paragraphs is for general guidance only.

5.6.1

Detector Unit Location Check. The purpose of this check is to determine the uniformity not only of direction, but also the strength of the earth's field at points of the compass base which will correspond to the locations of detector units, e.g. in the vertical stabiliser, or wing tips of an aircraft. The strength of field is determined in order to obtain certain voltage values which must be set up in the calibrator control console unit~ during an 'electrical' swing procedure, to simulate the earth's field. The monitor and console control units are electrically interconnected by the appropriate cables and are set up within 3 to 4.6 metres (10 to 15 feet) of each other. Magnetic direction and strength is measured in both the vertical and horizontal planes.

(a) Measurements in the vertical plane are taken with the monitor mounted on its tripod, adjusted firstly to the minimum height position and then to the maximum height position. Direction is determined in each position of the tripod by setting the monitor on each of the four cardinal headings and noting the difference between these headings and the headings recorded on the control console unit. The average of the errors (the algebraic sum divided by four) is recorded as the monitor index error. The difference between the index errors at the minimum and maximum height positions is then calculated. If the difference exceeds the specified value (6 minutes is a typical value) the area surveyed is unsuitable. Field strength is determined at the minimum and maximum height positions of the monitor tripod, by setting the monitor to a heading of zero degrees and obtaining voltage values from settings made on the control unit.

(b) Measurements in the horizontal plane are carried out to determine the uniformity of the earth's field direction and strength over a circle of 1.5 metres (5 feet) radius, the centre of which is at the location of the flux detector unit on the aircraft. The readings are taken at the centre of the circle with the monitor tripod at its normal operating height and with the monitor set~ in turn, on each of the four cardinal headings. The differences between monitor and control console unit headings are then noted and a monitor index error obtained in the same manner as that described in (a). Readings are then taken on the cardinal headings with the monitor and tripod positioned, in turn, at four equidistant points on the perimeter of the circle and corresponding monitor index errors are obtained. The algebraic difference between these errors and the index error at the centre of the circle is then calculated and should be within the limits ±6 minutes. Field strength is determined by setting the monitor to a heading of zero degrees and obtaining voltage values with the monitor at the centre of the circle and at the four equidistant points on its perimeter.

5.6.2

Monitor Location Check. The purpose of this check is to select the location for the monitor in order to measure the earth's field during the 'electrical' swinging procedure. The location selected should be such that with an aircraft positioned on the base, the monitor readings will not be influenced by magnetic effects of the aircraft itself. A distance of 23 to 30 metres (75 to 100 feet) is normally sufficient. The direction and strength of the earth's field is determined at the selected location and corresponding monitor index error and voltage values, calculated. The bearing of a reference target at least 800m (one-half mile) distant from the location is also obtained. As this target is to be used during a compass swing it should be ensured that it will be visible from the monitor location when an aircraft is positioned on the base. The suitability of the selected monitor location is then determined by re-positioning the monitor at the flux detector unit location and measuring the direction and strength of the earth's field and then calculating the algebraic difference between values at each location. The readings at each location should be taken within a time of 30 minutes of each other to lessen the possibility of a change in the earth's field.

5.6.3

Marking of Base. On completion of the foregoing checks, markings must be permanently set out on the base to indicate the following:

(a) Location of Flux Detector Units. For aircraft the flux detector units of which are installed in the vertical stabiliser, the marking is made on the North-South line (see (c)) and for aircraft the flux detector units of which are installed in each wing tip, the markings are made each side of the North-South line at distances corresponding to those from the aircraft centre line.

(b) Location of Monitor. In addition to this marking, the bearing of the reference target used during the survey should also be marked at the monitor location.

(c) North-South Line. This line should be marked out from a point which is used as a reference in determining the flux detector unit location. The monitor is set up over this point and by lowering the monitor telescope so that its graticules are observed against the base, several other points are marked out; firstly, with the monitor on a corrected zero degree heading and then on a heading of 180°degrees. The points are then joined by a painted line.