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Aeronautical charts and compass

Rev. 47 — page content was rearranged 13 September 2013
Flight Planning and Navigation

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Pilots operating under the Visual Flight Rules in Australia are required to carry — and have readily accessible in the aircraft — the latest issues of the aeronautical charts and other aeronautical information relative to the flight. Such material is generally in a printed paper format.

The charts used for air navigation are overlaid with a coordinate reference graticule system showing the local meridians of longitude and the parallels of latitude. In aviation, surface locations are generally defined in terms of latitude and longitude, while chart directions in azimuth are referenced in relation to true north. Topographical charts also indicate terrain elevation by the use of contour lines and spot elevations, thus safe operating altitude above terrain can be derived.

Note: at September 2013 two Australian data service providers hold a CASA instrument of approval for some types of digitised aeronautical charts, so private VFR pilots are able to store those charts in an 'electronic flight bag [EFB] system so replacing paper charts, as the primary means of in-flight documentation. Even so, although an EFB is a paper replacement system, it is prudent to carry back-up paper charts.

The prime navigational direction instrument — the magnetic compass — aligns itself with the north magnetic pole and, in Australia, the variation between the direction to true north and that to magnetic north can be as much as 13°.


2.1 Defining position – latitude, longitude, altitude and time

Lateral dimensions
In aerial navigation any point on the Earth's surface may be precisely defined in terms of a latitude and longitude graticule reference.

Meridians of longitude are half 'great circles', perpendicular to the equator, that extend from pole to pole. The meridians are identified by the angle that they subtend, at the centre of the Earth, with the prime meridian. That angle is measured in degrees, minutes and seconds east or west from the prime meridian:
  • the WGS84 International Reference Meridian — or 0° longitude or prime meridian — passes about 102 metres east of the originally defined Greenwich meridian at the Greenwich Observatory, England
  • subsequent meridians are identified as degrees east or west around to 180°
  • there are 60 minutes of arc in a degree and 60 seconds of arc in a minute.

Parallels of latitude are 'small circles' drawn around the Earth starting from the equatorial plane, north and south of the equator and parallel with it and reducing in circumference toward the poles. For our purposes we can say that the parallels appearing on aviation charts are identified by the angle that they subtend with the equatorial plane, i.e. they are geodetic, measured in degrees, minutes and seconds and whether they lie north or south of the equator:
  • the north pole is a point position having a latitude of 90°N
  • the south pole is a point position having a latitude of 90°S
  • the equator has a latitude of 0° and is a great circle, in that it is formed by a plane that passes through the Earth's centre, bisecting the Earth's sphere.

One nautical mile is the length, at the Earth's mean sea level surface, of one minute of arc of a great circle. The International Nautical Mile is 1852 metres or 6076.1 feet. Consequently, one degree of latitude (measured along a meridian) has an equivalent surface distance of 60 nautical miles, and one second of latitude is about 31 metres, while 1/100th of a second is about 0.3 metres. Seconds of arc are generally not used in those aeronautical publications intended for navigation under the Visual Flight Rules; latitude and longitude is expressed in degrees plus minutes to (generally) one decimal place — about 185 metres. For example the reference point for Mount Beauty airstrip in Victoria is located at S36° 44.1' E147° 10.2'; aerodrome reference points (usually regarded as the centre of the airfield) are defined in degrees, minutes and tenths of minutes. However, when necessary, the location of a point position may be specified much more precisely; some point locations for instrument landings are required to be specified to 1/100th of a second.

Some systems may use degrees only, in which case the degrees may be expressed to five decimal places, e.g. S36.73499

Incidentally, a 'knot' is a speed of one nautical mile per hour.

It is logical to express 'Lat/long' coordinates with the direction from the equator/prime meridian first (e.g. S and E), then a numeral group representing the degrees followed by a group for the minutes. The symbols for degrees and minutes are omitted, e.g. S36 44.1 E147 10.2. That is the standard format for geographic locations in ERSA. However in the global navigation satellite system (GNSS), and other systems, the northern hemisphere latitude coordinates may be represented as a positive value and the southern hemisphere as a negative value, while the longitude coordinates for the western hemisphere have a negative value and those for the eastern hemisphere have a positive value, so S36 44.1 E147 10.2 is represented as −36 44.1 +147 10.2. The positive sign is usually omitted for the northerly and easterly coordinates.
Effect of continental drift on precise location
The Earth's latitude/longitude reference graticule is regarded as fixed relative to the Earth as a whole, but the continents are in motion. The Australian tectonic plate, for example, is moving north north-east towards the North Pacific at the rate of seven centimetres per annum*. So, during the last 14 years every fence post in Australia has moved one metre north north-east and their precise latitude and longitude reference position has changed, and will continue to change. Of course these tectonic plate movements have no discernible effect on aerial navigation but they do complicate land survey activities. In Australia, to overcome this the 1994 Geocentric Datum of Australia [GDA94] uses a reference meridian that is fixed relative to the Australian tectonic plate rather than the International Reference Meridian. The map projection for GDA94 is the Map Grid of Australia [MGA94].

*For comparison, it is estimated that the average fingernail growth is 3.5 cm per annum.
The third positional dimension – altitude
Contour lines and spot points on topographical maps provide an indication of terrain elevation — i.e. height above the Australian Height Datum. The aircraft's altimeter reading provides the aircraft's vertical position and thus the current height above the terrain indicated on the chart — height above ground level [AGL] or the terrain clearance — may be determined.
Universal Coordinated Time
Time is a most important dimension in aerial navigation; the reference time is Universal Coordinated Time (symbol UTC — a compromise between the initialisms of the preferred French and English names) rather than local times. UTC is the time at the International Reference Meridian and is an average of a large number of atomic clocks. The suffix 'Z' is used to identify dates and times as UTC, so it may be referred to as 'Zulu' time — the phonetic for 'Z'.

UTC and the 24-hour clock system — rather than local time — are used throughout the aviation information, communication and meteorological services. UTC is 10 hours behind Australian Eastern Standard Time, 9.5 hours behind Australian Central Standard Time and 8 hours behind Australian Western Standard Time. Add an additional hour in a daylight saving time period.

2.2 Defining the shape of the Earth – geoids and ellipsoids

The Australian chart elevation reference datum — the Australian height datum
The Australian height datum* [AHD] is based on the average local sea level as observed throughout 1966-1968 at each of 30 tidal gauges distributed around the coastline and is the zero elevation reference for Australian aeronautical charts; i.e. the elevations shown on the charts are height above the AHD or mean sea level [msl].

* A datum is the fixed reference or starting point of a scale or measurement system e.g. an aircraft weight and balance pre-flight check. In this context the plural is datums not data.
The Australian geoid
The Earth's density is not uniform throughout, thus gravity and its perpendicularity — and consequently msl (or AHD) distance from the geometric centre of the Earth — varies irregularly around the surface of the globe. A geoid is a notional surface of equal potential gravity within the Earth's gravity field, that describes that irregular shape and basically follows mean sea level over the oceans and extends through the continents. The current Australian geoid is AUSGeoid09 but for aerial navigation it can be regarded as equivalent to the Australian Height Datum ±0.5 m. Check Geoscience Australia for more information about geodetic datums.
The World Geodetic System 1984
A geoid is not the same as an ellipsoid (a smooth, slightly flattened sphere), which is a mathematically (rather than physically) derived representation of the Earth's underlying shape. The WGS84 ellipsoid is a mathematical representation of the Earth's underlying shape with an equatorial radius of 6 378 137.0 metres, a polar axis radius of 6 356 752.314 metres and a flattening ratio of roughly 1:300. There are many ellipsoids in use but WGS84 is of most interest to Australian aviators because it is the reference ellipsoid used by the Global Positioning System and is its basis for GPS altitude, whereas the Australian Height Datum (a 'geoid') is the basis for elevations on Australian navigation charts. GLONASS uses other ellipsoids.

For aerial navigation and cartography purposes the shape of the Earth is defined by the WGS84 ellipsoid providing the standard coordinate frame for navigation/cartography systems. Some Australian charts may also show the GDA94 as the datum, which is fixed relative to the Australian tectonic plate as mentioned above, however for navigation purposes, this is compatible with WGS84.
Geoid-ellipsoid separation and GPS altitude
The difference in elevation of a particular point on the Earth's surface — when measured against both the ellipsoid and the geoid — can be quite considerable, as much as ±100m ; this is known as the geoid-ellipsoid separation.

In Australia the degree of geoid-ellipsoid separation is quite unusual. The image below shows the substantial geoid undulation that slopes across Australia. In the south-west corner of the continent AUSGeoid09 is 33m below the WGS84 ellipsoid while at the tip of Cape York in the north-east corner it is 72m above the ellipsoid. As shown in the image the geoid and ellipsoid coincide (i.e. zero separation) on a rough line between Port Hedland and Melbourne.

AUS Geoid98 separation

(Image courtesy of Geoscience Australia).

The local value (known as the 'N-value') of the geoid-ellipsoid separation might be shown on aeronautical navigation charts but the values are not shown on Australian charts. The local N-value is of little significance to recreational aviators (although it should be noted that a GPS instrument may give an apparently incorrect height if the software doesn't adjust for the local N-value*) but may be of great significance to IFR pilots and designers of GPS approaches when the GNSS achieves sole-means navigation status for all flight phases. A table of the geoid-ellipsoid separation value for each cell of a roughly one nautical mile square grid covering Australia is produced by Geoscience Australia's National Geographic Information Group — previously known as AUSLIG. AUSGeoid09 provides the AHD-to-ellipsoid separations, see the AustGeoid09 on the Geoscience Australia site.

*Note: some GPS receivers may store just a single N-value for each 10° latitude/longitude graticule cell. As can be seen from the image above some 10 x 10 degree cells have a 40-50m variation diagonally across the cell. If the N-value is not used or just approximated, the calculated GPS altitude may be incorrect.

2.3 Aeronautical charts

Chart system basics
A chart system is built on three basics that must be defined for use:
  • the projection employed — generally the 'Lambert conformal conic projection' for air navigation.
  • the coordinate system — latitude and longitude for air navigation.
  • the geodetic datum — WGS84 [or GDA94] is the standard horizontal (area) datum for most Australian aeronautical charts and the Australian Height Datum is the vertical datum.
Note: when using a GPS receiver ensure that these three formats have been selected correctly, particularly the WGS84 datum.

A map intended for aerial or marine navigation is usually referred to as a 'chart'. The chart graticule is latitude and longitude, with the meridians more or less vertical on the sheet but converging slightly. As the Earth is ellipsoid, there has to be a technique to map the image of the surface of the three-dimensional ellipsoid onto a flat two-dimensional chart without overly distorting the represented areas. The most suitable projection technique for world aeronautical charts is the 'Lambert conformal conic projection'. Although this projection distorts areas a little, distances anywhere on the chart have the same scale. The great circle arc* — the shortest distance between two points on the surface of a sphere — can be represented reasonably accurately by the flight planner drawing a straight line between two points on the chart. However you will note that the angle at which that straight line crosses each meridian changes because of the convergence of the meridians.

*Note: the shortest distance between, say, Sydney and Perth, is a straight line (a tunnel) joining those cities and passing through the Earth. The great circle route follows that 'tunnel' on the surface.

The Lambert chart legend will indicate the latitudes of two 'standard parallels'. There is no scale distortion at these parallels, however scale distortion increases with distance from a standard parallel. For an explanation of standard parallels see www.icsm.gov.au/mapping/about_projections.html and look for the heading 'Multiple standard parallels or central meridians'.

Those meridians of longitude shown on Lambert conformal aeronautical charts are straight lines, that converge towards the poles*. On a southern hemisphere chart the meridian spacing between the meridian lines at the bottom of the sheet is a little less than that at the top — about 5 mm on an Australian 1:1 000 000 World Aeronautical Chart. A central meridian drawn on each chart is vertical and the others converge towards it. The parallels of latitude as shown on the chart are arcs of circles and cross all the meridians at right angles because of the slant of the meridians. If a straight line is drawn diagonally across the chart, the angle that this great circle route subtends with each meridian varies slightly across the chart. Aircraft flying very long legs would alter their heading slightly every 500 nm or so to maintain the great circle route and thus the shortest distance.

*Note: that convergence of the meridians is why the 'grid' on such charts is called a 'graticule'; the meridians and parallels do not form true rectangles, i.e. a 'grid'. If you joined a number of WACs together by matching parallels and the edge meridians the maps would form an arc.

On Mercator (a 16th century Flemish geographer) cylindrical projection charts, straight line plots are 'rhumb lines' and great circle plots are curved. A rhumb line is a line drawn so that it crosses the meridians of the Mercator projection at a constant angle, but it is not the shortest distance between two points; an aircraft flying a constant track heading would be following a rhumb line plot. The concept of choice between a great circle route or rhumb line route is interesting but inconsequential to a light aircraft navigator because a constant track heading (i.e. a rhumb line track) is usually flown for each leg; except, perhaps, if planning a direct route from Australia to New Zealand.

The scales used for aeronautical charts are the representative fractions 1:1 000 000, 1:500 000 and 1:250 000. The latter scale means that an actual distance of 2.5 km (250 000 centimetres) is represented by one centimetre on the chart. The 1:1 000 000 scale is a small-scale chart; i.e. it covers a large area but with minimum detail, one centimetre represents 10 km. The 1:500 000 and 1:250 000 are larger-scale charts that cover progressively smaller areas but with increasing detail.

The Australian Intergovernmental Committee on Surveying and Mapping's Fundamentals of Mapping is well worth visiting.

Recommended VFR charts
      The paper charts recommended for sport and recreational aviation VFR flight planning, in-flight navigation and sourcing VHF radiocommunications data are:
  • Planning Chart Australia: the PCA is a single sheet showing the coverage of the WACs (below), the meteorological area forecast [ARFOR] boundaries, the estimated FIS VHF coverage from both 5000 feet amsl and 10 000 feet (but not the frequencies), and the areas without FIS VHF coverage. The FIS HF communication frequencies are shown. The spot location of about 700 named airfields is indicated. PCA is designed to assist in initial VFR flight planning and it is amended semi-annually. It is of rather limited use in initial planning of flights below 5000 feet (i.e. most ultralight flights) in eastern Australia because straight-line tracks between departure point and destination may be precluded because of the topography, and there are no indications of such on the PCA. But it is generally okay for use west of the Dividing Range. Also it is the only chart that indicates FIS VHF coverage; essential knowledge if a flight is being planned into the less accessible areas of Australia.

  • World Aeronautical Charts: the 43 Australian WACs are small scale (1:1 000 000 or 1mm=1km), derived from aerial photography, and designed for pre-flight planning and pilotage. They are part of an ICAO international series. They do not indicate CTR or PRD, nor is there any FIA, radiocommunications or radionavigation information. As the reissue frequency is 2–4 years (i.e. 50% of the 43 maps are supposed to be re-issued every two years) the base can be slightly out of date, particularly in regard to the infrastructure. Amendment lists for each edition are published in AIP SUP but these amendments generally relate to location of airstrips and special activities rather than topography or infrastructure. Each WAC generally covers 6° of longitude and 4° of latitude. Sheet dimensions are about 70 × 60 cm and the scale is such that a real distance of one nautical mile is represented by less than 2 mm on the chart; thus WACs are really not suited to low-altitude navigation in slow aircraft, but it is wise to always have the latest edition of the WAC/WACs — relevant to the journey — in the cockpit.

  • Visual Navigation Charts: the VNCs are a larger scale at 1:500 000 and show airspace information and FIS detail laid over the topographic base. All VNCs are reissued at six-monthly intervals but the base topographic detail may not be up to date. They are far superior to the WACs for both flight planning and pilotage. VNC sheet dimensions are about 100 × 60 cm and contain the following airspace detail:

    • CTR, CTA dimensions and lower levels
    • Flight Information Area and Surveillance Information Service boundaries where available
    • Flight Information Service and Surveillance Information Service frequencies and providers
    • communication and navigation aid frequencies for licensed airfields
    • PRD and designated & remote areas.

    There are only 15 VNCs, those available covering the more populous areas of Australia — Tasmania to North Queensland, plus areas around Perth, Adelaide, Darwin and Tindal.

  • Visual Terminal Charts: the 25 or so VTCs provide both aeronautical and topographic information around major airports at a scale of 1:250 000. They are essential for VFR operations in the vicinity of such airports to avoid violating controlled airspace. In some cases, these charts show the details of tracks to be flown and significant landmarks to be used by pilots of VFR aircraft to avoid inadvertent entry into controlled airspace. All VTCs are amended and reissued every 6 months. The charts are based on the NATMAP 250K series maps and use the Universal Transverse Mercator [UTM] projection but with a latitude/longitude graticule rather than the normal UTM grid; their dimensions are around 90 × 50 cm and show the following details:

    • PRD areas
    • CTR and associated CTA dimensions including the lower levels of the CTA steps surrounding the airport, lanes of entry, ATC check points
    • Surveillance Information Service frequencies where available
    • communication and navigation aid frequencies for licensed airfields
    • VFR approach points.

  • En Route Chart (low level): the ERC-L series is drawn to various scales to accommodate significant air traffic route areas and shows controlled airspace, PRD areas, air routes and segment distances, ATS and radio-navigation services, ATS frequencies and location, plus communication and navigation aid frequencies for licensed airfields — but no topography. It also indicates those airfields where VHF radio contact with FIS is possible from the ground.

    The FIS area boundaries are shown together with an information box showing the provider of the flight information service (e.g. Brisbane Centre), the frequency and the location of the area transceiver.

    The series of eight sheets cover Australia and are intended primarily for IFR flights conducted below 20 000 feet. The multitude of air routes that radiate from major cities make the charts difficult to read but they are the only chart series that show all the FIS frequencies, PRD areas and give indications of sports aviation activities — thus they are an essential document for VFR navigation. Each route segment is a great circle route with the magnetic track angles measured at the end points rather than the middle of the segment, which is why there is an apparent discrepancy in the reciprocal track angles. Reissue frequency is twice per year.

      PCA, WAC, VNC, VTC and ERC-L can be purchased from the Airservices Australia online store. You can see the coverage for each sheet in each series by clicking 'Coverage Map' on their 'Aviation charts' page. These charts can also be ordered from pilot supply shops. Possibly the TPCs may be purchased from the National Mapping Division of Geoscience Australia.

Satellite and aerial images of the Earth's surface are also available via the Google Earth and Google Map geobrowsers and provide help in flight planning; for example, the ability to locate an unlisted airstrip and establish the exact lat/long coordinates for entry into a GPS.
Carriage of flight documentation
      AIP ENR 1.10 para 5.1 states:

'Pilots are required to carry, and have readily accessible in the aircraft, the latest editions of the aeronautical maps, charts and other aeronautical information and instructions, published:
a. in AIP, or
b. by an organisation approved by CASA,
that are applicable to the route to be flown, and any alternative route that may be flown, on that flight.'

(The AIP entry is an extract from CAR 233 'Responsibility of pilot in command before flight')
Digitised aeronautical charts
      The WAC, VNC, VTC and ERC-L charts, and others for flight under the instrument flight rules [IFR] , are also available in digitised format — raster or vector images — for use in tablet computers with flight planning software and for inflight use with portable electronic devices with moving map software. They have the same reissue frequency as the paper charts. This is discussed in the 'Electronic planning and electronic flight bag' module.

2.4 Map topography

Aircraft operating under the VFR must navigate by visual reference to the ground. The lower the level at which a flight is planned, the more important it is that the pilot is able to visualise a three-dimensional image of the terrain from the graphical details presented by the two-dimensional topographic chart — by the usage of colour, symbols and lettering. To assist this visualisation, WACs and VNCs display tinted topographic contours signifying surface areas between the 660 feet (200 m) and 1639 feet elevations, 1640+ feet (500 m), 3280+ feet (1000 m), 4920+ feet (1500 m) and 6560+ feet (2000 m) levels. The shape of the contours and the width between them indicates the form of the land and the gradient. The closer the contour lines (i.e. the narrower the colour bands) are to each other, the steeper the gradient.

Also the WAC utilises relief shading of elevated ranges and ridges so that they are more evident. In addition, spot elevations are shown and the highest spot elevation within each chart graticule is recorded in a bolder lettering than other spot elevations. The graticule on the WACs and VNCs is spaced at 30 minutes of latitude and 30 minutes of longitude: 30 nm in latitude and, for much of Australia, around 24 nm in longitude.

The contours on VTCs are at 500+, 1000+, 2000+, 3000+, 4000+ and 5000+ feet amsl, but in addition all areas are shaded purple where there is less than 500 feet of clearance between the terrain and the lower limit of the overlying controlled airspace. Like WAC and VNC, the highest spot elevation within each chart graticule is shown in a bolder type than other spot elevations. The graticule is spaced at 10 minutes of latitude and 10 minutes of longitude: 10 nm in latitude and around 8 nm in longitude. The VTCs generally cover an area within a 40–50 nm radius from the major airport and are the essential chart for visual navigation within that area.

Vegetation is usually not shown on WACs, nor are many structures except for towers and similar obstructions to low-flying aircraft; although grain silos — which are an excellent navigation aid usually associated with a railroad — are shown. Railroads, power transmission lines and some roads are depicted.

2.5 Defining direction — the aircraft direct reading magnetic compass

Sport and recreational navigation under the VFR is basically azimuth and distance and is also short-range i.e. each leg is usually less than 500 nm or so. Directions in azimuth are usually expressed as the angular distance from the north pole — true north — in whole degrees from 0° at north clockwise to 360°; i.e. north is both 0° and 360° (though is usually expressed as 360°). For example, the direction due east from any particular location is 090°. These directions may be described as bearings, headings, courses or tracks depending on the application. Direction is usually paired with distance expressed in nautical miles, thus the bearing and distance of a location 55 nm due east would be expressed as bearing 090°/55.

However, the prime navigational direction instrument — the magnetic compass — aligns itself with the north magnetic pole and, in Australia, the variation between the direction to true north and that to magnetic north can be as much as 13, so there is a need to define directions in terms of 'degrees true" or 'degrees magnetic'.

Civil Aviation Order 20-18 specifies just four mandatory flight and navigational instruments for flights under the day Visual Flight Rules. These basic instruments are:
  • an airspeed indicating system
  • an altimeter, with a readily adjustable pressure datum setting scale graduated in millibars
  • an accurate timepiece indicating the time in hours, minutes and seconds, which may be carried on the person of the pilot and
  • a direct reading magnetic compass.
If the aircraft is a Light Sport Aircraft for which a current special certificate of airworthiness or an experimental certificate has been issued it need not carry the individual instruments as defined above, if equipment is carried that provides a pilot with the same information, i.e. an electronic flight display.
Magnetic variation
The simple direct reading compass is essentially a bar magnet freely suspended in a lubricating fluid designed to damp out oscillations, vibrations and swings caused by aircraft accelerations. The bar magnet, which may be a needle or part of a circular compass card, aligns itself with the Earth's local magnetic lines of force with the north-seeking end pointing roughly north. The Earth's magnetic field is systematically surveyed so that the difference between the direction at which a compass points — magnetic north— and the direction of true north is measured. That difference is called variation, or declination if you are of a scientific bent, and is expressed in degrees of arc east or west of true north. The magnetic lines of force at any location may also be substantially varied by local magnetic anomalies — substantial iron ore deposits for example. Lines on a chart joining locations with equal magnetic variation are isogonals, or isogonic lines, and are shown on WACs and VNCs as dashed purple lines at half-degree intervals. The local variation may also be shown numerically on some charts. The isogonals on Australian charts vary from 3° west in the south-west corner of the continent to 13° east on the eastern coast.

This means that if you want to fly from A to B, the direction ascertained from the chart will be relative to true north — the true course — and let's say it is due west, 270°. If you then set 270° on the aircraft compass and fly that heading then your track over the ground will not be due west but will vary according to the variation. Let's say the variation is 10° east then the true course you are flying will be 280°. This small complication requires that when you have finally calculated the true course you have to fly to get from A to B, after allowing for the effects of wind, then you need to convert it to a magnetic heading. The conversion rule used for at least the past 70 years is: "Variation east, magnetic heading least; variation west, magnetic heading best". So if the local variation is 12° east the magnetic heading will be the true course minus 12°; e.g. true course 010°, magnetic heading 358°. If the variation is 2° west the magnetic heading will be the true course plus 2°; e.g. true course 010°, magnetic heading 12°.

For all wind velocities, given in meteorological forecasts and actuals, the directions are relative to true north, except if you happen to hear a broadcast from a CTR tower controller (or an Automatic Terminal Information System [ATIS] broadcast) who provides the wind direction as magnetic, because the airfield runway numbers are relative to magnetic north. The air route directions shown on ERC-L are also relative to magnetic north.
Compass deviation
Aircraft compasses are also deflected by magnetic fields within the aircraft, some related to ferrous engine/structural metals, others related to electrical currents. These aircraft magnetic fields produce heading errors — compass deviation — which vary according to the aircraft course, either reducing or increasing the Earth's magnetic field. These errors can be quite significant, 30° or more, and any magnetic field within about one metre of the compass may have a discernible effect. Mobile telephones in the cockpit may also affect the compass. Compass error is the combination of variation and deviation adjustment necessary to determine the compass heading that will provide the true course.

A bar magnet aircraft compass will have screw-adjustable compensating magnets to negate or at least reduce the effect of these magnetic fields. The compass and aircraft must be 'swung' to make these adjustments, and the residual deviation errors noted on a compass correction card displayed in the cockpit. Residual deviation errors should not exceed 10° at any compass point. The procedure for 'swinging the compass' is time-consuming and difficult but necessary. We will go further into compass deviation in the 'En route adjustments' module.

Airfield runway numbers are stated as their magnetic heading rounded off to the (supposedly) nearest 10°; thus an east-west runway will be numbered 09/27. The ERSA entry in the "Physical characteristics" section for the airfield usually shows the actual magnetic heading following the runway numbers, but only for one direction. For example at Dubbo aerodrome '05/23 043' indicates the actual magnetic heading for runway 05 is 043° magnetic, and consequently 223° for runway 23. Thus, when stationary and accurately lined up for take-off on such a runway, you can measure deviation on that heading; but make sure the compass has stopped moving. Flying to a few airfields and checking deviation at various runway headings is one way of producing a compass correction card. Always make sure the compass fluid level is okay. A vacuum chamber for de-aerating the compass fluid must be used in the re-filling process — using the proper fluid, not alcohol.

Bar magnet compasses are also affected by vibrations, aircraft accelerations and inertia when turning; thus they tend to be shifting constantly. Compass acceleration errors are most apparent when the aircraft is on an east/west heading and least apparent when on a north/south heading. The turning errors require the pilot to make an undershoot/overshoot adjustment when changing heading. To overcome these errors, normally the magnetic compass is accompanied by a gyroscopic instrument that indicates the direction in which the aircraft is heading, without being subject to external forces. This electrically or suction-operated directional gyro [DG] or direction indicator [DI] is initially aligned with the compass before take-off and needs to be realigned occasionally during flight; however, few ultralights are equipped with DGs.

Electronic flight information systems [EFIS or 'glass cockpits'] are now becoming much cheaper and thus a reasonable proposition for amateur-built light aircraft. These systems use solid-state electronic componentry plus software to present a cockpit display incorporating the functions of most single flight instruments. In such systems magnetic field strength sensors (magnetometers) are used to provide a three-dimensional magnetic compass that displays magnetic heading without acceleration, attitude or turning errors; thus it also incorporates the DG facility. The simple direct reading magnetic compass must still be part of the aircraft equipment.

Things that are handy to know

   •   There are other maps such as the Australian NATMAP 250K series. This is intended mainly for surface use, with the GDA 94 datum and using the Map Grid of Australia projection (MGA 94) which conforms with the Universal Transverse Mercator (UTM) projection with usually the metric UTM Eastings and Northings grid — rather than a latitude/longitude graticule. This coordinate system is more complex than latitude/longitude — the Earth's surface is divided longitudinally into 60 six-degree numbered zones and Eastings are measured in metres (to 3 decimal places) from the central meridian of each zone. Northings start from zero at the equator in the northern hemisphere but, for the southern hemisphere, start from a 10 000 000m base Northing at the equator. See en.wikipedia.org/wiki/UTM_coordinate_system.

Fortunately the digitised NATMAP 250K series are also available with a latitude/longitude graticule, so these larger-scale (1:250 000) maps could be used for the limited leg distance of recreational aircraft navigation, particularly with GPS. Each map covers an area of 1.5° longitude and 1° latitude. VTCs, being based on the NATMAP 250K, use the Transverse Mercator projection with a lat/long graticule. Some UTM maps may show the lat/long graticule* in one colour with the UTM grid* in another.

*Note: 'grids' are rectangular in shape; the 'graticule' is not — the meridian lines converge poleward.

The digitised NATMAP 250K series may be purchased from Geoscience Australia. The 513 maps of the NATMAP 250K series are available on DVD for about $100 which is less than 3% of the cost of the paper series and well worth having as home reference material — even if you don't use them for aeronautical navigation. They are in ECW format and software is supplied for viewing and for export to geoTIFF, TIFF, JPEG, PNG, bitmap or OziExplorer format. Image resolution is 200 dpi and the pixel size is around 30 metres with a positional accuracy of 127 metres. The 'Map Viewer' software supplied is currently (2012) confined to Windows operating systems. View 'About NATMAP Digital Maps 2008'.

Stuff you don't need to know
   •   Maps that lack contours, like street maps, are planimetric; i.e. flat.

   •   'Large scale' maps are those with a scale of 1:70 000 or less.

Groundschool – Flight Planning & Navigation Guide

| Guide content | 1. Australian airspace regulations | [2. Charts & compass] | 3. Route planning |

| 4. Effect of wind | 5. Flight plan completion | 6. Pre-flight safety and legality check | 7. Airmanship & flight discipline |

| 8. En route adjustments | 9. Supplementary techniques | 10. En route navigation using the GNSS |

| 11. Using the ADF | 12. Electronic flight planning & the EFB | 13. ADS-B surveillance technology |

Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing |

Next - route planning Section 3 of the Flight Planning & Navigation Guide discusses route planning

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