6.5 Earth Coordinates, UTC, Chart Projections, and Flight Instruments

Translate Position and Time into a Usable Flight Plan

A dispatcher must read a route consistently across charts, flight-plan coordinates, navigation displays, and weather periods. Latitude, longitude, time reference, and chart projection are basic syllabus topics. A plausible-looking coordinate or local-time estimate can put a route in the wrong hemisphere or apply the wrong forecast. The practical skill is to identify the reference and verify every conversion.

Coordinates and distance

Latitude measures angular position north or south of the Equator. Longitude measures angular position east or west of the Greenwich reference meridian. Include hemisphere designators and the correct degrees/minutes format. A coordinate entered in decimal degrees must be converted if the receiving field requires degrees and minutes; the decimal part is not itself a minutes value.

For example, 45.5 degrees north equals 45°30′ N, because 0.5 × 60 = 30 minutes. Do not read it as 45°50′. A longitude of 120° west is not interchangeable with 120° east. At high latitudes, a small longitudinal angle corresponds to less surface distance than near the Equator because the meridians converge.

A nautical mile corresponds approximately to one minute of latitude. That relationship does not mean every minute of longitude is one nautical mile at every latitude. For short operational exercises, use the chart scale or approved navigation computation rather than applying a latitude shortcut to all directions and distances.

Great circles, rhumb lines, and projections

A great circle follows the shortest surface path between two points on a spherical Earth model. A rhumb line crosses meridians at a constant angle, maintaining constant true course. A route selected for least time or fuel can differ from the geometrically shortest route because of winds, weather avoidance, airspace, performance, and airport constraints.

A Mercator projection represents rhumb lines conveniently as straight lines but distorts scale toward the poles. A Lambert conformal conic projection is useful across many midlatitude aeronautical charts and approximately represents great-circle segments over suitable distances. A gnomonic projection represents great circles as straight lines with its own distortion. The projection explains chart geometry; it does not authorize use of an outdated or inappropriate chart for IFR planning.

UTC and date rollover

Coordinated Universal Time (UTC) is the shared reference for aviation weather and many flight records. Local time zones and seasonal clock changes are not fixed offsets everywhere year-round. Use the appropriate conversion for scheduling, then retain UTC and date explicitly in the operational calculations.

A hypothetical flight departs at 2340Z on the 12th and has an estimated duration of one hour fifty minutes. Arrival is 0130Z on the 13th, not 2530Z on the 12th. The destination ETA ± one-hour window crosses the date boundary: 0030Z–0230Z on the 13th. Select TAF groups using the encoded day/hour dates, and include any amendment that changes those periods.

Crossing the International Date Line also changes the local calendar relationship. Elapsed time and UTC remain consistent. Do not count fuel endurance or dispatcher duty by subtracting two displayed local clock times without accounting for their dates and zones.

Heading, course, and wind

Course/track describes the desired or actual ground path. Heading describes the aircraft nose direction. Wind can require a heading different from course and makes groundspeed different from true airspeed. Magnetic variation changes the relationship between true and magnetic directions; compass deviation is an aircraft-specific magnetic-instrument effect.

For a supplied true course of 100° and east variation of 12°, the corresponding magnetic course is 88°. If the approved wind calculation supplies a true heading of 108° for that course, the magnetic heading is 96°. Keep the references separate; do not apply variation twice or confuse heading with track. Use the stated weather/ATC direction reference rather than assuming every wind report uses the same convention.

Airspeed and altimetry errors

IAS is indicated airspeed. CAS corrects instrument and position error. EAS also addresses compressibility. TAS describes speed through the air, while groundspeed adds the wind-vector effect. High density altitude changes the relationship between indicated and true speeds; a chart can require a particular speed type. Read its labels before substituting an FMS groundspeed for an aerodynamic input.

Pitot/static blockage can make airspeed, altimeter, or vertical-speed indications misleading depending on which openings are blocked and the drain/configuration state. Position error, instrument error, nonstandard temperature, and incorrect pressure settings are different error sources. A plausible altitude indication can therefore coexist with inadequate true obstacle clearance in cold air.

Airborne navigation instruments and automation

Modern transports can combine EFIS, FMS, inertial reference/navigation, GNSS, VOR/DME, and other sensors. The FMS database and active route must be checked against current charts and clearance. An inertial system can accumulate error; a GNSS position can lose integrity; a display can retain a route that no longer matches ATC authorization. Redundant displays do not prove independent navigation sources.

Transponders and altitude encoding support surveillance but do not themselves establish accurate onboard navigation. A dispatcher considers a defect's MEL restrictions, route approvals, required surveillance/navigation capability, and airport procedures. During flight, the crew uses approved abnormal procedures and available alternatives; dispatch supplies relevant route, weather, navaid, and airport information.

Verification exercise

A fictional flight plan contains an east/west coordinate reversal, a local arrival time lacking a date, and a stale navigation database indication. Identify each independent defect before computing fuel or assigning alternates. Correct the coordinate using its actual source, establish the UTC date/time, and verify the approved database/procedure basis. Then rebuild the route and relevant weather windows. Correct arithmetic performed on those wrong references would still be an incorrect dispatch solution.

Official References

Source check: 2026-10-12. FAA AIM navigation guidance; FAA aeronautical handbooks and current addenda; Current AC 91-70 oceanic guidance. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.