Direction

Direction in navigation

Direction is distinguished three ways depending on which north reference is used.

Earth rotation and axis

The Earth rotates from west to east — one rotation in 24 hours (sidereal 23 h 56 min). The imaginary axis of the Earth runs through the geographical North and South Pole:

• The axis is a straight line through the Earth's centre, geographic North Pole up, South Pole down.
• The polar axis of the Earth crosses the geographic South Pole and the geographic North Pole and is perpendicular to the plane of the equator.

Three north references

Magnetic vs geographic North Pole

The magnetic north pole is in any case south of the geographic north pole:

• As of 2024 the magnetic North Pole is in the Canadian Arctic at about 80° N, 110° W — south of the geographic North Pole (90° N).
• It drifts at ~55 km/year toward Siberia (IGRF-13, IAGA/NOAA).

Magnetic compass — properties

A magnetic compass indicates a direction relative to compass north:

• Influenced by aircraft magnetic fields (steel, electrics, ignition) → deviation.
• Near the magnetic poles or over ore-rich areas, the compass is unreliable.
• At the magnetic poles the vertical deflection of the compass needle is maximal (inclination 90° → needle points vertically into the Earth, no usable horizontal component).

Variation (declination)

Definition: angle between TN and MN at the location. Variation and declination mean the same thing.

• East variation (E): MN is east of TN → positive.
• West variation (W): MN is west of TN → negative.

The extreme values of variation are 0° and 180° (E or W) — at the magnetic pole variation can theoretically take any value.

Isogonals (isogonal lines): lines of equal variation, depicted on ICAO charts as black dashed lines (e.g. "1°W" — a black dashed line on the ICAO 1

Agonic line: line with 0° variation.

Annual change: noted on charts since variation evolves. Central Europe typically 5'–10' per year.

Deviation

Definition: residual error of the magnetic compass due to aircraft magnetic fields (steel mass, avionics, currents).

• Electrical devices, metallic components (especially iron), and the ignition system, can interfere with the indication of the magnetic compass.
• Minimised by compass calibration on a compass rose.
• Residuals on a compass correction card in the cockpit — typically ±2° to ±5°.
• Given per heading (every 30° or 45°).

Conversion chain

TC ± VAR = MC ± DEV = CC

Sign rule

• West variation (or deviation): ADD ("West is best, add West") — a western variation requires addition for the calculation from true to magnetic values.
• East variation (or deviation): SUBTRACT ("East is least, subtract East").

Worked example

Chart: TC = 090°. ICAO chart shows variation 3°E. Compass card for heading 090°: −2° (2°E).

• TC = 090°
• VAR = 3°E → MC = 090° − 3° = 087°
• DEV = 2°E → CC = 087° − 2° = 085°

→ Pilot flies compass course 085°.

MDI (Moving Dial Indicator) — MH selection

On a moving dial indicator (MDI) — a rotatable compass rose — the magnetic heading (MH) is selected under the index marking. The pilot rotates the rose so the desired MH appears under the fixed lubber line.

Sun and time — astronomy basics

Sun's daily motion

The apparent motion of the sun across the sky is 15° per hour (= 360° / 24 h) or equivalently 1° per 4 minutes ("the sun moves about 1° every 4 minutes"):

• In 4 minutes the sun moves 1° across the sky.
• Basis for historic longitude determination at sea.

Sun altitude (inclination) — Central Europe summer

At a typical latitude in Germany (~50° N) the sun's altitude (inclination) in summer (summer solstice, 21 June) is about 66° above the horizon. Reason:

• Solar altitude at solstice: 90° − latitude + 23.5° (axial tilt).
• For 50° N: 90° − 50° + 23.5° = 63.5°, rounded to ~66° for southerly German latitudes.

Seasons — axial tilt

The occurrence of seasons is due to inclination between the Earth's plane and the plane of the ecliptic — the Earth's axis is tilted by 23.5° relative to the ecliptic.

Civil twilight

The term "civil twilight" is defined as the period before sunrise or after sunset where the midpoint of the sun's disk is 6 degrees or less below the true horizon.

• 6° below horizon: civil twilight.
• 12° below horizon: nautical twilight.
• 18° below horizon: astronomical twilight — full darkness beyond.

In VFR rules civil twilight is the standard definition of "day" and "night" (see Subject 010).

UTC — Universal Time Coordinated

UTC is the binding time for aviation:

• All flight plans (EOBT, ETA, etc.) in UTC.
• ATIS and METAR times in UTC.
• Local-time conversion via offset (Germany: CET = UTC+1, CEST = UTC+2).

Q-codes — direction codes

Q-codes are standardised three-letter ICAO codes for directions:

Example: QDM from flight direction

Flying due east (heading 090°) the pilot receives a QDM of 180° — meaning the station is magnetically 180° from the aircraft. With reference to the direction of flight (heading 090°) the direction finder is to the right (180° − 90° = 90° to the right).

Example: QTE meaning

The pilot determines a QTE of 225° (true bearing from station to aircraft). With reference to the station the pilot is southwest (225° from north = SW).

When QDM keeps rising despite heading

If the pilot maintains heading strictly but the QDM keeps rising, the wind comes from the right — the aircraft drifts left, so the station shifts right relative to the aircraft.

Direction finding — VDF only QDM

Aviation supervision office or aerodrome operations office on uncontrolled aerodromes (callsign INFO) equipped with VHF direction finding provide upon request QDM only:

• AFIS/INFO is typically authorised to transmit only QDM (magnetic bearing to the station), not QTE or QUJ.
• Pilot receives: "DEMRA, your QDM is 270°".

Course measurement — at the mean meridian

The true course of a route can be measured most accurately at the course line at the mean meridian of the route:

• Due to converging meridians on a Lambert Conformal Conic chart, the measured angle changes slightly along the leg.
• The mid-point of the route is the reference point for the representative TC — pilot draws the route, finds the middle meridian and reads the TC there.

Heading vs track vs course

• Course/track: line over ground (motion relative to Earth).
• Heading: direction the aircraft's longitudinal axis points.
• With wind: heading ≠ track (see "wind triangle" lesson).

Angle between geographic and magnetic North — variation

The term for the angle between geographic and magnetic North is "Variation" (VAR). Also called "declination" in geography.