ATPL General Navigation (061) common mistakes
ATPL General Navigation (061) · Understand the traps and how to avoid them.
When converting a magnetic bearing to a true bearing (or the reverse), candidates add variation and deviation in the wrong order or with the wrong sign, then carry the error straight into the plotted track.
The two corrections are taught with memory aids (e.g. 'variation east, magnetic least') but candidates apply them to the wrong pair of quantities, or they forget that deviation is compass-to-magnetic and variation is magnetic-to-true, so they swap which correction belongs where.
What to do instead
Always write the full chain in one line before touching numbers: True → (apply variation) → Magnetic → (apply deviation) → Compass, and reverse it going the other way. Draw a small compass rose with the three norths marked and label each arrow with the correction that connects it. Practise with mixed east/west variation and east/west deviation until the sign handling is automatic, then check by working the problem backwards from the answer.
When plotting a position given in latitude/longitude on a Lambert conformal conic or Mercator chart, candidates measure from the wrong parallel of latitude or use the scale at the bottom of the chart instead of the scale for the latitude concerned.
On conic and Mercator projections the scale is not constant across the chart, and candidates treat the printed representative fraction as valid everywhere. They also measure latitude along a meridian starting from the wrong reference parallel.
What to do instead
Before measuring, identify the projection and its scale behaviour: constant scale along the standard parallels on a Lambert conic, and scale increasing with latitude on Mercator. Use the latitude scale on the meridian at the same latitude as the point for distance measurement, and always measure latitude from the nearest labelled parallel, interpolating with the minute divisions on the border. Do a quick sanity check: one minute of latitude is one nautical mile anywhere on the chart.
In the triangle of velocities, candidates draw the wind vector in the wrong direction (pointing where the wind is going instead of where it comes from) or add the vectors head-to-tail in the wrong order, producing a ground speed and track that are internally consistent but wrong.
Weather reports give wind direction as the direction the wind comes from, while the vector triangle requires the direction the air mass moves toward. The 'from' versus 'to' convention is the single most common source of sign errors, and candidates rush the sketch.
What to do instead
Convert wind direction to the 'to' direction (add 180°) before drawing the wind vector, and label it 'W/V to'. Draw the triangle in a fixed order every time: true heading and TAS from the start point, then the wind vector from the end of that line, then close back to the start to get track and groundspeed. Check that the wind vector points downwind on your sketch and that the resulting groundspeed is greater than TAS when there is a tailwind component.
Candidates compute a great-circle distance or initial track using a rhumb-line formula, or plot a great circle as a straight line on a Mercator chart and then read off the mid-point track as if it were constant.
Both great circles and rhumb lines appear as straight lines on some charts (great circles on a gnomonic, rhumb lines on a Mercator), and candidates conflate the two properties. The initial great-circle track changes continuously along the route, which is easy to forget.
What to do instead
Memorise the chart/property matrix: great circle = shortest distance, curved on Mercator, straight on gnomonic, track changes continuously; rhumb line = constant track, straight on Mercator, longer than the great circle except on the equator or a meridian. For great-circle problems use the spherical formulae (or the approximate method) and state the initial track and final track separately. When asked for the track at an intermediate point, recompute rather than interpolating linearly.
On time-zone and LMT/UTC problems, candidates add or subtract the zone offset in the wrong direction, or mix up local mean time with standard time and forget daylight-saving adjustments.
The sign convention ('east is ahead, west is behind') is simple but easy to invert under pressure, and candidates forget that zone boundaries follow political lines rather than exact 15° meridians, and that some regions use half-hour or 45-minute offsets.
What to do instead
Convert everything to UTC first: LMT = UTC ± longitude in time (15° = 1 hour, 1° = 4 minutes), then apply the standard-time offset for the zone. Write the direction of each conversion explicitly ('longitude east → LMT ahead of UTC'). For zone questions, look up the actual zone offset rather than assuming the nominal meridian, and check whether daylight saving is in force at the date given.
When computing leg time and fuel, candidates use groundspeed for one leg and TAS for the next, or forget to apply the wind to each leg separately, so the fuel plan is internally inconsistent.
Candidates build a table of legs but fill in heading, track, TAS and groundspeed piecemeal, losing track of which speed belongs to which leg, and they carry a single wind for the whole route when the forecast wind changes with position and altitude.
What to do instead
Use a fixed leg table with columns for track, wind, heading, TAS, groundspeed, distance, time and fuel, and fill every column for every leg before doing any arithmetic. Recompute the wind triangle for each leg using the forecast wind at that leg's position and level. Cross-check total time against total distance divided by an average groundspeed, and total fuel against endurance.
Candidates treat GNSS as error-free and ignore the difference between the aircraft's true position and the RNAV system's computed position, or they confuse the various accuracy, integrity and alert-limit concepts.
RNAV and GNSS questions look like general-knowledge questions, so candidates answer from vague memory rather than from the definitions of required navigation performance, actual navigation performance, integrity monitoring and the alert limits that trigger a warning.
What to do instead
Learn the definitions precisely: the navigation specification states the required accuracy (e.g. RNP 1 means total system error must stay within 1 NM for 95% of the time), while integrity monitoring ensures the system detects when that is exceeded and alerts the crew. Distinguish accuracy (how close to truth) from integrity (confidence in that accuracy) and from continuity and availability. When a question mentions an alert limit, identify which limit (alert vs. containment) is being described.
On visual navigation and map-reading questions, candidates identify a feature by its shape alone without cross-checking scale, orientation or the surrounding features, and they misjudge the aircraft's position when the chart is not north-up or when the map is folded.
Visual navigation is tested with descriptions of features and their chart symbols, and candidates rely on a single cue (a lake, a road junction) rather than a set of cues. They also forget that the chart may be a different scale from the one assumed, or that magnetic north on the chart differs from true north.
What to do instead
Use a deliberate feature-matching routine: identify at least three independent cues (shape, relative position, size/scale, linear features, relief) before committing to a position. Check the chart's scale bar and the direction of grid or true north before measuring. When estimating position along a track, use time and groundspeed to narrow the possible area, then confirm with the features.
Candidates compute a position line from a radio aid or visual bearing but forget to convert the bearing to the chart's reference (true vs. magnetic) or to allow for the station's position error, and they plot the position line as a fixed position rather than a line of possible positions.
Position-line questions combine bearing conversion, plotting and the concept of a fix, and candidates collapse the line into a point or forget that a single position line only constrains position in one direction.
What to do instead
Convert every bearing to true before plotting (apply variation, and deviation if the bearing is compass-derived), and mark the station position first. Draw the position line through the station at the reciprocal of the observed bearing. Remember that one position line gives a line of position, two give a fix, and a running fix uses a transferred position line plus the course and distance run.
On Earth-shape and coordinate-system questions, candidates use a spherical Earth where an ellipsoid is implied (or vice versa), or they confuse geodetic and geocentric latitude and the definition of the nautical mile.
The Earth is modelled as an oblate spheroid, and the relationships between geodetic latitude, geocentric latitude, the ellipsoid and the definition of the nautical mile are subtle; candidates memorise isolated facts without the underlying geometry.
What to do instead
Learn the model: an oblate spheroid flattened at the poles, with geodetic latitude measured from the ellipsoid normal and geocentric latitude from the line to the centre, differing by up to about 11.5 minutes at 45°. Know that one minute of latitude is approximately one nautical mile (1,852 m) by definition of the nautical mile, and that the length of a degree of longitude decreases with latitude. When a question gives an ellipsoid, use ellipsoidal relationships; when it says 'assume a sphere', use the spherical formulae.