CAA NZ ATPL Theory - Flight Navigation General common mistakes
CAA NZ ATPL Theory - Flight Navigation General · Understand the traps and how to avoid them.
Applying magnetic variation with the wrong sign when converting between true and magnetic, e.g. adding variation at a station where it is east instead of subtracting it, so the heading flown is wrong by twice the variation.
Candidates memorise 'variation east, magnetic least' as a slogan but never build a physical picture of where magnetic north lies relative to true north, so under exam pressure they pick the arithmetic that 'feels' right rather than deriving it from the diagram.
What to do instead
Draw a three-line sketch every time: true north line, magnetic north line labelled east or west of true, then the track. Read the correction direction off the sketch rather than from memory. Check with the slogan afterwards: if the sketch and the slogan disagree, the sketch wins. Practise ten conversions per session until the sketch takes under 15 seconds.
Using the actual wind velocity directly in the triangle of velocities instead of the wind vector, and mixing units (knots of wind speed with a TAS in km/h, or minutes with decimal hours) inside the same calculation.
The triangle of velocities requires all three vectors in the same units and drawn head-to-tail; candidates who have only ever used a flight computer by rote lose track of which quantity is which when the question is worded differently from the textbook example.
What to do instead
Before computing, write down the three vectors explicitly (heading/TAS, wind direction/speed, track/groundspeed) and convert everything to knots and whole degrees first. Solve one worked example both by scale drawing and by the sine rule, and confirm the answers agree. In the exam, sanity-check the result: groundspeed must lie between TAS minus wind speed and TAS plus wind speed.
Confusing heading, track, drift and wind correction angle when the question asks for the heading to steer to make good a required track, and giving the track as the answer.
The words 'track' and 'heading' are used loosely in everyday speech, and candidates read the question quickly, see a required track and a wind, and report the first angle they compute.
What to do instead
Underline the words 'heading to steer' or 'track made good' in every question before starting. Remember that heading equals track plus or minus drift, with the sign chosen so the aircraft crabs into wind. After solving, ask whether the answer would put the nose into or away from the wind; if it is away, the sign is wrong.
Treating DME slant range as equal to ground distance when the aircraft is close in and high, and using the displayed distance as a fix without allowing for the height offset.
DME measures line-of-sight range, and the error is small at long range so candidates never notice it; near the beacon at high altitude the difference becomes operationally significant but the formula is rarely practised.
What to do instead
Use the approximation that ground distance equals the square root of (slant range squared minus height above the beacon squared), with both in the same units. Work one example at 30,000 ft and 10 nm slant range to see the size of the error. In exam questions, check whether the aircraft is within about 10 nm of the beacon and above 20,000 ft; only then does the correction matter.
Mixing up VOR radials with bearings to or from the station, so a reported 'radial 090' is treated as a bearing of 090 degrees to the station rather than from it, giving a reciprocal error of 180 degrees.
The OBS and CDI present information from the aircraft's perspective, while the radial is defined from the station, and candidates who have not flown the aid mentally invert the two.
What to do instead
Fix the definition in one sentence: a radial is the magnetic bearing from the station to the aircraft. Convert every radial to a bearing to the station by adding or subtracting 180 before plotting. When using the CDI, remember that with a FROM indication the selected course equals the radial; with TO it equals the reciprocal.
Calculating local mean time or solar time without correctly converting longitude to time, using 15 degrees per hour in the wrong direction, or forgetting that east is later.
The arithmetic is simple but the sign convention is easy to invert, and candidates rush the division of degrees and minutes by 15 without checking whether the answer should be ahead of or behind UTC.
What to do instead
Convert longitude to time at 15 degrees per hour, or 4 minutes per degree, and write 'east = ahead of UTC, west = behind UTC' at the top of the working. Always convert to a single reference (UTC) first, then apply the longitude correction, then the equation of time if it is given. Do a reality check: a place at 120 degrees east should be about eight hours ahead of UTC.
Ignoring the difference between indicated altitude, true altitude and pressure altitude when planning terrain clearance or computing true altitude from a forecast QNH and temperature, and assuming the altimeter reads true height.
The altimeter is calibrated to ISA, so cold air makes the aircraft lower than indicated; candidates learn the phrase 'high to low, look out below' but do not apply the temperature correction quantitatively in planning questions.
What to do instead
Remember that in air colder than ISA the aircraft is lower than the altimeter shows, roughly 4 per cent of height per 10 degrees Celsius of ISA deviation. Compute true altitude as indicated altitude plus or minus that correction, and apply it whenever the question gives an temperature at the level or a forecast ISA deviation. State the direction of the error explicitly before choosing an answer.
Using the forecast wind at the wrong level, or averaging winds over a leg when the question provides a single mean wind, and then reporting a groundspeed or fuel figure that does not match the stated conditions.
Long-range and oceanic questions often supply a wind at a specific flight level or a mean wind for the whole leg; candidates grab the first wind figure on the page without checking its level or validity period.
What to do instead
Before any fuel or groundspeed calculation, write down the level, the validity time and whether the wind is a spot wind or a mean. If the question gives a mean wind, use it directly for the whole leg; if it gives winds at several levels, use the one matching the planned level. Re-read the stem to confirm the units of fuel flow and the reserve policy.
Plotting or reading positions with latitude and longitude reversed, or misreading the scale on a chart so that a distance is taken in the wrong units, leading to a gross error in time or fuel.
Latitude and longitude are conventionally written latitude first, but many questions present them in the other order, and chart scales differ between small-scale planning charts and large-scale approach charts.
What to do instead
Always label the axes on the working: latitude north or south first, longitude east or west second, and check that latitude never exceeds 90 degrees. Confirm the chart scale from the legend before measuring, and convert nautical miles to time at the planned groundspeed rather than at TAS. Estimate the answer roughly before measuring so a gross error is obvious.
Assuming an RNAV or GNSS system is approved for the procedure in the question without checking the required performance, and treating all RNAV as equivalent when answering PBN questions.
RNAV, RNP and the various navigation specifications look interchangeable, and candidates answer from the equipment name rather than from the specification, its accuracy value and its containment requirements.
What to do instead
Learn the distinction that RNP includes on-board performance monitoring and alerting while RNAV does not, and know that each specification (for example RNAV 5, RNAV 1, RNP 4, RNP AR APCH) carries a specific lateral accuracy and, where applicable, a vertical accuracy. For any question, identify the specification named, then check whether the stated equipment and approval meet it before answering.