CPL Navigation (Sri Lanka CAA Pilot Licence Examinations) common mistakes
CPL Navigation · Understand the traps and how to avoid them.
Using indicated airspeed or TAS directly in the flight-planning leg-time calculation instead of first converting to groundspeed with the forecast wind.
Candidates memorise the triangle-of-velocities formula but practice it only in isolation, so under time pressure they treat the cruise TAS as the speed over the ground and get leg times and fuel burns that are systematically short on headwind legs.
What to do instead
Build a fixed worksheet order for every leg: TAS and heading from the flight plan, then wind velocity, then solve the wind triangle to get groundspeed and true track, then apply variation and deviation. Practise ten mixed headwind, tailwind and crosswind legs until groundspeed is always written down before any time or fuel figure.
Applying magnetic variation with the wrong sign, or adding variation and deviation to the wrong quantity, so headings are off by twice the variation.
The memory aid 'variation east, magnetic least' is recalled but not understood, and candidates forget that deviation is a property of the aircraft compass on a given heading, not of the route, so they apply it as a fixed route correction.
What to do instead
Write the conversion chain explicitly on every calculation: true track, apply variation to get magnetic track, apply deviation to get compass heading, and reverse the signs when going from compass back to true. Do worked examples both directions and check with a sketch of isogonals so the sign is derived, not remembered.
Plotting a position from a bearing and distance without converting the magnetic bearing to true before measuring on the chart, or measuring the distance with the wrong scale.
Chart work is learned as a sequence of steps that candidates follow mechanically; when a question gives a magnetic bearing from a beacon they measure it straight onto the protractor, and they use the latitude scale for distance only sometimes.
What to do instead
Adopt a rule that no magnetic value touches the chart until converted. Measure all distances on the latitude scale at the same latitude as the leg, and re-measure the scale bar each time to confirm the chart edition. Do five position plots per study session and annotate each step with true or magnetic.
Treating the drift and heading calculations as independent, so the wind correction angle is added to track without recomputing the along-track component of wind.
Candidates use the approximate 'max drift times sine of wind angle' shortcut and then assume groundspeed equals TAS, which is acceptable only in light winds; in strong crosswinds the error in time and fuel becomes significant and compounds over a long leg.
What to do instead
Use the exact wind triangle for every leg in the exam, and reserve the max-drift approximation only as a quick sanity check. After solving, verify that the crosswind component equals groundspeed times the sine of the drift angle, so an arithmetic error is caught immediately.
Misreading the altimeter subscale setting or failing to apply the correct pressure setting when converting between indicated altitude, true altitude and height.
The relationship between QNH, QFE, QNE and true altitude is studied as definitions but not practised as arithmetic, so candidates cannot compute the altimeter error when flying from a high-pressure to a low-pressure area or from warm to cold air.
What to do instead
Learn the two rules quantitatively: about 30 ft of altimeter error per hectopascal of pressure difference, and roughly 4 per cent of height per 10 degrees Celsius of temperature error, with the direction 'high to low, look out below'. Work numerical examples for both pressure and temperature corrections until the sign and magnitude are automatic.
Planning fuel using only trip fuel and ignoring contingency, alternate, final reserve and taxi fuel, or computing reserve as a percentage of the wrong figure.
Candidates treat fuel planning as one multiplication of burn rate by time, having memorised a reserve rule without tracking what each component is measured against, so totals come out low and they cannot answer questions that ask for fuel at a specific point.
What to do instead
Draw a fuel ladder for every question: taxi, trip, contingency, alternate, final reserve, extra, total. Label which quantity each percentage applies to and compute fuel remaining at each waypoint by subtraction. Check that the sum of components equals total fuel before moving on.
Assuming a flight-planning question uses the same units throughout, mixing knots, kilometres per hour and miles per hour, or mixing nautical miles with statute miles on the chart.
Charts and manuals use different units for different quantities, and candidates read numbers without reading the unit label, especially where a question deliberately mixes a km/h wind with a knot airspeed.
What to do instead
Convert every given value to a single working set of units (knots and nautical miles) as the first action of the question, writing the conversion factor used. Keep a conversion card with 1 nm equals 1.852 km and 1 kt equals 1.852 km/h, and re-read the unit of every answer option before selecting.
Failing to account for the effect of wind on the outbound and inbound legs of a there-and-back route, treating the average groundspeed as the still-air TAS.
Candidates average the two groundspeeds arithmetically instead of computing total distance over total time, which overstates average speed whenever there is wind, and they then understate total time and fuel.
What to do instead
Always compute time leg by leg and divide total distance by total time to get average groundspeed. Verify with a simple case: 100 kt TAS, 50 nm each way, 20 kt wind gives 2.08 hours, not 2.0. Practise until the harmonic nature of the average is understood rather than memorised.
Reading a position or bearing off a chart without checking the chart's graticule orientation, or measuring a track from the nearest meridian when the route crosses several degrees of longitude.
On a Lambert conformal conic chart, straight lines are not rhumb lines and great-circle tracks change direction along the route; candidates measure once at the departure point and apply that track for the whole leg.
What to do instead
For each leg, measure the initial true track at the departure meridian and the final track at the arrival meridian, and if they differ, split the leg or use the mid-meridian value as directed by the question. State on the answer sheet which meridian the track was measured from.
Skipping the final plausibility check, so answers that are internally consistent but physically impossible, such as a groundspeed lower than the headwind component or a drift angle above 90 degrees, are submitted.
Under exam time pressure candidates stop after the arithmetic and do not compare the result with the rough mental estimate they could have formed in seconds, so a misplaced decimal or reversed sign goes undetected.
What to do instead
Before every answer, do a 10-second estimate: groundspeed must lie between TAS minus wind speed and TAS plus wind speed, drift cannot exceed the wind angle, and fuel remaining must decrease monotonically. If the computed answer falls outside these bounds, redo the triangle rather than adjusting the answer.