ATPL Navigation (Sri Lanka CAA Pilot Licence Examinations) common mistakes
ATPL Navigation · Understand the traps and how to avoid them.
When converting between true, magnetic, and compass headings, candidates add the variation or deviation in the wrong direction, or apply it twice, so the final heading is off by twice the variation (for example 10°E applied as 20°).
The mnemonic 'variation east, magnetic least' is memorised as words rather than understood as a vector relationship, and under time pressure candidates cannot reconstruct which quantity is larger, so they guess the sign.
What to do instead
Draw a small compass rose for every conversion and place True at the top, then rotate to Magnetic by the variation and to Compass by the deviation. State the arithmetic aloud: 'True to Magnetic, easterly variation, subtract.' Do ten mixed-direction drills until the sign is automatic rather than recalled.
Candidates compute great-circle distance or initial track using a flat or Mercator approximation, or use the departure formula (departure = dlong × cos lat) for a long leg and treat the result as a great-circle distance.
The departure formula is the last thing taught and is easy to apply, and on a Mercator chart a straight line looks like the shortest path, so the error is invisible unless the candidate checks whether the route is actually a great circle.
What to do instead
Before computing, classify the problem: rhumb line or great circle, and short or long leg. Use the departure formula only for rhumb-line east-west distance at a single latitude. For great-circle legs use the spherical cosine (haversine) formula and the initial-track formula, and sanity-check with the fact that a great circle always curves poleward of the rhumb line in the northern/southern hemisphere.
Candidates answer a sunrise/sunset or twilight question using the time at the departure aerodrome, or forget to convert UTC to local mean time (or standard time) at the position in question.
The question gives a departure time and a route, and candidates anchor on that time instead of on the position where the event occurs; the UTC/LMT conversion is then skipped or applied with the wrong sign.
What to do instead
Always identify the position of the event first, compute the LMT there from longitude (15° = 1 hour, 1° = 4 minutes, east positive), then convert to UTC and finally to the required standard time. Write the chain LMT → UTC → standard time explicitly on the working sheet before touching the tables.
In radio navigation, candidates confuse the CDI deflection with the aircraft's actual track error: they read a full-scale deflection as a fixed number of degrees regardless of the selected range, or they misidentify which side of the needle means 'fly toward' the course.
The instrument is drawn as a static picture and candidates memorise 'needle right, turn right' without understanding that deflection represents angular deviation from the selected radial, and that full-scale deflection corresponds to a specified angle that differs between systems.
What to do instead
Learn the deflection sensitivity for each aid (for example a VOR CDI is typically ±10° full scale, an ILS localiser ±2.5°) and always ask 'what radial am I on, and what radial did I select?' before interpreting the needle. Practise with a sketch of the aircraft, the station, and the selected radial drawn together.
Candidates treat DME distance as slant range equal to ground distance, or use DME range as if it were a position fix along the track without correcting for aircraft altitude and station offset.
DME is presented as a distance readout and the slant-range error is small at long range, so candidates assume it is negligible at all ranges and altitude combinations.
What to do instead
Remember DME measures slant range; the error grows at high altitude and close range. Compute ground distance from slant range and height when the question involves close-in geometry, and never assume a DME readout places the aircraft on the airway centreline. Treat DME as one line of position, not a fix.
In flight planning, candidates compute fuel or time for the whole route using still-air figures and then forget to apply the wind for each leg, or apply a single average wind to every leg of a multi-leg route.
The wind triangle is worked once at the start and then the candidate reuses the resulting groundspeed for the entire route, because the legs are perceived as similar.
What to do instead
Build a leg-by-leg table: track, TAS, wind, heading, groundspeed, distance, time, fuel. Compute the wind triangle separately for each leg where the wind or track changes, and total the columns only at the end. Cross-check the total time against distance divided by an average groundspeed.
Candidates confuse indicated, calibrated, true, and Mach airspeed, or use IAS in a flight-planning groundspeed calculation without applying position error, compressibility, and density-altitude corrections.
The terms appear interchangeable in casual reading, and the correction chain (IAS → CAS → EAS → TAS) is long, so candidates skip intermediate steps and treat the airspeed indicator as giving TAS.
What to do instead
Write the correction chain in order and apply it step by step: IAS plus instrument/position error gives CAS; CAS corrected for compressibility gives EAS; EAS corrected for density altitude gives TAS; TAS with wind gives groundspeed. Only groundspeed enters distance/time/fuel calculations.
In instrument questions, candidates mix up the indications of a failed pitot tube versus a blocked static source, or attribute an altimeter or airspeed anomaly to the wrong instrument.
Both failures produce airspeed and altitude errors, and candidates memorise symptoms as isolated facts without linking them to which pressure source is affected.
What to do instead
Reason from the pressure source: a blocked pitot affects the dynamic pressure and therefore airspeed only (altimeter and VSI unaffected); a blocked static affects static pressure and therefore airspeed, altimeter, and VSI together. For each failure, state which pressures are trapped and predict each instrument's response before checking the answer.
Candidates compute mass and balance using the wrong datum direction, adding a negative arm as positive, or they forget to include the empty-mass moment and only sum the variable loads.
The datum is sometimes at the nose and sometimes elsewhere, and arms can be negative; the moment table is filled in quickly and the sign convention is not re-checked.
What to do instead
Sketch the datum and fuselage, mark each arm with its sign, and compute total moment as the sum of mass × arm including the empty aircraft. Divide total moment by total mass to get the centre of gravity, and verify the result lies between the forward and aft limits before answering.
Candidates answer a centre-of-gravity shift question by comparing the new CG with the old CG in the wrong direction, or they assume adding mass forward always moves the CG forward without computing the moment change.
The intuition that 'weight at the front moves the CG forward' is usually right but fails when the added mass is behind the datum or when fuel is burned from a tank ahead of the CG, and candidates do not test the sign.
What to do instead
Always compute the moment change: added mass × its arm relative to the datum, added to the existing total moment, divided by the new total mass. Compare the resulting CG numerically with the old CG and state whether it moved forward or aft, rather than predicting from intuition.