Brunei DCA CPL(A) General Navigation common mistakes
Brunei DCA CPL(A) General Navigation · Understand the traps and how to avoid them.
When converting a magnetic heading to a true heading (or vice versa), candidates add the variation in the wrong direction, or apply it to the wrong quantity - for example, applying variation to a true track when the question actually asks for the compass heading, or treating east variation as subtract when converting from magnetic to true.
The mnemonic 'variation east, magnetic least' is memorised as a phrase without a physical model, so under time pressure candidates cannot decide which direction applies. Mixing up 'least/greatest' with 'add/subtract' and losing track of which value is the reference produces sign errors.
What to do instead
Always write the conversion chain explicitly before calculating: True → (apply variation) → Magnetic → (apply deviation) → Compass, and reverse the arithmetic when going the other way. Draw a small compass rose sketch with the variation arrow so the direction is visible, and check the answer by asking whether the magnetic value should be larger or smaller than the true value for the given easterly or westerly variation.
Candidates compute a great-circle distance or initial track using a flat-chart approximation, or use departure (nm) directly as a distance along a great circle, especially on long legs or high-latitude routes.
The scale of a Mercator chart varies with latitude, and candidates carry over the comfortable '1 minute of latitude = 1 nm' rule without noticing that longitude spacing is not constant. The departure formula (change of longitude × cos latitude) is applied at the wrong latitude or confused with great-circle distance.
What to do instead
For anything beyond short legs, use the spherical formulas deliberately: great-circle distance from the haversine or cosine formula, initial track from the sine/cosine rule, and always use the mean or vertex latitude when computing departure. Sanity-check by comparing the rhumb-line and great-circle answers - the great circle should be shorter, with the difference growing with latitude and change of longitude.
Candidates convert between UTC, local mean time and zone time by adding or subtracting the zone offset in the wrong direction, or by using the standard meridian offset instead of the actual longitude for LMT.
The relationship 'UTC = LMT ± longitude/15' and 'UTC = zone time ± zone offset' are both signed operations, and candidates lose the sign convention. They also forget that LMT depends on actual longitude while zone time depends on the zone's standard meridian, so they mix the two.
What to do instead
Adopt one fixed convention: convert everything to UTC first, then convert out to the required time. For LMT, compute the longitude correction as (longitude in degrees)/15 hours, with east longitude meaning LMT is ahead of UTC. For zone time, use the zone offset only. Write the intermediate UTC value on the working paper so each step is checkable.
When computing sunrise or sunset, candidates use the wrong latitude (e.g., the departure aerodrome instead of the position at the time of the event), ignore the equation of time, or apply the standard refraction/semidiameter correction incorrectly.
The almanac tabulation is for a reference latitude and the candidate treats it as directly usable. The equation of time is small and easily dropped, and the 34-minute refraction-plus-semi-diameter allowance is applied with the wrong sign or omitted.
What to do instead
Work through a fixed sequence: determine the latitude at the event, extract the tabulated LMT for that latitude, apply the equation of time with its correct sign, then convert LMT to UTC and to zone time. State the assumed refraction and semi-diameter correction explicitly in the working so the sign is visible, and cross-check that the answer lies in a plausible part of the day.
Candidates compute drift and groundspeed by treating the wind vector as a simple scalar addition or subtraction along track, rather than resolving it into along-track and cross-track components.
The wind triangle is a vector problem, but candidates default to 'headwind equals wind speed' and ignore the crosswind component. They also confuse track with heading when drawing the triangle, so the drift angle is applied to the wrong side.
What to do instead
Draw the wind triangle to scale every time: true track and true airspeed as one vector, wind direction and speed as the second, and the resultant as groundspeed and drift. Resolve the wind into head/tail and cross components using sine and cosine of the angle between wind and track, then compute drift = arcsin(crosswind/TAS) and groundspeed from the along-track component. Verify that drift is zero for a pure headwind or tailwind.
On dead-reckoning leg calculations, candidates mix units - using minutes of arc as nautical miles without checking latitude, or using kilometres, statute miles and nautical miles interchangeably in fuel and ETA computations.
The exam mixes unit systems and the candidate works from memory rather than converting explicitly. Fuel-flow figures given per hour are combined with times given in minutes without a conversion step.
What to do instead
Write the unit beside every number in the working and convert to a single system (usually nautical miles and hours) before any arithmetic. For fuel, compute time in hours first, then multiply by hourly flow; for ETA, convert the leg time to minutes only at the final step. Check the magnitude of the answer against a rough mental estimate.
Candidates treat chart projections as interchangeable, assuming a Mercator chart preserves great-circle tracks as straight lines or that a Lambert conformal conic chart has a constant scale everywhere.
The properties of each projection are learned as a list without linking them to the navigation use. The candidate remembers 'Mercator is conformal' but not that rhumb lines are straight and great circles are curved on it.
What to do instead
Build a comparison table with columns for shape, scale, straight-line track type and typical use: Mercator (conformal, rhumb lines straight, great circles curved, poor near poles), Lambert conformal conic (conformal, great circles approximately straight, constant scale along standard parallels), and polar stereographic (used near the poles). Answer projection questions by naming the property the question actually tests.
In RNAV, GNSS and PBN questions, candidates confuse the roles of RNP and RNAV, assume all RNAV systems provide on-board performance monitoring and alerting, or treat GNSS integrity (RAIM) as unrelated to the navigation specification.
The terminology overlaps and the candidate learns the acronyms without the defining distinction: RNP requires on-board performance monitoring and alerting, RNAV does not necessarily. The relationship between the navigation specification, the required performance and the integrity monitoring is not modelled.
What to do instead
Anchor each concept to its defining characteristic: RNAV is the ability to fly a desired path, RNP adds on-board performance monitoring and alerting, and PBN is the framework linking the specification to the airspace. For GNSS questions, state whether the issue is accuracy, integrity, continuity or availability, and link RAIM to integrity monitoring in the RNP context.
Candidates answer mass-and-balance questions by checking only that the total mass is within limits, without verifying that the centre of gravity lies within the forward and aft limits for the actual loading, or they compute the moment arm from the wrong datum.
The total-mass check is quick and feels sufficient, so the CG envelope is skipped. Moment arms are taken from the wrong reference point, or the candidate forgets that fuel burn and load distribution shift the CG during the flight.
What to do instead
Always complete both checks: sum masses and moments to get total mass and CG position, then plot or compare against the envelope at the relevant mass. Confirm the datum and arm for every item before multiplying, and for questions involving fuel burn, recompute the CG at the end of the flight as well as at take-off.
Candidates misidentify the fix in radio-navigation position-fixing questions, for example confusing a radial from a VOR with a bearing to the station, or treating a DME distance as a slant range equal to the ground distance.
Radials are measured from the station, bearings to the station, and the candidate applies the reciprocal without noticing. DME slant range exceeds ground distance at high altitude and close range, and the candidate uses it directly as a ground distance.
What to do instead
For each fix, state explicitly whether the quantity is measured from or to the station, and convert radials to bearings (or vice versa) by taking the reciprocal. For DME, note that ground distance equals slant range only at long range and low altitude; where the question requires it, apply the slant-range correction or state the assumption. Cross-check a two-station fix by sketching the geometry.