DGCA CPL Air Navigation common mistakes
DGCA CPL Air Navigation · Understand the traps and how to avoid them.
Converting a true track to magnetic by adding easterly variation instead of subtracting it, then using that value as the magnetic heading; the arithmetic looks plausible and the sign error survives into the answer.
The mnemonic 'variation east, magnetic least' is memorized as words rather than as a vector relationship, so the rule is applied backwards under exam time pressure.
What to do instead
Write the relationship explicitly as M = T − E (or M = T + W) and verify with a sketch: easterly variation means magnetic north lies east of true north, so the magnetic bearing is smaller. Do several conversions in both directions and check each against the sketch.
Computing a local time of arrival across the 180° meridian and applying the calendar-day change in the wrong direction, or omitting it, so the answer carries the wrong date.
Candidates treat the date line as an ordinary time-zone boundary and add or subtract hours without tracking the calendar day, especially on Pacific examples.
What to do instead
Draw a meridian strip from 0° to 180° with UTC labels and mark the date change explicitly: crossing westward, add a day; crossing eastward, subtract a day. State the UTC date alongside the time in every answer.
Measuring a straight line between two points on a Mercator chart and quoting it as a great-circle distance, ignoring that the straight line is a rhumb line that is longer than the great circle at high latitudes.
Mercator is the most familiar chart, and its straight-line appearance encourages treating it as geometric truth rather than as a projection with known distortion.
What to do instead
Identify the projection before measuring. On Mercator, great circles curve poleward, so plot intermediate points or use the great-circle formula; on a Lambert/conic chart a straight line is closer to a great circle. State the assumption made.
Drawing the wind vector in the direction the wind is blowing toward, then deriving groundspeed and drift from that, which reverses the sign of the wind component.
Weather reports give wind direction 'from', but vector diagrams often draw the arrow 'to'; mixing the two conventions is easy when rushing the triangle.
What to do instead
Always label the wind arrow with 'from' at its tail and check the result against TAS: a headwind must give GS < TAS, a tailwind GS > TAS. Use a standard triangle with TAS, wind vector and GS drawn to scale.
Converting fuel mass to volume with 1 kg = 2.2 lb applied in the wrong direction, or mixing US gallons and Imperial gallons, so the planned fuel is out by about 20 percent.
Conversion factors are memorized as bare numbers without units, and questions deliberately mix kg/lb and US/Imperial gallons.
What to do instead
Carry units through every step (kg × 2.2046 = lb; 1 US gal = 3.785 L; 1 Imp gal = 4.546 L) and do a magnitude check. Write each conversion as a fraction so the unwanted units cancel.
Reading a relative bearing from an RBI and treating it directly as the magnetic bearing to the station, without adding the aircraft heading.
The instrument presents a number that looks like a bearing; candidates forget that relative bearing is measured from the nose, not from magnetic north.
What to do instead
Use QDM = heading + relative bearing (mod 360) and verify with a sketch of aircraft and station. Remember the error is invisible when heading is 000°, which is why it survives practice.
Confusing the selected OBS course with the radial the aircraft is actually on, so an aircraft on the 090 radial is placed west of the station or the TO/FROM flag is misread.
VOR radials are defined from the station outward, while the cockpit instrument shows the selected course, not the aircraft's position relative to the station.
What to do instead
Draw a compass rose centred on the station: radial 090 means the aircraft lies east of the station and the bearing to the station is 270°. Confirm with the TO/FROM flag: FROM with 090 selected means outbound east.
Using DME distance as ground distance when close to a station at high altitude, accepting a position that is off by more than a nautical mile.
DME measures line-of-sight slant range; at 30,000 ft and 10 NM ground distance the error exceeds a mile, but the readout looks like a precise range.
What to do instead
Apply the slant-range correction: ground distance ≈ √(DME² − height²) with height in NM (1 NM ≈ 6,076 ft). Remember DME is most accurate at long range and least accurate overhead the station.
Assuming RAIM is available for an RNAV approach simply because the receiver shows enough satellites, without checking a RAIM prediction for the approach time and location.
RAIM is a receiver function that depends on satellite geometry; a good satellite count does not guarantee fault detection, and the prediction changes with time.
What to do instead
Use the receiver's RAIM/FDE prediction tool for the planned approach time and location, and know that RAIM detects but does not necessarily exclude a faulty satellite unless FDE is available. Record the prediction in the flight plan.
Treating CDI full-scale deflection as a fixed angular value on all RNAV/RNP legs, or assuming every waypoint must be overflown before turning.
CDI scaling changes with phase of flight (for example 1 NM en route to 0.3 NM terminal), and ARINC 424 leg types determine whether the aircraft turns early or overflies.
What to do instead
Learn the scaling table for the avionics in use and the difference between fly-by and fly-over legs; check the leg type on the flight plan and brief turn anticipation. Interpret deviation using the correct scaling for the phase of flight.