Brunei DCA CPL(A) Performance common mistakes
Brunei DCA CPL(A) Performance · Understand the traps and how to avoid them.
Adding the headwind or subtracting the tailwind component directly to the aeroplane's published take-off or landing ground roll instead of applying the wind correction to the chart-derived distance, and doing so without regard to the chart's stated wind basis.
Candidates treat the AFM ground roll as a purely kinematic figure and assume a 10 kt headwind simply removes 10 kt worth of speed from the roll, when in fact wind affects take-off distance roughly as the square of the airspeed-to-groundspeed ratio and the chart already encodes this non-linear relationship. The error persists because wind correction is the last step in most worked examples and gets compressed into a quick mental percentage.
What to do instead
Always derive the still-air distance first, then apply the chart's wind correction (percentage per knot, or the wind grid on the chart) to that figure. Memorise that a 50% headwind gives about a 25% reduction in ground run and a 50% tailwind about a 100% increase for the same airspeed, and state the correction basis explicitly in every calculation line.
Entering the density altitude or pressure altitude chart with QNH or field elevation directly, skipping the pressure-altitude conversion, or using QFE-derived altitude when the chart is calibrated to QNH.
Pressure altitude and density altitude look interchangeable in a hurry, and candidates who have memorised 'high, hot, humid' as a slogan often jump straight from elevation to the temperature line. The 1013.25 hPa reference is easy to forget when the actual QNH happens to be near standard, reinforcing the habit.
What to do instead
Write the conversion as a fixed first line of every performance calculation: pressure altitude = elevation + 30 ft per hPa of (1013 - QNH). Then apply the ISA temperature deviation to get density altitude. Never let a chart entry occur before this line is on the page.
Computing the take-off weight limit from field length alone and declaring the take-off feasible, without separately checking the obstacle-clearance limit, the climb-gradient limit and the tyre-speed or brake-energy limit.
Field length is the most visually prominent line on the chart and gives a satisfying single number, so candidates stop there. The multiple-limit concept is understood in principle but not operationalised into a checklist, so the second and third limits are only discovered when a marker points them out.
What to do instead
Adopt a fixed four-box routine for every take-off: field length, obstacle, climb (WAT) and structural/tyre. Take the lowest of the four as the regulated take-off weight and show all four numbers even when one is obviously dominant. Practise with at least one question per limit so the routine becomes automatic.
Using the take-off distance available for the landing case, or confusing landing distance available with the landing distance required plus the regulatory factor, and applying the 1.67 or 1.92 factor to the wrong quantity.
TORA, TODA, ASDA and LDA share a common look on the aerodrome chart and differ only in the stopway and clearway definitions. Candidates who learned the definitions as a list rather than as a diagram map them onto the wrong strip, and the multiplier is then applied to a number that was never the scheduled landing distance.
What to do instead
Sketch the four declared distances to scale for every aerodrome in a question, labelling stopway and clearway. State 'LDR required (factored) must not exceed LDA' as a written inequality before substituting numbers, and check which factor the question's regulation demands rather than assuming one.
Reading a multi-segment climb chart or table at the wrong flap configuration or at the wrong segment (take-off, en-route, landing), particularly when the chart's reference speed or configuration is printed in a footnote.
Climb charts are dense and the configuration header sits far from the data block, so under time pressure the eye goes straight to the grid. Candidates also assume the climb gradient they need for obstacle clearance is the same as the en-route gradient, when the segments have different requirements and different limiting speeds.
What to do instead
Before touching the grid, write down the segment, configuration, speed and reference conditions from the chart title and footnotes on a single line. Then read the value and check it against the regulatory minimum for that segment (for example the take-off flight path, en-route and landing gradients) so a misread is caught by the sanity check.
Treating the centre of gravity position as irrelevant to performance once the weight is within limits, and ignoring that a forward CG increases the take-off and landing distances and the trim drag while an aft CG reduces static stability.
Weight and balance is taught as a loading exercise separate from performance, and the CG envelope is checked only for legality. The performance consequence of CG position is a small print item in many manuals, so it is never linked to the distance charts the candidate is using.
What to do instead
Whenever a question gives a CG position as well as a weight, ask explicitly whether the chart is based on the forward or aft limit and apply the manual's CG correction. Learn the direction of the effect (forward CG worsens take-off distance, climb gradient and trim drag) so the correction can be sanity-checked rather than blindly added.
Interpolating between chart grid lines by eye and rounding the result to the nearest convenient value, or interpolating on the wrong axis when the chart has non-linear scales.
Charts are printed with widely spaced lines and the candidate is working without a ruler or is reading from a small reproduction. Because the answer options are often close together, a rounded eye-read lands on a plausible but wrong option, and the candidate never notices because the figure 'looked about right'.
What to do instead
Interpolate arithmetically: read the bracketing values off the chart, write them down, and compute the proportional value with the actual numbers. Mark the chart with a straight edge and record the two bracketing figures in the working so the interpolation can be checked. Never round a chart read before the final answer.
Applying a contaminated-runway correction as a flat percentage without checking whether the manual's contaminated data is already a complete distance, and mixing up a braking-action coefficient with a friction coefficient.
Contaminated runway data appears in several formats (separate tables, multiplicative factors, or a note that the data is unfactored), and candidates carry over a percentage remembered from a different type. The terminology of mu values and braking action is superficially similar, encouraging substitution of one for the other.
What to do instead
For each type, note in the manual study whether the contaminated figures are factored or unfactored and whether they replace or modify the dry figures. Keep a one-line definition of braking action versus friction coefficient and check the units and the runway condition code before using any number.
Forgetting to convert between indicated and true airspeed, or between calibrated and true airspeed, when using a cruise or descent chart, and using IAS in a range or endurance calculation.
The cruise chart's speed axis may be TAS while the cockpit reference is IAS, and the conversion is buried in the chart's small print. Candidates who are focused on fuel flow take the speed at face value and produce a range figure that is internally inconsistent with the wind and altitude columns.
What to do instead
Write the speed basis next to every speed used ('TAS 240 kt') and perform the IAS-to-TAS conversion explicitly at the given altitude and temperature before entering the chart. Cross-check range by dividing TAS by fuel flow so an inconsistent speed is exposed by the units.
Assuming that a high-altitude or hot-day take-off is limited by the same factor as a sea-level one, and failing to notice that the limiting factor has switched from field length to climb or obstacle clearance as density altitude rises.
Candidates learn one worked example at sea level where field length dominates, then apply the same reasoning at 6000 ft where the climb-limited weight is now the controlling figure. The switch is invisible unless the candidate computes all the limits, so the error is systematic rather than random.
What to do instead
For every hot-and-high question compute the field-length, obstacle and climb-limited weights and state which one is lowest and why. Track how the controlling limit migrates with density altitude across a few worked examples so the pattern is recognised rather than memorised.