ATPL Performance (Australian CASA Pilot Theory Exams) common mistakes
ATPL Performance · Understand the traps and how to avoid them.
Using the take-off distance required from the runway-distance table without first adding the clearway/stopway corrections, then comparing that figure directly against the full runway length (TORA).
Candidates treat the published table value as a single 'take-off distance' and forget that TODA, TORA and ASDA are different declared distances; the table is usually entered with obstacle or clearway data and the result must be matched to the correct declared distance.
What to do instead
Before touching the chart, write down TORA, TODA and ASDA from the aerodrome data. Decide which one the question asks about, label the chart output accordingly, and only compare like with like. If clearway exists, remember it can extend TODA but not TORA or ASDA.
Interpolating take-off chart values linearly between two weight or temperature lines when the chart lines are curved, producing a figure several hundred feet off.
Pressure-altitude and temperature lines on take-off charts are non-linear, and candidates split the difference arithmetically instead of reading the curvature or using the printed grid proportionally.
What to do instead
Interpolate only between adjacent lines on the same family, and judge the spacing visually at the point of intersection rather than assuming equal increments. Where a chart provides a separate correction grid, apply corrections sequentially (weight first, then temperature, then wind) instead of combining them in one guess.
Applying a tailwind or headwind correction with the wrong sign, or applying the wind correction to the ground roll instead of the whole take-off distance.
Wind correction grids are often expressed as a percentage increase or decrease, and candidates lose track of whether the reference is zero wind and whether the correction applies to the air distance or total distance.
What to do instead
State the rule explicitly: headwind reduces, tailwind increases, and the correction applies to the total distance to 50 ft (or the screen height specified). Check whether the chart already assumes a wind; if so, correct from that baseline, not from zero.
Ignoring the effect of flaps or thrust setting on the take-off chart entry and using the default configuration.
Charts are often printed for one flap setting and one thrust rating, and the question may specify a different configuration buried in the text; candidates read the chart before reading the conditions.
What to do instead
Extract every condition from the question into a short list (weight, pressure altitude, temperature, wind, slope, surface, flaps, thrust) before opening the chart. If the chart does not match the configuration, look for the alternate chart or the correction factor.
Computing climb gradient in percent and then comparing it directly with a required gradient in feet per nautical mile (or vice versa) without converting.
Gradient is quoted in both units, and the conversion factor (approximately 1% equals 60 ft/NM) is easily forgotten under time pressure.
What to do instead
Standardise on one unit at the start of the calculation. Use gradient (%) = (height gained / horizontal distance) x 100, and ft/NM = gradient (%) x 60. Write the conversion factor on the working paper before starting.
Using the gross climb gradient from the flight manual without applying the net-to-gross reduction required for obstacle clearance.
Certification gradients are gross; the net gradient used for obstacle accountability is reduced (typically by 0.8% for twins and 0.9% for quads in the second-segment case), and candidates quote the gross figure as if it were net.
What to do instead
Identify which segment is being assessed and apply the published net-to-gross decrement before comparing with the required obstacle gradient. State the decrement explicitly in the answer so the examiner can see it was applied.
Treating cruise altitude as a free choice and ignoring the crossover altitude where a lower altitude gives a better specific range because of the change from constant IAS to constant Mach.
Candidates assume higher is always more efficient, forgetting that above the crossover altitude the achievable speed schedule changes and wind and temperature alter the optimum; also long-range cruise tables are read at the wrong weight.
What to do instead
Compare specific range at two or three candidate levels using the cruise chart at the planned weight, and check the crossover altitude for that weight. Choose the level that minimises trip fuel for the wind and temperature actually forecast, not the highest available level.
Reading the landing distance chart at the actual landing weight but forgetting to apply the fact that landing charts are usually based on full flap and maximum braking, and then not applying the operational factor.
The chart gives a demonstrated or scheduled distance under specific configuration and braking assumptions; candidates compare it directly with the LDA and declare the landing legal without the 1.67 (dry) or 1.92 (wet/contaminated) factor or the operator factor.
What to do instead
Confirm the chart's assumed configuration and braking action, then multiply the chart result by the required operational factor before comparing with LDA. If the runway is wet or contaminated, use the appropriate factor and check reverse thrust is not assumed.
Mixing units between the fuel plan (kg or lb) and the fuel flow chart (kg/hr or lb/hr) or between US gallons and litres, producing a fuel figure out by a factor of 2.2 or 3.8.
Different charts and aircraft use different mass and volume units, and candidates carry a figure forward without checking the unit label on the axis or the conversion constant.
What to do instead
Write the unit next to every number in the working. Use the standard constants (1 kg = 2.205 lb, 1 US gal = 3.785 litres, jet fuel density about 0.8 kg/litre) and convert to a single system before any addition.
Forgetting to include the contingency, alternate, final reserve and taxi fuel in the total, or adding the reserve into the trip fuel when computing the alternate fuel.
The fuel plan has a defined build-up order, and candidates either omit a component or double-count the reserve when working backwards from the required fuel.
What to do instead
Build the plan in a fixed order every time: taxi, trip, contingency, alternate, final reserve, extra. When working backwards from a fuel figure, subtract in reverse order and check each subtraction against the total.