GCAA ATPL Theory common mistakes
GCAA ATPL Theory · Understand the traps and how to avoid them.
Computing a crosswind or headwind component using the full wind speed or the wrong angle, e.g. treating a 30° crosswind as a 90° crosswind or forgetting to convert magnetic to true wind before applying it to a true-course track.
Candidates memorise 'sin for crosswind, cos for headwind' but lose track of which angle is which and which reference (true vs magnetic) the wind is given in, especially when the runway heading and wind direction come from different sources.
What to do instead
Always sketch the wind triangle or vector, label the angle between wind direction and runway/track explicitly, and decide sine vs cosine from the sketch rather than memory. Convert all directions to a common reference (usually true) before computing, then convert the final heading back to magnetic for the answer.
Mixing up pressure altitude, density altitude, indicated altitude and true altitude when answering altimetry questions, or applying the wrong lapse rate to the wrong layer.
The terms sound similar and candidates answer the question they expect rather than the one asked; also the 27 ft/hPa and 30 ft/hPa rules and the 2 °C per 1000 ft temperature correction get applied out of context.
What to do instead
For every altimetry question, write down which altitude is being asked for and which is given, then apply only the rule linking those two. Use 27 ft/hPa for low-level pressure error, 30 ft/hPa only for flight-level/transition calculations, and state the ISA deviation before applying temperature corrections.
Treating mass and balance as a single CG check and forgetting that maximum take-off mass, maximum landing mass, maximum zero-fuel mass and the CG envelope must all be satisfied simultaneously, including the effect of fuel burn en route.
Candidates compute one take-off CG and stop, because that is the only number the question seems to ask for, and they forget that fuel burn moves the CG forward or aft and can put the landing case outside the envelope.
What to do instead
For every loading problem, tabulate moment arms for each item, compute both take-off and landing mass and CG, and explicitly check MTOM, MLM, MZFM and the forward and aft CG limits at both states. If any limit fails, state which limit and by how much.
Confusing the four forces and their points of application in questions on stability and control, e.g. stating that the centre of pressure is fixed or that the tailplane always produces a downforce regardless of CG position.
Textbook simplifications are memorised as absolutes; candidates do not realise that CP moves with angle of attack and that tail load direction depends on CG relative to the neutral point.
What to do instead
Learn the relationship between CP movement, angle of attack and pitch stability as a causal chain, and practise explaining what happens to tail load as CG moves forward or aft of the neutral point. Answer stability questions by reasoning from the CG-neutral point relationship, not from a memorised rule.
Answering ATC and airspace questions from general knowledge of the home country rather than from ICAO Annex 2, Annex 11 and the stated airspace classes, e.g. assuming a clearance limit or position report requirement that does not exist in the class quoted.
Candidates have practical flying experience in one state and assume the rules are universal; the exam tests ICAO standard provisions and the specific class or service quoted in the stem.
What to do instead
For each ATC question, identify the airspace class and the service (ATS, ATC, FIS, alerting) being asked about, then recall the ICAO table for that class. Where a state difference exists, answer the ICAO standard unless the stem specifies a state.
Reading a navigation question as a pure calculation and ignoring convergence, chart scale or the difference between rhumb line and great circle, leading to answers that are arithmetically right but geometrically wrong.
Candidates reach for the CRP-5 or formula immediately and do not first decide whether the problem is a great-circle, rhumb-line or chart-scale problem, so the wrong model is applied.
What to do instead
Before calculating, classify the problem: is it a track/distance on a great circle, a constant-heading rhumb line, or a chart measurement? Then choose the method. For great-circle problems, always check that the answer's initial and final tracks differ by the expected convergence.
Confusing the indications and failure modes of gyroscopic instruments, e.g. attributing a vacuum failure to the attitude indicator and heading indicator while expecting the turn coordinator to fail, or misreading a suction gauge.
Candidates learn 'vacuum drives the gyros' without distinguishing which gyros are vacuum-driven and which are electrically driven, and without linking the real instrument to its power source.
What to do instead
Build a table of each instrument, its power source, its gimbal freedom and its failure indication, and learn it as a set. When a failure is described, first identify the power source lost, then list which instruments are affected and what each will show.
Applying the human performance 'rule of thumb' without the underlying mechanism, e.g. stating that hypoxia begins at a fixed altitude for everyone, or that hyperventilation and hypoxia are distinguished by a single symptom.
Candidates memorise thresholds and symptom lists but not the physiology, so they cannot answer questions that vary the individual, the rate of ascent or the time of useful consciousness.
What to do instead
Learn hypoxia in terms of the alveolar oxygen partial pressure curve and time of useful consciousness as a function of altitude, and learn the shared symptoms of hypoxia and hyperventilation plus the discriminating test (response to increased CO2 or to oxygen). Answer by mechanism, not by threshold.
Answering performance questions with the wrong configuration or the wrong correction order, e.g. applying a wind correction before a temperature correction, or using take-off distance instead of take-off run.
Performance charts have a prescribed sequence and candidates apply corrections in a convenient order, or they conflate the definitions of take-off run, take-off distance and landing distance.
What to do instead
Learn the definitions of each performance distance precisely, then follow the chart's correction order exactly, writing each intermediate value down. State the configuration (flap setting, mass, pressure altitude, temperature, wind, slope, surface) before starting, and check that the final answer's units and definition match what was asked.
Treating radio navigation aids as interchangeable and answering a question about one aid using the characteristics of another, e.g. giving a VOR radial as a bearing to the station, or confusing ILS glidepath and localiser frequencies.
Candidates learn each aid in isolation and never build a comparison, so under time pressure the features blur, particularly radial vs bearing, DME slant range vs ground distance, and the 90 Hz/150 Hz modulation logic.
What to do instead
Build a comparison table for VOR, DME, NDB/ADF, ILS and GNSS covering what is measured, what is displayed, the frequency band and the common errors. For each question, identify the aid and the quantity asked for, then answer from the table rather than from memory of a single aid.