PPL Flight Performance and Planning common mistakes
PPL Flight Performance and Planning · Understand the traps and how to avoid them.
Using the take-off or landing distance straight from the chart as the runway requirement, without applying the factor for runway surface, slope, wind or the operator's safety margin.
Candidates treat the chart output as the final answer because it is the last number read. The chart represents a specific reference condition (usually a hard, dry, level runway at a stated pressure altitude), and the factors are applied afterwards, often on a separate page or in the operations manual. Under time pressure the second step is skipped entirely.
What to do instead
Before touching the chart, write the four modifiers on the answer sheet: surface, slope, wind, and required margin. Read the chart, then apply each modifier in the order the manual specifies, and only then compare with the available distance. Practise with a fixed template where the final answer box is labelled 'factored distance', not 'chart distance'.
Reading the performance chart at the wrong pressure altitude because the candidate uses aerodrome elevation instead of pressure altitude, or forgets to set the subscale to 1013 hPa.
Pressure altitude and elevation are equal only at standard sea-level pressure. On a low-pressure day the pressure altitude is higher than elevation, and the resulting error in take-off distance is large. The mistake is invisible on the answer sheet because the chart is read correctly, just from the wrong input.
What to do instead
Make the first line of every calculation 'PA = elevation + 30 x (1013 - QNH)'. Write the computed pressure altitude on the working before opening the chart. When a question gives QNH and elevation together, that is a deliberate signal to compute pressure altitude.
Interpolating between chart lines in the wrong direction, for example moving towards shorter distance when temperature or altitude increases.
Chart grids are dense and the direction of the trend is not always obvious, particularly on combined wind and slope graphs or on landing charts where the axes are non-linear. Candidates interpolate mechanically between the nearest lines without checking which way the value should move.
What to do instead
Before interpolating, state the physical trend aloud: higher temperature, higher altitude, higher mass, downslope and tailwind all increase distance. Sanity-check the interpolated result against the trend, and if the number moves the wrong way, re-read the axes. Build the habit of bracketing: read the value at the two nearest grid lines and confirm the answer lies between them.
Mixing units, such as using a wind component in knots where the chart expects metres per second, or a mass in pounds where the chart is in kilograms.
PPL performance charts appear in both imperial and metric versions, and questions sometimes give data in one system while the chart uses the other. Candidates convert only the obvious quantity and leave the rest, or apply a conversion factor in the wrong direction.
What to do instead
At the top of the working, write the units of every chart axis next to the input value. Convert all inputs to the chart's units before reading, and convert the output back only at the end. Memorise the few conversions needed (1 kt = 0.514 m/s, 1 kg = 2.205 lb, 1 ft = 0.305 m) and practise questions that deliberately mix systems.
Computing the headwind or crosswind component using the full wind speed, or using the wrong angle, instead of resolving the wind into components relative to the runway.
The wind velocity in a question is given as direction and speed relative to magnetic north, while the runway is identified by its magnetic heading. Candidates subtract the runway number from the wind direction but then apply the sine and cosine to the wrong side, or forget that the wind direction is 'from' not 'to'.
What to do instead
Use the standard method: angle = wind direction minus runway magnetic heading (take the smaller angle to 180 degrees), headwind = speed x cos(angle), crosswind = speed x sin(angle). Draw a small compass sketch before calculating. Check the sign: a wind from ahead gives a positive headwind, from behind a negative one.
Forgetting to apply the mass-and-balance envelope check after computing the loaded mass, or checking only the total mass and not the centre of gravity position.
Candidates compute the zero-fuel mass and take-off mass and stop, treating the envelope as a formality. The envelope is a two-dimensional boundary in mass and CG, so an aircraft can be within mass limits but outside the forward or aft CG limit, which is the more dangerous condition.
What to do instead
Always finish with a two-part statement: mass within limit yes/no, and CG within the forward and aft limits at the relevant mass. Plot or calculate the CG for zero fuel, take-off and landing, since fuel burn moves the CG. If either check fails, state the required action (offload, redistribute, add ballast) rather than leaving the answer as 'out of limits'.
Using the moment arm or station number as the moment, or adding moments without converting units consistently (kg-m versus lb-in).
The moment is mass multiplied by arm, and candidates under time pressure copy the arm into the moment column, or mix a kilogram mass with an inch arm taken from a different chart. The arithmetic then looks plausible but the CG is wrong.
What to do instead
Write the formula 'moment = mass x arm' at the top of the table and compute each row explicitly. Keep one unit system throughout the table and state it in the header. As a check, the CG must lie between the smallest and largest arm in the table; if it does not, an arithmetic error has occurred.
Reading the fuel planning question as a simple fuel-flow multiplication and ignoring reserve fuel, taxi fuel, contingency or the difference between trip fuel and block fuel.
The question often gives a route distance, a cruise TAS and a fuel flow, which invites a single division. Reserve, alternate and taxi fuel are stated separately in the question or implied by the regulation, and candidates omit them because the arithmetic already 'works'.
What to do instead
Build the fuel plan in a fixed column order: taxi, trip, contingency, alternate, final reserve, extra, total. Compute trip fuel from the cruise calculation, then add each subsequent item. State the regulatory reserve basis used (for example 30 minutes or 45 minutes) so the examiner can see the reasoning.
Treating a climb or cruise performance question as a single-segment calculation and ignoring the effect of wind, temperature deviation or the difference between indicated and true airspeed.
Candidates substitute the given IAS directly into a TAS-based calculation, or use still-air figures when a wind is given. The climb and cruise charts are usually entered with pressure altitude, temperature and mass, and the output is TAS or fuel flow, which then needs correcting for wind.
What to do instead
Separate the calculation into three labelled steps: find TAS from the chart, apply the wind to get ground speed, then compute time and fuel from ground speed. Convert IAS to TAS using the chart or the rule-of-thumb correction for altitude and temperature before any wind is applied.
Leaving the answer as an unrounded chart value or failing to state the operational decision, for example giving a take-off distance of 743 m without saying whether the runway is adequate.
The question asks for a decision or a limiting value, but candidates stop at the arithmetic. Marking schemes award the final mark for the comparison and conclusion, and an unrounded figure also hides whether the margin is positive.
What to do instead
End every performance answer with an explicit comparison: 'required 743 m, available 820 m, margin 77 m, take-off permitted'. Round to a sensible operational precision (usually the nearest 10 m or 10 ft) and always state the direction of the margin, not just the number.