CPL Flight Performance and Planning common mistakes
CPL Flight Performance and Planning · Understand the traps and how to avoid them.
Using the pressure altitude read off the altimeter subscale setting directly as the chart entry for takeoff and landing distance, without first converting indicated altitude to pressure altitude using the aerodrome elevation and QNH.
Candidates conflate 'altitude' with 'pressure altitude' because both are in feet and the chart axis is simply labelled 'altitude'. The chart is actually tabulated against pressure altitude, so entering with aerodrome elevation or with the altimeter reading introduces an error of roughly 30 ft per hPa of QNH deviation from 1013.
What to do instead
Before touching the chart, compute pressure altitude = aerodrome elevation + 30 x (1013 - QNH in hPa), or set 1013 on the subscale and read the altimeter. Write the number down as 'PA' and use only that value on the chart axis. Practise ten conversions in both directions until the sign convention is automatic.
Interpolating takeoff and landing distances linearly between chart columns without applying the associated corrections for wind, slope, surface condition and clearway/stopway separately, then reporting the raw chart value as the required distance.
The chart value is a baseline for still air, level, dry, hard surface. Candidates treat it as final because the corrections are listed in notes rather than built into the grid, and under exam time pressure the notes get skipped.
What to do instead
Adopt a fixed sequence: read baseline distance, apply wind correction as a percentage, apply slope correction as a percentage, apply surface factor, then compare against available runway, clearway and stopway. Apply percentage corrections multiplicatively in the order the notes specify, and always state which distances (TODA, ASDA, TODR, LDR) the corrected figure must be compared with.
Computing the centre of gravity using only the empty mass and payload, omitting fuel, and then comparing the result against the forward and aft limits without checking the zero-fuel case separately.
Fuel is treated as a minor item because it burns off, but it is often the largest variable mass and its moment arm can move the CG several percent of MAC. Candidates also forget that the most critical case may be zero fuel rather than takeoff mass.
What to do instead
Build a full mass-and-moment table with one row per item (empty, crew, front pax, rear pax, baggage, fuel, any ballast), sum mass and moment, divide for CG. Then repeat with fuel set to zero to test the zero-fuel CG. Check both results against the envelope at the correct mass, and state explicitly which case is limiting.
Reading the CG envelope as a rectangle in mass-versus-CG coordinates and accepting any point inside it, ignoring that the forward limit varies with mass and that some envelopes have a stepped or sloping boundary.
Textbook diagrams are often simplified rectangles, so candidates carry that mental model into real envelopes where the forward limit moves aft at low mass and the aft limit may change with fuel or configuration.
What to do instead
For every loading check, plot or tabulate the point and then read the limit at that specific mass from the envelope, not the extreme limit. Where the envelope has a sloping forward boundary, verify the point against the boundary line at that mass, and re-check after fuel burn because the mass change moves the point vertically.
Planning fuel by summing only trip fuel and reserve, omitting taxi, contingency, alternate and final reserve, or applying the reserve as a percentage of the wrong quantity.
Candidates memorise 'trip plus reserve' as a formula and lose track of which components are mandatory in which regulation and which are operator policy. Percentage-based contingency is especially prone to being applied to total fuel rather than trip fuel.
What to do instead
Write the fuel ladder in a fixed order every time: taxi, trip, contingency, alternate, final reserve, extra, total. Label each line with its basis (percentage of trip fuel, fixed minutes at holding flow, etc.). Recompute the total from the ladder rather than adding components ad hoc, and cross-check that landing fuel never falls below final reserve.
Selecting an alternate without verifying that it is actually usable: checking only distance, not weather minima, runway suitability, operating hours, or the additional fuel needed to reach it from the missed approach point.
The alternate is often chosen early as a convenient nearby aerodrome and never re-validated. Candidates also compute alternate fuel from destination overhead rather than from the missed approach point, which understates the requirement.
What to do instead
For each alternate, list distance from the missed approach point, forecast weather against that aerodrome's minima, runway length and surface against the landing distance required, and opening hours. Compute alternate fuel from the missed approach point at the planned alternate flight level. If any item fails, choose another alternate and redo the fuel.
Mixing indicated airspeed, true airspeed and ground speed in the same calculation, for example using TAS to compute a climb gradient in still air and then applying wind to it, or using IAS for a descent rate calculation.
The three speeds differ by density and wind, and exam questions deliberately supply one while requiring another. Candidates grab whichever number appears in the question without tracking which speed the formula expects.
What to do instead
Annotate every speed in the working with its type (IAS, CAS, TAS, GS). Convert IAS to TAS using the density ratio before any gradient, radius of turn, or true-distance calculation; apply wind only when converting TAS to GS. As a check, climb gradient in still air depends on TAS and rate of climb, never on wind.
Treating a headwind and a tailwind as symmetric in takeoff and landing calculations, subtracting the same percentage for a tailwind as is added for a headwind.
The wind correction note on most charts is explicitly asymmetric and often caps the tailwind component, but candidates apply a single symmetric percentage because it feels arithmetically tidy.
What to do instead
Read the wind correction note literally: apply the headwind percentage only for headwind components, apply the (usually larger) tailwind percentage only for tailwind components, and check the maximum permitted tailwind. If the chart provides a graph, enter it with the actual component rather than a rounded percentage.
Ignoring the difference between the gradient quoted on a departure or approach chart and the gradient the aircraft can actually achieve at the planned mass, altitude and temperature, and assuming performance is adequate because the aircraft type is approved for the procedure.
Procedure design gradients are published as minimums for the procedure, not guarantees for the aircraft. Candidates assume approval equals capability, forgetting that obstacle clearance depends on achieving the gradient to the specified height.
What to do instead
For each significant obstacle or published gradient, compute the aircraft's achievable gradient from the rate-of-climb and TAS at the relevant mass, altitude and temperature, then compare with the required gradient. If the achievable gradient is lower, plan a reduced mass, a different departure procedure, or a higher acceleration altitude, and record the decision.
Using a chart or table beyond its tabulated range by extrapolating, or interpolating between rows that are already corrected for a condition the question has changed.
The required value falls just outside the printed grid and extrapolation is quicker than finding the correct table or applying the correction. Candidates also fail to notice that a table's header specifies a configuration or condition that no longer matches the scenario.
What to do instead
Before entering any table, read the header and the footnotes to confirm the conditions match the question. If the value lies outside the tabulated range, do not extrapolate; use the conservative limit of the table and state that it is limiting, or switch to the correct table. When interpolating, interpolate only between rows that share identical conditions.