ATPL Flight Performance and Planning common mistakes
ATPL Flight Performance and Planning · Understand the traps and how to avoid them.
Using the take-off distance available (TODA) as if it were the take-off run available (TORA) when computing the field-length-limited mass, or mixing up TORA, TODA, ASDA and LDA on the declared distances line.
The four declared distances look similar and are often printed together; candidates grab the largest number because it gives a more flattering mass, forgetting each segment of the take-off must be checked against its own distance.
What to do instead
Before touching the chart, write the four distances in a labelled block and state which segment uses which: ground roll to TORA, to 35 ft to TODA, accelerate-stop to ASDA. Recompute the limiting mass for each and take the lowest. Never substitute one distance for another, even if the chart axis is labelled only 'distance'.
Forgetting to apply corrections for pressure altitude, temperature, slope, wind, runway surface and bleed-air/anti-ice configuration in the right order, or applying a percentage correction to the wrong base value.
Charts are built on a reference condition and corrections are multiplicative or additive depending on the chart; candidates apply them in the order they remember rather than the order the chart specifies.
What to do instead
Read the chart's correction notes before entering the graph. Adopt a fixed routine: pressure altitude and temperature first to get base distance, then wind, then slope, then surface, then configuration. Apply each correction to the value the note says (some are per cent of the gross gradient, not of the distance). Sanity-check the final figure against a rough rule of thumb.
Treating the climb gradient required to clear an obstacle as a net gradient when the published performance is gross, or ignoring the 0.9 (or 0.8 for two-engine) net-to-gross factor and the segment structure (first, second, final take-off segments; en-route).
The obstacle-limited climb and the certification climb segments use different references; candidates conflate 'achievable gradient' with 'required net gradient' and drop the factor.
What to do instead
Always state which gradient the question asks for. Convert gross to net by the applicable factor before comparing with the obstacle gradient. For take-off, check the obstacle against the second-segment net gradient with gear up and the critical engine failed; for en-route, use the net en-route gradient with the critical engine inoperative and the aircraft at the relevant configuration.
Using the landing mass instead of the landing mass at the destination alternate, or failing to check the landing distance at the most limiting of destination and alternate with the correct wind, slope, surface and temperature.
Candidates compute one landing case and assume it covers the flight; alternates often sit at higher elevation or hotter temperature and become limiting.
What to do instead
Build a small table with a row for destination and each alternate, columns for pressure altitude, temperature, wind, slope, surface and configuration, and compute the required landing distance for each. Apply the regulatory factor (for example 1.67 for dry, or the applicable wet/contaminated factor) and confirm the LDA at each is sufficient. The lowest permissible landing mass governs.
Ignoring contaminated-runway penalties, or applying a dry-runway factor to a runway reported wet or with standing water, slush or snow.
Contaminated performance is usually a separate chart or table and candidates default to the dry chart because it is familiar; they also forget that contaminant depth and type change the braking coefficient.
What to do instead
Check the runway condition report first and select the matching chart or table. Note that on contaminated runways the accelerate-stop distance and the screen height may change, and crosswind/braking limits may apply. If the contaminant depth exceeds the chart's validity, treat the runway as unusable rather than extrapolating.
Confusing mass and weight units, or mixing kg, lb and the 'mass' used by the chart, when converting fuel volume to mass using density.
Some charts and load sheets are in kg, some in lb; fuel is given in litres or US gallons and must be converted with a density that varies with temperature. Candidates multiply by the wrong factor or forget the density step entirely.
What to do instead
Write the unit next to every number as you go. Convert fuel volume to mass with the stated density (kg/L or lb/US gal) and keep the same unit system through the whole calculation. If the question gives a density, use it; if not, state the assumption. Carry units into the final answer and check the magnitude is plausible.
Computing the centre of gravity using arms or moments from the wrong datum, or omitting items such as crew, unusable fuel, or the mass of the fuel in the tanks at the moment of interest.
The datum is defined by the type and items are listed in different orders in different documents; candidates drop the zero-fuel mass items or double-count fuel.
What to do instead
Lay out a moment table with columns: item, mass, arm, moment. Start from basic empty mass, add crew, payload, then fuel, and sum moments before dividing. Check the zero-fuel mass and the take-off mass separately against their limits, and confirm the CG falls inside the envelope at both zero-fuel and take-off states.
Planning fuel with the trip fuel only, or applying the contingency and reserve percentages to the wrong base (for example taking contingency as a percentage of total fuel rather than trip fuel).
The fuel plan has several named components with specific bases; candidates remember the percentages but not what they attach to, and reserve fuel is often treated as usable for the trip.
What to do instead
Build the fuel plan in the order the regulation defines: taxi, trip, contingency (a percentage of trip fuel), alternate, final reserve, additional/extra. Sum them for the required fuel and compare with the fuel on board at start-up. Keep reserve and alternate fuel ring-fenced and never plan to burn them.
Selecting an alternate without checking its weather, runway suitability, approach availability and the fuel/time to reach it, or forgetting the additional fuel needed when no suitable alternate exists.
Candidates treat the alternate as a name on the flight plan rather than a performance and weather decision, and forget the planning minima that apply to alternates.
What to do instead
For each candidate alternate, check forecast weather against the alternate planning minima, runway length and condition against the landing mass, and the approach aids available. Compute the fuel and time from destination to alternate at the planned level. If no alternate meets the criteria, plan the additional fuel required by the rules for a no-alternate case.
Treating the computer flight plan as authoritative and not cross-checking it against the operational flight plan, or failing to replan and monitor fuel in flight when actual burn differs from plan.
The computer plan is generated with assumed winds and temperatures; candidates accept it without comparing to the operational plan figures or noticing a divergence once airborne.
What to do instead
Compare the computer plan's trip fuel, time and levels with the operational flight plan and reconcile differences before departure. In flight, compare actual fuel at waypoints with the plan, compute the fuel margin over the next leg, and if the trend shows a shortfall, replan (level, speed or destination) before the reserve is eroded. Record the decision point.