NCAA CPL Flight Performance & Planning common mistakes
NCAA CPL Flight Performance & Planning · Understand the traps and how to avoid them.
Using the basic empty mass and moment from the aircraft documentation without adding the mass and moment of the specific aircraft's installed optional equipment, then computing the CG from that incomplete total.
Candidates memorise one 'BEM' figure and treat it as fixed across the fleet, forgetting that the CPL syllabus aircraft (typically a light twin or complex single) carries a listed equipment inventory whose mass and arm must be summed to give the operational empty mass actually used on the day.
What to do instead
Before any loading calculation, build a one-page worksheet that starts from the basic empty mass and moment, adds each item on the equipment list with its arm, and only then adds crew, fuel, payload and baggage. Recompute the empty mass and arm for the exact registration in the question and carry that figure forward.
Adding masses to get a total but averaging the arms (or ignoring moment entirely) instead of summing moments and dividing total moment by total mass to find the CG position.
Candidates who are comfortable with the mass arithmetic assume the CG is 'somewhere in the middle' and eyeball it, or they average two arms because averaging feels like a natural way to combine numbers.
What to do instead
Write every loading problem as a three-column table: mass, arm, moment. Sum the moment column, sum the mass column, and divide. Never state a CG without showing the quotient; treat any CG reached without a division as an unfinished answer.
Checking only that the take-off CG is inside the envelope and never re-checking the landing CG after fuel burn, or checking the zero-fuel CG and assuming the landing case is covered.
The forward and aft limits are not parallel, and fuel is often carried in a tank whose arm lies outside the useful range, so burning fuel can move the CG out of the envelope in either direction. Candidates stop at the first satisfactory plot.
What to do instead
Plot three points on the envelope for every loading question: take-off, zero fuel, and landing. Where the question gives a fuel-burn sequence or a long sector, also check the point of minimum fuel. State explicitly which of the three is most critical and why.
Reading pressure altitude straight off the altimeter setting or using aerodrome elevation as pressure altitude, without applying the 30 ft per hPa (or the millibar-to-inch conversion) correction.
The performance graphs are entered on pressure altitude, but the question stem usually gives elevation and QNH. Under exam time pressure the conversion step is the one most often skipped, and the error is small enough to look plausible.
What to do instead
Adopt a fixed entry sequence for every performance graph: outside air temperature, then pressure altitude (elevation corrected for QNH), then mass, then configuration. Write the corrected pressure altitude in the margin before touching the graph so the marker can see the step was done.
Interpolating performance figures linearly between published table rows or graph lines that are not linear, and rounding the intermediate result before the next step.
Candidates interpolate mechanically between the nearest two lines because that is what they were taught for simple tables, but take-off distance and climb gradients vary non-linearly with mass and temperature; early rounding compounds through two or three graph entries.
What to do instead
Interpolate only between adjacent published values, keep at least three significant figures through every intermediate step, and round only the final answer. Where the graph is visibly curved, interpolate on the vertical distance between the curves rather than on the axis values.
Using the still-air take-off or landing distance as the answer and omitting the factor for runway surface, slope, wind or the applicable safety factor.
The graph gives a headline distance and candidates stop reading once they have a number that matches an option. The correction factors sit in notes below the graph or in a separate table that is easy to miss under time pressure.
What to do instead
After reading the graph, run a mental checklist of every correction the question supplies - surface, slope, wind component, and any required factor - and apply them in the order the flight manual specifies. If a correction is given in the stem, it must appear in the working.
Computing the wind component along the runway by using the full wind speed, or by using the sine of the angle when the cosine is required, and then applying the wrong sign for head or tailwind.
The crosswind and headwind formulae look similar and are often memorised as 'sine for crosswind' without the candidate checking which component the question asks for; a tailwind entered as a headwind reverses the sign of the correction and can turn a legal take-off into an illegal one.
What to do instead
Draw the runway and the wind vector on the working paper every time, label the along-runway and across-runway components, and compute both with the cosine and sine respectively. State 'headwind' or 'tailwind' in words next to the number before applying it.
Planning fuel as trip fuel plus a fixed percentage reserve and omitting the contingency, alternate, final reserve and taxi fuel that the operational flight plan requires.
Candidates carry over a light-aircraft habit of 'trip plus 45 minutes' and do not rebuild the fuel plan from the five components the syllabus expects; the question then offers a distractor that matches the incomplete sum.
What to do instead
Build the fuel plan in a fixed vertical list: taxi, trip, contingency, alternate, final reserve, extra. Add them in that order and show the running total. Check the total against the maximum fuel the tanks can hold and against the maximum landing mass at the destination.
Treating the alternate fuel as a fixed time and distance regardless of the alternate's own wind, temperature and routing, or assuming the alternate is always the nearest aerodrome.
The alternate leg is computed from a different direction and often a different altitude from the destination leg, so the same still-air figures do not apply. Candidates reuse the destination leg's groundspeed because it is already on the page.
What to do instead
Compute the alternate leg separately: its own track, its own wind component, its own altitude and its own fuel flow. Where the question gives an alternate in a different direction, recalculate the groundspeed from scratch rather than reusing the inbound figure.
Filling in the operational flight plan with the planned figures and never reconciling them against actual fuel on board at the diversion or destination, so the in-flight fuel check is treated as a formality.
The flight-planning questions are answered as a paper exercise, and candidates do not connect the planned figures to the monitoring task of comparing actual against planned consumption at waypoints and deciding whether the flight can continue.
What to do instead
For every monitoring question, compute the difference between planned fuel at the checkpoint and actual fuel, convert it to a time equivalent at the current fuel flow, and state the resulting endurance. Then compare that endurance with the fuel required to reach the destination plus the final reserve and alternate.
Using maximum take-off mass as the limiting mass without checking whether the performance-limited mass, the maximum landing mass at destination, or the zero-fuel mass is the actual constraint.
Candidates equate 'MTOM' with 'the most I can carry' and stop there, but the binding limit on a short runway or a hot day is usually the performance-limited mass, and on a short sector it can be the maximum landing mass because the fuel cannot be burned off.
What to do instead
Compute all four candidate masses - MTOM, performance-limited take-off mass, maximum landing mass plus trip fuel, and MZFM plus payload - and take the lowest. State in the answer which one is limiting and why the others are not.