EASA 031 Mass & Balance common mistakes
EASA 031 Mass & Balance · Understand the traps and how to avoid them.
Treating mass and weight as interchangeable and mixing kg with daN or litres of fuel without converting, then reporting take-off mass in the wrong unit.
Everyday language uses 'weight' for mass, and the exam deliberately mixes units (kg, lb, daN, litres). Candidates plug numbers into formulas from memory without tracking the unit of each input, so a fuel volume is added directly to a mass or a force is compared with a mass limit.
What to do instead
Adopt a fixed working procedure: write every given quantity with its unit, convert volume to mass using the stated density (kg = litres x density in kg/l), convert mass to weight only when a force is required (1 kg = 9.81 N, so 1 kg = 0.981 daN), and state the unit of the final answer before selecting an option. Practise conversions in both directions until they are automatic.
Confusing dry operating mass with empty mass, or zero-fuel mass with landing mass, and therefore including or excluding crew, crew baggage, pantry/consumables and unusable fuel in the wrong total.
The definitions overlap in everyday usage; candidates memorise the acronyms but not what each includes. The distinction between DOM (empty mass plus crew, crew baggage, catering/consumables, unusable fuel, engine oil) and the zero-fuel mass (take-off mass minus usable fuel) is easily blurred under time pressure.
What to do instead
Build a one-page hierarchy and redraw it from memory before each practice session: empty mass + crew and crew items + unusable fuel and oil = dry operating mass; DOM + traffic load = zero-fuel mass; ZFM + usable fuel = take-off mass; TOM - trip fuel = landing mass. For every question, identify which of these four totals is being asked for before doing any arithmetic.
Comparing the calculated take-off mass only with MTOM and ignoring MLM, MZFM and performance-limited masses, so the answer is 'within limits' when a later or earlier limit is actually exceeded.
MTOM is the most familiar limit and candidates stop checking once it is satisfied. They forget that landing mass must be checked against MLM (which can be critical on short sectors with little fuel burn) and that zero-fuel mass must be checked against MZFM (critical when a large payload is carried with little fuel).
What to do instead
Use a fixed four-check sequence for every load problem: TOM vs MTOM, LM vs MLM, ZFM vs MZFM, and TOM/LM vs any performance-limited mass given in the question. If any check fails, the load must be reduced; state which limit is binding and by how much. Treat the lowest of the structural and performance values as the operative limit.
Treating the CG envelope as a single forward and aft limit and ignoring that the permitted CG range varies with mass, so a CG that is legal at one mass is declared legal at another.
Candidates read the forward and aft limits as fixed numbers and do not look at the shape of the envelope, where the forward limit often moves aft at high mass and the aft limit may be restricted at low or high mass depending on type.
What to do instead
Always plot or read the CG against the mass on the envelope provided in the question, not against a remembered number. When no envelope is given, check whether the question supplies mass-dependent limits and apply them at the actual mass. In explanations, state the CG and the mass together, never the CG alone.
Getting the sign or direction of a CG shift wrong when mass is added, removed or moved, and reporting a shift in the wrong direction or with the wrong magnitude.
The moment equation is simple but the intuition is not: removing mass from ahead of the CG moves the CG forward, which feels counter-intuitive, and candidates also forget that moving a mass changes the CG by (mass moved x distance moved) / total mass, not by the distance moved.
What to do instead
Apply the rule 'the CG moves toward the added mass and away from the removed mass' as a sanity check on every answer, then verify with moments. For a transfer, use delta CG = (moved mass x shift) / new total mass and confirm the sign by recomputing total moments about a fixed datum. Do two or three numerical drills per study session.
Using the wrong datum or arm convention, for example mixing arms measured from the nose with arms measured from the leading edge or from a point aft of the datum, and producing a CG that is out by a constant offset.
Loading tables, graphs and nomograms in questions use different references, and candidates carry over an arm from one source to another without re-referencing. Negative arms ahead of the datum are also dropped.
What to do instead
Before calculating, write down the datum and the sign convention explicitly, and convert every arm to that datum. When a graph or nomogram is used, read the input axis label and the reference point first. Check plausibility: a CG should fall within a few metres of the values in the envelope, and a result far outside indicates a datum error, not a loading error.
Assuming fuel burn does not change the CG, or assuming it always moves the CG forward, instead of computing the effect from the tank positions relative to the CG.
Fuel is a large mass and its tanks may be ahead of, behind, or near the CG; candidates generalise from one aircraft type or from the common statement that 'burning fuel moves the CG forward' and apply it universally.
What to do instead
Treat fuel as a mass removal at the tank arm: compute the moment of the fuel burned at the tank arm and subtract it, then recompute the CG. Compare the tank arm with the current CG to predict the direction before calculating, and check the landing CG against the landing envelope, not just the take-off CG.
Reading nomograms and loading graphs imprecisely, for example reading a value off the wrong axis, interpolating between the wrong lines, or ignoring the graph's scale and units.
Graph questions are answered quickly by eye, and candidates read the nearest line rather than interpolating between lines, or forget that the graph's axes may be in different units from the data given in the text.
What to do instead
Practise a fixed reading routine: identify both axes and their units, locate the input on the correct axis, follow the line to the second axis, interpolate between adjacent lines and state the value to the precision the graph allows. Cross-check the graph result with an approximate moment calculation; if the two disagree by more than the reading tolerance, re-read the graph.
Ignoring floor load limits, area load limits and load spreading when assessing whether a hold load is acceptable, and checking only the total mass and CG.
Candidates focus on the mass and balance arithmetic and treat the hold as a single point; the structural limits per unit area and per running metre are seen as a separate subject and not applied to the loading problem.
What to do instead
For any cargo question, run three checks: total mass against the hold limit, mass per unit area against the floor limit, and mass per running metre against the linear limit. If the area limit is exceeded, calculate the spreading needed (contact area = mass / permitted area load) and state the number of pallets or the spreader dimensions required.
Mis-stating who is responsible for what in loading documentation, for example assuming the flight crew compute the load sheet or that the loading supervisor signs the trim sheet, and confusing the load sheet with the trim sheet.
The roles overlap in practice and the documents look similar, so candidates answer from general impression rather than from the defined responsibilities: the loading supervisor controls and confirms the load, the load sheet records the mass and balance, and the trim sheet is the flight-crew document used to set the trim and confirm the CG.
What to do instead
Learn the documents as a chain: loading instruction (what to load), load sheet (what was loaded, with mass and CG, signed by the loading supervisor), trim sheet (flight crew's summary and trim setting). For responsibility questions, ask 'who physically controls the load' versus 'who accepts the aircraft', and remember the commander retains final responsibility for the mass and balance.