Mass and Load Terminology - ATPL 031 — Mass & Balance

Every mass-and-balance document, every load sheet and every exam question in this subject is written in a small, precise dictionary of terms. Learn the dictionary once, properly, and the calculations that fill the rest of the subject become bookkeeping; guess at it, and every question hides a trap. This lesson defines the mass terms, the load and fuel terms, shows how they chain together into one loading sum, and finishes with the fuel-conversion arithmetic that turns litres and densities into the kilograms the load sheet needs.

The mass terms

The mass terms describe the aeroplane at successive stages of loading, each term a snapshot with a strict definition of what is and is not on board:

Basic empty mass (BEM) — the mass of the aeroplane itself plus its standard items: unusable fuel and other unusable fluids, lubricating oil in the engines and auxiliary units, fire extinguishers, pyrotechnics, emergency oxygen equipment and supplementary electronic equipment. It is the mass established by weighing the aircraft, and it changes only when the aircraft is modified.
Dry operating mass (DOM) — the total mass of the aeroplane ready for a specific type of operation, excluding usable fuel and traffic load. On top of the BEM it includes the crew and their baggage, the catering and removable passenger-service equipment, and the potable water and lavatory chemicals: everything the operation needs that is neither payload nor fuel.
Operating mass (OM) — the DOM plus the take-off fuel: the aircraft ready to fly, but with nobody and nothing paying for the trip on board yet.
Zero fuel mass (ZFM) — the DOM plus the traffic load, with no usable fuel: the loaded aircraft as it would be with the tanks dry.
Ramp mass (taxi mass) — the mass at the start of taxi, at departure from the loading gate: everything on board, including the fuel that will be burned before the runway.
Take-off mass (TOM) — the mass of the aeroplane, including everything and everyone in it, at the start of the take-off run.
In-flight mass (gross mass) — the mass at any given moment airborne: the take-off mass minus the fuel burned since the start of the take-off run.
Landing mass (LM) — the mass at touchdown: in planning terms, the take-off mass minus the trip fuel.

Notice what the two towers in the figure share and where they differ. ZFM and OM both sit on top of the DOM; the difference is only which of the two remaining ingredients, traffic load or fuel, has been added. That makes them complements: whichever one a question gives you, adding the missing ingredient produces the take-off mass. Each of these masses also carries its own maximum, and the reasons the maxima exist are different enough that the next lesson is devoted to them.

The load terms

Two load terms join the dictionary:

Traffic load (also called the payload) — the total mass of the passengers, their baggage and the cargo, including any non-revenue load such as company mail or staff travelling on duty: the part of the loading the operator is in business to carry.
Useful load — a related term appearing in some documents: the traffic load plus the usable fuel, or equivalently everything between the dry operating mass and the take-off mass.

Traffic load earns money, fuel makes the trip possible, and the useful load is the two together: the aircraft's total carrying ability beyond its own operational self.

The fuel terms

Fuel gets its own vocabulary because a flight does not carry one quantity of fuel; it carries a stack of quantities, each with its own purpose and its own rules about when it may be burned. The full planning logic belongs to subject 033, Flight Planning; what this subject needs is the terms, because they appear as lines on every load sheet:

Block fuel — the total usable fuel on board when the aircraft moves off the blocks, the wheel chocks at the parking stand (which is where block fuel and off-blocks time get their names): the whole stack, taxi fuel included.
Taxi fuel — the fuel expected to be burned before the take-off run begins: engine start, the auxiliary power unit, and the taxi out.
Take-off fuel — the fuel on board at brake release, the moment the take-off run begins: the block fuel minus the taxi fuel.
Trip fuel — the fuel planned to be burned from the start of the take-off run to touchdown at the destination: take-off, climb, cruise, descent and the approach.
Contingency fuel — the buffer for the unforeseen: winds worse than forecast, a different cruising level, a longer routing. It is typically planned as a percentage of the trip fuel.
Alternate fuel — the fuel to fly a missed approach at the destination and continue to a landing at the alternate aerodrome.
Final reserve fuel — the last defence, intended never to be burned in normal operations: for a jet aeroplane, enough fuel to hold for 30 minutes at 1,500 ft above the aerodrome in standard conditions.
Additional fuel — required only when the standard scheme above does not cover a specific situation, such as an isolated destination with no alternate or a diversion after an engine failure or depressurisation at the most critical point of the route.
Extra fuel — whatever the commander chooses to add on top of every requirement, at their own discretion.

Together, the contingency, alternate, final reserve and additional fuel form the reserve fuel: the quantities carried for what is not planned to happen. The figure below stacks the whole vocabulary into one bar.

One loading sum, several subtotals

The relationships between all these terms are a single loading sum read at different points, and the exam tests them as identities. Each line below is worth being able to write from memory in both directions:

Ramp mass = DOM + traffic load + block fuel — everything on board at the gate.
TOM = ramp mass − taxi fuel — the taxi burn is gone before the take-off run starts.
TOM = ZFM + take-off fuel = OM + traffic load — the two towers of the figure: the same total reached via either subtotal.
ZFM = DOM + traffic load — the loaded aircraft without its usable fuel.
OM = DOM + take-off fuel — the fuelled aircraft without its traffic load.
LM = TOM − trip fuel — the planned mass at touchdown.
Useful load = TOM − DOM — traffic load and usable fuel together.

Any one component can be recovered from the others by walking these identities, and that is precisely what the exam asks: given four of the numbers, find the fifth. The habit that prevents errors is to place every given number on the ladder before calculating, because the classic trap is a question that gives block fuel where the chain needs take-off fuel, or ramp mass where it needs TOM, and lets the taxi fuel silently poison the sum.

Fuel by volume, mass and density

The load sheet works in mass, but fuel is pumped, gauged and sold by volume, and the bridge between the two is the density: the mass of one unit of volume, for jet fuel usually quoted in kilograms per litre. The same idea appears as specific gravity, the density relative to water, which for a fuel of 0.79 kg per litre is simply 0.79. Density is not a constant: it falls as fuel warms, and it differs between batches, so the figure to use is the one on the fuel slip or, failing that, the standard value from the operations manual. The conversion itself is one multiplication:

Aviation mixes unit systems freely, so the conversions between them are part of the syllabus. The ones worth memorising: one US gallon is 3.785 litres, one imperial gallon is 4.546 litres, and one kilogram is 2.205 pounds. A density given in one system converts to another by chaining these factors, exactly as the example below does.

Define every mass term: BEM, DOM, OM, ZFM, ramp/taxi mass, TOM, in-flight (gross) mass and landing mass, and know exactly what each includes and excludes

Define traffic load (payload) and useful load, and the fuel terms from block fuel through the reserve components to extra fuel

Write the identities linking the terms in both directions: ramp, TOM, ZFM, OM, LM and useful load

Calculate any missing component from the others, watching for the taxi-fuel and block-versus-take-off-fuel traps

Convert fuel between volume, mass and density in litres, gallons, kilograms and pounds

The mass terms

Check you understood

The load terms

The fuel terms

One loading sum, several subtotals

Fuel by volume, mass and density