The previous lesson built the loading ladder; this one puts a ceiling over every rung. Some ceilings are structural, cast into the airframe at certification and printed in the flight manual; one is set fresh for every flight by the performance of the day; and the number the crew actually works to is always the lower of the two. The lesson ends inside the cargo compartments, where two further limits protect the floor itself, because a compartment can be overloaded in ways a total mass figure never shows.
The structural maxima
Four structural maximum masses cap the ladder, each protecting a different load case:
Maximum zero fuel mass (MZFM) — the maximum permissible mass of the aeroplane with no usable fuel on board. Everything loaded above the MZFM must therefore be fuel, and the reason is in the wing structure: the bending story told below.
Maximum ramp mass (maximum taxi mass) — the structural limit on the mass of the aeroplane at the commencement of taxi. It sits slightly above the maximum take-off mass, the difference being an allowance for the fuel burned between the gate and the runway.
Maximum structural take-off mass (MSTOM) — the maximum permissible total mass at the start of the take-off run, the figure certified against the flight and gust load cases of the previous module's lesson on why limits exist.
Maximum structural landing mass (MSLM) — the maximum permissible total mass on landing in normal circumstances, certified against the landing-gear and airframe loads of a touchdown at the design sink rate (the certificated descent rate at the moment of touchdown).
The odd one out in that list is the MZFM, because it caps a mass the aircraft never actually flies at. It exists because of where lift and weight act. Lift is produced along the wings and bends them upward; the fuselage's weight hangs at the centre and bends them the same way, like two hands lifting a loaded tray by its ends. Fuel carried in the wings pushes down exactly where the lift pushes up, cancelling part of the bending before it ever reaches the wing root. That relief is why mass above the MZFM is reserved for fuel: a kilogram of cargo loads the fuselage and adds to the root bending, while a kilogram of wing fuel largely carries itself. Cap the fuselage-loaded mass at the MZFM, and the wing-root bending stays inside what the structure was certified for, however much fuel is added on top.
Check you understood
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
1. Warm-up
The maximum ramp mass (maximum taxi mass) is:
2. Applying it
A small, very dense machine part on a tiny pallet is loaded into a compartment that is otherwise almost empty. Which limit is threatened first?
3. Pushing it
A load is repacked so that the same total mass rests on the same floor contact area but is spread along twice the fuselage length. What happens to the running load and the floor load?
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What you are looking at: the same aeroplane from the front, producing the same total lift in both panels, with the same take-off mass. In the left panel all the disposable load is in the fuselage: the wing carries the full difference between its distributed lift and the concentrated centre weight, and the bending at the wing root is large, shown by the curved arrows. In the right panel part of the load is fuel in the wing tanks: the fuel's weight presses down along the wing, directly against the lift, and the root bending shrinks. The maximum zero fuel mass caps the left-panel kind of loading; only fuel, the load that relieves its own bending, may be added beyond it.
The performance and regulated limits
The structural maxima never change; the aerodromes of the day do. A short, hot, high runway can make the performance-limited take-off mass (PLTOM), the take-off mass subject to the departure aerodrome's limitations, far lower than anything the structure would allow, and the destination sets a performance-limited landing mass (PLLM) in the same way from the landing aerodrome's limitations. The numbers the crew must actually respect are the regulated masses, and their definitions are pure pessimism:
Regulated take-off mass (RTOM) — the lower of the performance-limited take-off mass and the maximum structural take-off mass.
Regulated landing mass (RLM) — the lower of the performance-limited landing mass and the maximum structural landing mass.
Whichever consideration bites first, structure or performance, wins. On a long runway at sea level on a cool day the structural figure usually governs; from a short runway in summer at a high-elevation aerodrome, performance does. The next lesson turns these definitions into the exam's favourite calculation: finding the allowed take-off mass and the traffic load it leaves room for.
Cargo compartment limits
A cargo compartment can be overloaded with its total mass nowhere near any limit above, because floors fail locally before airframes fail globally. Two limits guard against that, and both are printed in the loading manual for every compartment:
Maximum floor load — the maximum load per unit of floor area, quoted in kilograms per square metre (or pounds per square foot). It protects the floor panels themselves: a small, dense crate standing on a tiny footprint can press harder on each square metre than the floor structure can bear, while weighing less than almost anything else in the compartment.
Maximum running load — the maximum load per unit of fuselage length, quoted in kilograms per metre. It protects the fuselage frames and the floor beams that span between them: mass concentrated along a short stretch of fuselage loads the few frames under it far harder than the same mass spread along the compartment.
What you are looking at: one cargo compartment drawn twice. On the left, from above: two loads of equal mass, one spread across a large footprint and one concentrated on a small pallet. The floor pressure, mass divided by contact area, is far higher under the small footprint, and that is what the maximum floor load caps. On the right, from the side: the same compartment along the fuselage, with the load's mass divided by the length of fuselage it occupies. Stacking the mass along a short stretch drives up the load per metre on the frames beneath it, and that is what the maximum running load caps.
The two limits answer different questions: floor load asks how hard the load presses on each square metre under it, running load asks how much mass each metre of fuselage length is asked to carry. A load can violate either one alone, which is why both are always checked. Here the syllabus only asks what the limits are; the arithmetic of contact areas and spreader floors belongs to this course's cargo handling lesson, where it is worked in full.
Define MZFM, maximum ramp/taxi mass, maximum take-off mass and maximum landing mass, and explain the wing-bending-relief logic behind the MZFM