Aircraft Mass Limitations: Structural, Performance and Regulated
Structural limits
MTOM, MLM and MZFM are certification (structural) limits that may never be exceeded in normal operation. MTOM protects the wing, fuselage and landing gear in flight and on the take-off run and sets the climb capability required at certification. MLM limits the loads at touchdown and the energy the brakes and structure must absorb; it may also reflect go-around performance. MZFM limits the bending moment at the wing root and fuselage loads with the wings full of fuel but no fuel load to relieve them - it applies whenever the aircraft is at that mass, including in flight and at take-off.
Performance-limited masses
Performance-limited take-off mass is the highest mass from which the aircraft can meet the applicable field-length, obstacle-clearance, climb-gradient, tyre-speed and brake-energy requirements for the actual conditions (pressure altitude, temperature, runway, wind, slope, contamination). It varies with every departure. The mass actually used must not exceed the lowest of the structural limits and the performance-limited values. Landing mass is similarly checked against the landing field length and the approach/climb requirements, and against MLM.
Regulated versus operational
The regulated (certificated) masses are fixed in the type-certificate data: MTOM, MLM, MZFM. The operational masses - DOM, TOM, LM, ZFM - are what the flight actually achieves and are limited by the certificated figures and by performance.
Worked example
MTOM 70 000 kg, MLM 64 000 kg, MZFM 55 000 kg, DOM 40 000 kg, take-off fuel 12 000 kg, trip fuel 9 000 kg, traffic load 14 000 kg. ZFM = 54 000 kg (below 55 000). TOM = 66 000 kg (below 70 000). LM = 57 000 kg (below 64 000). However, the performance-limited TOM for the day is 62 000 kg, so 4 000 kg must be offloaded - normally payload first, since fuel is needed for the flight.
Traps that cost marks
Checking MZFM only at landing; comparing LM with MTOM; forgetting that reserve fuel is still on board at landing; treating a performance-limited mass as if it were structural and fixed; offloading fuel when a payload reduction would do.