Aircraft Mass Limitations (MTOM, MLM, MZFM, MRW)
The structural limits
Four structural mass limits dominate CPL questions:
- MTOM (maximum take-off mass) - the greatest mass permitted for the start of the take-off run.
- MLM (maximum landing mass) - the greatest mass permitted at touchdown; often lower than MTOM.
- MZFM (maximum zero-fuel mass) - the greatest mass permitted with usable fuel exhausted, protecting the wing root and fuselage structure from an over-heavy payload.
- MRW (maximum ramp mass) - the greatest mass on the apron before taxi; equals MTOM plus taxi fuel allowance.
Some types also publish a maximum taxi mass (MTM) and maximum payload.
How they interact
Because MLM < MTOM on most aircraft, a flight may need to reduce take-off mass so that landing mass after burning trip fuel stays within MLM. This is the classic "landing mass limited" scenario. Conversely, a short flight with heavy payload may be MZFM limited before MTOM is reached, because little fuel is burned.
Traps
- MRW is always ≥ MTOM; if a question gives MRW < MTOM, re-read the data.
- MZFM limits payload, not fuel; you can trade fuel for payload but never exceed MZFM.
- Performance-limited masses (e.g. field-length-limited TOM) may be below structural MTOM; the actual limit is the lowest of all applicable limits.
- Do not confuse MTOM with maximum all-up mass if a question uses older terminology.
Worked example
MTOM 15,000 kg, MLM 14,000 kg, MZFM 11,500 kg, trip fuel 1,400 kg. If TOM = 15,000 kg, LM = 13,600 kg - within MLM. If trip fuel were only 600 kg, LM = 14,400 kg, exceeding MLM by 400 kg, so TOM must be reduced to 14,600 kg.
Rule of thumb
Check all limits; the governing limit is the lowest of structural, performance and landing-derived masses.