Mass limitations

031 Mass & BalanceChapter 2

Aircraft may only be loaded up to the lowest applicable limit — structural, performance, or regulatory. Know the abbreviations and what each one restricts.

Structural vs regulated

Definition — Max certified (structural) mass

Set by the manufacturer in the AFM/loading manual. Must not be exceeded under normal operations (MSLM may be exceeded in emergencies).

Definition — Max regulated mass

The lower of the structural limit and the performance-limited mass for the actual conditions (runway, temperature, altitude).

Key structural limits

Limit	Meaning
MSTOM	Max structural take-off mass
MSLM	Max structural landing mass
MZFM	Max zero fuel mass
MSTM	Max structural taxi/ramp mass

Definition — Max zero fuel mass (MZFM)

Structural limitation at the wing root — max mass without usable fuel. Calculated for +2.5G load factor and maximum bending moment at the root. MZFM = DOM + max traffic load (no fuel). Limits traffic load on short flights where fuel weight is small.

Definition — Max taxi/ramp mass

Fixed structural value — max mass before engine start. Governed by undercarriage and structural considerations.

Definition — Max landing mass (MLM)

Restricted by structural strength, performance (runway length, tyre temperature), and runway PCN. Landing above performance-limited mass risks: insufficient runway, tyre limits exceeded, structural damage, go-around not achievable.

Regulated take-off mass

Formula — Regulated TOM — lowest wins

Regulated TOM = lowest of:
  • RTOM  (regulated/performance-limited TOM)
  • RLM + trip fuel
  • MZFM + take-off fuel

Key fact

RTOM = lower of MSTOM and PLTOM (performance-limited TOM). RLM = lower of MSLM and PLLM (performance-limited landing mass).

Key fact

MSTOM limits traffic load on long flights (fuel-heavy). MZFM limits traffic load on short flights (little fuel to “spend” before ZFM ceiling).

Landing above limits

Key fact

If landing mass exceeds the regulated landing mass: insufficient runway, tyre temperature exceeded, possible structural damage, go-around may be impossible. Fuel jettison reduces mass to a safe level at or below regulated landing mass.

Floor & surface loading

Formula — Floor loading

Floor loading  = Mass ÷ Area
Surface load   = Load ÷ (length × width)   [kg/m² or kg/ft²]
Running load   = Mass ÷ length (longest side)   [kg/m or N/m]

Key fact

For lowest floor loading in exam questions, use the two largest sides to maximise area. For lowest running load, use the longest side as length. Keep units consistent.

Definition — Max floor load

Maximum load per unit area permitted on a cargo compartment floor — specified in the AFM.

Effects of higher TOM

Key fact

Higher take-off mass increases: Vmu (minimum unstick speed), stall speed, gliding speed (range unchanged), fuel flow, stick force, AOA, drag, landing distance, tyre wear. Decreases: manoeuvrability, range, rate of climb, safety margin.

Formula — Stall speed scaling with mass

Vs₁ = Vs₀ × √(W₁ ÷ W₀)

Vs new = Vs old × √(New mass ÷ Old mass)

Key fact

When mass increases, lift must increase to maintain flight (L = ½ρV²SCL). Stall speed rises with the square root of the mass ratio — not proportionally.

Exam trap

Regulated TOM uses MZFM + take-off fuel, not trip fuel. RLM + trip fuel is a separate candidate limit. Pick the lowest of all three.

Exam trap

MZFM is about wing root bending at zero fuel — inner wing tanks often fuelled first to keep root bending down. Do not confuse with MLM.