Mass definitions

031 Mass & BalanceChapter 1

EASA Mass & Balance questions hinge on knowing exactly what each mass term includes and excludes. Draw a mental stack from empty aircraft to ramp mass.

Basic empty mass (BEM)

Definition — Basic empty mass (BEM)

Determined by the manufacturer; includes no additional equipment beyond the standard role. Already includes unusable fuel and full operating fluids (oil, hydraulic fluid). Component of DOM.

Key fact

BEM = OM − crew − fuel load (when crew and fuel are stripped from operating mass). Also referred to as basic weight in some manuals.

Key fact

The operator must establish BEM from the manufacturer or an AMO. Recorded in the weighing schedule and amended for modifications without always re-weighing.

Dry operating mass (DOM)

Definition — Dry operating mass (DOM)

No traffic load, no usable fuel. Includes crew and their baggage, operating items, and BEM. DOM = TOM − traffic load or BEM + operating items.

Definition — Variable load (VL)

Role equipment, crew, and crew baggage — effectively DOM − BEM.

Zero fuel mass (ZFM)

Definition — Zero fuel mass (ZFM)

Traffic load present, no fuel. ZFM = TOM − take-off fuel. Represents the heaviest the aircraft can be with pax/cargo but tanks empty.

Operating & take-off masses

Definition — Operating mass (OM)

No traffic load, with fuel. OM = DOM + fuel (fuel on board but no pax/cargo revenue load).

Definition — Take-off mass (TOM)

Everything on board at brake release. TOM = DOM + traffic load + take-off fuel.

Definition — Landing mass (LM)

TOM − trip fuel. Mass at touchdown after en-route burn.

Definition — Ramp mass (RM)

Mass on the ramp before taxi — includes taxi fuel in the total fuel figure.

Traffic load & useful load

Definition — Traffic load (TL)

Revenue and non-revenue payload: passengers, baggage, cargo. TL = TOM − OM. Must be fixed during flight so only planned fuel-burn CG shifts occur.

Definition — Useful load (UL)

TL + usable fuel, or TOM − DOM. On light aircraft: pilots, operating items, passengers, baggage, cargo, and usable fuel.

Fuel mass breakdown

Key fact

Take-off fuel (TOF) = block fuel − taxi fuel (also: total fuel − unusable fuel for loading purposes). Ramp fuel = trip + taxi + alternate + extra/contingency/reserve.

Key fact

Contingency & reserve fuel — used for unforeseen circumstances en route. Some operators fly at regulated TOM with full traffic load and full fuel; on other routes, traffic load may be limited by desired range.

Key fact

Fuel volume conversion uses standard fuel density values from the operations manual. Typical SG: AVGAS 0.71, Jet A-1 0.80.

Quick reference relationships

Formula — Core mass relationships

TOM     = DOM + TL + TOF
LM      = TOM − trip fuel
TL      = TOM − OM
UL      = TOM − DOM  =  TL + usable fuel
ZFM     = TOM − TOF  =  DOM + TL
OM      = DOM + fuel (no TL)
DOM     = TOM − TL
BEM     = OM − crew − fuel load

Unit conversions (exam favourites)

Key fact

USG × 6 ≈ lb (approximate). USG × 3.7852 = litres. lb × 0.454 = kg. kg × 2.205 = lb. inch × 2.54 = cm. ft × 12 = inches.

Exam trap

DOM includes crew and their baggage — a classic wrong answer is “crew not included.” BEM has no crew, no usable fuel, no traffic load. ZFM has traffic load but no fuel.

Exam trap

If the question does not ask for a limiting mass, simply calculate: TOM = DOM + TL + take-off fuel. Do not apply MTOM/MZFM/MLM unless asked for maximum or regulated TOM.