Mass terms
| Abbreviation | Meaning | Description |
|---|---|---|
| MTOM (MTOW) | Maximum Take-Off Mass | Largest mass at take-off (per AFM) |
| MLM | Maximum Landing Mass | Largest mass at landing; often < MTOM |
| MZFM | Maximum Zero-Fuel Mass | Largest mass without usable fuel |
| MRM (MRW) | Maximum Ramp Mass | MTOM + fuel burn during start/taxi |
| OEM / BEM | Operating / Basic Empty Mass | Empty mass: aircraft + non-consumable fluids + standard equipment |
| DOM | Dry Operating Mass | OEM + crew + service items (without payload/fuel) |
| Useful Load | Net useful margin | = MTOM − OEM (max for pax + baggage + fuel) |
| Payload | Paying load | = pax + baggage (or cargo) |
Basic Empty Mass — complete components
The Basic Empty Mass (BEM) of an aircraft is defined under CS-23.25 and the EASA CRI (Certification Review Item) as the sum of the following components:
- Mass of the empty aircraft — airframe, wings, engine(s), standard equipment, avionics.
- Fixed ballast — permanently installed weights for CG trimming (on some types).
- Unusable fuel — the fuel residue in the tanks that cannot be burnt (e.g. sump fuel below the pickups).
- Undrainable oil — oil residue that cannot be removed via the drain valve (in lubrication circuits).
- Total quantity of engine coolant — for liquid-cooled engines (e.g. Rotax 912 with cooling system); on air-cooled engines (Continental, Lycoming O-series) this item is absent.
- Total quantity of hydraulic fluid — in brake, gear, and flap systems, depending on configuration.
- Built-in instruments and equipment — all fixed cockpit instruments, avionics, antennas, etc. The mass of built-in instruments is included in the empty mass.
NOT included: usable fuel, passengers, baggage, removable equipment (survival kit, personal items).
Determination: BEM is determined by actually weighing the aircraft — typically every 4 years and after major modifications. The result (BEM + corresponding CG position) is documented in the weight and balance report of the aircraft and entered into the AFM/POH section Mass and Balance.
CG terms
| Term | Meaning |
|---|---|
| Datum | Reference point set in AFM (often spinner nose or firewall) — all arms count from here |
| Arm / Balance Arm | Distance from datum to the centre of mass of an item (positive aft). The "balance arm" of a mass is the distance from the datum to the CG of that mass. |
| Moment | Product of mass and balance arm — torque about the datum. In M&B context: Moment = Mass × Balance Arm. |
| CG | Σ(moments) ÷ Σ(masses) — distance from datum to the resulting overall CG. Definition: the centre of gravity is the point at which the total mass of the aeroplane is considered to act. |
| CG Envelope | Diagram in AFM: allowed CG range as a function of total mass |
| Forward Limit | Forwardmost permitted CG position |
| Aft Limit | Aftmost permitted CG position |
Equilibrium conditions in flight
In unaccelerated steady flight the aerodynamic force balance holds:
- Thrust = Drag — no longitudinal acceleration.
- Lift = Weight (gravity) — no vertical acceleration.
In an M&B context: with the aircraft suspended on weighing scales, the aircraft is in static equilibrium — the sum of all masses (Σm) and sum of all moments (Σm·x) are the relevant quantities for CG calculation.
Notes
- The DOM must be known before every flight — it is documented in the aircraft technical log and the weight-and-balance report.
- On foreign flights or with other operators: use the same definitions — some AFMs use BEW (Basic Empty Weight) instead of OEM.
- BEM components (especially unusable fuel, undrainable oil, fixed ballast) are to be taken from the AFM and not estimated by the pilot — they are uniquely defined and part of certification.
Weight and Balance / Equipment List* — type-dependent.*