AIRCRAFT MASS AND BALANCE

AIRCRAFT MASS AND BALANCE

The main purposes, of monitoring the mass and balance of aircraft, are to maintain safety and to achieve efficiency in flight. The position of loads such as passengers, fuel, cargo and equipment will alter the position of the Centre of Gravity (CG) of the aircraft.

Incorrect loading will affect the aircraft rate of climb, manoeuvrability, ceiling, speed and fuel consumption. If the CG were too far forward, it would result in a nose-heavy condition, which could be potentially dangerous on take-off and landing. If the CG is too far aft, the tail-heavy condition will increase the tendency of the aircraft to stall and make landing more difficult.

Stability of the aircraft will also be affected with the CG outside the normal operational limits. Provided the CG lies within specified limits, the aircraft should be safe to fly. The unit of measurement for mass and balance are normally dictated by the aircraft manufacturer and can be either Metric or Imperial terms. Specific definitions for mass and balance ensure they are correctly interpreted.

• Datum: The datum is an imaginary vertical plane from which horizontal measurements are taken. The locations of items such as baggage compartments, fuel tanks, seats and engines are relevant to the datum. There is no fixed rule for the location of the datum. The manufacturer will normally specify the nose of the aircraft, but it could be at the front main bulkhead or even forward of the aircraft nose

• Arm: The horizontal distance from an item or piece of equipment to the datum. The arm's distance is usually measured in inches (or millimetres) and may be preceded by a plus (+) or a minus (-) sign. The plus sign indicates that the distance is aft of the datum and the minus sign indicates distance is forward of the datum

• Moment: The product of a force multiplied by the distance about which the force acts. In the case of mass and balance, the force is the mass (kg/lb) and the distance is the arm (m/in). Therefore, a mass of 40 kilograms, at 3 metres aft of the datum will have a moment of 40 x 3 = 120 kg/m. It is important to consider whether a value is positive (+ve) or negative (-ve) when moments are calculated and the following conventions are used:

Distances horizontal: aft of the datum (+), forward of the datum (-).
Weight: added (+), removed (-).

• Centre of Gravity (CG): This is the point about which all of the mass of the aircraft or object is concentrated. An aircraft could be suspended from this point and it would not adopt a nose-down nor a tail-down attitude.

• Centre of Gravity Balance Limits: For normal operation of the aircraft, the CG should be between the Forward and Aft limits as specified by the manufacturer. If the CG is outside these limits, the aircraft performance will be affected and the aircraft may be unsafe.

• Dry Operating Mass: The total mass of the aeroplane, ready for a specific type of operation, excluding all usable fuel and traffic load. This mass includes crew and crew baggage, catering and removable passenger service equipment, potable water and lavatory chemicals.

• Maximum Zero Fuel Mass: The maximum permissible mass of an aircraft with no usable fuel.  Fuel contained in certain tanks must be included if this is explicitly mentioned in the aircraft’s Flight Manual limitations.

• Maximum Structural Take-Off Mass (MTOM): The maximum permissible total aeroplane mass at the start of the take-off run.

• Maximum Structural Landing Mass: The maximum permissible total aeroplane mass upon landing under normal circumstances.

• Traffic Load: This includes the total mass of passengers, baggage and cargo, including any non-revenue load.


Mass and Balance Documentation

The Mass and Balance documentation used by an operator must include certain basic information, which is listed below. Subject to the approval of the authority, some of this information may be omitted.

A. Aeroplane registration and type
B. Flight identification number and date
C. Identity of the commander
D. Identity of the person who prepared the document
E. Dry operating mass and the corresponding CG of the aeroplane
F. Mass of the fuel at take-off and the mass of trip fuel
G. Mass of consumables other than fuel
H. Load components that include passengers, baggage, freight and ballast
I. Take-off Mass, Landing Mass and Zero Fuel mass.
J. The load distribution
K. Aeroplane CG positions
L. Limiting mass and CG values


FREQUENCY OF WEIGHING

Aircraft must be weighed before entering service, to determine the individual mass and CG position. This should be done once all manufacturing processes have been completed. The aircraft must also be re-weighed within four years from the date of manufacture, if individual mass is used, or within nine years from the date of manufacture, if fleet masses are used.

The mass and CG position of an aircraft must be periodically re-established. The maximum interval between one aircraft weigh and the next, must be defined by the operator, but not exceed the four/nine year limits

15.7 CALCULATION OF MASS AND CG OF ANY SYSTEM

The position of the CG of any system (refer to Fig. 1) may be found using the following process:

• Total Mass is calculated, by adding the mass of each load (plus the mass of the beam)

• The moment of each load is calculated, by multiplying the mass by the arm (distance from the reference datum)

• ALL the moments are added together, to provide the Total Moment

• Total Moment is divided by the Total Mass to give CG position.