Aircraft Weight and Performance

• Weight is a crucial factor in aircraft construction and operation, affecting efficiency and safety.
• Excessive weight reduces efficiency and narrows the safety margin in emergencies.
• Aircraft are designed to be as light as structurally possible, with wings/rotors supporting max allowable weight.
• Increased weight requires additional lift and structural support for both static and dynamic loads.
• Modern aircraft are often designed such that they can be overloaded if all seats are occupied, baggage compartments are full, and fuel tanks are full.
• Overloading an aircraft can cause multiple problems

Problems Caused by Overloading

• Higher take-off speed and longer take-off run
• Reduced rate and angle of climb
• Lowered service ceiling
• Reduced cruising speed and range
• Decreased manoeuvrability
• Longer landing roll due to higher landing speed
• Excessive loads on the structure

Key Elements in Weight and Balance

• Total weight must not exceed the maximum allowed by the Type Certificate
• The Centre of Gravity (CG) must be within the allowable range.

Determining Basic Empty Weight

• An aircraft must be weighed to determine the Basic Empty Weight and CG position

Datum

• An aircraft datum is a reference point set by the manufacturer for CG determination.
• All length measurements are taken from the datum
• The datum location is chosen by the manufacturer and used consistently for all CG calculations
• The aircraft datum can be located anywhere.
• Common locations: Leading edge of the wing or a specified distance from an easily identified location.
• Helicopter datum: Centre of the rotor mast or at/ahead of the nose

Arms and Weight

• An arm is the distance between the CG of an object and the datum, measured in mm or inches.
• For pilots, the object can be fuel, passenger or luggage.
• For AMEs, wheel-weighing points represent the CG of part of the aircraft
• Arms ahead of or to the left of the datum are negative (-).
• Arms behind or to the right of the datum are positive (+).
• A datum ahead of the aircraft minimizes errors.
• Weight is measured in kilograms or pounds.

Weight Considerations

• Removed weight is negative (-).
• Added weight is positive (+).

Moments

• A moment is a force about a point and that point is calculated by multiplying the arm and weight.
• Weight and balance problems are based on the physical law of the lever.
• Positive moments cause an aircraft to nose up, while negative moments cause it to nose down.
• A lever is balanced when the weight on one side of the fulcrum multiplied by its arm equals the weight on the opposite side.
• A lever is balanced when the algebraic sum of the moments about the fulcrum is 0.
• The forces that try to rotate it clockwise have the same magnitude as those that try to rotate it anti-clockwise.
• Aircraft balance refers to the location of the CG along the longitudinal axis of the aircraft.
• CG is expressed in inches or millimetres from the datum.
• Aircraft engineers compute the CG and the forward and rearward CG limits.
• The distance between these limits is the CG range.
• Operation with the CG outside its range is prohibited.

Finding the Centre of Gravity

• Measure the arm of each weight in inches from the datum.
• Multiply each arm by its weight in pounds to determine the moment in pound-inches of each weight.
• Determine the total of all weights and all moments.
• Divide the total moment by the total weight to determine the CG in inches from the datum.

Balancing

• Balance is achieved by placing loads so that the average arm of the loaded aircraft falls within the CG range.
• The exact CG location range is specified for each type of aircraft in the flight manual.

Empty Weight Determination

• The empty weight and corresponding CG of all civil aircraft must be determined at the time of certification.

Balance Report

• The manufacturer is required to weigh one aircraft out of each 10 produced
• The condition of the aircraft at the time of determining empty weight must be well defined and easily repeated.
• The starting point for weight computation is the weight of the aircraft before passengers, cargo and fuel are added.
• The Basic Empty Weight includes the weight of the standard aircraft, optional equipment, fixed ballast and unusable fuel.
• It also includes full operating fluids (engine oil, hydraulic fluid, etc.) EXCEPT potable water, lavatory pre-charge water and water intended for injection in the engines.

Information Sources

• Information regarding the weight and balance data for a particular aircraft can be found in the following documents:
• Aircraft specifications
• Aircraft operating limitations
• Aircraft flight manual
• Aircraft Weight and Balance Report
• Aircraft Type Certificate Data Sheet (TCDS).

Weight and Balance Report Requirements

• Certain procedures must be followed in order to make the report accurate, this includes understanding weight and balance terms, how to prepare the aircraft for weighing, and how to set up and use the necessary equipment.
• Other regulations that apply to general and commercial aviation must be adhered to.
• Each application for a Certificate of Airworthiness or Restricted Certificate of Airworthiness must include a Weight and Balance Report with a loading schedule.

Basic Empty Weight

• Aircraft certified prior to March 1, 1978, might use the term Licensed Empty Weight
• Licensed Empty Weight is similar to Basic Empty Weight except that it does not include full engine oil.
• For these aircraft, the Licensed Empty Weight includes only the weight of undrainable oil.
• Check the aircraft TCDSs when in doubt about what operating fluids are included in the empty weight.
• Potable and toilet waste tanks must be empty and fuel drained to pump pick-up.
• Aircraft must be washed, their interior cleaned and a checklist of fixed equipment carried out to ensure it is fitted.

Sources For Weight and Balance Data

• Aircraft specifications
• Aircraft operating limitations
• Aircraft flight manual
• Aircraft Weight and Balance Report
• Aircraft Type Certificate Data Sheet (TCDS).

Exceeding 5700 kg (12 500 lb) MTOW

• Before a Certificate of Airworthiness can be issued for an aircraft exceeding 5700 kg (12 500 lb) Maximum Take Off Weight (MTOW), a Weight and Balance Report must be prepared by an Approved Organisation.
• The Wieght and Balance Report records the essential loading data to enable a particular aircraft to be correctly loaded and is the primary document used during weighing.

Weight and Balance Report Breakdown

• Part A - Basic Weight
• Part B - Variable Load
• Part C - Loading Information.

Part A Includes

• Reference number and date
• Designation, manufacturer's number, nationality and registration marks of the aircraft
• Weighing Record
• Weight and Centre-of-Gravity Schedule
• Basic Equipment List
• Diagram and description of the datum points used for weighing and loading.

Part B Includes

• Weight of crew and their baggage
• Weight of passengers and their baggage
• Weight of removable items
• Weight of other equipment whose presence depends on the intended use of the aircraft, e.g. aerobatics
• Weight of removable items, e.g. stretchers, medical stores, de-icing fluid, water injection.
• A weight and balance report must be submitted for each different operation.

Part C Includes

• Disposable load intended to be carried
• The lever arm for a passenger in each seat
• Mean lever arm for cargo bays
• Any significant change in CG, e.g. during landing gear retraction or extension
• Lever arm of fuel and oil in each tank
• Maximum total useable capacities of consumable fuel in the tanks.

Pilot Responsibilities

• The pilot must certify that they are satisfied the aircraft is safely loaded by signing the form used to calculate the CG and ensure it is within range prior to take-off.
• It is filled out by load control and signed by the Pilot in Command.

Key Definitions

• Disposable load is the weight of all persons and items of load, including fuel, cargo and baggage carried, that are not part of Basic Empty Weight and Variable Load.
• CG limits are defined as the extreme forward and aft CG locations.

CG and Aircraft Stability

• When the aircraft is loaded, the CG must fall within the forward and aft limits.
• Maintaining the CG within allowable limits ensures adequate longitudinal stability and control.
• The CG limits may be found in the aircraft Type Certificate, Weight and Balance Report or aircraft specifications.

Exceeding the Aft Centre of Gravity Limit

• If the CG is too far aft, it will be too near the centre of lift
• If the unstable aircraft ever enters a spin, the spin could become flat and recovery would be difficult or impossible.

Exceeding the Forward Centre of Gravity Limit

• Can lead to lack of sufficient elevator authority
• Increased tail load results in higher angle of attack and increased drag
• Elevator might not produce enough nose-up force to rotate, and on landing, there may not be enough elevator force to flare the aeroplane.
• Both take-off and landing runs are lengthened if the CG is too far forward.

Calculating the CG

• Subtract the tare weight, from the scale reading to determine the net weight at each weighing point.
• Multiply each net weight by its arm to determine its moment, and then determine the total weight and total moment.
• The arm and moment forward of the datum are negative.
• The arm and moment aft of the datum are positive.

Table Method

• A major advantage of the table method is that one table is applicable to nose-wheel aircraft and tail-wheel aircraft with the datum in any location.
• Disadvantages include; a slow process and there is risk of error when transcribing large values of moment into the table and when determining which sign to apply to an arm.

EWCG Formula

• An EWCG formula is a faster and less error-prone alternative to the table method.
• The formula method calculates the position of the CG relative to the main-wheel weighing points.
• The CG is forward of the main wheels on a nose-wheel aircraft, and aft of the main wheels on a tail-wheel aircraft.

EWCG Steps

• The only moment that must be calculated is the moment of the third wheel (nose wheel or tail wheel) relative to the main wheels, i.e. the distance between main wheels and third wheel multiplied by the weight at the third wheel weighing point.
• This moment is divided by total aircraft weight to calculate the distance to the CG forward or aft of the main wheels.
• This distance is then added to or subtracted from the distance between the main wheels and the datum to determine the distance and direction of the CG from the datum.
• To further reduce the risk of error, the AME is only required to feed positive magnitude values of distance into the formula.
• There is no requirement to determine the sign of the distance.
• The appropriate sign is applied internally within the formula.
• However, the use of magnitude values requires four separate formulas, with one each for:
• Nose-wheel aircraft with datum: Forward of the main wheels Aft of the main wheels
• Tail-wheel aircraft with datum: Forward of the main wheels Aft of the main wheels.
• The appropriate formula for the aircraft being weighed is provided in its maintenance data, accompanied by a worked example.

Percentage of Mean Aerodynamic Chord (MAC)

• The CG is an aerodynamic consideration.
• Its position is sometimes specified as a percentage of the Mean Aerodynamic Chord (MAC) of the wing.
• MAC is also referred to as Standard Mean Chord (SMC).
• It is the chord of an imaginary aerofoil that has all of the aerodynamic characteristics of the actual aerofoil.

Aircraft Engineer Responsibilities

• The aircraft engineer is primarily concerned with the location of the CG relative to the datum, an identifiable physical location from which measurements can be made.
• Wings such as tapered wings, express the allowable CG range in percentages of the MAC.
• It is important for longitudinal stability that the CG be located ahead of the centre of lift of a wing.
• The location of the leading edge of the MAC, the Leading Edge MAC (LEMAC), is a specified number of inches from the datum.
• The relative positions of the CG and the aerodynamic centre of lift of the wing have critical effects on the flight characteristics of the aircraft aft of the datum.
• In order to relate the percentage of MAC to the datum, all weight and balance information includes two items:
• The length of the MAC in inches
• The location of the LEMAC in inches from the datum.
• CG in % MAC = ((A-B)/C)*100
• It is sometimes necessary to determine the location of the CG in inches from the datum when its location in percentage of MAC is known.
• CG in inches from Datum= B+((C*CG%MAC)/100)
• The weighing data are used to calculate the loaded weight of the aircraft.

Other Important Weight Categories

• Maximum Ramp Weight
• Maximum Zero-Fuel Weight.
• Maximum Ramp Weight is the maximum weight approved for ground manoeuvre and includes the weight of start, taxi and engine run-up fuel.
• Maximum Zero-Fuel Weight is the approved maximum weight of an aircraft without fuel, and it includes the aircraft and everything that will be carried on the flight.
• Weights in excess of zero-fuel weight must consist of useable fuel.
• Basic Empty Weight + Payload = Zero-Fuel Weight
• Zero-Fuel Weight + Useable Fuel = Ramp Weight
• Ramp Weight - Fuel for Start, Taxi, etc. = Take-Off Weight
• It is important to know the Maximum Weight of an aircraft for the two critical flight phases:
• Maximum Take-Off Weight (MTOW)
• Maximum Landing Weight (MLW).
• Maximum Take-Off Weight is the maximum weight approved for the start of the take-off run.
• Maximum Landing Weight is the maximum weight approved for the landing touchdown.

Type Certificate Data Sheet (TCDS)

• The TCDS loading graph illustrates the forward and aft CG limits. -CG moves aft when landing gear is retracted. -Calculated CG must fall within the forward and aft limits.
• The calculated CG is checked on the graph and must be within limits.
• The limit changes and becomes a smaller range with higher weight.
• Info is then entered on the load data sheet.

Repairs, Alterations and CG Changes

• The weight of a repair or alteration and added or removed component must be known before the aircraft weight and balance calculations can be accurately made.
• As components are installed or structural repairs are made, the CG changes according to the following:
• The weight of the object or repair
• Their distance from the datum.
• Whenever something is added to an aircraft, such as a beacon or strobe light, the centre of gravity changes.
• Because of this, a Weight and Balance Report should be calculated before a repair or installation is accomplished to ensure that the CG will fall within its specified range.
• Determine the CG change by making a chart.

Ballast

• Often it is necessary to install ballast, or weight, in an aircraft to bring the CG into its desired range.
• There are two classifications for ballast: permanent and temporary.
• Permanent ballast is permanently installed in an aircraft and must not be removed.
• Temporary ballast, on the other hand, is typically installed to bring the CG within its range for a specific flight condition.
• Permanent ballast must be attached rigidly so it cannot be dislodged by flight manoeuvres or a rough landing.

Calculating Ballast

• To determine the amount of ballast needed to bring the CG within limits, two things must be known:
• The amount the CG is out of limits
• The distance between the ballast location and the limit that is affected.
• The formula for calculating the desired weight of ballast is:
• Ballast Weight=Aircraft Empty Weight x Dist out of Limits / Distance between Ballast and new CG
• Anytime you need to shift the CG rearwards, ballast must be installed aft of the datum.

Preparation of Aircraft Weighing

• All tare weight, that is, the weight of any chocks or devices used to hold the aircraft on the scales, is subtracted from the scale reading, and the net weight from each wheel weighing point is entered on a chart.
• Tare weight also includes items used to level the aircraft.
• The arms of the weighing points are specified in the Type Certificate Data Sheet (TCDS) for the aircraft in terms of stations.
• Stations are distances, in inches or millimetres, from the datum.

Two Basic Scales Used To Weigh Aircraft

• Mechanical
• Electronic load.
• The aircraft is lowered or rolled onto mechanical scales.
• Pressure-sensitive cells are placed between the aircraft jack and the jack pads on the aircraft.
• Jack-mounted load cells allow the aircraft attitude to be adjusted for weighing.
• Roll-on type ramp electronic load cells are also used.
• Sometimes a jack type load cell is used at the nose or tail, and a ramp type is used under the main wheels, where levelling is a requirement.
• Normally, an aircraft must be at a level attitude before it can be weighed.
• Airbus Attitude Monitor: Some aircraft have spirit levels with square grids or two normal type spirit levels.
• Aircraft must be weighed in a straight and level position, levelled by jack extension if using jack load pads, or by adjusting strut and tyre pressures if on platform scales and load cells.