Weight & Balance.pdf

8/19/2019 Weight & Balance.pdf

1/37

MALAYSIAN AVIATION TRAINING ACADEMY

Apprentice Course

For training purposes only Rev. 0 Issued 01 Dec 06 Malaysian Aviation Training Academy All rights reserved

MIP/TECH/TN/055

INSTITUTE OF AEROSPACE TECHNOLOGY

AIRFORCE COLLEGE

TECHNICIAN COURSE

AIRFRAME

WEIGHT & BALANCE


2/37


Apprentice Course


MIP/TECH/TN/055

RMAF TECHNICIAN

ENHANCEMENT PROGRAMME

NAME :

CLASS :


3/37


Apprentice Course


MIP/TECH/TN/055

WARNING

This training note is intended for training purposes

only. The information it contains is as accurate as

possible at the time of issue, and it is not subjected to

amendment action. Where the information contained in

this training note is at variance with official documents,

the latter must be taken as the overriding authority. The

contents in this training note shall not be reproduced inany form without the expressed permission of


(MATA) SDN BHD


4/37

Ai rf rame Royal Malaysian Air Force Int roduct ionWeight & Balance 9.6.1 - HO - 1 _____________________ Apprent ice Course - Technic ian ___________________ _

WEIGHT AND BALANCE

INTRODUCTION

Because of the complex and varied loads that aircraft may carry, it is necessary toensure that these loads are not excessive and that they are satisfactorily distributed.For safe flight and that they are satisfactory distributed. For safe flight the ensure ofgravity (C of C) of the aircraft must remain within the specified limits.

At specified intervals the aircraft is weighed and the Basic Weight and the positionof the Centre of Gravity is established.

The method of calculating the C of G position is based on the principle of turningmoments.

EFFECTS OF INCORRECT LOADING

C OF G FORWARD OF FORWARD LIMIT

a Aircraft nose heavy

b Reduces effective elevator up travel

c May cause the aircraft to "nose in" on take off with consequent longer

take off run

d Increases the difficulty in lowering the tail on landing

e More power required for a given speed, resulting in increased fuelconsumption and decrease in range

f May cause pilot fatigue, particularly during instrument flying

For training purposes only Rev. 01 Issued 01 Dec 06 Malaysian Aviation Training Academy All rights reservedMIP/TECH/TN/055


5/37


C OF G AFT OF AFT LIMIT

a Aircraft tail heavy

b Reduces effective elevator down travel

c Increases the tendency to stall

d Increases difficulty in raising the tail on take off, in the case of tailwheeled aircraft

e More power is required for a given speed resulting in increased fuel consumption and decrease in range

f Possible damage to the tail structure on landing

g May cause pilot fatigue particularly during instrument flying

OVERLOADING

a Increases the stalling speed

b Longer take off and landing runs

c Increases gliding speed

d Reduces rate of climb

e Reduces manoeuvrability

f More power required for a given speed resulting in increased

fuel consumption and a decrease in range

g Reduces the structural safety factor

h Reduces the tyre safety factor

i Increases the wear on undercarriage and brakes



6/37


SAFETY FACTOR AND EFFECT OF OVERLOADING

Generally speaking aircraft have a 1.5 factor of safety e.g. a maximum loading whichwould be experienced in normal flight multiplied by 1.5 i.e. a 50% safety overloadmargin.

In normal cruising flight all parts of the aircraft and contents are subject to gravita-tional loading of 1g.

If you weigh 70 kgf you exert a loading of 70 kgf on the aircraft in level flight. Load-ing is carried in to airframe through seat and floor structure.

If aircraft is manoeuvred into a 2g banked turn your 70 kgf body will load seat andsupporting structure at 140 kgf instead of 70 kgf.

Since 2g x 70 kgf = 140 kgf

A manoeuvre that double body load also doubles load applied to wing and otherparts of aeroplane structure. In a turn greater lift is required to keep the aircraft at aconstant angle of bank and height so a g loading developed.



7/37


REQUIREMENTS

2.1 The requirements relating to the weighing of aircraft and the establishment ofa Weigh and Balance Schedule are prescribed in British Civil Airworthiness Requirements (BCAR) Section A, Chapter A5-1. An interpretation of thoseparts of Chapter A5-1 which are pertinent to this Leaflet is given below.

2.2 Aircraft must be weighed to determine the Basic Weight and thecorresponding e.g. position when all the manufacturing processes havebeen completed. Aircraft, the MTWA of which exceeds 5700 kg (12500 lb)must be re-weighed within two years after the date of manufacture and, after

this, a check weighing must be carried out at intervals not exceeding fiveyears and at such times as the CAA may require. Aircraft, the MTWA ofwhich does not exceed 5700 kg (12500 lb) must be re-weighed as requiredby the CAA.

2.3 In making decisions on weighing, the CAA considers the history of theaircraft, its flying performance, and the probable effect on the weight aftera major overhaul, or embodiment of a modification, repair of replacement.

2.4 Certain types of aircraft may be weighed on a sampling basis (i.e. arepresentative aircraft, as weighed, would be acceptable for others of the

same standard) by agreement with the CAA.

2,5 An alternative arrangement to the periodical check weighing of individualaircraft is for the operator to establish a fleet mean weight (i.e. BasicWeight) and fleet mean centre-of-gravity position. The initial fleet meanweight is based on the mean of the weights of all the aircraft of the sametype in the fleet which is revised annually by sample weighing (see BCARSection A, Chapter A-1, Appendix No. 1).

2.6 When an aircraft is weighed, the equipment and other items of load suchas fluid in the tanks must be recorded. This recorded load should not

differ significantly from the Basic Equipment List associated with the Weightand Centre-of-Gravity Schedule (see paragraph 2.9). In circumstanceswhere there is a significant difference between the Basic Weight of theaircraft and the operating weight (i.e. Basic Weight plus the Variable Load)not accountable to structural changes brought about by modifications/repairs,the CAA may require that the actual weights of the Variable Load items beascertained.



8/37


2.7 All records of the weighing, including the calculations involved, must beavailable to the CAA. The records are retained by the aircraft manufacturer,overhauled or operator, and when the aircraft is weighed again, the previousweighing records must not be destroyed but retained with the aircraft records.Operators must maintain records of all known weight and e.g. changeswhich occur after the aircraft has been weighed.

DEFINITIONS

BASIC WEIGHT AND CENTRE OF GRAVITY POSITION

Basic weight is the weight of the aircraft and all its basic equipment and that of thedeclared quantity of unusable fuel and unusable oil. The centre of gravity will begiven as an arm from centre of gravity datum.

VARIABLE LOAD

Variable load includes the weight of crew and their baggage plus removable unitsand other equipment, the carriage of which depends upon the role for which the op-erator intends to use the aircraft for a particular flight.

DISPOSABLE LOAD

Disposable load is the weight of all persons and items of load, including fuel andother consumable fluids carried in the aircraft, other than the basic equipment andvariable load.

ZERO FUEL WEIGHT

Is the total weight of an aircraft ready for take-off including crew, passengers freight

etc., but excluding fuel.

LANDING WEIGHT

Is the maximum weight at which the aircraft can land safely considering the ambientconditions and runway in use.

TAKE-OFF WEIGHT

Is the actual total weight for a particular take-off.



9/37


DETERMINATION OF TAKE-OFF WEIGHT

When determining the take-off weight, the weight and position of all variable loadsmust be added to the aircraft basic weight, together with their arms and moments.Variable loads include such items as crew, crew baggage, passenger seats, drinkingwater, life raft, emergency transmitter and service equipment (food toiletries etc) asnecessary for the particular role of the aircraft.

MAXIMUM AUTHORISED WEIGHT, MAXIMUM TOTAL WEIGHT TOTAL WEIGHT AUTHORISED (M.T.W.A.)

The maximum total weight of the aircraft and its contents at which the aircraft maytake off anywhere in the world, in the most favourable circumstance in accordancewith the Certificate of Airworthiness or Flight Manual.

The load sheet is compiled in the following order:-

Basic Weight + Variable Load + Disposable Load + Fuel Load required for journey.The weight and C of G moment is calculated at each stage to give the pilot his Cen-tre of Gravity under a variety of conditions, i.e. take off and landing.

THE PRINCIPLES OF AIRCRAFT WEIGHT AND BALANCE

PRINCIPLES OF BALANCE

The theoretical principle of the weight and balance of aircraft is basically very simple,and can be compared with that of the familiar scale (as depicted in Figure 1) which,when in balance will rest horizontally on the fulcrum in perfect equilibrium providedthat the two pans suspended from the beam are of equal weight and distance fromthe fulcrum.

In aeronautical terms the fulcrum can be equated to the aircraft e.g. and the weights,with the loads imposed thereof on the structure.

Because of the design tolerances built into aircraft, the Weight and Balance is not ascritical as that of the scales in Figure 1, although it is important that they remainwithin those tolerances for reasons of safety, performance, and economy.

From Figure 1 it can be understood that the influence of weight, in relation to bal-ance, is directly dependent upon the distance of the weight from the fulcrum.

Unlike the scales in Figure 1, aircraft, (apart from some helicopters) cannot practi-cably be suspended in such a way as to determine the relative weight, balance, ande.g. However, it can be achieved mathematically.



10/37


The steelyard shown in Figure 2 has a known weight "D' and, a known weight "C"set at a specific distance "c". Under normal circumstances to determine the distancerequired to balance "C", the known weight "D" is moved along the beam until theweight of "D" and its accompanying lever arm are equal to the weight of "C" thereforealigning the beam with the balance mark. Once achieved the distance "d" can thenbe read from the graduated scale.

Mathematically the distance can be found as follows :-

d = Cc

D

where C = 50 lb

c = 10 inches

D = 20 lb

C = 50 x 10c

D 20

d = 25 in

Thus as the Resultant Moment is clockwise the C of G must be to the right of X.

C of G relative to X = Resultant Moment Total Weight

Total Weight = 10 + 25 + 40 + 45 = 120 lbs

Thus C of G position = 200 = 1ft. 8in.120

i.e. the C of G is 1ft. 8in. to the right of X.

A uniform beam 60 cm. long and weighing 8 kgs, has weights of 2 kg, 10 kg, 20 kgand 30 kg, at distances of 6 cm, 14 cm, 23 cm and 36 cm from the left hand and re-spectively. Find C of G of beam.(Fig. 3 )



11/37


PREPARATION OF AIRCRAFT FOR WEIGHING

1 Ensure that the aircraft is equipped in accordance with the loading andDistribution Schedule, or the Weight and Centre of Gravity Schedule.

2 Ensure that only unusable fuel and oil is in the appropriate tanks. Seealso definition "Basic Weight".

3 Ensure that the hydraulic system reservoir is topped up to the correctlevel.

4 Ensure that the aircraft is clean and dry.

5. The aircraft should be weighed indoors.



12/37


Fi . 1 Sim le Scale



13/37




14/37


Fig. 3



15/37

Ai rf rame Royal Malaysian Air Force Aircraft WeighingWeight & Balance 9.6.2 - HO - 1 _____________________ Apprent ice Course - Technic ian ___________________ _

AIRCRAFT WEIGHING

It is first necessary to find the weight of the aircraft acting at the undercarriage posi-tions or the main jacking positions. The aircraft must be longitudinally and laterallylevel; it may be possible to reduce the pressures in the tyres or shock absorbers toachieve this.

The aircraft is weighed by placing the wheels on weighing machines of if jacking theaircraft, by placing a weighing machine between each jack and the aircraft jackingpoints. When weighing the aircraft at the undercarriage positions, mechanical orelectrical scales are used. These may be permanently fitted in the hangar floor, butare usually portable units with a suitable ramp so that the aircraft can easily be rolled

up on to them. If jacking the aircraft, hydrostatic units (based on hydraulic principles)or electrical units, (based on the strain gauge principle) are used. The hydrostatictype may not give a direct reading, the indications may have to be converted using achart applicable to that type.

The capacity of the weighing equipment must be correct for the aircraft beingweighed. All weighing equipment should be checked at periods not exceeding oneyear.

AIRCRAFT WEIGHT AND CENTRE OF GRAVITY

The weight and c.g. of an aircraft is calculated in the same way as for the loadedbeam. The Basic Weight and c.g. of the aircraft corresponds to the weight and c.g.of the beam, and the Variable and Disposable Loads correspond to the beam loads.Further more before each flight the total weight and moment of these items must bedetermined, and the c.g. of the aircraft calculated to ensure the aircraft remainswithin the approved limits. If for example, the c.g. was too far forward, it would resultin a nose-heavy condition which could be potentially dangerous (particularly duringtake-off and landing), cause a general reduction in the performance of the aeroplane,and effect an increase in fuel consumption as a result of the drag caused by exces-sive balancing of the elevator trim. Where rotorcraft are concerned, a c.g. too far

forward could result in excessive strain on the main rotor shaft and a general lack ofcontrol. The c.g. too far aft results in a tail-heavy condition which, with the tendencyof the aeroplane to stall, makes landing more difficult, may result in a reduction inperformance, and cause an increase in fuel consumption. In the case of rotorcraft itwill reduce the forward speed and also the range of effective control.

The operational limitations for the fore and aft positions of the c.g. are defined in theaircraft Flight Manual or other document associated with the Certificate of Airworthi-ness, such as the Owner's Manual. Where no such document exists, the limitationsare specified in the Certificate of Airworthiness.



16/37


Fortunately it is not necessary for an aircraft to be perfectly balanced to achieve sta-ble flight, i.e. to an exact c.g. position. The permissible variation is called the Centre-of-Gravity Range. This is specified by the manufacturer for each aircraft type and isdetermined by the need to comply with various airworthiness design requirements..

STANDARD MEAN CHORD (S.M.C.) ( Ref. Fig. 4&5) Standard Mean Chord is known as MEAN AERODYNAMIC CHORD in the U.S.A.In normal practice the longitudinal zero station is at or near the nose of the aircraft. Itis chosen as the aircrafts C of G reference datum, so that the moment of any item onthe aircraft may be calculated from its weight and distance from the longitudinal zero

station.

Since the C of G is an aerodynamic consideration (C of G to C of P relationship) itsposition is sometimes specified as a percentage of the S>M>C> of the wing, meas-ured aft from its leading edge.

X x 100 = % S.M.C S.M.C. 1

x = arm (C OF G) = ARM (L.E.)

% S.M.C. =- ARM (C of G) - ARM (L.E.) x 100 S.M.C. 1

Given that the length of the S.M.C. = 200 cm the leading edge is 229 cm from theaircraft datum and the aircrafts C of G position is 269 cm aft of the datum. Find theC of G position as a percentage of S.M.C.

% S.M.C. = 269 - 229 x` 100

200 1

= 40 x 100 200

C of G = 20% of S.M.C.



17/37




18/37




19/37

Ai rf rame Royal Malaysian Air Force Weighing EquipmentWeight & Balance 9.6.3 - HO - 1 _____________________ Apprent ice Course - Technic ian ___________________ _

WEIGHING EQUIPMENT

GENERAL

There are four main types of weighing equipment which may be used for weighingaircraft, weighbridge scales, hydrostatic weighing units, electrical and electronicweighing equipment based on the strain gauge principle. Since considerable errorcan arise if small loads are checked with equipment designed for heavy loads, andscales may be calibrated in increments too coarse for accurate calculation, the ca-pacity of the weighing equipment should be compatible with the load.

All weighing apparatus should be checked, adjusted and certified by a competent au-

thority at periods not exceeding one year and, in addition, the zero indication shouldbe checked for accuracy before any weighing is commenced.

WEIGHBRIDGE SCALES

Ref. Fig 6

This equipment consists of a separate weighing platform for each wheel or bogey onthe aircraft, the weight at each reaction point being recorded directly on the balancearm. On some equipment a dial indicator is also provided. Large aircraft are nor-

mally weighed in a hangar, using either portable weighbridge scales or weighbridgesset permanently into the floor at appropriate positions with their platforms level withthe floor. The aircraft may then be rolled directly onto the platforms without the needfor special equipment.

NOTES:-

1 Care should be taken when moving portable weighbridge scales to prevent them becoming out of balance

2 It is advisable to set the approximate load on each balance arm before

releasing it. Failure to do this could cause damage to the knife edge.



20/37




21/37


Fig 7 - Hydraulic Weighing Unit



22/37


HYDROSTATIC WEIGHING UNITS

The operation of these units is based on the hydraulic principle that the fluid pres-sure in a cylinder in which a piston is working depends on the area of the piston andthe load applied to it. The units are interposed between the lifting jacks and the air-craft jacking points, the weight at each position being recorded on a pressure indica-tor. The indicator may record directly in units of weight or may be a multi-rotationaltype where the readings are converted to weight by means of a conversion table pe-culiar to each particular unit.

It is important that the lifting jacks are exactly vertical and the units correctly posi-

tioned, otherwise side loads may be imposed on the weighing units and may affectthe accuracy of the readings.

Using hydraulic weighing unit-this is positioned between the aircraft and lifting jackand measures the pressure applied to hydraulic fluid inside the unit. The pressurereading is then converted to a chart.

(a) Position jacks under each jacking point on the aircraft.

(b) Place a weighing unit and suitable adaptors on each jack.

(c) Ensure that the weighing unit pressure indicator reads zero.

(d) Release aircraft brakes.

(e) Raise aircraft clear of the ground with the jacks and note the reading ateach jack and convert to units of weight.

(f) Add the weights together to obtain the total aircraft weight.

ELECTRICAL WEIGHING EQUIPMENT

Ref. Fig. 8

Equipment of this type incorporates three or more weighing cells, each of which con-tains a metallic element of known electrical resistance. Aircraft load is measuredwith the variation in resistance with elastic strain by means of a galvanometer, the

scale of which is calibrated in units of weight. As with the hydrostatic weighing units,the weighing cells are interposed between the lifting jacks and the aircraft jacking

points and similarly care is necessary to ensure that no side loads are imposed uponthem.



23/37


Fig. 8 - Electrical Weighing Unit



24/37


ELECTRONIC WEIGHING EQUIPMENT

Ref. Fig. 9

This type of weighing equipment combines elastic strain load cells as described inparagraph into weighbridge-type platforms which are placed either as a single unit orcombination of units beneath the wheels of the aircraft undercarriage.

Each platform, is electrically connected to an instrumentation unit, which digitallydisplays the selected platform load. The number of platforms required to weigh anaircraft by this method is determined by the size of the aircraft. For example, a very

large transport aircraft may require as many as 18 or more platforms to accommo-date the wheel multiples of the undercarriage. The number of units that can be usedis, however, limited by the terminal facility of the instrumentation unit.

As there is generally a requirement for aircraft weighing equipment to be portable,the platforms are normally constructed of high strength lightweight materials, with theload cells interposed between the platform table, and the base unit. Where a plat-form is unevenly loaded (because of structural movement or undercarriage position-ing), a greater load imposed on one side of the platform will be automatically com-pensated for by the lesser load on the other side.

NOTE:

The displayed load (or reaction) on the instrumentation unit for each platform, is adedicated computation of all load cell inputs from that particular platform.

The positioning of aircraft onto electronic weighbridge platforms may be accom-plished by one of the following methods:-

a by towing the aircraft directly onto platforms permanently set into thehangar floor (sometimes in specific appropriate positions).

b by supporting the aircraft on jacks and, where facilities allow, lowering thehangar floor, positioning the platforms below the extended undercarriageand then raising the hangar floor until all the weight of the aircraft issupported by the platforms, or

c by towing the aircraft up purpose-made ramps onto the platforms.



25/37


The function of the instrumentation unit is to :-

a compute and display the loads imposed upon on each platform.

b provide a facility for the fine calibration of the platforms to a zero datum

c record and print out the indicated data.

CHANGE IN BASIC WEIGHT

ESTABLISHING THE BASIC WEIGHT AND C OF G POSITION AFTER AMODIFICATION

When any items of basic equipment are added, removed or repositioned in an air-craft, calculations must be made to determine the effect on basic weight and Centreof Gravity. This information is used in preparing a revised loading and DistributionSchedule or Weight and Centre of Gravity Schedule as appropriate.Ref. Fig. 10

EXAMPLE 1

An aircraft weighs 5000 lbs and its C, of G, is 20 in. aft of the datum. An item ofequipment weighing 40 lbs and fitted 10 in. forward of the datum is to be removedand refitted 39 in. aft of the datum. Find the new C of G.Ref. Fig.11 - Example 1

PREPARATION OF A LOAD SHEET

To prepare a load sheet the pilot or loading officer will require information from sev-eral sources.

The loading and Distribution Schedule or the Weight and C of G schedule will give

the Basic Weight and C of G position. It will also give the lever arms of the seats,fuel tanks and cargo compartments, fuel capacity, weight and lever arms of standardequipment etc.

The Flight Manual (or C of A, if there is no Flight Manual) will specify the M.T.W.A.and the permissible C of G range.



26/37


Limitations will also be given in the Flight Manual concerning the loading of cargoholds or compartments, giving for example the maximum load per square foot offloor area. Other relevant information such as access to each compartment will alsobe included.

The loading of very large aircraft is complex and may require a team of specialists,particularly if the lateral C of G is also required to be established.



27/37


Fig. 9- Aircraft Weighing ( Electronic)



28/37


Fig. 10 Negative and Positive Weight , Arm And Moment Relationship



29/37




30/37

Ai rf rame Royal Malaysian Air Force Passenger Aircraf tWeight & Balance 9.6.4 - HO - 1 _____________________ Apprent ice Course - Technic ian ___________________ _

PASSENGER AIRCRAFT

In respect of passenger transport aircraft exceeding 5700 kg M.T.W.A. or where 12or more passengers are to be carried (see Air Navigation General Regulations) theweight of each passenger may be assumed to be not less than as follows:-

Adult Male 75 kg (165 lbs)

Adult Female 65 kg (145 lbs)

Children (2 to 11 inc.)` 39 kg ( 85 lbs)

Infants under 2 8 kg ( 17 lbs)

Figures are also laid down for cabin and hold baggage.

If the weight of passengers is assumed, it must be stated on the Load Sheet. TheCommander of the aircraft can always insist on passengers and baggage beingweighed.

LARGE PASSENGER AND CARGO AIRCRAFT

With large aircraft the moment of items such as fuel, passengers and cargo are con-siderable and the procedures for determining a particular loading become compli-cated. In addition to the longitudinal c.g. calculation it is also usually necessary toensure that distribution of fuel and cargo is satisfactory in a transverse (lateral) direc-tion. Most airlines employ a specialist section dealing with loading calculations,whose responsibility it is to produce a load sheet for each flight.

The main items of variable moment during flight is the fuel, and although correctmanagement of the fuel system will minimise c.g. movement, some variations willremain due to the impracticability of locating all fuel near the c.g. on modern sweptwing aircraft. The critical points in the c.g. envelope are caused by fuel usage and

variations in specific gravity, these variations are calculated and applied to the enve-lope to curtail its boundaries.



31/37


The c.g. limitations are further curtailed by fixed allowances for other variable itemssuch as the following:-

a Seating allowance, which is calculated to provide for out-of balanceseating loads resulting from empty seats or passenger weight variation

b Flight allowance, which is provided to allow for the normal movement ofcrew and passenger during flight

c Moment changes due to operation of the landing gear or flaps

Weights and moments of passengers and cargo are then calculated, the cargo beingarranged within the fuselage or holds in such a way that the total weight and moment

of the loaded aircraft fall within the curtailed limitations. The heavier pieces of cargoor pallets are normally located close to the c.g. to restrict their effect, due attentionbeing paid to floor loading limitations, strength, and number of lashing points, etc.

On some aircraft it is also necessary to predetermine the order of loading fuel, cargoand passengers, in order to ensure that the structural limits are not exceeded, by ex-cessive out-of-balance forces tending to tip the aircraft on its tail.

LOADING GRAPHS AND C.G. ENVELOPES

Ref. Fig 12

The weight and balance computation system, commonly called the loading graphand c.g. envelope system, is an excellent and rapid method for determining the c.g.location for various loading arrangements. This method can be applied to any makeand model of aircraft.

Aircraft manufacturers using this method of weight and balance computation preparegraphs similar to those shown in Figure 12 and 13 for each make and model aircraftat the time of original certification. The graphs become a permanent part of the air-craft records. Along with the graphs are the data for the empty weight arm and mo-

ment (index number) for that particular make and model aircraft.

The loading graph illustrated in Figure-13 is used to determine the index number ofany item or weight that may be involved in loading the aircraft. To use this graph,find the point on the vertical scale that represents the known weight. Project a hori-zontal line to the point where it intersects the proper diagonal weight line (i.e. pilot,copilot, baggage, etc). From the point of intersection, read straight downward to thehorizontal scale to find the moment or index number.



32/37




33/37




34/37


After the moment for each item of weight has been determined, all weights areadded and all moments are added. With knowledge of the total weight and moment,project a line from the respective point on the c.g. envelope shown in figure13, andplace a point at the intersection of the two lines. If the point is within the diagonallines, the loading arrangement meets all balance requirements.

The following is an actual weight and balance computation using the graphs in Fig-ure13 For this example, assume that the aircraft has an empty weight of 1,386.0pounds and a moment of 52,772.0 pound-inches. The index number for the emptyweight of the aircraft is developed by dividing the empty-weight moment by 1,000.This gives an index number of 52.8 for the airplane's empty-weight moment. Load

the aircraft to determine whether the c.g. will fall within the diagonal lines of figure13. Arrange item weights and index numbers in an orderly form to facilitate adding.

----------------------------------------------------------------------------------------------------Moment

Weight (thousands Item (lbs) of lb in)

-----------------------------------------------------------------------------------------------------

Acft. EW 1,386.0 52.8

Oil 19.0 - 0.4

Pilot & Copilot 340.0 12.2

Rear passenger (two) 340.0 24.1

Baggage 20.0 1.9

Fuel 245.0 11.8---------- ------

TOTAL: 2,350.0 102.4---------- ------

The total airplane weight in pounds is 2,350.0, and the moment is 102.4. Locate thispoint (2,350 @ 102.4) on the c.g. envelope illustrated in Figure 3-14. Since the pointfalls within the diagonal lines, the loading arrangement meets all weight-and-balancerequirements.



35/37


ON-BOARD WEIGHING AND BALANCING SYSTEM

Some larger aircraft have weight sensors built into the wheel axles. These sensorssend out signals (electronic) which are fed into a on-board computer, interpreted andconverted to Gross Weight and position of Centre of Gravity and in some cases per-centage Standard Mean Chord (% S,M.C.) or Mean Aerodynamic Chord (%M.A.C.). at any time during loading or taxying of the aircraft.

The advantages of this system over the previously described system i.e. a calcula-tion of Basic Weight, Crew Weight, Passenger and Cargo Weight then Fuel Weight,manually, which may require adjustment of the loading several times, are :-

a Gross Weight and Centre of Gravity position are available, in seconds,

at any time during loading or taxying.

b Flight safety is increased. The system provides a guarantee againsterrors which could cause overloading or dangerous Centre of Gravityposition.

c Centre of Gravity control, during loading, aids attainment of the idealposition, achieving a minimum trim requirement and thus reducing fuelconsumption.

d The aircraft can operate consistently at Maximum Total Weight Authorised

because uncertainties over average or assumed weights of passengers orbaggage, inaccuracies of procedures or fuel gauges, fuel density variationsand the possibility of errors is eliminated.

LARGE PASSENGER AND CARGO AIRCRAFT

Fig. 14

With large aircraft the moment of items such as fuel, passengers and cargo are con-siderable and the procedures for determining a particular loading become compli-

cated. In addition to the longitudinal c.g. calculation it is also usually necessary toensure that distribution of fuel and cargo is satisfactory in a transverse (lateral) direc-tion. Most airlines employ a specialist section dealing with loading calculations,whose responsibility it is to produce a load sheet for each flight.

The main item of variable moment during flight is the fuel, and although correct man-agement of the fuel system will minimise c.g. movement, some variations will remaindue to the impracticability of locating all fuel near the c.g. on modern swept wing air-craft. The critical points in the c.g. envelope are caused by fuel usage and variationsin specific gravity, these variations are calculated and applied to the envelope to cur-tail its boundaries.



36/37


Fig. 14



37/37


The c.g. limitations are further curtailed by fixed allowances for other variable itemssuch as the following:-

a Seating allowance, which is calculated to provide for out-of-balance seatingloads resulting from empty seats or passenger weight variation

b Flight allowance, which is provided to allow for the normal movement ofcrew and passengers during flight

c Moment changes due to operation of the landing gear or flaps

Weights and moments of passengers and cargo are then calculated, the cargo beingarranged within the fuselage or holds in such a way that the total weight and momentof the loaded aircraft fall within the curtailed limitations. The heavier pieces of cargo

or pallets are normally located close to the c.g. to restrict their effect, due attentionbeing paid to floor loading limitations, strength, and number of lashing points, etc.

On some aircraft it is also necessary to predetermine the order of loading fuel, cargoand passengers, in order to ensure that the structural limits are not exceeded, by ex-cessive out-of-balance forces tending to tip the aircraft on its tail.

Weight & Balance.pdf

Documents

Transcript of Weight & Balance.pdf