INSTITUTE OF AEROSPACE TECHNOLOGY AIRFORCE COLLEGE

TECHNICIAN COURSE AIRFRAME WEIGHT & BALANCE

MALAYSIAN AVIATION TRAINING ACADEMY

Apprentice Course For training purposes only Rev. 0 Issued 01 Dec 06 MIP/TECH/TN/055

Malaysian Aviation Training Academy All rights reserved

RMAF TECHNICIAN ENHANCEMENT PROGRAMME

NAME : CLASS :




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 in any form without the expressed permission of MALAYSIAN AVIATION TRAINING ACADEMY (MATA) SDN BHD




Airframe Royal Malaysian Air Force Introduction Weight & Balance 9.6.1 - HO - 1 _____________________ Apprentice Course - Technician ____________________ WEIGHT AND BALANCE INTRODUCTION Because of the complex and varied loads that aircraft may carry, it is necessary to ensure 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 of gravity (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 position of the Centre of Gravity is established. The method of calculating the C of G position is based on the principle of turning moments.

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 fuel consumption and decrease in range

f

May cause pilot fatigue, particularly during instrument flying

For training purposes only Rev. 01 Issued 01 Dec 06 MIP/TECH/TN/055


Airframe Royal Malaysian Air Force Introduction Weight & Balance 9.6.1 - HO - 2 _____________________ Apprentice Course - Technician ____________________

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 tail wheeled 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




SAFETY FACTOR AND EFFECT OF OVERLOADING Generally speaking aircraft have a 1.5 factor of safety e.g. a maximum loading which would be experienced in normal flight multiplied by 1.5 i.e. a 50% safety overload margin. In normal cruising flight all parts of the aircraft and contents are subject to gravitational loading of 1g. If you weigh 70 kgf you exert a loading of 70 kgf on the aircraft in level flight. Loading 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 and ing 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 other parts of aeroplane structure. In a turn greater lift is required to keep the aircraft at a constant angle of bank and height so a g loading developed.




REQUIREMENTS 2.1

The requirements relating to the weighing of aircraft and the establishment of a Weigh and Balance Schedule are prescribed in British Civil Airworthiness Requirements (BCAR) Section A, Chapter A5-1. An interpretation of those parts 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 the corresponding e.g. position when all the manufacturing processes have been 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 five years and at such times as the CAA may require. Aircraft, the MTWA of which does not exceed 5700 kg (12500 lb) must be re-weighed as required by the CAA.

2.3

In making decisions on weighing, the CAA considers the history of the aircraft, its flying performance, and the probable effect on the weight after a 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. a representative 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 individual aircraft is for the operator to establish a fleet mean weight (i.e. Basic Weight) and fleet mean centre-of-gravity position. The initial fleet mean weight is based on the mean of the weights of all the aircraft of the same type in the fleet which is revised annually by sample weighing (see BCAR Section A, Chapter A-1, Appendix No. 1).

2.6

When an aircraft is weighed, the equipment and other items of load such as fluid in the tanks must be recorded. This recorded load should not differ significantly from the Basic Equipment List associated with the Weight and Centre-of-Gravity Schedule (see paragraph 2.9). In circumstances where there is a significant difference between the Basic Weight of the aircraft and the operating weight (i.e. Basic Weight plus the Variable Load) not able to structural changes brought about by modifications/repairs, the CAA may require that the actual weights of the Variable Load items be ascertained.




2.7

All records of the weighing, including the calculations involved, must be available to the CAA. The records are retained by the aircraft manufacturer, overhauled or operator, and when the aircraft is weighed again, the previous weighing records must not be destroyed but retained with the aircraft records. Operators must maintain records of all known weight and e.g. changes which 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 the declared quantity of unusable fuel and unusable oil. The centre of gravity will be given as an arm from centre of gravity datum.

VARIABLE LOAD Variable load includes the weight of crew and their baggage plus removable units and other equipment, the carriage of which depends upon the role for which the operator 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 and other consumable fluids carried in the aircraft, other than the basic equipment and variable load.

ZERO FUEL WEIGHT Is the total weight of an aircraft ready for take-off including crew, engers freight etc., but excluding fuel.

LANDING WEIGHT Is the maximum weight at which the aircraft can land safely considering the ambient conditions and runway in use.

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




DETERMINATION OF TAKE-OFF WEIGHT When determining the take-off weight, the weight and position of all variable loads must be added to the aircraft basic weight, together with their arms and moments. Variable loads include such items as crew, crew baggage, enger seats, drinking water, life raft, emergency transmitter and service equipment (food toiletries etc) as necessary 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 may take off anywhere in the world, in the most favourable circumstance in accordance with 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 Centre 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 provided that the two pans suspended from the beam are of equal weight and distance from the fulcrum. In aeronautical 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 as critical as that of the scales in Figure 1, although it is important that they remain within those tolerances for reasons of safety, performance, and economy. From Figure 1 it can be understood that the influence of weight, in relation to balance, is directly dependent upon the distance of the weight from the fulcrum. Unlike the scales in Figure 1, aircraft, (apart from some helicopters) cannot practicably be suspended in such a way as to determine the relative weight, balance, and e.g. However, it can be achieved mathematically.



Airframe Royal Malaysian Air Force Introduction Weight & Balance 9.6.1 - HO - 7 _____________________ Apprentice Course - Technician ____________________ 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 distance required to balance "C", the known weight "D" is moved along the beam until the weight of "D" and its accompanying lever arm are equal to the weight of "C" therefore aligning the beam with the balance mark. Once achieved the distance "d" can then be 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

Cc D

=

50 x 10 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 =

200 120

=

1ft. 8in.

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 kg and 30 kg, at distances of 6 cm, 14 cm, 23 cm and 36 cm from the left hand and respectively. Find C of G of beam.(Fig. 3 )




PREPARATION OF AIRCRAFT FOR WEIGHING 1

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

2

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

3

Ensure that the hydraulic system reservoir is topped up to the correct level.

4

Ensure that the aircraft is clean and dry.

5.

The aircraft should be weighed indoors.




Fig. 1 Simple Scale







Fig. 3



Airframe Royal Malaysian Air Force Aircraft Weighing Weight & Balance 9.6.2 - HO - 1 _____________________ Apprentice Course - Technician ____________________

AIRCRAFT WEIGHING It is first necessary to find the weight of the aircraft acting at the undercarriage positions or the main jacking positions. The aircraft must be longitudinally and laterally level; it may be possible to reduce the pressures in the tyres or shock absorbers to achieve this. The aircraft is weighed by placing the wheels on weighing machines of if jacking the aircraft, by placing a weighing machine between each jack and the aircraft jacking points. When weighing the aircraft at the undercarriage positions, mechanical or electrical scales are used. These may be permanently fitted in the hangar floor, but are 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 hydrostatic type may not give a direct reading, the indications may have to be converted using a chart applicable to that type. The capacity of the weighing equipment must be correct for the aircraft being weighed. All weighing equipment should be checked at periods not exceeding one year.

AIRCRAFT WEIGHT AND CENTRE OF GRAVITY The weight and c.g. of an aircraft is calculated in the same way as for the loaded beam. 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 be determined, and the c.g. of the aircraft calculated to ensure the aircraft remains within the approved limits. If for example, the c.g. was too far forward, it would result in a nose-heavy condition which could be potentially dangerous (particularly during take-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 excessive 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 of control. The c.g. too far aft results in a tail-heavy condition which, with the tendency of the aeroplane to stall, makes landing more difficult, may result in a reduction in performance, and cause an increase in fuel consumption. In the case of rotorcraft it will 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 the aircraft Flight Manual or other document associated with the Certificate of Airworthiness, such as the Owner's Manual. Where no such document exists, the limitations are specified in the Certificate of Airworthiness.




Fortunately it is not necessary for an aircraft to be perfectly balanced to achieve stable flight, i.e. to an exact c.g. position. The permissible variation is called the Centreof-Gravity Range. This is specified by the manufacturer for each aircraft type and is determined 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. It is chosen as the aircrafts C of G reference datum, so that the moment of any item on the 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) its position is sometimes specified as a percentage of the S>M>C> of the wing, measured aft from its leading edge.

X S.M.C.

x

100 1

=

% S.M.C

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 the aircraft datum and the aircrafts C of G position is 269 cm aft of the datum. Find the C of G position as a percentage of S.M.C.

% S.M.C.

C of G

=

269 - 229 200

=

40 x 100 200

=

20% of S.M.C.


x`

100 1








Airframe Royal Malaysian Air Force Weighing Equipment Weight & Balance 9.6.3 - HO - 1 _____________________ Apprentice Course - Technician ____________________

WEIGHING EQUIPMENT GENERAL There are four main types of weighing equipment which may be used for weighing aircraft, weighbridge scales, hydrostatic weighing units, electrical and electronic weighing equipment based on the strain gauge principle. Since considerable error can arise if small loads are checked with equipment designed for heavy loads, and scales may be calibrated in increments too coarse for accurate calculation, the capacity of the weighing equipment should be compatible with the load. All weighing apparatus should be checked, adjusted and certified by a competent authority at periods not exceeding one year and, in addition, the zero indication should be 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 on the aircraft, the weight at each reaction point being recorded directly on the balance arm. On some equipment a dial indicator is also provided. Large aircraft are normally weighed in a hangar, using either portable weighbridge scales or weighbridges set permanently into the floor at appropriate positions with their platforms level with the floor. The aircraft may then be rolled directly onto the platforms without the need for 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.







Fig 7 - Hydraulic Weighing Unit




HYDROSTATIC WEIGHING UNITS The operation of these units is based on the hydraulic principle that the fluid pressure in a cylinder in which a piston is working depends on the area of the piston and the load applied to it. The units are interposed between the lifting jacks and the aircraft jacking points, the weight at each position being recorded on a pressure indicator. The indicator may record directly in units of weight or may be a multi-rotational type where the readings are converted to weight by means of a conversion table peculiar to each particular unit. It is important that the lifting jacks are exactly vertical and the units correctly positioned, otherwise side loads may be imposed on the weighing units and may affect the accuracy of the readings. Using hydraulic weighing unit-this is positioned between the aircraft and lifting jack and measures the pressure applied to hydraulic fluid inside the unit. The pressure reading 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 at each 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 contains a metallic element of known electrical resistance. Aircraft load is measured with 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 upon them.




Fig. 8 - Electrical Weighing Unit




ELECTRONIC WEIGHING EQUIPMENT Ref. Fig. 9 This type of weighing equipment combines elastic strain load cells as described in paragraph into weighbridge-type platforms which are placed either as a single unit or combination of units beneath the wheels of the aircraft undercarriage. Each platform, is electrically connected to an instrumentation unit, which digitally displays the selected platform load. The number of platforms required to weigh an aircraft 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 accommodate the wheel multiples of the undercarriage. The number of units that can be used is, 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 the load cells interposed between the platform table, and the base unit. Where a platform is unevenly loaded (because of structural movement or undercarriage positioning), a greater load imposed on one side of the platform will be automatically compensated for by the lesser load on the other side. NOTE: The displayed load (or reaction) on the instrumentation unit for each platform, is a dedicated computation of all load cell inputs from that particular platform. The positioning of aircraft onto electronic weighbridge platforms may be accomplished by one of the following methods:a

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

b

by ing the aircraft on jacks and, where facilities allow, lowering the hangar floor, positioning the platforms below the extended undercarriage and then raising the hangar floor until all the weight of the aircraft is ed by the platforms, or

c

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




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 A MODIFICATION When any items of basic equipment are added, removed or repositioned in an aircraft, calculations must be made to determine the effect on basic weight and Centre of Gravity. This information is used in preparing a revised loading and Distribution Schedule 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 of equipment weighing 40 lbs and fitted 10 in. forward of the datum is to be removed and 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 several 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 standard equipment 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.




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




Fig. 9- Aircraft Weighing ( Electronic)




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






Airframe Royal Malaysian Air Force enger Aircraft Weight & Balance 9.6.4 - HO - 1 _____________________ Apprentice Course - Technician ____________________

ENGER AIRCRAFT In respect of enger transport aircraft exceeding 5700 kg M.T.W.A. or where 12 or more engers are to be carried (see Air Navigation General Regulations) the weight of each enger 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 engers is assumed, it must be stated on the Load Sheet. The Commander of the aircraft can always insist on engers and baggage being weighed.

LARGE ENGER AND CARGO AIRCRAFT With large aircraft the moment of items such as fuel, engers and cargo are considerable and the procedures for determining a particular loading become complicated. In addition to the longitudinal c.g. calculation it is also usually necessary to ensure that distribution of fuel and cargo is satisfactory in a transverse (lateral) direction. 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 correct management of the fuel system will minimise c.g. movement, some variations will remain due to the impracticability of locating all fuel near the c.g. on modern swept wing 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 envelope to curtail its boundaries.




The c.g. limitations are further curtailed by fixed allowances for other variable items such as the following:a

Seating allowance, which is calculated to provide for out-of balance seating loads resulting from empty seats or enger weight variation

b

Flight allowance, which is provided to allow for the normal movement of crew and enger during flight

c

Moment changes due to operation of the landing gear or flaps

Weights and moments of engers and cargo are then calculated, the cargo being arranged 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 cargo or pallets are normally located close to the c.g. to restrict their effect, due attention being 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, cargo and engers, in order to ensure that the structural limits are not exceeded, by excessive 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 graph and 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 make and model of aircraft. Aircraft manufacturers using this method of weight and balance computation prepare graphs similar to those shown in Figure 12 and 13 for each make and model aircraft at the time of original certification. The graphs become a permanent part of the aircraft records. Along with the graphs are the data for the empty weight arm and moment (index number) for that particular make and model aircraft. The loading graph illustrated in Figure-13 is used to determine the index number of any 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 horizontal 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 the horizontal scale to find the moment or index number.










After the moment for each item of weight has been determined, all weights are added 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, and place a point at the intersection of the two lines. If the point is within the diagonal lines, the loading arrangement meets all balance requirements. The following is an actual weight and balance computation using the graphs in Figure13 For this example, assume that the aircraft has an empty weight of 1,386.0 pounds and a moment of 52,772.0 pound-inches. The index number for the empty weight 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 figure 13. 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

19.0

- 0.4

Pilot & Copilot

340.0

12.2

Rear enger (two)

340.0

24.1

20.0

1.9

245.0 ---------2,350.0 ----------

11.8 -----102.4 ------

Oil

Baggage Fuel TOTAL:

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




ON-BOARD WEIGHING AND BALANCING SYSTEM Some larger aircraft have weight sensors built into the wheel axles. These sensors send out signals (electronic) which are fed into a on-board computer, interpreted and converted to Gross Weight and position of Centre of Gravity and in some cases percentage 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 calculation of Basic Weight, Crew Weight, enger 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 against errors which could cause overloading or dangerous Centre of Gravity position.

c

Centre of Gravity control, during loading, aids attainment of the ideal position, achieving a minimum trim requirement and thus reducing fuel consumption.

d

The aircraft can operate consistently at Maximum Total Weight Authorised because uncertainties over average or assumed weights of engers or baggage, inaccuracies of procedures or fuel gauges, fuel density variations and the possibility of errors is eliminated.

LARGE ENGER AND CARGO AIRCRAFT Fig. 14 With large aircraft the moment of items such as fuel, engers and cargo are considerable and the procedures for determining a particular loading become complicated. In addition to the longitudinal c.g. calculation it is also usually necessary to ensure that distribution of fuel and cargo is satisfactory in a transverse (lateral) direction. 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 management of the fuel system will minimise c.g. movement, some variations will remain due to the impracticability of locating all fuel near the c.g. on modern swept wing 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 envelope to curtail its boundaries.




Fig. 14




The c.g. limitations are further curtailed by fixed allowances for other variable items such as the following:a b c

Seating allowance, which is calculated to provide for out-of-balance seating loads resulting from empty seats or enger weight variation Flight allowance, which is provided to allow for the normal movement of crew and engers during flight Moment changes due to operation of the landing gear or flaps

Weights and moments of engers and cargo are then calculated, the cargo being arranged 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 cargo or pallets are normally located close to the c.g. to restrict their effect, due attention being 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, cargo and engers, in order to ensure that the structural limits are not exceeded, by excessive out-of-balance forces tending to tip the aircraft on its tail.



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