Early heavier-than-air machines weighed just a few hundred pounds. In the early days of ultralights, the general agreement among enthusiasts was that the maximum empty weight of an ultralight was to be 150 pounds.
A Cessna Turbo Skylane's "typically equipped empty weight"--no fuel, no payload--is 2082 lbs; Takeoff weight--the maximum this plane can weigh in flight--is 3100 lbs. They figure that by the time you do all your checks and taxi out to the runway, you'll have burned off a couple gallons of fuel.
A Boeing 737-800 has a maximum takeoff weight of 187,700lbs. They don't give the typically equipped empty weight on those because no two of those planes are alike. If ABC Air offers "all business class service" and DEF Air offers "great rates to vacation destinations" because the whole plane is coach class using smaller seats and more of them, ABC Air's planes and DEF Air's planes will have different weights...and if GHI Air is a cargo carrier who has no seats on his plane at all, their planes will weigh something radically different from the first two airlines' jets.
Get ready for takeoff!
Course Target
The general target of the course is, that after following the course, the student will have achieved the basic theoretical knowledge which is necessary in order to be able to follow the practical course for weighing an aircraft.
The student should be able to state and define the course targets and contents.
The student should have the basic knowledge regarding weight and balance.
The student should be able to define the definition of weight and weight limitations
The student should be fully aware of the instructions to be followed when weighing an aircraft.
The student should be able to describe the basic procedures used for the weighing of an aircraft.
The student should be able to describe the operational aspects and have insight as to the construction of a Weight and Balance manifesto.
Course Content
Preface Aircraft weighing
Course target
Introduction
Weighing methods
Effects of weighing
General information for weighing an aircraft
Weighing process
Moment theory
Basic principle
Datum
The moment arm
Filling in a weighing report
Determine the Centre of Gravity
FLowchart aircraft weighing
Appendix – A Technical talk
Appendix – B Weighing report ( example )
Appendix - C Short review video about aircraft weighing
Questionair
Introduction
The reason for weighing aircraft is not immediately obvious. It would be natural to assume that an aircraft's weight remains fairly constant throughout its life. However this is not the case and a number of factors affect the weight and weight distribution.
The weight of an aircraft continues to change daily. Accumulations of trapped moisture, dust, dirt, modifications, repair schemes all contribute to weight change. The amount of paint, for instance, required to coat a 747 is around 8-9 tonnes! Although a strict control may be applied to modification embodiment, the other factors involved rapidly accumulate. Therefore, to keep track of this weight growth, periodic weighing of the aircraft is necessary to advise the flight operations department or operator of the current weight and, more importantly, the position of the centre of gravity.
For light aircraft and helicopters, weight distribution is more critical. The balance of helicopters for instance, can be radically affected simply by the addition of equipment such as cameras. Military aircraft are usually weighed on a more frequent basis and their balance is affected by factors such as missile payload and distribution.
The frequency of weighing is determined by the regulatory authority of the country in which the aircraft is registered. As well as regular weight checks, typically every 2 or 3 years, weighing must usually be carried out to determine the centre of gravity after any critical modifications .This will be determined by the appropriate regulatory body and the operator in the interests of safety.
There are a number of recognised ways of weighing aircraft and strict procedures must be adopted to ensure accurate results are obtained. As a result, weighing is usually carried out by certified aircraft weighing companies. All weighing must be done on level surfaces in sheltered conditions-usually indoors. (Any slight upcurrents of air will render weight data invalid.)
Weight defenitions & limitations.
Weights:
In order to make the weighing, centre of gravity calculations and operation of the aircraft simpler, international agreements have been made regarding the definition of aircraft weight.
These definitions simply define what should or should not be done when weighing an aircraft. The weight definition therefore simply defines the configuration of the aircraft in relation to a certain weight. All documentation that has anything to do with the weighing of aircraft, centre of gravity definitions and/or aircraft operation (thus not only Fokker documentation) refers to these definitions.
Nearly all documentation with regard to these definitions is in the English language.
Manufacturers Empty Weight (MEW)
This is the weight of structure, powerplant, furnishings, systems and other items of equipment that are an integral part of a particular aircraft configuration (It is essentially a "dry" weight, including only those fluids contained in closed systems).
Standard Basic Empty Weight (SBEW)
This is the MEW plus standard items.
Standard items are:
‑ Unusable fuel and other unusable fluids
- Engine oil
- Toilet fluid and chemicals
- Fire extinguishers, pyrotechnics, portable oxygen equipment
- Structure in galley, buffet, bar
- Supplementary equipment
Basic Empty Weight (BEW)
This is the SBEW plus or minus weights of standard variations. Standard item variations are items that the operator adds, deducts or changes.
Operational Empty Weight (OEW)
The OEW is the BEW plus operational items. Operational items are personnel, equipment and supplies necessary for a particular operation but not included in BEW. These items may vary for a particular aircraft and may include, but are not limited to, the following:
‑ Crew and baggage
‑ Manuals and navigational equipment
‑ Removable service equipment for cabin, galley, and bar
‑ Food and beverages, including liquor
- Usable fluids, other than those is useful load
- Life rafts, life vests and emergency transmitters
- Aircraft cargo handling systems and cargo containers
Delivery Empty Weight (DEW)
The DEW is the MEW, minus any shortages, plus those standard items and operational items at the time of delivery.
Actual Zero Fuel Weight (ZFW)
The ZFW is the OEW plus traffic-load.
Traffic-load
The traffic-load is the weight of passengers, cargo and baggage. (These may be revenue and/or non-revenue).
Useful load
The useful load is the difference between Operational Take-off Weight and OEW. It includes traffic-load, usable fuel and other usable fluid not included as operational items.
Weight limitations:
There are a number of structural weight limitations which are based on the strength of an aircraft and/or the components used. These limitations are included in the chapter. Any other operation weight limitations are excluded from this course (Performance calculations). These limitations can lie considerably lower depending upon the runway and meteorological conditions.
Maximum Design Taxi Weight (MTW)
Maximum weight for ground manoeuvring as limited by aircraft strength and airworthiness requirements. It includes weight of tax and run-up fuel.
Maximum Design Take-Off Weight (MTOW)
The MTOW is the maximum weight for take-off as limited by aircraft strength and airworthiness requirements. It is the maximum allowed weight at the start of the take-off run.
Maximum Design Landing Weight (MLW)
The MLW is the maximum weight for landing as limited by aircraft strength and airworthiness requirements.
Maximum Design Zero Fuel Weight (MZFW)
The MZFW is the maximum weight allowed before usable fuel and other specified usable agents must be loaded in defined sections of the aircraft as limited by strength and airworthiness requirements.
Limiting Design Weight
The aircraft shall be certified for operation at the following gross weights*:
Datum
A vertical plane or line to which the Centre of Gravity position and all Lever Arm measurements relate.
Weighing methods.
The two most common methods are load cell and platform .
LOAD CELL
Believe it or not , most aircraft, even the largest passenger jets, can be jacked up on stands at 3 or more points. Load cells can be fitted in between the jacks and the aircraft in order to provide weight information. This method has the advantage that the aircraft can be set up and weighed in its normal flight attitude. This weights measured by each load cell can be summed to give the total weight and individually used to compute the centre of gravity. The complete weighing kit can fit inside a large suitcase making it highly portable. However there are important safety issues to be considered with this method.
PLATFORM
The platform system is now well established for aircraft weighing. Shallow ramps are placed in front of each undercarriage wheel with a weighing platform placed at the head of each ramp. The aircraft is usually towed or pushed up the ramps onto the platforms by tug or tractor. For large aircraft, the platforms required are large and special transport equipment is utilised.
The process of aircraft weighing with loadcells or platform scales is shown in de video's below
Weighing with loadcells
Weighing with platform scales
Effects of Weighing
WEIGHT AND BALANCE CONTROL
Weight is a measure of the attractive force of the earth's gravity upon a material body. It is an indication of the mass or heaviness of the body. Weight is also one of the greatest enemies of the flyer. It is a factor which must be respected if flight is to be conducted safely. The force of gravity acting on the mass of the aircraft continuously attempts to pull it down from flight. The force of lift which is generated by the airfoils of the aircraft is the only force available to counteract weight and keep the aircraft in flight. However, the airfoils can produce only a limited amount of lift for use in resisting gravity; therefore, any increase in aircraft weight is to be avoided if possible. The total lift of the aircraft depends on the design of the airfoils, the speed and angle of attack of the airfoils as they move through the air, and the density of the air through which the airfoils are moving. If the generated lift does not equal aircraft weight, level flight cannot be maintained and the aircraft must descend.
WEIGHING SET
EFFECTS OF WEIGHT
Any object aboard the aircraft which increases the total weight significantly is an undesirable object as far as flight is concerned. However, aviators must accept a compromise and load some heavy objects in the fuselage or wings to make flight possible. Fuel is an example of a heavy but necessary item. It is always easier to fly when the aircraft is light and more difficult and dangerous when the aircraft is heavy. Therefore, it has always been a primary rule of flight to make the machine as light as possible without sacrificing strength or safety and to include only those loads essential for the particular flight. The total weight of a vehicle changes as the contents (passengers, fuel, or cargo) are varied. If care is not taken, the vehicle can be weighted down with objects to a point where it can no longer function efficiently as a mover of loads. The operator of the vehicle and especially the pilot of an aircraft should always be aware of the consequences of overloading. An overloaded boat might sink, a truck or automobile might not be able to climb a hill, and an aircraft may not be able to leave the ground. Each vehicle has its limits, beyond which excessive weight leads to inferior operation and possible disaster. Of all common vehicles, the aircraft is most susceptible to trouble if weight considerations are disregarded; its limits are most easily exceeded. Furthermore, when the aircraft has weight problems, the initial indication of poor performance will be during takeoff; an unfortunate place for the vehicle and the pilot to be in trouble. Excessive weight reduces the flying ability of an airplane in almost every respect. The most important performance deficiencies of the overweight airplane are:
Higher takeoff speed.
Longer takeoff run.
Reduced rate and angle of climb.
Lower maximum altitude.
Shorter range.
Reduced cruising speed.
Reduced maneuverability.
Higher stalling speed.
Higher landing speed.
Longer landing roll. Excessive weight on the nose and / or main wheels.
The pilot must appreciate the effect of excessive weight on the performance of the aircraft. Every preflight check should include a study of performance charts to see if the aircraft weight may contribute to hazardous flight conditions. Most pilots have been trained to recognize and avoid such aircraft performance reducing factors as: High density altitude, frost on the wings, low engine power, and severe or uncoordinated maneuvers. Excessive weight reduces the safety margins available to the pilot when these conditions are encountered. The pilot must also consider the consequences of an overweight aircraft if emergency conditions arise. If an engine fails on takeoff or ice forms at low altitude, it is usually too late to reduce the aircraft's weight to help keep the machine in the air.
WEIGHT CHANGES
The weight of the aircraft can be changed easily by varying the payload (passengers, baggage, and cargo). But, if weight has to be decreased by reducing the payload, the flight will be less profitable. Weight can also be changed by altering the fuel load. Gasoline or jet fuel has considerable weight - 30 gallons may weigh more than a paying passenger. But, if weight is lowered by reducing fuel, the range of the aircraft is shortened. Fuel burn is normally the only weight change that takes place during flight. As fuel is used, the aircraft becomes lighter and performance is improved; this is one of the few good things about the consumption of the fuel supply. Changes of fixed equipment also have a major effect upon the weight of the aircraft. Many aircraft are overloaded to a dangerous degree by the installation of extra radios or instruments. Repairs or modifications usually add to the weight of the aircraft; it is a rare exception when a structural or equipment change results in a reduction of weight. As with people, when an aircraft ages, it just naturally puts on weight. The total effect of this growth is referred to as "Service Weight Pickup." Most service weight pickup is the known weight of actual parts installed in repair, overhaul, and modification. These parts should have been weighed or the weight calculated when they were installed. In addition, an unknown weight pickup results from the collection of trash and hardware, moisture absorption of soundproofing, and the accumulation of dirt and grease. This pickup can only be determined by the accurate weighing of the aircraft as a unit
.
BALANCE, STABILITY, AND CENTER OF GRAVITY
Balance refers to the location of the c.g. (Centre of gravity) of an aircraft. It is of primary importance to aircraft stability and safety in flight. Pilots should never fly an aircraft if they are not personally satisfied with its loading and the resulting weight and balance condition. The c.g. is the point about which an aircraft would balance if it were possible to support the aircraft at that point. It is the mass Centre of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. The c.g. must be within specific limits for safe flight. The prime concern of aircraft balancing is longitudinal balance, or the fore and aft location of the c.g. along the longitudinal axis. Location of the c.g. with reference to the lateral axis, however, is also important. The design of the aircraft is such that lateral symmetry is assumed to exist as far as weight is concerned. In other words, for each item of weight existing to the left of the fuselage centerline, there is generally an equal weight existing at a corresponding location on the right.
This lateral mass symmetry, however, may be upset by unbalanced lateral loading. The position of the lateral c.g. Is not computed, but the operating crew must be aware that adverse effects will certainly arise as a result of a laterally unbalanced condition.
Lateral unbalance will occur if the fuel load is mismanaged by supplying the engine(s) unevenly from tanks on one side of the aircraft . The airplane pilot can correct the resulting wing-heavy condition by the use of aileron tab adjustment or by holding a constant lateral control pressure. However, this puts the aircraft controls in an out-of-streamline condition and results in a lowered operating efficiency. Since lateral balance is relatively easy to control and longitudinal balance is most critical, further reference to c.g. in this handbook will mean longitudinal location of mass balance.
The c.g. Is not necessarily a fixed point; its location depends on the distribution of items loaded in the aircraft. As variable load items are shifted or expended, there is a resultant shift in c.g. location. The pilot should realize that if the mass Centre of an aircraft is displaced too far forward on the longitudinal axis, a nose-heavy condition will result. Conversely, if the mass Centre is displaced too far aft on the longitudinal axis, a tail-heavy condition will result. It is possible that an unfavorable location of the c.g. could produce such an unstable condition that the pilot could lose control of the aircraft. In any event, flying an aircraft which is out of balance, either in a tail-heavy or a nose-heavy direction, may produce increased pilot fatigue with obvious effects on the safety and efficiency of flight. The pilot's natural correction for longitudinal unbalance is a change of trim to remove the excessive control pressure. However, excessive trim has the effect of reducing primary control travel in the direction the trim is applied.
EFFECTS OF ADVERSE BALANCE
Adverse and abnormal balance conditions affect the flying ability of an airplane with respect to the same flight characteristics as those mentioned for an excess weight condition. In addition, there are two essential airplane attributes which may be seriously reduced by improper balance; these are STABILITY and CONTROL. Loading in a nose-heavy direction causes problems in controlling and raising the nose, especially during takeoff and landing. Loading in a tail-heavy direction has a most serious effect upon longitudinal stability even to the extent of reducing the airplane's ability to recover from stalls and spins. Limits for the location of the aircraft's c.g. are established by the manufacturer. These are the fore and aft limits beyond which the c.g. should not be located for flight. The limits are published for each aircraft in the FAA Aircraft Type Certificate Data Sheets or Specifications. If, after loading, the c.g. does not fall within the allowable limits, it will be necessary to shift loads before flight is attempted. The forward c.g. limit is often established at a location determined by the landing characteristics of the aircraft. It may be possible to maintain stable and safe cruising flight with the c.g. ahead of the prescribed forward limit, but since landing is one of the most critical phases of flight, the forward c.g. limit is placed at a relatively rear position to avoid damage to the aircraft structure when landing.
A restricted forward c.g. limit is also specified to assure that sufficient elevator deflection is available at minimum airspeed. When structural limitations or large stick forces do not limit the forward c.g. position, it is located at the position where full-up elevator is required to obtain a high angle of attack for landing. The aft c.g. limit is the most rearward position at which the c.g. can he located for the most critical maneuver or operation.
As the c.g. moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or after disturbances by gusts . For some aircraft, the c.g. limits, both fore and aft, may be specified to vary as gross weight changes. They may also be shifted for certain operational procedures, such as acrobatic flight, retraction of the landing gear, or the installation of special loads and devices that change the flight characteristics. The actual location of the c.g. can be altered by many variable factors - usually under control of the pilot.
Placement of baggage and cargo items can both determine c.g. and be used to control c.g. In addition, the assignment of seats to particular passengers can be used as a means of obtaining the most favorable balance. If the aircraft is tail-heavy, it is only logical "horse sense" to place a heavy passenger in a front seat. The loading and selective use of fuel from various tank locations can have a decided effect on aircraft balance. Large aircraft must have fuel loaded in a particular manner determined by the total load, and then the tanks must be selected in a sequence that will keep the load in balance. Swept wing aircraft have special problems along these lines. Fuel in outboard tanks has a tendency to rotate the aircraft in a tail-heavy direction and fuel in inboard tanks adds to a nose-heavy condition . The use of fuel from swept wing tanks must be carefully managed to keep c.g. under control.
SHIFTING OF LOOSE CARGO
The shifting of cargo or baggage during flight can result in several hazards, not the least of which is a dangerous balance condition. If the c.g. of an aircraft is already near the forward or aft limit, a significant longitudinal shift of cargo may make control difficult or impossible. This hazard is most likely to occur in aircraft having cargo poorly secured in the main cabin. Particular care must be taken to restrain this type load with proper tiedown devices.
MANAGEMENT OF WEIGHT AND BALANCE CONTROL
Weight and balance control is a matter of serious concern to all pilots and to many people on the ground who are involved in the support of flight. The pilot has control over the loading and fuel management within established limits for the particular aircraft. The pilot has weight and balance information available in the form of aircraft records and operating handbooks. Loading information is also available in the form of placards in baggage compartments and on tank caps. The aircraft owner or operator should make certain that up to date information is available in the aircraft for the pilot's use. The owner or operator of the aircraft should insure that maintenance personnel make appropriate entries in the aircraft records when repairs or modifications have been accomplished. Weight changes must be accounted for and proper notations made in weight and balance records. Without such notations, the pilot has no foundation upon which to base calculations and decisions.
The aircraft manufacturer and the FAA (Federal Aviation Administration) EASA (European Aviation Safety Agency ) have major roles in designing and certificating the aircraft with a safe and workable means of controlling weight and balance. If the prototype aircraft has weight and balance control problems which are potentially dangerous, design changes are made before the aircraft is type certificated.
General information for weighing an aircraft.
Weighing an aircraft is a very important and exacting phase of aircraft maintenance and must be carried out with accuracy and good workmanship. Thoughtful preparation saves time and prevents mistakes.
To begin, assemble all the necessary equipment, such as:
1. Scales, hoisting equipment, jacks, and leveling equipment.
2. Blocks, chocks, or sandbags for holding the airplane on the scales.
3. Straight edge ruler, spirit level, plumb bobs, chalk line, and a measuring tape.
4. Applicable Aircraft Specifications and weight and balance computation forms. 5. Fuel drain equipment, tractor or tug, towbar.
Aircraft should be weighed in a closed building where there are no air currents to cause incorrect scale readings. An outside weighing is permissible if wind and moisture are negligible.
picture left-Platform system picture middle-Loadcell picture right-Compleet weighing set
Drain the fuel system until the quantity indication reads zero, or empty, with the aircraft in a level attitude. If any fuel is left in the tanks, the aircraft will weigh more, and all later calculations for useful load and balance will be affected. Only trapped or unusable fuel (residual fuel) is considered part of the aircraft empty weight. Fuel tank caps should be on the tanks or placed as close as possible to their correct locations, so that the weight distribution will be correct.
In special cases, the aircraft may be weighed with the fuel tanks full, provided a means of determining the exact weight of the fuel is available. Consult the aircraft manufacturer's instructions to determine whether a particular model aircraft should be weighed with full fuel or with the fuel drained.
If possible, drain all engine oil from the oil tanks. The system should be drained with all drain valves open. Under these conditions, the amount of oil remaining in the oil tank, lines, and engine is termed residual oil and is included in the empty weight. If impractical to drain, the oil tanks should be completely filled.
The position of such items as spoilers, slats, flaps, and helicopter rotor systems is an important factor when weighing an aircraft. Always refer to the manufacturer's instructions for the proper position of these items.
Unless otherwise noted in the Aircraft Specifications or manufacturer's instructions, hydraulic reservoirs and systems should be filled; drinking and washing water reservoirs and lavatory tanks should be drained; and constant speed drive oil tanks should be filled.
Inspect the aircraft to see that all items included in the certificated empty weight are installed in the proper location. Remove items that are not regularly carried in flight. Also look in the baggage compartments to make sure they are empty. Replace all inspection plates, oil and fuel tank caps, junction box covers, cowling, doors, emergency exits, and other parts that have been removed. All doors, windows, and sliding canopies should be in their normal flight position. Remove excessive dirt, oil, grease, and moisture from the aircraft.
Properly calibrate, zero, and use the weighing scales in accordance with the manufacturer's instructions.
Some aircraft are not weighed with the wheels on the scales, but are weighed with the scales placed either at the jacking points or at special weighing points. Regardless of what provisions are made for placing the aircraft on the scales or jacks, be careful to prevent it from falling or rolling off, thereby damaging the aircraft and equipment. When weighing an aircraft with the wheels placed on the scales, release the brakes to reduce the possibility of incorrect readings caused by side loads on the scales.
All aircraft have leveling points or lugs, and care must be taken to level the aircraft, especially along the longitudinal axis. With light, fixed wing airplanes, the lateral level is not as critical as it is with heavier airplanes. However, a reasonable effort should be made to level the light airplanes around the lateral axis. Accuracy in leveling all aircraft longitudinally cannot be overemphasized.
left picture - Aircraft on Jacks right picture - aircraft on scales(ramp wheel scales)
Weighing proces
Weighing proces.
Weighing the aircraft:
Weighing aircraft with accurately calibrated scales is the only sure method of obtaining an accurate empty weight and CG location.
All scales for aviation use, manuel or electronic, must be protected when stored, shipped, and they must be checked periodically for accuracy.
The maximum period between calibration checks is twelve months, this period can be reduced by any airworthiness authority dependent on the condition of use.
In general, weight procedures may vary with the aircraft and types of weight equipment employed.
The weight procedure contained in the manufacturer's maintenance manual should be followed for each particular aircraft.
Scale preparation.
Mechanical and electronic scales shall be inspected prior to use and set to zero.
This is done by adding and removing a weight, then rechecking for zero. This proces should be repeated until a steady zero setting is obtained.
The scales should be located in the same environment in which they are to be used and allowed to come up to temperature at least 2 hours prior to use.
Weigh clean aircraft inside hangar:
The aircraft should be weighed inside a hangar where wind, hangar heaters and or ventilation cannot blow over the surface and cause fluctuating or false scale readings.
The aircraft should be clean inside and outside, with special attention to ensure that no water, debris is trapped inside several area's.
The outside of the aircraft should be free as possible of mud and dirt.
Equipment List:
You must use only the correct equipment list that belongs to the aircraft yor going to weigh.
All of the required equipment must be properly installed, and ther should be no equipment installed that is not included in the equipment list.
If such equipment is installed, the weight and balance record must be corrected to indicate it.
Ballast:
All required permanent ballasts must be properly secured in place.
All temporary ballasts must be removed before the weighing proces.
Standard weights:
Standard weights are established weights for numerous items involved in weight and balance computations.
These weights should not be used if actual weights are available.
Draining fuel, oil and other fluids
Fuel:
Draining fuel from the tanks in the manner specified by the aircraft manufacturer.
If there are no specific instructions, drian the fuel until the fuel quantity gauges read empty when teh aircraft is in a level-flight attitude.
Any fuel remaining in the system is considered residual or unusable fuel and is part of the aircraft empty weight.
The amount of residual fuel and its arm are normally found in the section of the type certificatedata sheets.
If it is not feasible to drain the fuel, the tanks can be topped off to be sure of the qantity they contain and the aircraft weighed with full fuel.
After weighing is completed, the weight of the fuel and its moment are substracted from those of the aircraft as weighed.
To correct the empty weight for residual fuel, add its weight and moment.
Note:
The difference in weight as temperatures change is small.
Although this change is a very small amounth per gallon, it could end up in a significant total weight when dealing with large quantites of fluids, suchs as found in commercial aircraft.
OIL:
To weigh an aircraft that does not include the engine lubricating oil as part of the empty weight, place it in a flight level attitude, open the drain valves and allow the oil to drain out.
Any remaining is undrainable oil and is part of the empty weight.
If it is impractictical to drain the oil, the reservoir can be filed to the specified level and the weight and moment of the oil can be substracted from the weight and arm of the the weighed.
Other fluids:
The hydraulic fluid reservoir and all other reservoirs containing fluids for normal operations of the aircraft should be full.
Fluids not to be considered to be part of the empty weight of the aircraft are potable water(drinkable water) lavatory precharge water, and water for injection into the engines must be drained from the aircraft before it is weighed.
Configuration of the aircraft.
Configuration of the aircraft:
Consult the aircraft service manual regarding position of the landing gear shock struts and the control surfaces for weighing.
When weighing a helicopter, the main rotor must be in its correct position accordance ATA-8 weight and balance of the specific helicopter.
For the correct cabin and safety items consult the equipment list.
Jacking the aircraft:
Aircarft are often weighed with the use of platform scales (ramp scales).
The aircraft is then rolling them on the platform scales or on to ramps in which load cells are embedded, this eliminates the problems associated with jacking the aicraft of f the ground.
However many aircraft are still weighed by jacking the aircraft up and then lowering them onto scales or load cells.
Extra care must be used when raising an aircraft on jacks for weighing.
For some aircraft stress panels or plates must be installed before the aircraft is raised with wing jacks to distribute the weight over the jack pad.
Be sure to follow the recommendations of the aircraft manufacturer in detail anytime an aircraft is jacked.
As jacks are raised, keep the safety collars screwed down against the jack cylinder to prevent the aircraft from tilting if one of the jacks should lose hydraulic pressure.
Leveling the aircraft:
When an aircraft is weighed, it must be in its level flight attitude so that all of the components are at there correct position and distance from the datum.
Some aircraft rquire a plumb line to be dropped from a specific location so that the point of the weight (the BOB) hangs directly above an identifiable point.
Other aircraft specify that a spirit level to be placed across two leveling lugs, often special srews on the outside of the fuselage.
Other aircraft call for a spirit level to be placed on the upper door sill.
Lateral level is not specified for light all aircraft, but provisions are made on helicopters and multi engine aircraft for determining both longitual and lateral level.
This may be done by build-in leveling indicators, or plumb bob that shows the conditions of both logitudinal and lateral level.
When weighing from the wheels leveling is normally done by adjusting the pressure in the nosewheel shock strut.
Special Safety Considerations:
1.Stress plates must be installed under the jack pad if the manufacturer specifies them.
2.If anyone is rquired to be in the aircraft while it is being jacked, there must be no movement.
3.The jacks must be straight under the jack pads before beginning to raise the aircraft.
4.All jacks must be raised simultaneously and the safety devices are against the jack cylinder to prevent the aircraft from tipping if any jack should lose hydraulic pressure.
Not all jacks have screw collars, some use drop pins or friction locks.
Moment theory.
Moment theory
Balance in an object concerns the size, the force exercised on the object and place where the forces are applied.
Imagine a see-saw.
When the fulcrum is in the middle of the see-saw and two children of equal weight sit on it the see-saw is balanced. Only the removal of the children will bring the see-saw into motion. Put a heavier child on one side of the see-saw and the see-saw will become unbalanced. The heavier child will fall to the ground and the lighter child will go into the air.
Put the two children of equal weight back onto the see-saw but put one child further forward than the other. The child sitting farthest from the fulcrum will fall to the ground and the other will go into the air.
There is therefore a relationship between the balance of the bodies and the forces being exercised and the area where the forces are being applied.
This connection is recorded in the so called moment theory.
A moment is the product of force and arm.
The theory is as follows:
“Whenever an object is balanced, then the sum of the forces that are necessary to rotate that object in one direction, around a fulcrum are equal to the forces that are necessary to rotate that object in the opposite direction”.
In order to avoid any confusion it has been agreed upon that the forces travelling to the right (clockwise) are the positive (+) forces and the forces travelling to the left (anti-clockwise) are the negative (-) forces.
If the sum of the forces of the working moments is equal to 0 then the object is in balance.
This is mostly expressed as:
ΣM = 0
Σ is the Greek letter sigma, which means the sum of all the forces, negative and positive.
The forces are attacking the beam in such a way that the beam is in balance.
Anyone who can understand this simple example works understands the theory behind the Weight and Balance of an aircraft.
Compare the downward forces on the beam with that of the weight of an aircraft, the fuel, the crew, the passengers etc. and the upward force produced by the aircraft wings.
Below a second example.
Basic principle.
Basic principle
The centre of gravity in an aircraft can be seen, if you use the “balance” calculation as the point where the “nose heavy” (-) and “tail heavy” (+) forces are equal. If the aircraft was to be hung up then the aircraft would be balanced.
Or, in other words, when the upward force “lift” is applied at this point then the aircraft will not have the compulsion to fly nose-up of nose-down.
Aircraft hung up in centre of gravity.
The centre of gravity of an aircraft is dependent upon the weight and the situation of everything onboard. Any extra baggage added relatively near to the rear of the centre of gravity will automatically move the centre of gravity further to the rear and vice versa.
If an aircraft were to have no trim then the aircraft would have to be so loaded that the centre of gravity would fall at the same point as the lift. If not, it would be constantly necessary for the pilot or auto-pilot to fly the aircraft at the correct altitude.
This movement has to take place within a certain “range”. This range is called the centre of gravity range.
This range has both a forward and backward limit.
The centre of gravity range give the forward and backward centre of gravity where the aircraft fulfils the necessary performance and inspection requirements on the basis of how it has been certified
These limits are accurately laid down during the design and test-phase of the aircraft and are recorded in the Type Specification, certification documentation and the Flight Manual (limitations). The CG range is normally located around a point a 1/3 of a wing chord behind the wing nose section.
At all times during the flight of the aircraft should the centre of gravity be found to be within these limits.
In some aircraft types different limits will be used during different phases e.g. start, cruising and landing.
These limits are expressed in % MAC (Mean Aerodynamic Chord). The MAC is the mean average chord of the wing which is also used for aerodynamic calculations during the aircraft design phase.
Datum.
Datum:
In weight and balance calculations it is not only the total weight onboard the aircraft that is important but also where the weight is situated.
The relative location of all loaded weight is expressed using a datum line.
This datum line, or in simpler terms the datum, is an imaginary vertical line that can be found in or outside of an aircraft, it is always vertical.
There is no fixid rule for the location of the datum line, except that it must be a location that will not change during the life of the aircraft. For example, it would be not be a good idea to have the datum be the tip of the propellor spinner or the front edge of the pilot seat, because changing to a new design of the spinner or moving the seat of the the pilot would change the the datum.
The datum is mostly to be found at a recognisable position in the aircraft, the nose, leading edge or any other bulkhead.
The manufacturer has the choise of locating the datum where it is most convenient for measurement , equipment location and weight & balance computation
The figure below shows an aircraft with the leading edge of the wing being the datum.
The datum line through the nose of the aircraft.
The chosen datum line for weight and balance calculation purposes is through the nose of the aircraft, which makes the centre of gravity calculations easier to calculate. We have already seen that all clockwise moments/forces are positive and all anti-clockwise moments/forces are negative. If the datum line runs through the nose of the aircraft then all other moments to the datum line are therefore positive.
The figure below shows an aircraft with the datum line through the nose of the aircraft.
The moment arm ( H-arm).
The moment arm
A manufacturer or operator may use sometimes the Moment Arm or H-arm(both are correct), and it is the horizontal distance between the datum line in the nose of the aircraft and the centre of gravity of the aircraft or an object in or outside of the aircraft.
See the diagram below.
A moment
As we have seen earlier a moment/force is the product of weight and moment arm. Moments in respect of the datum line in an aircraft are calculated by multiplying the weight by the moment arm (H-arm).
Aircraft manufacturers are using kilograms and pounds to express weight.
Moment arms are expressed as millimeters or inches.
Therefore moments/forces dimensions are expressed as mm.kg or in.lbs and are therefore dependent on the preferred options of the operator.
Filling in a weighing report
Leveling
Level the airplane ( deflating the nose wheel tire to center bubble on level.
Weighing - Airplane Basic Empty Weight
With the airplane level and brakes released, record the weight shown on each scale. Deduct the tare, if any, from each reading.
AIRPLANE AS WEIGHED (lncluding full oil and operating fluids but no fuel).
Scale Position and Symbol
Scale Reading
Tare
Net Weight
Nose Wheel (N)
Right Main Wheel (R)
Left Main Wheel (L)
Weight, as Weighed (T)
WEIGHING FORM
The centre of gravity
Center of Gravity
The following geometry applies to the PA-38-112 airplane when it is level. Refer to Leveling paragraph Chapter-8 of the Maintenance manual.
2.
The empty weight center of gravity (as weighed including optional equipment, full oil and operating fluids) can be determined by the followingformula:
C.G. Arm = N (A) + (R + L) (B) inches
T
Where: T = N + R + L
3.
Basic Empty Weight
Video about aircraft weighing
Watch the short instruction video below about aircraft weighing in general.
Flow Chart: Aircraft Weighing
Appendix:
Appendix - A: Technical Talk
Why do we need to weigh an aircraft
An aircraft has a maximum authorised weight that it must not exceed. In order to calculate its safe payload the weight in its basic empty configuration must be determined periodically; on a commercial aircraft that’s normally a requirement every 4 years under EASA/JAR OPS.
The aircraft must be loaded so that its Centre of Gravity (CG) falls within the stated forward and aft limits in order to retain full control surface authority e.g. aileron movement in either direction.
Why do aircraft get heavier?
This may be caused by several factors:
1.
Moisture retention; typically when the aircraft descends and the cabin depressurises the moisture present in the atmosphere forms water droplets that are absorbed by the aircraft’s soft furnishings and soundproofing.
2.
Retention of dirt in the cabin, under the floor and in some of the aircraft compartments that may be exposed to the elements.
3.
Operators’ and manufacturers’ modifications and/or repairs to the airframe.
4.
Paint schemes. Remember that light coloured paint generally needs to be thicker than dark to cover other dark colours.Some aircraft are painted 4 or 5 times with different paint schemes.
Why re-weigh after a new paint scheme?
It is difficult and time consuming to determine accurately how much paint is used on the aircraft and in what proportions it was applied over the various affected surfaces; this may on occasion include the interior. The colour scheme and finish chosen could result in a localised or overall build up of several layers e.g. stripes or lacquer. The difference between a complete paint strip and a rub down can also have a significant effect.
Do aircraft get lighter?
An airframe may get lighter occasionally. For example, if it has had several layers of paint removed prior to re-painting or it is on a long maintenance programme in a warm dry hangar and the items that retain moisture have had a chance to dry out.
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2022-03-22 21:13:43
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The empty weight center of gravity (as weighed including optional equipment, full oil and operating fluids) can be determined by the followingformula:
