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.)
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.
A typical weighing procedure for light aircraft is given as an example
Annex
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.
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.
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
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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) 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.
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