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Figure 1.1 Mass and Balance calculations must be carried out before every flight.

Mass and Balance calculations affect both the safety and performance of your aircraft and must never be neglected. An overloaded aircraft may fail to lift off during the take-off run while an aircraft whose centre of gravity is out of limits may be unstable and difficult to control. In the subject of Mass and Balance, the word mass refers, of course, to the mass of the aircraft, either empty or loaded, and the word balance refers to the position of the centre of gravity.

During the early stages of your flying training, your flying instructor will take responsibility for the Mass and Balance calculations for the aircraft. But, in the later stages of your training, and, certainly, in order to pass the ground examinations and skills test for the Private Pilot’s Licence, you will be required to demonstrate your understanding of the principle of Mass and Balance by successfully answering questions in a multiple-choice examination paper, and by practically presenting your instructor and examiner with Mass and Balance data for actual flights. This chapter and the chapter on ‘Centre of Gravity Calculations’ aim to provide you with the theoretical knowledge that you require to achieve these important objectives.

During flying training and on the skills test, your flying instructor or examiner has the obligation to ensure that you correctly perform the Mass and Balance calculations for the aircraft; but once you have obtained your licence, the responsibility is yours alone. Both good airmanship and regulations require that the Pilot-in-Command of an aircraft registered in the United Kingdom satisfy himself that, before the aircraft takes off, the weight (mass) of the aircraft is within limits, and that any load is so distributed that the centre of gravity of the aircraft is also within limits.

The Pilot’s Operating Handbook for your aircraft will provide you with the specific information that you need to ensure that your aircraft is safely loaded.

MASS OR WEIGHT?

You will often hear the subject Mass & Balance referred to as Weight & Balance although there is a crucial conceptual difference between the concepts of mass and weight. The change of ground subject title occurred when pilot licences changed

The pilot-in- command has a legal

responsibility for the safe loading of his aircraft.

Mass is the amount of

matter in a body. Weight

is the force which acts on a mass within a gravitational field.

from purely UK CAA licences to JAA licences. The word Mass was substituted for Weight in the title of the subject, but the authorities gave no explanation of why the change was made.

As we explain below, although when considered scientifically, weight and mass are two different concepts, as far as the subject Mass and Balance is concerned, mass and weight are treated as having the same meaning. Some text books call the subject Weight and Balance, others call it Mass and Balance. The following paragraphs explain why mass and weight may be considered as meaning the same thing for aircraft balance and performance considerations.

The difference between mass and weight is fully explained in the Principles of Flight volume in this series. Basically, however, mass is the amount of matter in a body, while weight is the force which acts on a mass when the mass is

located in a gravitational field. A body will always possess mass but if there is no gravity the body will have no weight. (See Figure 1.2)

For us Earth dwellers, weight is the force acting on a body of a given mass due to the Earth’s force of gravity; the direction of the weight force is always towards the centre of the Earth. In Figure 1.3 on page 6, you see that the aircraft’s weight is acting vertically downwards.

When we consider aircraft accelerations in the horizontal plane, it is mass which is the relevant concept; when we consider lifting the mass of the aircraft against the force of gravity, it is the concept of weight which is in play.

Of course, in a gravitational field of constant strength (as we may consider the Earth’s to be in the realms where aircraft fly) weight is directly proportional to mass, so we can use the concepts of weight and mass interchangeably and get away with it.

Nevertheless, the subject might more accurately have been styled ‘Weight, Mass and Balance’. But Mass and Balance is the title of the subject we have been given, so in this section we use the words weight and mass largely interchangeably.

Be aware, when referring to aircraft handbooks, that American handbooks are almost certain to refer to an aircraft’s weight, whereas European handbooks will invariably refer to mass. British aircraft handbooks are likely to use either mass or weight. JAA/EASA refers almost exclusively to mass, and, so, that will be our practice here. Just remember that for this groundschool subject, Mass and Balance, mass and weight are treated as the same concept.

Where the distinction between mass and weight is important for your understanding, the difference will be pointed out.

A number of fundamental definitions are used when referring to standard aircraft mass. The fundamental definitions are:

Ba s i c Em p t y Ma s s .

The Basic Empty Mass of an aircraft refers to the mass of the airframe, engine, all standard, fixed equipment, unusable fuel, full oil contents, and the of any other item which is used on all flights of the aircraft concerned.

The Basic Empty Mass, which does not include the weight of the pilot, payload

(referred to as traffic load by the JAA), ballast or usable fuel, is the mass of the aircraft which a pilot uses as the starting point for all load and centre of gravity calculations, prior to a flight.

T h e Em p t y Ma s s .

The Empty Mass is the same as the Basic Empty Mass except that the mass of the oil contents is limited to the unusable oil. The Empty Mass of the aircraft is the mass used for the initial calculation of the Centre of Gravity entered in the aircraft’s Flight Manual or Pilot’s Operating Handbook.

G r o s s Ma s s ( Al l - U p Ma s s ) .

The Gross Mass or All-Up Mass of the aircraft is the total mass of the aircraft and all its contents at any given time. The Gross Mass must never exceed Maximum TakeOff Mass for any given take-off, whether structurally or performance limited, nor must it exceed the Maximum Landing Mass for any given landing, again irrespective of whether that landing is limited by structural or performance considerations. (A given aircraft may have different maximum gross mass limitations depending on what category – e.g. Normal or Utility – it is permitted to operate in.)

An increase in Gross Mass will cause an increase in stall speed, and an increase in the take-off and landing run required.

T h e Z e r o F u e l Ma s s .

The Zero Fuel Mass is equal to an aeroplane’s Gross Mass less the usable fuel in the aircraft’s tanks. Zero Fuel Mass, then, does include pilot, payload and ballast, but only unusable fuel.

MASS AND BALANCE LIMITATIONS.

Aircraft are designed to carry a prescribed maximum mass. Furthermore, the force acting on the aircraft’s mass (i.e. its weight) must also act through a point (the aircraft’s centre of gravity), which lies within prescribed limits along the fore and aft axis of the airframe structure.

The point through which the weight acts is called the Centre of Gravity .

To operate legally in the United Kingdom (UK), an aircraft must have a valid Certificate of Airworthiness or Permit to Fly. These documents, directly, or by reference to an approved Flight Manual or Pilot’s Operating Handbook, lay down various limitations on mass, and the range of Centre of Gravity (CG) positions, within which the aircraft must be operated.

An increase

in gross mass (weight) will

lead to an

increase in an aircraft’s stall speed, take-off run and landing run.

Increasing an aircraft’s gross weight will reduce rate of climb.

Figure 1.3 Weight is the force acting on the aircraft’s mass. Weight acts vertically downwards through the aircraft’s Centre of Gravity.

The limitations on mass are set in order to ensure adequate margins of safety with regard to the strength of the airframe structure and so that the aircraft can meet the in-flight performance specified by the designer.

Limitations on the CG position ensure adequate stability and controllability of the aircraft.

Flight performance is also affected by CG position because of changes in drag which arise as CG position changes.

If an aircraft exceeds the mass limit, the aircraft is said to be overloaded. If the CG is not within the specified range, the CG is said to be out of limits. Both situations would be detrimental to flight safety.

The effects of overloading an aircraft are as follows.

•Structural safety margins will be reduced.

•Reduced acceleration and increased speed and distance required on takeoff.

•Flight performance is reduced, with reductions in rate and angle of climb, service ceiling, maximum speed, range and endurance.

•Manoeuvrability and controllability will be impared.

•Stalling speed of the aircraft will be increased.

•Increased landing speed and landing run.

•Excessive loading on the undercarriage will also increase wear on tyres and brakes.

Because of the dangerous consequences of overloading an aircraft, maximum mass or weight limits are applied to aircraft, depending on their structural strength, their operational role, and even for specific manoeuvres.

The Pilot-in-Command has a legal obligation to ensure that mass limitations are not exceeded and that the aircraft’s CG is within limits.

MAXIMUM TAKE-OFF MASS (MTOM).

The maximum limits of mass for flight which are permitted for a particular aircraft will be stated in the Certificate of Airworthiness, Permit to Fly or Pilot’s Operating

Handbook.

Figure 1.4 Maximum Take-Off Mass (MTOM) is the maximum permissible total mass of the aircraft at the beginning of the take-off run.

The Maximum Take-Off Mass (MTOM) is defined as the maximum permissible total aircraft mass at the beginning of the take-off run.

It may be permissible for the MTOM to be exceeded by a small amount, while the aircraft is parked on the ramp or while taxing to the take-off point. This level of mass is known as Maximum Ramp Mass. The extra mass, above MTOM, is carried as fuel. During taxi, this fuel is burnt off and the aircraft arrives at the start of the takeoff run at its prescribed MTOM. Not all aircraft have a specified Maximum Ramp

Mass, usually only commercial aircraft; but you need to be aware of the term and its definition.

NB.: Taking off from a given aerodrome in given runway, atmospheric and weather conditions may impose a performance-related Maximum TakeOff Mass, quite independent of the structurally-imposed Maximum Take-Off Mass.

Remember that operations from grass airfields, especially where the grass is long and/or wet, will impose performance limitations which must be considered along with MTOM. If a runway has a pronounced slope and the temperature and altitude are high, further performance considerations are imposed.

The Pilot-in-

Command has a legal

obligation

to ensure that the aircraft’s Maximum Authorised Take-Off Mass is not exceeded, and that the CG is within limits.

All definitions

of mass limitations

may be

examined in the PPL ground examination.

Figure 1.6 Aircraft with CG outside its forward limit.

Furthermore, with the CG outside the forward limit, the elevators may have to be permanently displaced upwards in order to provide sufficient balancing download from the tailplane for straight and level flight. This situation leads to increased form drag from the control surfaces and, because of the increased wing lift required to balance the tailplane download, increased induced drag, too.

Reductions in aircraft performance resulting from a CG outside its forward limit are:

•Stalling speed will be increased because of the increased wing-lift required.

•Longitudinal stability is increased, leading to a requirement for higher stick forces in pitch.

•Range and endurance are decreased because of the increased form and induced drag.

When an

aircraft’s CG is outside

its forward

limit, longitudinal stability is increased leading to a requirement for higher stick forces.

When an aircraft’s CG

is outside its forward limit,

its nose-up pitch range is decreased.

•The nose-up pitch range is decreased because some elevator deflection has already been used to trim the aircraft for straight and level flight.

•Decreased elevator authority, especially at low forward speed. This is especially important for rotation and round-out on take-off and landing.

CG O u t s i d e Af t Li m i t .

With the CG further aft than the aft limit the tailplane’s moment arm is shorter than usual, making the tailplane less effective. The aircraft’s longitudinal stability is, therefore, decreased, and the aircraft will, consequently, become more sensitive in the pitching plane when the pilot operates the elevators, especially at low airspeed.

Figure 1.7 Aircraft with CG outside of its aft limit.

Furthermore, with the CG outside its aft limit, the elevators may have to be permanently displaced downwards in order to provide sufficient balancing upload from the tailplane for straight and level flight. This will increase form drag.

The principal effects on performance from a CG outside the aft limit are:

When an aircraft’s CG

is outside its aft limit, elevator forces will be light,

the aircraft may be unstable and show a greater tendency to spin.

If an aircraft’s CG is outside

limits, its range will be

decreased.

•Longitudinal stability is reduced.

•If the CG is too far aft, the aircraft will become sensitive in the pitching plane. Stick forces in pitch will be light, and a small elevator movement will produce a larger than normal change in pitch attitude.

•The aircraft will tend to pitch up at low speed and high power setting. This may lead to over-rotation on take-off, or to an inadvertent stall in the climb.

•The aircraft will be difficult to trim, especially at high power settings.

•The aircraft will be more difficult to recover from a spin.

•Range and endurance will usually decrease due to the increased form drag from the elevator.