Introduction
Compliance with the weight and balance limits of any aircraft
is critical to flight safety. Operating above the maximum
weight limitation compromises the structural integrity of
an aircraft and adversely affects its performance. Operation
with the center of gravity (CG) outside the approved limits
results in control difficulty.
Weight Control
As discussed in Chapter 5, Aerodynamics of Flight, weight
is the force with which gravity attracts a body toward the
center of the Earth. It is a product of the mass of a body and
the acceleration acting on the body. Weight is a major factor
in aircraft construction and operation and demands respect
from all pilots.
The force of gravity continuously attempts to pull an aircraft
down toward Earth. The force of lift is the only force that
counteracts weight and sustains an aircraft in flight. The
amount of lift produced by an airfoil is limited by the airfoil
design, angle of attack (AOA), airspeed, and air density. To
assure that the lift generated is sufficient to counteract weight,
loading an aircraft beyond the manufacturer’s recommended
weight must be avoided. If the weight is greater than the lift
generated, the aircraft may be incapable of flight.
Weight and Balance
Chapter 10
Lateral unbalance will cause wing heaviness.
Empty Full
Excess baggage
Longitudinal unbalance will cause
either nose or tail heaviness.
Figure 10-1. Lateral and longitudinal unbalance.
Effects of Weight
Any item aboard an aircraft that increases the total weight is
undesirable for performance. Manufacturers attempt to make
an aircraft as light as possible without sacrificing strength
or safety.
The pilot should always be aware of the consequences of
overloading. An overloaded aircraft may not be able to leave
the ground, or if it does become airborne, it may exhibit
unexpected and unusually poor flight characteristics. If not
properly loaded, the initial indication of poor performance
usually takes place during takeoff.
Excessive weight reduces the flight performance in almost
every respect. For example, the most important performance
deficiencies of an overloaded aircraft 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 approach and landing speed
• Longer landing roll
• Excessive weight on the nose wheel or tail wheel
The pilot must be knowledgeable about the effect of weight
on the performance of the particular aircraft being flown.
Preflight planning should include a check of performance
charts to determine if the aircraft’s weight may contribute
to hazardous flight operations. Excessive weight in itself
reduces the safety margins available to the pilot and becomes
even more hazardous when other performance-reducing
factors are combined with excess weight. The pilot must
also consider the consequences of an overweight aircraft if
an emergency condition arises. If an engine fails on takeoff
or airframe ice forms at low altitude, it is usually too late to
reduce an aircraft’s weight to keep it in the air.
Weight Changes
The operating weight of an aircraft can be changed by
simply altering the fuel load. Gasoline has considerable
weight—6 pounds per gallon. Thirty gallons of fuel may
weigh more than one passenger. If a pilot lowers airplane
weight by reducing fuel, the resulting decrease in the range
of the airplane must be taken into consideration during flight
planning. During flight, fuel burn is normally the only weight
change that takes place. As fuel is used, an aircraft becomes
lighter and performance is improved.
Changes of fixed equipment have a major effect upon the
weight of an aircraft. The installation of extra radios or
instruments, as well as repairs or modifications, may also
affect the weight of an aircraft.
Balance, Stability, and Center of Gravity
Balance refers to the location of the CG of an aircraft, and is
important to stability and safety in flight. The CG is a point
at which the aircraft would balance if it were suspended at
that point.
The primary concern in balancing an aircraft is the fore
and aft location of the CG along the longitudinal axis. The
CG is not necessarily a fixed point; its location depends on
the distribution of weight in the aircraft. As variable load
items are shifted or expended, there is a resultant shift in
CG location. The distance between the forward and back
limits for the position of the center for gravity or CG range
is certified for an aircraft by the manufacturer. The pilot
should realize that if the CG is displaced too far forward
on the longitudinal axis, a nose-heavy condition will
result. Conversely, if the CG is displaced too far aft on the
longitudinal axis, a tail heavy condition results. It is possible
that the pilot could not control the aircraft if the CG location
produced an unstable condition. [Figure 10-1]
Location of the CG with reference to the lateral axis is also
important. For each item of weight existing to the left of
the fuselage centerline, there is an equal weight existing at
a corresponding location on the right. This may be upset
by unbalanced lateral loading. The position of the lateral
CG is not computed in all aircraft, but the pilot must be
aware that adverse effects arise as a result of a laterally
unbalanced condition. In an airplane, lateral unbalance occurs
if the fuel load is mismanaged by supplying the engine(s)
unevenly from tanks on one side of the airplane. The pilot
can compensate for the resulting wing-heavy condition by
adjusting the trim or by holding a constant control pressure.
This action places the aircraft controls in an out-of-streamline
condition, increases drag, and results in decreased operating
efficiency. Since lateral balance is addressed when needed in
the aircraft flight manual (AFM) and longitudinal balance is
more critical, further reference to balance in this handbook
means longitudinal location of the CG.
Flying an aircraft that is out of balance can 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. Excessive trim, however, has the effect of reducing
not only aerodynamic efficiency but also primary control
travel distance in the direction the trim is applied.
Effects of Adverse Balance
Adverse balance conditions affect flight characteristics in
much the same manner as those mentioned for an excess
weight condition. It is vital to comply with weight and
balance limits established for all aircraft. Operating above
the maximum weight limitation compromises the structural
integrity of the aircraft and can adversely affect performance.
Stability and control are also affected by improper balance.
Stability
Loading in a nose-heavy condition causes problems in
controlling and raising the nose, especially during takeoff
and landing. Loading in a tail heavy condition has a serious
effect upon longitudinal stability, and reduces the capability
to recover from stalls and spins. Tail heavy loading also
produces very light control forces, another undesirable
characteristic. This makes it easy for the pilot to inadvertently
overstress an aircraft.
Stability and Center of Gravity
Limits for the location of the CG are established by the
manufacturer. These are the fore and aft limits beyond
which the CG should not be located for flight. These limits
are published for each aircraft in the Type Certificate Data
Sheet (TCDS), or aircraft specification and the AFM or
pilot’s operating handbook (POH). If the CG is not within the
allowable limits after loading, it will be necessary to relocate
some items before flight is attempted.
The forward CG limit is often established at a location that
is determined by the landing characteristics of an aircraft.
During landing, one of the most critical phases of flight,
exceeding the forward CG limit may result in excessive loads
on the nosewheel, a tendency to nose over on tailwheel type
airplanes, decreased performance, higher stalling speeds, and
higher control forces.
Control
In extreme cases, a CG location that is beyond the forward
limit may result in nose heaviness, making it difficult or
impossible to flare for landing. Manufacturers purposely
place the forward CG limit as far rearward as possible to
aid pilots in avoiding damage when landing. In addition to
decreased static and dynamic longitudinal stability, other
undesirable effects caused by a CG location aft of the
allowable range may include extreme control difficulty,
violent stall characteristics, and very light control forces
which make it easy to overstress an aircraft inadvertently.
A restricted forward CG limit is also specified to assure
that sufficient elevator/control deflection is available at
minimum airspeed. When structural limitations do not limit
the forward CG position, it is located at the position where
full-up elevator/control deflection is required to obtain a high
AOA for landing.
The aft CG limit is the most rearward position at which the
CG can be located for the most critical maneuver or operation.
As the CG moves aft, a less stable condition occurs, which
decreases the ability of the aircraft to right itself after
maneuvering or turbulence.
For some aircraft, both fore and aft CG limits may be
specified to vary as gross weight changes. They may also
be changed for certain operations, 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 CG can be altered by many variable
factors and is usually controlled by the pilot. Placement of
baggage and cargo items determines the CG location. The
assignment of seats to passengers can also be used as a means
of obtaining a favorable balance. If an aircraft is tail heavy,
it is only logical to place heavy passengers in forward seats.
Fuel burn can also affect the CG based on the location of the
fuel tanks. For example, most small aircraft carry fuel in the
wings very near the CG and burning off fuel has little effect
on the loaded CG.