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moisture absorbed in the cabin insulation. This growth in
weight is normally small, but it can only be determined by
accurately weighing the aircraft.
1–
Changes of fixed equipment may have a major effect upon
the weight of the aircraft. Many aircraft are overloaded by
the installation of extra radios or instruments. Fortunately,
the replacement of older, heavy electronic equipment with
newer, lighter types results in a weight reduction. This
weight change, however helpful, will probably cause the
CG to shift and this must be computed and annotated in
the weight and balance record.
Repairs and alteration are the major sources of weight
changes, and it is the responsibility of the A&P mechanic
or repairman making any repair or alteration to know the
weight and location of these changes, and to compute the
CG and record the new empty weight and EWCG in the
aircraft weight and balance record.
If the newly calculated EWCG should happen to fall
outside the EWCG range, it will be necessary to perform
adverse loading check. This will require a forward and
rearward adverse-loading check, and a maximum weight
check. These weight and balance extreme conditions
represent the maximum forward and rearward CG position
for the aircraft. Adverse loading checks are a deliberate
attempt to load an aircraft in a manner that will create the
most critical balance condition and still remain within
the design CG limits of the aircraft. If any of the checks
fall outside the loaded CG range, the aircraft must be
reconfigured or placarded to prevent the pilot from loading
the aircraft improperly. It is sometimes possible to install
fixed ballast in order for the aircraft to again operate within
the normal CG range.
The A&P mechanic or repairman conducting an annual or
condition inspection must ensure the weight and balance
data in the aircraft records is current and accurate. It
is the responsibility of the pilot in command to use the
most current weight and balance data when operating the
aircraft.
Stability and Balance Control
Balance control refers to the location of the CG of an
aircraft. This is of primary importance to aircraft stability,
which determines safety in flight.
The CG is the point at which the total weight of the aircraft
is assumed to be concentrated, and the CG must be located
within specific limits for safe flight. Both lateral and
longitudinal balance are important, but the prime concern
is longitudinal balance; that is, the location of the CG
along the longitudinal or lengthwise axis.
An airplane is designed to have stability that allows it to
be trimmed so it will maintain straight and level flight with
hands off the controls. Longitudinal stability is maintained
by ensuring the CG is slightly ahead of the center of lift.
This produces a fixed nose-down force independent of
the airspeed. This is balanced by a variable nose-up force,
which is produced by a downward aerodynamic force on
the horizontal tail surfaces that varies directly with the
airspeed. [Figure 1-1]
Figure 1-1. Longitudinal forces acting on an airplane in flight.
If a rising air current should cause the nose to pitch up, the
airplane will slow down and the downward force on the
tail will decrease. The weight concentrated at the CG will
pull the nose back down. If the nose should drop in flight,
the airspeed will increase and the increased downward tail
load will bring the nose back up to level flight.
As long as the CG is maintained within the allowable
limits for its weight, the airplane will have adequate
longitudinal stability and control. If the CG is too far aft,
it will be too near the center of lift and the airplane will
be unstable, and difficult to recover from a stall. [Figure
1-2] If the unstable airplane should ever enter a spin, the
spin could become flat and recovery would be difficult or
impossible.
Figure 1-2. If the CG is too far aft at the low stall airspeed, there
might not be enough elevator nose-down authority to get the nose
down for recovery.
If the CG is too far forward, the downward tail load will
have to be increased to maintain level flight. This increased
tail load has the same effect as carrying additional weight;
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the aircraft will have to fly at a higher angle of attack, and
drag will increase.
A more serious problem caused by the CG being too far
forward is the lack of sufficient elevator authority. At slow
takeoff speeds, the elevator might not produce enough
nose-up force to rotate and on landing there may not be
enough elevator force to flare the airplane. [Figure 1-3]
　
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