Figure 1-1. Longitudinal forces acting on an airplane in flight.
Lift
Variable
Nose-up force
dependent upon airspeedFixed
Nose-down force
independent of airspeed
CG
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.
Lift
Insufficient elevator
Nose-down force
CG too far aft
CG
Figure 1-3. If the CG is too far forward, there is not enough elevator
nose-up force to flare the airplane for landing.
Lift
Insufficient elevator
Nose-up force
CG too far forward
CG
the aircraft must be reconfigured or placarded to prevent the
pilot from loading the aircraft improperly. It is sometimes
possible to install a fixed ballast in order for the aircraft to
operate again within the normal CG range.
The FAA-certificated 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 PIC 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 is a
factor in flight safety. 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 to 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]
If a rising air current should cause the nose to pitch up, the
airplane slows and the downward force on the tail decreases.
The weight concentrated at the CG pulls the nose back down.
If the nose should drop in flight, the airspeed increases and
the increased downward tail load brings the nose back up
to level flight
As long as the CG is maintained within the allowable limits
for its weight, the airplane has adequate longitudinal stability
and control. If the CG is too far aft, it is too near the center
of lift; the airplane is unstable and difficult to recover from
a stall. [Figure 1-2] If the unstable airplane should enter a
spin, the spin could become flat making recovery difficult
or impossible. If the CG is too far forward, the downward
tail load needs to be increased to maintain level flight. This
increased tail load has the same effect as carrying additional
weight; the aircraft must fly at a higher angle of attack and
drag increases.
A more serious problem caused by the CG being too far
forward is the lack of sufficient elevator authority. At low
takeoff speeds, the elevator might not produce enough
nose-up force to rotate; on landing there may not be enough
elevator force to flare the airplane. [Figure 1-3] Both takeoff
and landing runs are lengthened if the CG is too far forward.
The basic aircraft design is such that lateral symmetry is
assumed to exist. For each item of weight added to the left
of the center line of the aircraft (also known as buttock
line zero or BL -0), there is generally an equal weight at a
corresponding location on the right.
The lateral balance can be upset by uneven fuel loading
or burnoff. The position of the lateral CG is not normally
computed for an airplane, but the pilot must be aware of
Figure 1-4. Lateral imbalance causes wing heaviness, which may
be corrected by deflecting the aileron. The additional lift causes
additional drag, and the airplane flies inefficiently.
Empty Full
Additional lift and drag
Additional weight
Figure 1-5. Fuel in the tanks of a swept-wing airplane affects both
lateral and longitudinal balance. As fuel is used from an outboard
tank, the CG shifts forward.
Inboard fuel: nose heavy
Outboard fuel: tail heavy
the adverse effects that result from a laterally unbalanced
condition. [Figure 1-4] This is corrected by using the aileron
trim tab until enough fuel has been used from the tank on
the heavy side to balance the airplane. The deflected trim tab
deflects the aileron to produce additional lift on the heavy
side, but it also produces additional drag, and the airplane
flies inefficientl
Helicopters are more often affected by lateral imbalance
than airplanes. If a helicopter is loaded with heavy occupants
and fuel on the same side, it could be out of balance enough
to make it unsafe to fly. It is also possible that if external
loads are carried in such a position that requires large lateral
displacement of the cyclic control to maintain level fl ght, the
fore-and-aft cyclic control effectiveness is limited.
Swept-wing airplanes are more critical due to fuel imbalance
because as the fuel is used from the outboard tanks, the CG
shifts forward. As fuel is used from the inboard tanks, the CG
shifts aft. [Figure 1-5] For this reason, fuel-use scheduling
in swept-wing airplanes operation is critical.
Weight Control for Aircraft Other Than
Fixed and Rotor-wing
Some light aircraft utilize different methods of determining
weight and balance from the traditional fixed and rotor-wing
aircraft. These aircraft achieve flight control by methods
different from the fixed-wing airplane or helicopter. Most
notable of these are weight-shift control (WSC) aircraft (also
known as trikes), powered parachutes, and balloons. These
aircraft typically do not specify either an EWCG or a CG
range. They require only a certified or approved maximum
weight. To understand why this is so, a look at how flight
control is achieved is helpful.
Airplanes and WSC aircraft control flight under the influenc
of the same four forces (lift, gravity, thrust, and drag), and
around the same three axes (pitch, yaw, and roll). However,
each aircraft accomplishes this control in a very different
manner. This difference helps explain why the fixed-wing
airplane requires an established weight and a known CG,
whereas the WSC aircraft only requires the known weight.
The fixed-wing airplane has movable controls that alter lift
on various airfoil surfaces to vary pitch, roll, and yaw. In
turn, these changes in lift affect the characteristics of the
flight parameters. Weight normally decreases in flight due
to fuel consumption, and the airplane CG changes with this
weight reduction. An airplane utilizes its variable fligh
controls to compensate and maintain controllability through
the various flight modes and as the CG changes. An airplane
has a CG range or envelope within which it must remain if
the flight controls are to remain effective and the airplane
safely operated.
Weight-Shift Control Aircraft
The WSC aircraft has a relatively set platform wing without
a tail. The pilot achieves control by shifting weight. In the
design of this aircraft, the weight of the airframe and its
payload is attached to the wing at a single point in a pendulous
arrangement. The pilot, through the flight controls, controls the
arm of this pendulum and thereby controls the aircraft. When
a change in flight parameter is desired, the pilot displaces the
aircraft’s weight by the appropriate distance and direction.
This change momentarily disrupts the equilibrium between the
four forces acting on the aircraft. The wing, due to its inherent
stability, then moves appropriately to reestablish the desired
relationship between these forces; the wing flexes and alter
its shape. As the shape is changed, lift is varied at different
points on the wing to achieve the desired flight parameters
The flight controls primarily affect the pitch-and-roll axes.
Since there is no vertical tail plane, there is minimal or no
yaw control. Unlike in an airplane, the CG experienced by
the WSC aircraft wing remains constant. Since the weight
of the airframe acts through a single point (the wing attach
point), the range over which the weight may act is fixed at
the pendulum arm or length. Even though weight decreases
as fuel is consumed, weight remains focused at the wing
attach point. Because the range is fixed, there is no need to
establish a calculated range. The pilot should consult the
POH or Aircraft Operating Instructions (AOI) for maximum
takeoff weight and minimum and maximum seat weight
limits prior to each flight
Powered Parachutes
The powered parachute is also a pendulum-style aircraft. Its
airframe CG is fixed at the pendulum attach point. It is more
limited in controllability than the WSC aircraft because it
lacks an aerodynamic pitch control. Pitch (and lift) control
is primarily a function of the power control. Increased power
results in increased lift; cruise power amounts to level flight;
decreased power causes a descent. Due to this characteristic,
the aircraft is basically a one-airspeed aircraft. Once again,
because the CG is fixed at the attach point to the wing, there
is no CG range. As with WSC, the pilot should consult the
POH or AOI for maximum takeoff weight and minimum and
maximum seat weight limits prior to each flight
Roll control on a powered parachute is achieved by changing
the shape of the wing. The change is achieved by varying
the length of steering lines attached to the outboard trailing
edges of the wing. The trailing edge of the parachute is pulled
down slightly on one side or the other to create increased
drag along that side. This change in drag creates roll and
yaw, permitting the aircraft to be steered.
Balloons
The balloon is controlled by the pilot only in the vertical
dimension; this is in contrast to all other aircraft. He or she
achieves this control through the use of lift and weight.
Wind provides all other movement. The CG of the gondola
remains constant beneath the balloon envelope. As in WSC
and powered-parachute aircraft, there is no CG limitation.
Underestimating the Importance of
Weight and Balance
Many pilots, from sport pilot to commercial pilot, tend to
underestimate the importance of proper weight and balance
of their aircraft. Load sheets are taken for granted and hasty
calculations are made of the aircraft’s CG. Unfortunately,
each year there are a number of accidents related to weight
and balance issues. Many of these occurrences could have
been avoided had more attention been given to weight and
balance.
Every student pilot is taught how to work a weight and
balance problem and that it is important to make sure
every flight is loaded “within the envelope” (no more than
maximum gross weight) for both takeoff and landing. But
does he or she really understand just why this is so and the
disastrous effect of being out of the envelope? Two examples
of documented cases are provided below in an effort to
indicate the serious nature of maintaining the proper weight
and balance. In case studies when weight and balance was
listed as the major factor of the accident, many were fatal.
For instance, a small aircraft was loaded with hunters, gear,
and dogs (none of the dogs were secured inside the aircraft).
During takeoff, all the dogs went to the aft of the airplane.
This shifted the CG well aft of its allowable limit. The
airplane stalled and crashed. The airplane was destroyed
with casualties.
Another accident occurred when a group of skydivers were
sitting on the floor toward the aft portion of the airplane
(they were unsecured). During takeoff, the CG was again
well beyond its aft limit. The airplane stalled and crashed.
The airplane was destroyed with casualties.
There is a safety factor built into the formula for maximum
gross weight. Any airplane can fly when it takes off at a
weight greater than maximum gross weight if the runway
is long enough and the density altitude is low enough.
However, landing is a different matter. All airplanes are built
to withstand an occasional hard landing, but what would
happen if the hard landing were combined with a substantially
overweight airplane? Something would probably break at that
time or the structure would be weakened enough to break
sometime in the future when everything might seem normal to
a pilot unaware of the previous situation. Even more disastrous
than an overweight, hard landing is reaching or exceeding
the structural integrity of the metal and/or composite design
values when maneuvering or when turbulence is encountered.
Hidden damage could result, causing an unexpected
catastrophic failure at some future time.
If an airplane is certificated with a maximum gross weight
of 6,000 pounds (its weight on the ground) and is rolled into
a 60° bank, the forces exerted make it feel as if it weighed
12,000 pounds. At its maximum certificated gross weight,
there is no problem because the aircraft is operated within its
certificated maneuvering loads. But loaded to 8,000 pounds
with a 60° bank or an abrupt pullup, it suddenly weighs
16,000 pounds and might not be able to perform! Even if it
could, there would probably be internal stress damage that
would show up on future flights