Aircraft Preparation for Weighing

Aircraft Preparation for Weighing

The major considerations in preparing an aircraft for
weighing are discussed below.

Weigh Clean Aircraft Inside Hangar

The aircraft should be weighed inside a hangar where wind
cannot blow over the surface and cause fluctuating or false
scale readings.

The aircraft should be clean inside and out, with special
attention paid to the bilge area to be sure no water or
debris is trapped there, and the outside of the aircraft
should be as free as possible of all mud and dirt.
Equipment List

All of the required equipment must be properly installed,
and there should be no equipment installed that is not
included in the equipment list. If such equipment is
installed, the weight and balance record must be corrected
to indicate it.

Ballast

All required permanent ballast must be properly secured in
place and all temporary ballast must be removed.

Draining the Fuel

Drain fuel from the tanks in the manner specified by the
aircraft manufacturer. If there are no specific instructions,
drain the fuel until the fuel quantity gauges read empty
when the aircraft is in level-flight attitude. Any fuel
remaining in the system is considered residual, or unusable
fuel and is part of the aircraft empty weight.

If it is not feasible to drain the fuel, the tanks can be
topped off to be sure of the quantity they contain and the
aircraft weighed with full fuel. After weighing is complete,
the weight of the fuel and its moment are subtracted from
those of the aircraft as weighed. To correct the empty
weight for the residual fuel, add its weight and moment.

The amount of residual fuel and its arm are normally found
in NOTE 1 in the section of the TCDS, “Data pertaining to
all Models.” See “Fuel Capacity” on page 2-10.

When computing the weight of the fuel, for example
a tank full of jet fuel, measure its specific gravity (sg)
with a hydrometer and multiply it by 8.345 (the nominal
weight of 1 gallon of pure water whose s.g. is 1.0). If the
ambient temperature is high and the jet fuel in the tank
is hot enough for its specific gravity to reach 0.81 rather
than its nominal s.g. of 0.82, the fuel will actually weigh
6.76 pounds per gallon rather than its normal weight of
6.84 pounds per gallon. The standard weight of aviation
gasoline (Avgas) is 6 pounds per gallon.

Oil

The empty weight for aircraft certificated under the CAR,
part 3 does not include the engine lubricating oil. The
oil must either be drained before the aircraft is weighed,
or its weight must be subtracted from the scale readings
to determine the empty weight. To weigh an aircraft that
does not include the engine lubricating oil as part of the
empty weight, place it in level flight attitude, then open the
drain valves and allow all of the oil that is able, to drain
out. Any remaining is undrainable oil, and is part of the
empty weight. Aircraft certificated under 14 CFR parts
23 and 25 include full oil as part of the empty weight. If
it is impractical to drain the oil, the reservoir can be filled
to the specified level and the weight of the oil computed
at 7.5 pounds per gallon. Then its weight and moment are
subtracted from the weight and moment of the aircraft as
weighed. The amount and arm of the undrainable oil are
found in NOTE 1 of the TCDS, and this must be added to
the empty weight.

Other Fluids

The hydraulic fluid reservoir and all other reservoirs
containing fluids required for normal operation of the
aircraft should be full. Fluids not considered to be part of
the empty weight of the aircraft are potable (drinkable)
water, lavatory precharge water, and water for injection
into the engines.

Configuration of the Aircraft

Consult the aircraft service manual regarding position of
the landing gear shock struts and the control surfaces for
weighing; when weighing a helicopter, the main rotor must
be in its correct position.

Jacking the Aircraft

Aircraft are often weighed by rolling them onto ramps
in which load cells are embedded. This eliminates the
problems associated with jacking the aircraft off the
ground. However, many aircraft are weighed by jacking
the aircraft up and then lowering them onto scales or load
cells.

Extra care must be used when raising an aircraft on jacks
for weighing. If the aircraft has spring steel landing gear
and it is jacked at the wheel, the landing gear will slide
inward as the weight is taken off of the tire, and care must
be taken to prevent the jack from tipping over.

For some aircraft, stress panels or plates must be installed
before they are raised with wing jacks, to distribute
the weight over the jack pad. Be sure to follow the
recommendations of the aircraft manufacturer in detail
anytime an aircraft is jacked. When using two wing jacks,
take special care to raise them simultaneously, keeping
the aircraft so it will not slip off the jacks. As the jacks are
raised, keep the safety collars screwed down against the
jack cylinder to prevent the aircraft from tilting if one of
the jacks should lose hydraulic pressure.

Leveling the Aircraft

When an aircraft is weighed, it must be in its level
flight attitude so that all of the components will be at
their correct distance from the datum. This attitude is
determined by information in the TCDS. Some aircraft
require a plumb line to be dropped from a specified
location so that the point of the weight, the bob, hangs
directly above an identifiable point. Others specify that a
spirit level be placed across two leveling lugs, often special
screws on the outside of the fuselage. Other aircraft call
for a spirit level to be placed on the upper door sill.
Lateral level is not specified for all light aircraft,
but provisions are normally made on helicopters for
determining both longitudinal and lateral level. This may
be done by built-in leveling indicators, or by a plumb bob
that shows the conditions of both longitudinal and lateral
level.

The actual adjustments to level the aircraft using load cells
are made with the jacks. When weighing from the wheels,
leveling is normally done by adjusting the air pressure in
the nose wheel shock strut.

Equipment for Weighing

Equipment for Weighing

There are two basic types of scales used to weigh aircraft:
scales on which the aircraft is rolled so that the weight is
taken at the wheels, and electronic load cells type where a
pressure sensitive cell are placed between the aircraft jack
and the jack pads on the aircraft.

Some aircraft are weighed with mechanical scales of the
low-profile type similar to those shown in Figure 3-1.
Large aircraft, including heavy transports, are weighed
by rolling them onto weighing platforms with electronic
weighing cells that accurately measure the force applied
by the weight of the aircraft.

Electronic load cells are used when the aircraft is weighed
by raising it on jacks. The cells are placed between the
jack and the jack pad on the aircraft, and the aircraft is
raised on the jacks until the wheels or skids are off the
floor and the aircraft is in a level flight attitude. The weight
measured by each load cell is indicated on the control
panel.

Mechanical scales should be protected when they are not
in use, and they must be periodically checked for accuracy
by measuring a known weight. Electronic scales normally
have a built-in calibration that allows them to be accurately
zeroed before any load is applied.

Manufacturer-Furnished Information

Manufacturer-Furnished Information

When an aircraft is initially certificated, its empty weight
and EWCG are determined and recorded in the weight and
balance record such as the one in Figure 2-21. Notice in
this figure that the moment is expressed as “Moment (lbin/
1000).” This is a moment index which means that the
moment, a very large number, has been divided by 1,000
to make it more manageable. Chapter 4 discusses moment
indexes in more detail.

An equipment list is furnished with the aircraft, which
specifies all the required equipment, and all equipment
approved for installation in the aircraft. The weight and
arm of each item is included on the list, and all equipment
installed when the aircraft left the factory is checked.
When an aircraft mechanic or repairman adds or removes
any item on the equipment list, he or she must change
the weight and balance record to indicate the new empty
weight and EWCG, and the equipment list is revised to
show which equipment is actually installed. Figure 2-22
is an excerpt from a comprehensive equipment list that
includes all of the items of equipment approved for this
particular model of aircraft. The POH for each individual
aircraft includes an aircraft specific equipment list of
the items from this master list. When any item is added
to or removed from the aircraft, its weight and arm are
determined in the equipment list and used to update the
weight and balance record.

The POH/AFM also contains CG moment envelopes and
loading graphs. Examples of the use of these handy graphs
are given in chapter 4.

Weight and Balance Documentation

Weight and Balance Documentation

FAA-Furnished Information

Before an aircraft can be properly weighed and its emptyweight
center of gravity computed, certain information
must be known. This information is furnished by the
FAA to anyone for every certificated aircraft in the Type
Certificate Data Sheets (TCDS) or Aircraft Specifications
and can be accessed via the internet at: www.faa.gov
(home page), from that page, select “ Regulations and
Policies,” and at that page, select “Regulatory and
Guidance Library.” This is the official FAA technical
reference library.

When the design of an aircraft is approved by the FAA,
an Approved Type Certificate and TCDS are issued. The
TCDS includes all of the pertinent specifications for the
aircraft, and at each annual or 100-hour inspection, it is
the responsibility of the inspecting mechanic or repairman
to ensure that the aircraft adheres to them. See pages 2-
7 through 2-9, for examples of TCDS excerpts. A note
about the TCDS: aircraft certificated before January 1,

1958, were issued Aircraft Specifications under the Civil
Air Regulations (CARs), but when the Civil Aeronautical
Administration (CAA) was replaced by the FAA, Aircraft
Specifications were replaced by the Type Certificate Data
Sheets. The weight and balance information on a TCDS
includes the following:

Data Pertinent to Individual Models
This type of information is determined in the sections
pertinent to each individual model:

CG Range

Normal Category

(+82.0) to (+93.0) at 2,050 pounds.
(+87.4) to (+93.0) at 2,450 pounds.
Utility Category
(+82.0) to (+86.5) at 1,950 pounds.
Straight-line variations between points given.

Shifting the CG

Shifting the CG

One common weight and balance problem involves
moving passengers from one seat to another or shifting
baggage or cargo from one compartment to another to
move the CG to a desired location. This also can be
visualized by using a board with three weights and then
working out the problem the way it is actually done on an
airplane.

Solution by Chart

The CG of a board can be moved by shifting the weights
as demonstrated in Figure 2-10. As the board is loaded,
it balances at a point 72 inches from the CG of weight A.

Determining the CG

Determining the CG

One of the easiest ways to understand weight and balance
is to consider a board with weights placed at various
locations. We can determine the CG of the board and
observe the way the CG changes as the weights are moved.
The CG of a board like the one in Figure 2-4 may be
determined by using these four steps:

1. Measure the arm of each weight in inches from the
datum.

2. Multiply each arm by its weight in pounds to determine
the moment in pound-inches of each weight.

3. Determine the total of all weights and of all the
moments. Disregard the weight of the board.

4. Divide the total moment by the total weight to
determine the CG in inches from the datum.

The Law of the Lever

The Law of the Lever

The weight and balance problems are based on the
physical law of the lever. This law states that a lever is
balanced when the weight on one side of the fulcrum
multiplied by its arm is equal to the weight on the opposite
side multiplied by its arm. In other words, the lever is
balanced when the algebraic sum of the moments about the
fulcrum is zero. [Figure 2-2] This is the condition in which
the positive moments (those that try to rotate the lever
clockwise) are equal to the negative moments (those that
try to rotate it counter-clockwise).

Aircraft Arms, Weight, and Moments

Aircraft Arms, Weight, and Moments

The term arm, usually measured in inches, refers to the
distance between the center of gravity of an item or object
and the datum. Arms ahead of, or to the left of the datum
are negative(-), and those behind, or to the right of the
datum are positive(+). When the datum is ahead of the
aircraft, all of the arms are positive and computational
errors are minimized. Weight is normally measured in
pounds. When weight is removed from an aircraft, it is
negative(-), and when added, it is positive (+).

The manufacturer establishes the maximum weight and
range allowed for the CG, as measured in inches from the
reference plane called the datum. Some manufacturers
specify this range as measured in percentage of the mean
aerodynamic chord (MAC), the leading edge of which is
located a specified distance from the datum.

The datum may be located anywhere the manufacturer
chooses; it is often the leading edge of the wing or some
specific distance from an easily identified location. One
popular location for the datum is a specified distance
forward of the aircraft, measured in inches from some
point, such as the nose of the aircraft, or the leading edge
of the wing, or the engine firewall.
The datum of some helicopters is the center of the rotor
mast, but this location causes some arms to be positive
and others negative. To simplify weight and balance
computations, most modern helicopters, like airplanes,
have the datum located at the nose of the aircraft or a
specified distance ahead of it.

A moment is a force that tries to cause rotation, and is the
product of the arm, in inches, and the weight, in pounds.
Moments are generally expressed in pound-inches (lb-in)
and may be either positive or negative. Figure 2-1 shows
the way the algebraic sign of a moment is derived. Positive
moments cause an airplane to nose up, while negative
moments cause it to nose down.

Weight and Balance Theory

Weight and Balance Theory

Two elements are vital in the weight and balance
considerations of an aircraft.

• The total weight of the aircraft must be no greater
than the maximum weight allowed by the FAA for the
particular make and model of the aircraft.
• The center of gravity, or the point at which all of the
weight of the aircraft is considered to be concentrated,
must be maintained within the allowable range for the
operational weight of the aircraft.

Weight Control for Aircraft other than Fixed and Rotorwing

Weight Control for Aircraft other than Fixed and Rotorwing

Some light aircraft utilize different methods of determining
weight and balance from the traditional fixed and
rotorwing aircraft. These aircraft achieve flight control
differently than 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 empty
weight center of gravity or a center of gravity 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.

As an example, airplanes and WSC aircraft both control
flight under the influence 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 center of gravity, whereas
the WSC aircraft only requires the known weight.

The fixed-wing airplane has moveable controls that
alter the lift on various airfoil surfaces to vary pitch,
roll, and yaw. These changes in lift, in turn, change the
characteristics of the flight parameters. Weight normally
decreases in flight due to fuel consumption, and the
airplane center of gravity changes with this weight
reduction. An airplane utilizes its variable flight controls
to compensate and maintain controllability through the
various flight modes and as the center of gravity changes.
An airplane has a center of gravity range or envelope
within which it must remain if the flight controls are to
remain effective and the airplane safely operated.

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 in 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 re-establish
the desired relationship between these forces. This happens
by the wing flexing and altering 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
axis. Since there is no vertical tail plane, minimal or no
ability exists to directly control yaw. However, unlike the
airplane, the center of gravity experienced by the wing
remains constant. Since the weight of the airframe acts
through the single point (wing attach point), the range
over which the weight may act is fixed at the pendulum
arm or length. Even though the weight decreases as fuel is
consumed, the weight remains focused at the wing attach
point. Most importantly, because the range is fixed, the
need to establish a calculated range is not required.

The powered parachute also belongs to the pendulumstyle
aircraft. Its airframe center of gravity 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-air speed aircraft. Once again, because the
center of gravity is fixed at the attach point to the wing,
there can be no center of gravity range.

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.

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 center of gravity
of the gondola remains constant beneath the balloon
envelope. As in WSC and powered-parachute aircraft,
there is no center of gravity limitation.

Aircraft can perform safely and achieve their designed
efficiency only when they are operated and maintained in
the way their designers intended. This safety and efficiency
is determined to a large degree by holding the aircraft’s
weight and balance parameters within the limits specified
for its design. The remainder of this handbook describes
the way in which this is done.