Introduction
Chapter 2, Weight and Balance Theory, explained the theory
of weight and balance and gave examples of the way the
center of gravity (CG) could be found for a lever loaded
with several weights. In this chapter, the practical aspects
of weighing an airplane and locating its CG are discussed.
Formulas are introduced that allow the CG location to be
measured in inches from various datum locations and in
percentage of the mean aerodynamic chord (MAC).
Weighing the Aircraft and
Determining the Empty Weight
Center of Gravity
Chapter 3
Figure 3-1. Platform scales.
Requirements
Regulations do not require periodic weighing of privately
owned and operated aircraft. Such aircraft are usually
weighed when originally certificated or after major alterations
that can affect the weight and balance. The primary purpose
of aircraft weight and balance control is safety. Manufacturers
conduct extensive flight tests to establish loading limits
for their aircraft because limit information is critical for
safe flight. A secondary purpose is to aid efficiency during
flight. Overloading of the aircraft is not the only concern;
the distribution of the weight is important also. The aircraft
has CG limits, and any loading that places the CG outside
the established limits seriously impairs controllability of
the aircraft.
Weight and balance is of such vital importance that each
Federal Aviation Administration (FAA) certificated
mechanic or repairman maintaining an aircraft must be fully
aware of his or her responsibility to provide the pilot with
current and accurate information for the actual weight of the
aircraft and the location of the CG. The pilot in command
(PIC) is responsible for knowing the weight of the load, CG,
maximum allowable weight, and CG limits of the aircraft.
The weight and balance report must include an equipment
list showing weights and moment arms of all required and
optional items of equipment included in the certificate
empty weight.
Weight and balance records used in accounting for and
correcting the CG location are reliable for only limited
periods of time. For this reason, periodic aircraft weighing is
desirable. An aircraft should be reweighed and a new weight
and balance record should be started after the aircraft has
undergone extensive repair or major alteration, when the pilot
reports unsatisfactory flight characteristics (e.g., nose or tail
heaviness), and when recorded weight and balance data are
suspected to be in error.
Repairs and alterations are major sources of weight change.
The airframe and powerplant (A&P) FAA-certificated
mechanic or repairman who is responsible for making any
repair or alteration must:
1. Establish by computation that the authorized weight
and CG limits as shown in the type certificate data
sheet (TCDS) and aircraft specifications are not
exceeded, and
2. Record the new empty weight center of gravity
(EWCG) data in the current approved aircraft flight
manual or issued operating limitations.
When an aircraft has undergone extensive repair or major
alteration, it should be reweighed and a new weight and
balance record started. The A&P FAA-certificated mechanic
or repairman responsible for the work must provide the pilot
with current and accurate aircraft weight information and
location of the EWCG.
Equipment for Weighing
Weighing aircraft with accurately calibrated scales is the only
sure method of obtaining an accurate empty weight and CG
location. The two basic types of scales used to weigh aircraft
are platform and load cell.
Platform scales [Figure 3-1] or ramp wheel scales
[Figure 3-2] (usually a form or modified version of the
platform scale) are low profi e, easy to handle, safe, and
reliable. Tow or push the aircraft wheels or skids onto the
scale pad at ground level. With one scale per wheel, each
device should be capable of measuring up to at least 60,000
pounds since the weight on each wheel rarely exceeds this
figure
Load cell scales [Figure 3-3] are also a reliable means to
weigh aircraft and are typically cheaper than the platform
type.Using load cell scales allows for the aircraft to be set
up and weighed in its level flight attitude. With this method,
the aircraft is placed on jacks with electronic load cells
placed between the jack and the jack pad on the aircraft.
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. Jacking an aircraft off the ground from all
load points can be an inconvenience, as well as a safety
risk, which some operators would rather avoid by opting for
more expensive—but simpler to use—platform equipment.
In addition, weighing with platform scales typically takes
only one-third of the time needed to weigh with load cells.
Figure 3-2. Ramp scales.
Figure 3-3. Load cell scales.
All scales for aviation use, manual or electronic, must be
protected when stored or shipped, and they must be checked
periodically for accuracy. The maximum recognized period
between calibration checks is 12 months; however, this period
may be reduced by an airworthiness authority dependent on
the conditions of use. Scales in daily use may require a shorter
interval and/or testing to determine the continued accuracy
of the unit. Scales should be returned to the manufacturer for
proper calibration and testing.
Preparation for Weighing
In general, weight procedures may vary with the aircraft
and types of weight equipment employed. The weighing
procedure contained in the manufacturer’s maintenance
manual should be followed for each particular aircraft. The
major considerations in preparing an aircraft for weighing
are described in the following paragraphs.
Scale Preparation
Mechanical and electronic scales shall be inspected prior to
use and set to zero. This is done by adding and removing
a weight, then rechecking for zero. This process should be
repeated until a steady zero setting is obtained. The scales
should be located in the same environment in which they
Figure 3-4. Standard fuels and weights with temperatures of 32 °F
and 59 °F.
Weighing Point
AVGAS (Aviation Gasoline)
JET A & A-1
Water
Oil
6.14
6.75
8.35
7.50
6.01
6.68
8.33
7.43
Pounds per U.S. Gallon
32 °F 59 °F
are to be used and allowed to come up to temperature at
least 2 hours prior to use. Scales should not be used in
temperature extremes below 40 °F or above 100 °F unless the
scale is specifically designed for use in those temperatures.
Electronic scales are very sensitive and, if subjected to
freezing temperatures, the liquid displays may be damaged
beyond use.
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 ensure that no
water or debris is trapped there. 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 ballasts must be properly secured in
place. All temporary ballasts must be removed.
Standard Weights
Standard weights are established weights for numerous items
involved in weight and balance computations. These weights
should not be used if actual weights are available. Some of
the standard weights are listed in Figure 3-4.
Note the difference in weight as temperatures change.
Although this change is a very small amount per gallon, it could
end up in a significant total weight gain when dealing with large
quantities of fluids, such as those found in commercial aircraft.
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.
The amount of residual fuel and its arm are normally found
in Note 1 in the section of the Type Certificate Data Sheets
(TCDS), “Data pertaining to all Models.” For additional fuel
capacity information, see Chapter 2, Weight and Balance
Theory.
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.
When computing the weight of the fuel (e.g., 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 sg 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 sg of 0.82, the
fuel actually weighs 6.76 pounds per gallon rather than its
normal weight of 6.84 pounds per gallon.
Oil
The empty weight for aircraft certificated under the Civilian
Air Regulations (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 the oil to drain out.
Any remaining is undrainable oil and is part of the empty
weight. Aircraft certificated under Title 14 of the Code of
Federal Regulations (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.