Weight Changes Caused by a Repair or Alteration

Weight Changes Caused by a Repair or Alteration

A typical alteration might consist of removing two pieces
of radio equipment from the instrument panel, and a power
supply that was located in the baggage compartment
behind the rear seat. In this example, these two pieces are
replaced with a single lightweight, self-contained radio.
At the same time, an old emergency locator transmitter
(ELT) is removed from its mount near the tail, and a lighter
weight unit is installed. A passenger seat is installed in the
baggage compartment.

Computations Using Weight, Arm, and Moment
The first step in the weight and balance computation is to
make a chart like the one in Figure 5-3, listing all of the
items that are involved.
The new CG of 36.4 inches aft of the datum is determined
by dividing the new moment by the new weight.

Weight and Balance Revision Record

Weight and Balance Revision Record

Aircraft manufacturers use different formats for their
weight and balance data, but Figure 5-2 is typical of a
weight and balance revision record. All weight and balance
records should be kept with the other aircraft records.
Each revision record should be identified by the date, the
aircraft make, model, and serial number. The pages should
be signed by the person making the revision and his or her
certificate type and number must be included.
The computations for a weight and balance revision are
included on a weight and balance revision form. The
date those computations were made is shown in the
upper right-hand corner of Figure 5-2. When this work is
superseded, a notation must be made on the new weight
and balance revision form, including a statement that these
computations supersede the computations dated “XX/XX/
XX.”
Appropriate fore-and-aft extreme loading conditions
should be investigated and the computations shown.
The weight and balance revision sheet should clearly
show the revised empty weight, empty weight arm and/or
moment index, and the new useful load.

Aircraft Weight and Balance Equipment List

Aircraft Weight and Balance Equipment List

A typical comprehensive equipment list is shown in Figure
2-22 on pages 2-12 and 2-13. The FAA considers addition
or removal of equipment included in this list to be a minor
alteration. The weights and arms are included with the
items in the equipment list, and these minor alterations
can be done and the aircraft approved for return to service
by an appropriately rated aircraft mechanic or repairman.
The only documentation required is an entry in the aircraft
maintenance records and the appropriate change to the
weight and balance record in the POH/AFM. [Figure 5-1]
Major Alteration and Repair

Within the following text, information concerning major
repairs or major alterations does not apply to any aircraft
within the light-sport category. This category of aircraft is
not eligible for major repairs or alterations.

Any major alteration or repair requires the work to be done
by an appropriately-rated aircraft mechanic or facility. The
work must be checked for conformity to FAA-approved
data and signed off by an aircraft mechanic holding an
Inspection Authorization, or by an authorized agent of
an appropriately rated FAA-approved repair station. A
repair station record or FAA Form 337, Major Repair and
Alteration, must be completed which describes the work.
A dated and signed revision to the weight and balance
record is made and kept with the maintenance records, and
the airplane’s new empty weight and empty weight arm or
moment index are entered in the POH/AFM.

Multiengine Airplane Weight and Balance Computations

Multiengine Airplane Weight and Balance Computations

Weight and balance computations for small multiengine
airplanes are similar to those discussed for single-engine
airplanes. See Figure 4-9 for an example of weight and
balance data for a typical light twin-engine airplane.

The airplane in this example was weighed to determine its
basic empty weight and EWCG. The weighing conditions
and results are:

Fuel drained -
Oil full -
Right wheel scales -1,084 lbs, tare 8 lbs
Left wheel scales - 1,148 lbs, tare 8 lbs
Nose wheel scales - 1,202 lbs, tare 14 lbs
Determine the Loaded CG

Beginning with the basic empty weight and EWCG and
using a chart such as the one in Figure 4-11, the loaded
weight and CG of the aircraft can be determined.

The aircraft is loaded as shown here:

Fuel (140 gal).................... 840 lbs
Front seats......................... 320 lbs
Row 2 seats....................... 310 lbs
Fwd. baggage.................... 100 lbs
Aft. baggage........................ 90 lbs

Determining the Aircraft Center of Gravity

Determining the Aircraft  Center of Gravity

When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.

Figure 3-3. The datum is located at the firewall.
Determine the CG by adding the weight and moment of
each weighing point to determine the total weight and total
moment. Then divide the total moment by the total weight
to determine the CG relative to the datum.

As an example of locating the CG with respect to the
datum, which in this case is the firewall, consider the
tricycle landing gear airplane in Figures 3-3 and 3-4.
When the airplane is on the scales with the parking brakes
off, place chocks around the wheels to keep the airplane
from rolling. Subtract the weight of the chocks, called
tare weight, from the scale reading to determine the net
weight at each weighing point. Multiply each net weight
by its arm to determine its moment, and then determine the
total weight and total moment. The CG is determined by
dividing the total moment by the total weight.

The airplane in Figures 3-3 and 3-4 has a net weight of
2,006 pounds, and its CG is 32.8 inches behind the datum.

Two Ways to Express CG Location

The location of the CG may be expressed in terms of
inches from a datum specified by the aircraft manufacturer,
or as a percentage of the MAC. The location of the leading
edge of the MAC, the leading edge mean aerodynamic
cord (LEMAC), is a specified number of inches from the
datum.

Safety Considerations

Safety Considerations

Special precautions must be taken when raising an aircraft
on jacks.

1. Stress plates must be installed under the jack pads if the
manufacturer specifies them.

2. If anyone is required to be in the aircraft while it is
being jacked, there must be no movement.

3. The jacks must be straight under the jack pads before
beginning to raise the aircraft.

4. All jacks must be raised simultaneously and the safety
devices are against the jack cylinder to prevent the
aircraft tipping if any jack should lose pressure. Not all
jacks have screw down collars, some use drop pins or
friction locks.

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.