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.