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weighed with full fuel. After the 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 2-11.
When computing the weight of the fuel, for example a tank
full of jet fuel, measure its specific gravity (s.g.) 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 nominal weight of 6.84 pounds
per gallon. The standard weight of aviation gasoline (Avgas)
is 6 pounds per gallon.
Oil
The empty weight of 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, to drain out. Any oil 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 the 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
Large 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.
But most smaller aircraft are actually lifted off the ground
onto scales or load cells.
You must exercise special care 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 level 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.
Permanent ballast: A weight
permanently installed in an aircraft
to bring its center of gravity into
allowable limits. Permanent ballast is
part of the aircraft empty weight.
Temporary ballast: Weights that
can be carried in a cargo
compartment to move the location of
the CG for a specific flight condition.
Temporary ballast must be removed
when the aircraft is weighed.
Undrainable oil: Oil that does not
drain from an engine lubricating
system when the aircraft is in the
normal ground attitude and the drain
valve is left open. The weight of the
undrainable oil is part of the empty
weight of the aircraft.
Residual fuel: Fuel that remains in
the sumps and fuel lines when the
fuel system is drained from the inlet
to the fuel metering system, with the
aircraft in level flight attitude. The
　
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