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empty weight, above, plus
operating fluids (fuel, full oil).
Flight Planning 159
·  The Maximum Takeoff Weight,
which is simply the Basic Empty
Weight plus the payload
(passengers, cargo), which will
not necessarily coincide with the
full maximum, for performance
reasons. The conditions at your
destination, for example (it may
be hotter and higher) may mean
taking off lighter so you can
land safely. Any weight less than
the full maximum due to
performance factors is known
as the Restricted (or Regulated)
Takeoff Weight (RTOW) and is
actually the starting point for
calculating payload (see below).
A couple of things to bear in mind
are that the Basic Weight and the
payload will not change during flight,
but the fuel load will. Another is
that, in small aircraft, passengers and
cargo should be weighed separately.
Distribution
Incorrect loading naturally affects
aircraft performance, and will
possibly prevent the thing from even
getting airborne. A Centre of Gravity
too far forward will make it more
difficult to raise the nose on take-off
(or landing), possibly overstress the
nosewheel as a result, and make the
flight less economical by excessive
use of trim tabs, which causes more
drag. There are certain advantages to
having the C of G towards the rear
(by making the tailplane contribute
to total lift, or at least not detract
from it, which also reduces the
power required and hence fuel used),
but too much will make the aircraft
less stable, more fatiguing to fly and
cause similar drag and nosewheel
problems (but in reverse) as
excessive forward C of G. Also, if
you don’t have the elevator
movement to get yourself out of a
stall, you could end up in a flat spin
you can’t get out of.
In a helicopter, if the C of G is too
far aft or forward of its ideal
position, there is a danger of running
out of cyclic control in the opposite
direction – one too far forward, for
example, will mean you will not be
able to pull the cyclic back far
enough to cope with certain stages
of flight (as fuel is consumed, for
example, when the C of G generally
moves forward), as a lot of its range
will be taken up with the unusual
attitude, although a forward position
is needed to counteract flapback.
Not being able to flare in an
autorotation could well ruin your day
(in fact, if your engine fails and you
don't have enough cyclic movement
to counteract the nose down
tendency, the airflow will meet the
disc edge-on and not go up through
it, so you will not enter autorotation,
and the RPM will decrease even
more – ouch!). As well, lateral C of G
may be affected with some loads,
such as when hoisting.
The reference datum is an imaginary
point from which all calculations
start and where some C of G ranges
are expressed (for example, 106" aft
of datum). Mostly, it is at, or slightly
forward of, the nose, but can be at
the rotor mast of a helicopter.
The arm is the distance from the
reference datum to the area in
question, such as a passenger seat or
the fuel tank. It may be measured in
Imperial or Metric units, and you
must use the same ones (the word
station may also be used). To get the
C of G of an aircraft, you multiply
160 JAR Private Pilot Studies
the weight of each item in it by the
arm to get the moment, or the amount
of leverage that item contributes.
The aircraft itself will have an arm
and a moment from when it was last
weighed, and this is where you start.
You can find it in the weight and
balance schedule (usually in the flight
manual), and it may be varied if you
add or take off various items of
equipment, such as the hook or hoist
in a helicopter.
Because you might end up using very
long numbers, sometimes you use a
moment index, the result of dividing
the moment by 1,000 to make the
figures more manageable. Here is a
simplified typical calculation for a
Bell 206 helicopter (the principles
are the same for larger machines):
Item Wt Arm Moment
Aircraft 1881 116.5 219137
Front pax 185 65 13000
Rear Pax 185 104 19240
Baggage 50 147.50 7375
Fuel 310 110.7 34273
Total 2611 112.22 293025
The total C of G for takeoff is
112.22, obtained by dividing the total
moment figure (293025) by the total
weight (2611). This particular
machine's fuel has a variable CG
　
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