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range, meaning that it has one all on
its own (that is, the arm will change
with the weight), so the figure of
110.7 will change with the amount,
for which check the flight manual.
The procedure is therefore to
multiply the weights by the arms to
get the moments, and divide the
total moments by the total weights
to get the C of G. Then you refer to
the flight manual to see if the figure
fits into the authorised range.
Look for a graph like this:
Simply take the all-up weight you
end up with, and the final C of G,
and line them up horizontally and
vertically. If they are inside the
envelope, you are OK, but don't
forget you have to land again! Your
C of G may well be fine for takeoff,
but check again after the fuel has
been used!
Lateral C of G works the same way,
except the figures are smaller and
easier to work with. In fact, they may
even be zeroed if the items are in the
centre, as the fuel tank might be.
Items left of the centreline have a
negative sign, and those on the right
are positive, so the lateral moments
for the front doors on a 206 would
be –12 for the left and +12 for the
right. Here’s an example lateral C of
G for the 206:
Item Wt Arm Moment
Aircraft 1881 .41 773
Pilot 185 14 2590
Front pax 185 -11 -2035
Left Rear Pax 185 -16 -2960
Centre Rear Pax 185 0 0
Right Rear Pax 185 16 2960
Baggage 50 147.50 7375
Fuel 310 0 0
Total 3166 2.75 8703
Flight Planning 161
Again, there will be a chart in the
Flight Manual to show you where
your plot lies. Fortunately, most of
the time it is something that only
helicopter pilots need to bother
about, which is why it's mentioned.
Flight Manuals often have helpful
charts with precalculated moment
figures for fuel and baggage (the arm
figures will be excluded). They are
quite simple to use, except that the
exam will require you to interpolate
here and there. However, you should
watch for special conditions, as with
any chart, especially for maximum
weights in particular locations. There
may also be a plan view of the
aircraft with the arms displayed next
to the locations they refer to.
Performance
The take-off and landing phases are
the most critical, demanding the
highest skills from crews and placing
the most strain on the aircraft.
Because of this, strict regulations
govern the information used for
calculating take-off or landing
performance. Of course, in the old
days (say during the war, or when
the trains ran on time), having
enough engines to lift the load was
all that mattered and no priority was
given to reserves of power and the
like. Now it's different, and
performance requirements will be
worked out before a C of A is
issued, over a wide range of
conditions. They are subsequently
incorporated in the Flight Manual,
which actually forms part of the C of
A. In addition, the ANO requires
you to ensure that your aircraft has
adequate performance for any
proposed flight.
It is your responsibility to decide
whether or not a safe takeoff (and
landing) can be made under the
prevailing conditions. This means, in
particular, that, although the
crosswind on a particular runway
may be within limits for your
aircraft, you can choose another one
if you are not happy.
Individual aircraft of a given species
will vary in performance due to such
variables as the age of the airframe
and engines, the standard of
maintenance, or the skill and
experience of the crews. What you
can do on one day under a given set
of circumstances may well be
impossible another time. The
original testing, of course, is done
with new aircraft and highly
experienced pilots. "Performance" is
therefore based on average values.
There are fudge factors applied to
unfactored figures to produce net
performance (and gross performance when
they're not), which are meant to
offset the effects of tired engines or
variations in pilot skill. Occasionally,
performance data in a flight manual
will already be factored, but you will
have to check the small print on the
chart, in case they surprise you.
Figures and graphs are based on
standard conditions which allow for
fixed reductions in pressure and
temperature with height. As we all
know, the real world isn't like that,
so these assumptions may not always
be true and due allowance must
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