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The group in which an aircraft
operates depends on its
Certification, Max All-Up Weight
and the number of passengers it
carries. Within these limits you can
choose which group to operate in,
and come under the appropriate
weather and weight limitations; it
may be more acceptable
commercially, for example, to
operate in a lesser group if it enables
you to take more payload, and make
more money – all you might need is
longer runways.
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. These results are
unfactored, and not all performance
data for foreign aircraft is actually
verified by the CAA, though they do
carry out spot checks. In fact, any
figures are a mixture of actual
readings and calculated (or
guesstimated) adjustments from
them. The "performance" of an
aircraft is therefore a set of average
values—particular machines may be
better or worse.
There are fudge factors applied to
unfactored figures to produce net
performance (and gross performance when
they're not). Occasionally,
performance data (as amended by
the CAA) 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
therefore be made for them (if your
aircraft is performing sluggishly, you
may find it's not the machine, but
the conditions it has to work under
that are at fault).
Performance A aircraft must (with
one engine out) clear all obstacles
under the departure track within a
defined area by a specified margin,
without relying on seeing and
avoiding them – in Canada, for
example, aircraft are expected to
climb at over 200 feet per nm from a
Operational Procedures 137
point 35 feet above the end of the
runway, because obstacles are
assessed inside a slope of 152 feet
per nm, which gives you a clearance
of 48 feet (if there are no obstacles,
the takeoff visibility can also be
lower). All the relevant data will be
in the graphs, but some groups have
no information at all in some areas.
For instance, an aircraft in
Performance Group C is assumed to
have all engines working until above
200 feet, under which height there is
no data for landing or take-off
(which is why the take-off minima
will rarely be below this, because you
must be visual to avoid any obstacles
should an engine fail). Sometimes,
there can be no specific provision
for engine failure at all.
Each group requires certain
conditions to be met, either in
standards of power available,
environment or special procedures.
For example, take-off, landing and
reject areas need to be prepared
surfaces for Class A helicopters,
which also have to achieve certain
net gradients at particular points in
the climb. Lower groups are more
relaxed, but still have limitations—
you need somewhere to land in
emergency, but for these you only
need to avoid risk to third parties
while meeting certain weather limits.
Keeping to the helicopter theme,
Class A (1) take-off procedures
involve a vertical and backwards
liftoff to a predetermined height
before going forward, which is
known as the Critical Decision Point
(or CDP), and gives you a choice of
action if an emergency happens
(actually, ICAO now call it the
Takeoff Decision Point, or TDP).
Having moved backwards, you still
have the take-off spot in sight and
it's therefore available for landing. At
CDP (or TDP), if you elect to carry
on to forward flight, you should be
able to clear the landing spot during
the steep dive you have to make to
achieve flying speed (that's why the
CDP is about 40 feet high). Things
happen in reverse on landing. This
procedure is not without its critics,
since prolonged hovering at high
engine power outputs is not good
　
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