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won't see the print so well at night
(see the Human Factors chapter).
Slide Rule
On the other side of both types,
there is a circular slide rule, with the
60 point on the inner scale
Flight Planning 333
conveniently marked to make speed
and time calculations easier:
It can be positioned against fuel
quantity or distance on the outer
scale to read time on the inner scale.
As with any slide rule, you need the
approximate answer to your problem
first, as a protection against gross
error and to give you an idea where
to put the decimal point (if you were
wondering how it works, you are
adding indices, which is also where
logarithms come from, but that is
outside the scope of this book).
The most common problems
concern time and distance. Just
move the inner scale until the 60
point is opposite the TAS or fuel
consumption, for example. Read the
time on the inner scale against
distance on the outer scale, or fuel if
you are checking how much is being
used. In the above picture, the speed
triangle (60) is opposite 120 (knots or
gallons) on the outer scale, which
means it will take 6 minutes to go 12
nautical miles, or 6.5 to use 13
gallons, and so on. Always reduce
hours (and proportions thereof) to
minutes for simplicity.
To multiply normally (e.g. 2 x 4),
place 10 on the inner scale against
one number on the outer scale, and
read the answer on the outer scale
opposite the other, which will be on
the inner scale.
Square roots can be found easily, too
(useful for VHF ranges). Find the
number you want the square root of
on the outer scale, then rotate the
inner one until the number opposite
10 is the same as the one against
your original number. For example,
400 will have 2 opposite, as well as
against 10 (figure out where the
decimal point should go mentally).
To find TAS, line up the
temperature against the pressure
altitude in a window in the rotating
slide rule (it may be labelled True Air
Speed), then read the TAS on the
outer scale against the RAS. Don't
forget to allow for compressibility at
speeds over 300 kts.
Conversions are done by lining up
arrows on both scales representing
the commodities concerned. For fuel
weights, you will need the specific
gravity, which is 1 for water, and used
as a common denominator. It will
vary from place to place, but that in
the Flight Manual is the one to use.
For example, if the s.g. of fuel is
taken as .8, how much does 1 gallon
weigh? The answer is 8 lbs (water
would weigh 10). Alternatively, how
many litres do you need to get from
the fuel guy if you can carry 2600 kg
and the s.g. is 8.2? Try 3170.
In the picture below, you will see
that the arrow labelled km on the
outer scale is opposite the one
marked statute (miles) on the inner
scale. All you do is read of the direct
equivalent on each scale – here,
112.5 km is equal to 70 statute miles.
334 Canadian Professional Pilot Studies
Note: Electronics are all very well,
but batteries run out and electrics
reserve the right to go wrong at the
drop of a hat, as any avionics
technician will tell you. My
recommendation, at least in flight, is
to use an E6B or a CR, because they
are easier to work without getting
your head stuck in the cockpit. The
alleged accuracy you get with
electronic computers is not worth
the bother (and the expense), since
you won't be able to read the
instruments that closely anyway.
There is absolutely nothing wrong in non-
ATP pilots who don't fly high speed
aircraft using circular slide rules, despite
what other books may say.
Weight & Balance
This must follow the Flight Manual,
to ensure an aircraft is safely loaded.
As the Flight Manual forms part of
the Certificate of Airworthiness (or
Permit to Fly), if its conditions are
not met, any insurance is invalid.
There are two aspects to Loading,
the weights themselves and their
distribution, and you sometimes get
some nasty surprises—fuel in wings
means unusually shaped fuel tanks,
so you won't get a straight line
variation; every fuel load will have a
different figure, principally because
the fuel tanks have a C of G system
all of their own, running separately
from the aircraft (even in small
helicopters, like the Bell 206 or 407).
In this case, it's not enough just to
subtract the closing fuel moment
from the starting one—for example,
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