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the interval for lights following
mediums in the same circumstances,
but will issue advisories.
86 Canadian Private Pilot Studies
Otherwise, a three minute interval
will apply in the wake of a heavy (or
if the projected flight path crosses),
or a light aircraft departing in the
wake of a medium if it starts to
takeoff from an intersection or a
point further along than the previous
aircraft, or if the controller thinks it
needs more runway for takeoff (and
will therefore encounter the wake).
You can request a pilot waiver of
separation, but it will never be
offered. It will not be granted for
intersection takeoffs of lights or
mediums behind heavy takeoff or
missed approach.
Helicopters
Rotor downwash is wake turbulence
from helicopters, which is easy to
forget when hovering near a runway
threshold or parked aircraft with
little wind (although it's quite useful
when crop spraying). Otherwise, the
effects are similar to fixed wing, in
that you get vortices from each side
of the rotor disc, but the lower
operating speed means they are
more concentrated. Downwash also
creates dust storms and can lift even
heavy objects into the air, instantly
presenting Foreign Object Damage
(FOD) hazards to engines, main and
tail rotor blades (so don't bolt your
FOD, it gives you ingestion!—old
RAF joke, on which I hope there's
no copyright). Plastic bags or
packaging sheets are FOD, too.
Generally speaking, the larger the
helicopter, the greater the potential
danger (obvious, really). Bell 212,
Sikorsky S76 and smaller machines
are Light, in terms of the above table,
but size is not significant when
creating vortices; use the table for
comparison purposes when avoiding
other types.
The Altimeter
This is simply an aneroid barometer
calibrated in feet rather than
millibars or inches of mercury (its
inner workings are described fully in
the Instruments chapter). It measures
the pressure at a given altitude,
which is subtracted from the
pressure at sea level. The difference
is converted to get a height readout.
You would be very lucky to hit the
standard atmosphere more than, say,
25% of the time, so you need a
means of adjusting any instruments
based on it to cope with the
differences. An altimeter has a setting
window in which you can adjust the
figures for the correct pressure on
the ground by turning a knob on the
front (this is actually part of a very
important preflight check, where you
make sure that if you turn the knob
to the right, the height readings
increase, and vice versa. You also need
to check that the reading given
coincides with the airfield elevation,
±50 feet, and that, if you’ve got two
altimeters, they are within ± 50 feet
of each other (in other words, they
can misread by nearly 100 feet and
still be useable).
If you didn’t adjust your
instruments, and were flying
between different areas of air
pressure, you would not be at the
height you thought you were, which
is not that much of a problem if
everybody else uses the same setting,
as your relative height to other
aircraft would be maintained, but it
wouldn’t with respect to hard
objects, that is, obstacles, such as
mountains, television masts, etc.
Weather 87
As an example, if you were flying
from high to low pressure, your
altimeter would be overreading
(from HIGH to LOW, your
instrument is HIGH), so you would
be lower than planned and liable for
a nasty surprise. It’s therefore much
safer to be going the other way (that
is, from LOW to HIGH, where your
instrument is LOW).
You can check what the difference is
with simple maths, using the figures
given above of 1" being equal to
1,000 feet. Remember that an
increase in pressure equals a decrease
in altitude, so if you start with 29.92,
then go to where it’s 30.92, the
altimeter reading would be 1,000 feet
less, even though the figures
themselves increase.
To convert from inches to millibars,
in case you have an old altimeter,
start at 29.92 and find the difference
between it and the current pressure.
Multiply that by 3.4 and apply it to
1013.2. For example, if the current
pressure is 30.02, that is, 1 above
29.92", add 3.4 mb and set 1016.6.
The standard atmosphere has a
temperature element that also affects
the altimeter. Remembering that, as
　
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