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poor braking action and aquaplaning
(see Hydroplaning, below), which is a
Flight Planning 165
condition where the built-up
pressure of liquid under the tyres at a
certain speed will equal the weight of
the aircraft.
When taxying
On the ground, you may need slower
taxying speeds and higher power
settings to allow for reduction in
braking performance and the
increase in drag from snow, slush or
standing water, so watch your jet
blast or propeller slipstream doesn't
blow anything into nearby aircraft.
Try not to collect snow and slush on
the airframe, don't taxi directly
behind other aircraft, and take
account of banks of cleared snow
and their proximity to wing- and
propeller-tips or engine pods. Delay
flap selection to minimise the danger
of damage, or getting slush on their
retraction mechanisms.
Hydroplaning
This occurs when liquid on the
runway tends to creep under the
tyres. Higher speeds will lift them
completely, leaving them in contact
with fluid alone, with the consequent
loss of traction, so there may be a
period during which, if one of your
engines stops on take-off, you will
be unable to either continue or stop
within the remaining runway length,
and go water-skiing merrily off the
end (actually, you're more likely to
go off the side, so choosing a longer
runway won't necessarily help). The
duration of this risk period is
variable, but will vary according to
your weight, the water depth, tyre
pressure and speed.
Dynamic hydroplaning is the basic sort,
arising from standing water. Viscous
hydroplaning involves a thin layer of
liquid on a slippery surface, such as
the traces of rubber left on the
landing area of a runway (one reason
why it's dangerous to drive after a
rain shower in Summer).
Reverted Rubber Hydroplaning happens
when a locked tyre generates enough
heat from friction to boil the water
on the surface and cause the
resulting steam to stop the tyre
touching the runway. The heat
causes the rubber to revert to its
basic chemical properties.
A rough speed at which aquaplaning
can occur is about 9 times the square
root of your tyre pressures, 100
pounds per square inch therefore
giving you about 90 kts (7.7 times if
the tyre isn't rotating)—if this is
higher than your expected take-off
speed you're naturally safer than
otherwise. The point to note is that
if you start aquaplaning above the
critical speed (for example, when
landing), you can expect the process
to continue below it, that is, you will
slide around to well below the speed
you would have expected it to start if
you were taking off.
Under-inflating tyres doesn't help—
each 2 or 3 lbs below proper
pressure will lower the aquaplaning
speed by 1 knot, so be careful if
you've descended rapidly from a
colder altitude.
Grass
For dry short grass (under 5"), the
TODR will increase by 20%, a
factor of 1.2. When it's wet, 25%—a
factor of 1.25. For dry, long grass (5-
10"), TODR will increase by 25%,
and 30% when wet (it's not
recommended that you operate
when the grass is over 10" high).
166 JAR Private Pilot Studies
For dry short grass (under 5 inches),
the LDR will increase by 20%, a
factor of 1.2. When it's wet, 30%—a
factor of 1.3. For dry, long grass (5-
10 inches), LDR will increase by
30%, and 40% when wet. For other
soft ground or snow, the increase
will be in the order of 25% or more
for take-off and landing.
Obstacles (The Climb)
The best Rate of Climb speed is
obtained when there is the greatest
difference between the power
required for level flight and that
available from the engines. In
turboprops, this will coincide with
the speed that gives the best
lift/drag ratio, since power output is
relatively constant. Turbojets,
however, produce more engine
power with speed, which is enough
to overcome the extra drag, so the
maximum differential between
power required and available
happens at a higher speed. There will
be performance tables to find time
and fuel required for climbs.
Remember that headwind and
tailwinds will change the distance
figures. To cope with this, work out
the groundspeed with no wind and
apply the corrections then. You can
use the whizzwheel to find out the
distance and time.
The Cruise
That part of the trip from the top of
　
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