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the average of the readings of
mechanical and electronic
decelerometers used on a runway to
gauge how slippery it is – the figure
1.0 means the maximum friction is
available. A typical reading in heavy
rain might be .2 or .3, so you will
take longer to stop and may even be
pushed sideways in a crosswind.
Thus, there are minimum indices for
certain strengths of crosswind – for
example, 10 kts requires a CRFI
reading of .3, while 15 kts requires
.4. The brakes are applied on a test
vehicle every 1,000m, within 10m
either side of the centreline.
The CRFI tables in the AIP (AIR
1.6) assume a minimum distance
landing from 50 feet from a 3°
glideslope with a firm touchdown
and a minimum delay in lowering the
nose (watch for noseovers) and
deployment of lift dumping devices,
using antiskid until stopped. The
CRFI figures required are along the
bottom of the crosswind chart.
A CRFI report will contain readings
for separate thirds of the runway,
designated A, B and C. However,
because of inaccuracy, CRFI
readings will not be available in
conditions of water and slush (see
AIR 1.6). In those circumstances, an
Aircraft Movement Runway Surface
Condition (AMRSC) report describes
the conditions in plain English.
Flight Planning 347
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.
Most factors that will assist you
under these circumstances are
directly under your control, and it's
even more important to arrive for a
"positive" landing at the required 50
feet above the threshold at the
recommended speed on the
recommended glideslope than for
normal situations. 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%,
　
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