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problem, crosswinds may also
increase the TODR if you need to
use the brakes to keep straight.
Forecast winds should be factored
by 50% for a headwind and 150%
for a tailwind. TODR and LDR will
increase by 20% for each tailwind
component of 10% of the lift-off
and landing speed (factor by 1.2).
The flight manual will state
maximum crosswinds for your
machine (try the limitations section).
A useful guide (for American
machines, anyway) is that the
maximum crosswind will be about
20% of Vso. When finding the angle
between the wind and the runway,
remember that runway headings are
magnetic and forecast winds are true.
You can also use the crosswind chart
to find a limiting wind, or the
maximum you can accept from any
given angle. Just draw a line upwards
from the maximum speed you can
accept, and stop when you reach the
line representing the wind direction.
The curved line at that point (or its
interpolation) is the maximum
windspeed you can take.
Surface
Performance information is based
on a dry, hard surface. A
"contaminated" runway has standing
water or slush more than 3mm thick,
or snow and ice anywhere along the
takeoff run or accelerate-stop
surface. However, your flight manual
may have different ideas.
The most important factors are loss
of friction when decelerating, and
displacement of (and impingement
drag when accelerating through)
whatever is on it, so it may be
difficult to steer, and take-off and
accelerate-stop distances may
increase due to slower acceleration,
Flight Planning 165
as will landing distance because of
poor braking action and aquaplaning
(see Hydroplaning, below), which is a
condition where the built-up
pressure of liquid under the tyres at a
certain speed will equal the weight of
the aircraft.
CRFI
The Canadian Runway Friction Index is
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 are not available in water
and slush (see AIR 1.6). In those
circumstances, an Aircraft Movement
Runway Surface Condition (AMRSC)
report gives a description of the
conditions in plain English.
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
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