曝光台 注意防骗
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gals, which is uncomfortably close to
the minimum figures below.
In IFR, journeys are split into
specific phases, such as start, checks,
and taxi (that is, before take off),
take-off and climb (another phase),
cruise and descent (yet another),
approach and landing, plus 10%,
plus missed approach and diversion
to the alternate. Then there's holding
at the alternate, unuseable and
contingency fuel, which covers errors in
forecast winds, navigation, ATC
restrictions and individual variations
from standard fuel consumption. By
arrangement, block figures can be
used which ignore the take-off and
climb. Fuel flow will have to be
adjusted if you plan to use
specialised equipment in flight, such
as heaters, or not use anything
essential, such as an engine.
By day, use the IFR figures as above,
but without missed approach and
loiter fuel, and the contingency fuel
can be 5% if over non-hostile
terrain, and 10% otherwise (nonhostile
in this context means where
fuel is available). By night, add loiter
fuel as well. Whatever happens, you
need 20 minutes’ worth at the
destination.
Jet and piston fuels mix differently
with contaminants (particularly
water), which is due to variations in
their specific gravities and
temperature. The s.g. of water, for
example, is so close to Avtur that it
can take up to 4 hours for it to settle
out, whereas the same process may
take as little as half an hour with
Avgas. As a result, there is always
water suspended in jet fuel, which
must be kept within strict limits,
hence two filtration stages, for solids
and water. The latter doesn’t burn,
of course, and can freeze, but it’s the
fungi that gather round the interface
between it and the fuel that is the
real problem – it turns into a darkcoloured
slime which clings to tank
walls and supporting structures,
which not only alters the fuel
chemically but will block filters as
well. Not much water is required for
this – trace elements are enough,
although, in reduced temperatures,
dissolved water will escape as free
water, and look like fog. Aviation
fuel is “clean” if a one-quart sample
is clear of sediment when viewed
through a clean, dry, clear glass
container, and looks clear and bright.
Note (again): It has been found
that when visible water is present in
Operational Stuff 95
jet fuel containing anti-icing additive,
the additive will separate from the
fuel and be attracted to the water.
After a certain amount, thought to
be about 15%,the density of the new
liquid changes so much that it is not
identified as water, and will therefore
pass through water filters, and not
be detected by water finding paste.
Where the ratio becomes 50%, as
much as 10% of whatever is going
through the filter could actually be
water, which is very likely to get to
the engine, since the filters on the
airframe itself are not as restrictive.
Drums are also flushed with a
solution containing sodium that
passes through filters.
While on the subject of filters, they
work on a certain psi, which may be
less sometimes, if you use a manual
pump, or more if you use an
electrical one. In other words, if the
pressure is not as designed, they will
not do their job properly.
Fuel is actually a combination of
various (very toxic) substances—
pound for pound, it’s more
explosive than dynamite. Jet A,
standard for commercial and general
aviation, is narrow-cut kerosene,
usually with no additives apart from
anti-icing chemicals. Jet A1 has a
different freezing point and possibly
something for dissipating static, used
for long haul flights where the
temperature gets very low. Jet B is a
wide-cut kerosene containing
naphtha, so is lighter and has a very
low flash point (it's actually 2/3
diesel and 1/3 naphtha, but in
emergency you can swap the
naphtha for avgas to get pretty much
the same thing). It contains static
dissipators and is widely used in
Canada. Try not to mix Jet A and Jet B -
the mixture can ignite through static
in the right proportions, as Air
Canada found when they lost a DC-
8 on the ramp in the 70s. The static
can come simply from the
movement of fuel through the lines.
Jet A weighs about 5% more per litre
than Jet B, but it gives you a longer
range, as turbines work on the
weight of the fuel they burn, not the
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The Helicopter Pilot’s Handbook(63)
