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are not appropriate, ‘NSC’ (no
significant cloud) is used instead.
Significant Changes
In addition to ‘FM’ and the time
(see above) significant changes
may be indicated by ‘BECMG’
(becoming) or ‘TEMPO’
Operational Procedures 73
(temporarily). ‘BECMG’ is
followed by a four-figure group
indicating the beginning and
ending of the period when the
change is expected. The change
is expected to be permanent,
and to occur at an unspecified
time within it. ‘TEMPO’ will
similarly be followed by a 4-
figure time group, indicating
temporary fluctuations in
forecast conditions. ‘TEMPO’
conditions are expected to last
less than 1 hour in each
instance, and in aggregate, less
than half the period indicated.
Probability
Probability of a significant
change, either 30 or 40%. The
abbreviation ‘PROB’ will
precede the percentage,
followed by a time group, or a
change and time group, e.g.
‘PROB 30 0507 0800FG
BKN004’, or ‘PROB40
TEMPO 1416 TSRA
BKN010CB’.
Amendments
The amended forecast will have
AMD between TAF and the
aerodrome identifier, and will
cover the rest of the validity
period of the original forecast.
Fuel, Oil and Water Methanol
Fuel and oil consumption rates and
weights for flight planning should be
in Part B (the Flight Manual). It's
usual to keep a check on the fuel
contents during flight to see if things
are going according to plan, thus
keeping track of fuel consumption;
it’s written down on the Flight
Progress Log (PLOG) every hour or
so on long trips, but not required
(for obvious reasons) in singleengined
unstabilised helicopters.
Special conditions apply under IFR
and, needless to say, you shouldn't
think of going IMC on VMC fuel
reserves, talking of which….
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. More
about Critical Points and Points of
No Return against "wet footprints"
are in International Operations.
Helicopters
Helicopters don't need aerodromes,
and minimum figures reflect this.
However, they are calculated for
level aircraft. Odd attitudes, say
when slinging, may cause a fuel
boost pump to become uncovered
and give you a nasty surprise just
when you don’t want it. On a 206,
the unuseable fuel after a boost
pump failure can be up to 10 US
gals, which is uncomfortably close to
the figures below. By day, use the
IFR ones as above, but without
missed approach and loiter fuel, and
74 Operational Flying
the contingency fuel can be 5% if
over non-hostile terrain, and 10%
otherwise (non-hostile in this
context means where fuel is
available). By night, add loiter fuel
as well.
Final Reserve Fuel
An emergency exists when your fuel
has reduced to the point where you
should land without delay. This is at
least the sum of:
· for VFR by day, 20 minutes at
best range speed;
· for IFR, or VFR and navigating
by other means than visual
landmarks or at night, 30
minutes at holding speed at
1500 ft above the destination
heliport in ISA, based on the
estimated mass on arrival above
· extra fuel, at your discretion.
This total is what you should have
left when you arrive at your
destination.
Isolated Heliports
Where an alternate does not exist the
fuel required is the sum of:
· taxy fuel
· trip fuel
· contingency fuel
· additional fuel if required, but
not less than 2 hours' worth,
based on normal cruise
consumption over the
destination, or 45 mins for
piston-engines, plus 15% of
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