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about 5 nm by day and 20 nm or
more by night. Their sphere of
influence is ±10° of the extended
centreline, up to 4 nm.
VASIS
The Visual Approach Slope Indicator
System is a group of four lights (2-
bar), which may be turned off if the
weather is down to minimums so
they don't confuse you.
The light bars are called upwind and
downwind. There's a middle one for the
3-bar version - normal aircraft use
the middle and downwind ones, and
widebodies use the middle and
upwind bars to get a glideslope of 3°
(usually).
202 JAR Private Pilot Studies
Normal means an eye-to-wheel height
of up to 25 feet (e.g. DC-8).
Widebodies have up to 45 feet.
When you are on the glideslope, you
should see red lights over white ones
("red on white, you're all right"). If
you are too low, you will see red
over red ("red on red, you're dead").
When the approach is correct, you
will have safe clearance from
obstructions within 6-9° either side
of the centreline up to 4 nm out,
with a safe wheel clearance over the
threshold.
A Tri-Color VASI uses red, green
and amber to indicate too low, on
the glideslope and too high,
respectively.
A Pulsating VASI (PVASI) uses a
single light source to project a twocolour
approach indication. When
very low or very high, the light
pulsates more in relation to your
distance away, otherwise it is steady
for on the glideslope (white) and just
below (red).
PAPI
The Precision Approach Path Indicator
does pretty much the same thing as
VASIS, but with 4 lights in a row:
When on the correct slope, the two
lights nearest the runway are red and
the two furthest ones show white.
Three whites and a red mean slightly
high, and three reds and a white
means slightly low. Four of each is
way too much.
T-VASIS
This uses 10 lights, with 4 horizontal
ones in the middle and the other 6 as
3 vertical groups above and below,
which only appear when you are low
or high. If you do things properly,
you will arrive at the threshold at 45
feet. All lights are white, except
where a gross undershoot is involved,
when they turn red.
Lights above the horizontal 4 are the
fly down lights, whereas those
appearing below are the fly up ones.
Visual Flight Rules
Although the airspace you fly in
comes in six varieties (see below), it
is essentially split into two types,
controlled or uncontrolled, although it’s
fair to say that, in UK, once you are
above 3000 feet, most airspace is
controlled in one form or another.
As the names imply, in the first you
do as you're told (by ATC), and in
the second, you are responsible for
the safe conduct of the flight, which
means avoiding obstacles and other
aircraft, which you can only do if
you can see them.
The official definition of a flight
under VFR is "one conducted under
Visual Flight Rules", conveniently
leaving out the bit that tells you what
the Rules are (1500' ceiling with an
associated ground visibility of 5 nm).
The Visual Flight Rules govern flight
in Visual Meteorological Conditions
Air Law 203
(VMC). When the weather gets so
bad that you can't see where you are
going, Instrument Meteorological
Conditions (IMC) apply, and you must
fly under Instrument Flight Rules (IFR),
discussed below, although you can
fly IFR at any time. The definition of
IMC is actually a negative one, being
"weather precluding flight in
compliance with Visual Flight
Rules". The point at which this
happens depends on the type of
airspace you are in.
Special VFR Flight
This is used when you can’t comply
with IFR in a control zone (in fact,
in UK, there is no VFR at night,
except Special VFR in a control
zone). It's a legal technicality, used to
allow VFR aircraft to go where the
law says only IFR aircraft may fly.
However, it is never volunteered -
you must request Special VFR. ATC will
provide separation between Special
VFR and IFR flights.
Aside from requiring 10 km inflight
visibility (as a PPL holder without
any special ratings), you must be
clear of cloud and in sight of the
surface, and obtain clearance from
ATC before doing so, which means
you must have radios. You are
therefore absolved from the 1500
foot rule (within 600 m – Rule 5),
but not from being able to glide clear
of a built-up area in an emergency.
　
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