Chapter 1.
FUNCTIONAL REQUIREMENTS
OF VISUAL GROUND AIDS
1 INTRODUCTION
The purpose of this chapter
is to provide engineering personnel with a general appreciation of the task
pilot‑in command in relation to the use of and reliance upon visual aids
and visual cues in approaching, landing and operating on the airport surface.
The information provided herein is for illustrative purposes only and is not
necessarily meant to imply ICAO approval or endorsement of the operational
practices and procedures described. For currently approved detailed operational
procedures and practices, reference should be made to pertinent operational and
training documents.
1.2 OPERATIONAL
FACTORS
The pilot's problem
1.2.1
Human beings are two‑dimensional animals. From the moment we first start
to crawl we interpret visual cues and use our sense of balance to travel over
the surface of the earth. This long and gradual learning process continues as
we later take charge of various types of mechanical transport on land or water.
by which time we have years of accumulated experience on which to draw. As soon
as we take to the air we have a third dimension to cope with, and this means
that all our years of experience in solving two‑dimensional problems are
of little use.
1.2.2 There are two ways of
controlling an aircraft 'in flight ‑ either by means of the automatic
pilot or manually. The pilot can effect manual control either by reference to
the instrument panel, with some judgements being made by a flight director
system, or by reference to the outside world, with all judgements being made by
the pilot using visual cues. The latter method presupposes adequate visibility
and a clearly defined horizon, which may be the actual horizon or an apparent
horizon due to the perceived gradients in the texture or the detail on the
earth's surface.
1.2.3 By far the most
difficult tasks when flying an aircraft visually are judging the approach to a
runway and the subsequent landing manoeuvre. During the approach, not only must
the speed be carefully controlled, but continuous simultaneous corrections in
all three dimensions are necessary in order to follow the correct flight path.
This may be defined as the intersection of two planes at right angles, the
vertical plane containing the extended centre line of the runway and the other
plane containing the approach slope.
1.2.4 Maintaining an
accurate approach slope without the aid of visual approach slope indicator
systems has been made even more difficult by the introduction of the jet
engine. Earlier propel ler‑driven aircraft had an almost instantaneous
reaction to an increase of power; the faster airflow over the wings from the
speeded‑up propellers provided an immediate increase in lift. The jet
engine is not only slower to respond to an advance in throttle setting, but
also has no direct effect on the airflow over the wing. Not until the whole
mass of the aircraft has been accelerated following an increase in thrust will
an increase in lift result.
1.2.5 It is essential that
aircraft cross the runway threshold with a safe margin of both height and
speed. In order to effect a smooth touchdown both the speed and the rate of
descent must be simultaneously reduced in the manoeuvre known as the landing
flare, so that the wheels touch the runway just prior to or as the wing stalls.
1.2.6 After touchdown the
pilot has a continuing requirement for directional guidance to keep the
aircraft along or near the middle of the runway (at touchdown speeds often in
excess of 255 km/hr or 138 knots). The pilot also needs information from which
an assessment can be made of the length of runway remaining and, once the
aircraft has slowed sufficiently, advance warning of a suitable runway exit,
its width clearly delineated where taxiway centre line lighting is not provided.
1.2.7 Once clear of the
runway, the pilot has to taxi the decidedly unwieldy vehicle along an often
complicated maze of taxiways to the correct parking/docking position on an
apron which may well be crowded. The pilot must be
given a clear indication of the route to follow and be prevented from crossing
any runway in use, as well as being protected from conflicting taxiing aircraft
and vehicles.
1.2.8 If we
examine the most critical case, that of large‑bodied jets, the taxiing
pilot has to control one of the largest, heaviest, and most inefficiently
powered tricycles ever made. The pilot is seated at least
1.2.9 The manner in which all the varied
operational requirements outlined in the preceding paragraphs are satisfied by
visual aids is described in detail in Section 1.4.
The four Cs
1.2.10 There are four main elements that
comprise the character of the complete airport lighting system as it has
evolved from research and development programmes and practical field experience
over a long period of time. These elements may be conveniently referred to as
the 'Your Cs" ‑ configuration, colour, candelas and coverage. Both
configuration and colour provide information essential to dynamic three‑dimensional
orientation. Configuration provides guidance information, and colour informs
the pilot of the aircraft's location within the system. Candelas and coverage
refer to light characteristics essential to the proper functioning of
configuration and colour. A competent pilot will be intimately familiar with
system configuration and colour and will also be aware of candela changes which
increase or decrease light output. These four elements apply to all airport
lighting systems, in greatly varying degrees, depending on such factors as the
size of the airport and the visibility conditions in which operations are
envisaged, and are reviewed in the paragraphs which follow.
Configuration
1.2.11 This concerns the location of components
and spacing of lights and markings within the system. Lights are arranged in
both longitudinal and transverse rows with respect to the runway axis, whereas
painted runway markings are aligned only longitudinally with the runway axis.
(The foreshortening effect of viewing transverse markings at approach angles
makes painted transverse markings impractical.)
1.2.12 Light spacing varies primarily with
regard to whether a longitudinal or transverse array is involved. It is
apparent that a pilot's perspective view of visual aid systems causes widely
spaced lights in a longitudinal row to assume a 1inear effect'. On the other
hand, close spacings are required to achieve a 1inear effect' with lights in a
transverse row. Another factor influencing light spacing is the visibility
under which the system is to be used. When operations are conducted in lower
visibilities, closer spacing are required, especially in longitudinal rows, to
provide adequate visual cues within the reduced visual range.
1.2.13 Locating lights
for the runway edge, threshold and runway end was never a problem since the
very words themselves denote location. Locating threshold lights was somewhat
complicated, however, by displaced threshold involvements. Development of
improved semi‑flush fittings solved the problem, and it is now possible
to locate displaced runway threshold lighting in a standard configuration
within runway pavement. Spacing of lights associated with runway edges has
changed very little since runways were first lighted because primary visual
guidance in low visibilities has shifted from the runway edge to the newly
developed runway centre line and touchdown zone lighting systems.
1.2.14 While
development of runway lighting has been fairly uncomplicated, approach light
research and development resulted in major differences in both location and
spacing for systems in various States. In contemplating operations for
precision approach category II runways, it was agreed that a standard
configuration was needed for at least the 300 m of the system before the
threshold. A co‑operative programme by ICAO States achieved this
objective.
Colour
1.2.15 The function of
coloured light signals is to identify the different lighting systems of the
aerodrome, to convey instructions or information, and to increase conspicuity.
Thus, runway edge lights are white, and taxiway edge lights are blue; the red,
white and green signals of the
traffic signalling lamp are used to instruct ground and air traffic; and red
obstacle lights are more readily seen against a background of white lights than are lights of other colours,
their red colour also indicating a hazard.
1.2.16 Although many colours
can be recognised when the coloured surfaces are large enough to be seen as an
area, only four distinct coloured light signals can be recognised when the
lights are seen alone and as "point sources.
1.2.17 With proper selection
of colour boundaries, the colours red, white or yellow, green and blue can be
recognised. White and yellow can be differentiated only when:
i)
the lights of the two
colours are shown simultaneously in adjacent parts of the same signal system;
or
b) the white and yellow are shown as successive phases
of the same signal (for example, the alternate white and yellow flashes of the
water aerodrome beacon); or
ii)
the signal has
appreciable size so that it does not appear as a point source.
Similarly, a pink VASIS
signal can be recognized because the other signal is either red or white.
Because of the limitation of recognizable colours, the colours have more than
one meaning, and the location and configuration of coloured lights provide the
required differentiation in meaning. Thus, the colour green is used for
threshold lights, for taxiway centre line lights and as a "go" signal
in the signalling lamp and in traffic control lights. Aircraft navigation and
anti‑collision lighting must be considered in developing aerodrome
lighting systems because aircraft lights may lead to identification problems
with some ground lighting aids
1.2.18 Coloured lights are
usually obtained by using an incandescent tungsten source in combination with
the appropriate light filter. This filter is usually glass and may be either an
additional component in a light fitting which would otherwise provide a white
signal, or an integral part of the optical system of the fitting. In either
case, the action of the filter consists of the removal of light of unwanted
wavelengths, not the addition of light of the desired wavelength. In addition,
some light of the desired wavelength is removed. Thus, the intensity of the
coloured fitting is less than it would be if the fitting were designed to emit
white light. The intensities of coloured signals are usually a percentage of
the intensity possible with a white signal, i.e. approximately 40 per cent for
yellow, 20 per cent for red and green and 2 per cent for blue.
1.2.19 However, it should be
noted that since the luminance threshold for red light is about half the I
luminance threshold for white light, the effective intensity of a red light
produced by adding a red filter to a white fitting is greater than the
percentage given above indicates. For example, the addition of a red filter
with a 20 per cent transmittance reduces the effective intensity to about 40
per cent instead of 20 per cent of the intensity of a white light.
Candelas
1.2.20 It is the
illumination at the observer's eye produced by a light which will determine if
the light will be seen. The illumination produced at a distance V by a light
source having an intensity I, measured in candelas, in an atmosphere having a
transmissivity (transmittance per unit distance) T is given by Allard's
law: v 2
E = (IT ) /v
When the luminance is equal
to E, the minimum perceptible luminance, the light can just be seen and V is
the visual range of the light. Values for the minimum perceptible luminance for
use in determining the visual range given in Annex 3, Attachment C are:
1.2.21 The relation between
transitivity T, distance V, and the ratio of intensity to luminance, ME, is
shown in Figure 1‑1. Intensities of lights used in aerodrome lighting
range from about 10 candelas to 2 000 000 candelas. The transitivity of the
atmosphere varies over an extreme range, from more than 0.95 per kilometre in
very clear weather to less than 10‑‑"(' in dense fog.
1.2.22 As is evident from
Figure 1‑1, when the atmosphere is clear, a light of relatively low
intensity can be seen a long distance. Consider, for example, night‑time
conditions in which the transmissivity is 0.90 per kilometre. Then, for a light
having an intensity of 80 candelas, I/E would be 801.8 or 100, and the visual
range would be about 7 kilometres. However, in fog, the law of diminishing
returns takes effect at relatively short distances. For example, if the
transmittance was 10‑2(' per kilometre (thick fog), a light with an
intensity of 80 candelas could be seen about 0.17 km, and a light having an
intensity of 80 000 candelas could he seen only about 0.3 km. Consequently, it
is often not possible to obtain sufficient guidance from runway edge lights in
category 11 and 111 conditions by increasing the intensities of lights which
were designed for use in clearer weather. Changes in configuration and
decreases in spacing are required. Therefore, touchdown zone and closely spaced
centre line lights were added to the runway lighting system in order to
decrease the distances at which lights were required to be seen.
1.2.23 Another effect of the
atmosphere which must be considered is the marked difference the atmospheric
transmittance makes in the appearance of lights, e.g. an 80 000 candela light
which could just be seen at
Coverage
1.2.24 Early aeronautical
ground lights consisted of hare lamps or bare lamps with clear glass covers.
Emitted light had essentially the same intensity in all directions. As the need
for higher intensifies developed, lights with reflectors, lenses or prisms were
put into service. By redirecting light emission from unnecessary directions
toward directions where needed, the intensity in desired directions was
increased without increasing power consumed. In addition, annoying glare from
nearby lights was reduced by redirecting some of the light emitted in
directions from which it is viewed only at very short distances to directions
from which it is viewed at greater distances at longer visibilities. The
narrower the beam produced by the optical system, the higher the intensity of
the light within the beam.
1.2.25 In theory it is possible to design an
optical system for a light so that, for any fixed line of approach and for any
given atmospheric transmissivity, the peak intensity of the light beam is
directed at the point at which the light will first be seen. As the distance
between the aircraft and the light decreases, the intensity in the direction of
the aircraft decreases, so that the brightness of the light is constant.
(Paths, which go directly toward the light, are excluded.) Thus, it is possible
to design a beacon so that, for any selected atmospheric transmissivity, the
flashes will have constant brightness when viewed from an aircraft flying
toward the beacon at a fixed height above it. Such a design minimizes the
amount of power required to obtain the desired visual range. However, aircraft
do not fly only one path in one visibility condition. Hence, it is necessary to
design the beam patterns of aeronautical ground lights to cover a range of
paths and of atmospheric transmissivities.
1.2.26 These principles have been observed in
the determination of the beam spreads of the lights specified in Annex 14,
Volume 1, Appendix 2.
1.2.27 It should be noted that when operational
requirements change, a redesign of the light beam is usually required in order
to obtain optimum performance. For example, with the advent of jet aircraft,
coverage by the aircraft beacon was required at altitudes ranging up to 9 000
m. A simple elevation of the beam of the beacon was not sufficient, as this
would decrease significantly the coverage at the altitudes used by small
propeller aircraft, as indicated in Figure 1‑2, curves a and b. Instead,
by means of a change in lamp design, the vertical beam spread of the beacon was
increased to provide the coverage shown by curve c. Similarly, modification of
the beam spread of approach and runway lights designed for category 1 operation
is required to obtain the desired coverage for category 11, and especially
category
The human element in ground visual aid use
1.2.28 There are numerous factors that
determine how effectively pilots react to visual aids‑ in sensing,
understanding and acting on elements of guidance and information viewed during
an approach. While it would not be possible to review the cause and effect of
all of the problems, the following are those associated with system design and
visual cues within the environment and the possibility of pilot error arising
during the approach and landing operation.
System
standardization
1.2.29 The pilot always views the approach and
runway lighting system in perspective, never in plan, and only in the better
meteorological conditions will the complete system be in view. The pilot, while
proceeding along the approach path, often has to interpret the guidance
provided by a "moving visual segment of lights which will move down the
wind‑shield. The length of this segment will vary according to aircraft
height and the slant visual range from the cockpit. (See Figure 1‑3.) The
amount of information that can be absorbed from a comparatively short length of
the approach light pattern when viewed at high speed in low visibility is
strictly limited. Since only a few seconds of time are available to see and
react to visual aids in the lower visibilities, simplicity of pattern, in
addition to standardization, is extremely important.
Individual
differences
1.2.30 Visual acuity and sensitivity to glare
vary from pilot to pilot and are partly determined by age, fatigue and
adaptation to prevailing light levels. Moreover, a given pilot's abilities,
reactions and responses will vary, depending on his temporary state. Also, the
visual guidance system must be able to accommodate the less proficient pilot as
well as the average and better than average pilot.
Mechanics of
seeing
1.2.31 In order for guidance to always be
presented to the pilot in the best possible manner, two important factors must
be considered. First, it is essential that the intensity setting is carefully
matched to ambient conditions. Second, the intensities of the various
individual sections comprising the whole system must also be carefully matched,
particularly where colour is used. These two factors ensure that the pilot
neither misses a vital cue, such as the green threshold lights, because the
signal is too weak, nor is dazzled because certain lights are too bright for
the prevailing conditions.
1.2.32 There are two reasons why approach and
runway lighting systems are provided with patterns that emphasize the centre
line. One obvious reason is that the ideal landing position is along the centre
of the runway. The other is that the fovea of the eye, the region of sharp
vision, is only about 1.5 degrees in width.
1.2.33 Studies have shown
that the average time required for a pilot to switch from outside visual cues
to instruments and back to outside cues is about 2.5 seconds. Since high
performance aircraft will travel about 150 m in this time period, it is
apparent that in so far as possible the visual aids should provide the utmost
in guidance and information, enabling the pilot to proceed without the
necessity of cross-checking the instruments. Other crew members are used to
announce critical instrument-derived information, a procedure enhancing the
safety of low visibility operations.
Visual workload
1.2.34 The data processing
capability of the pilot is great if certain conditions are met, particularly
when the situation unfolds as expected and succeeding cues confirm what has
gone before. In this case, the pilot can attend to a rapidly evolving pattern
of data, retain his ability to assess the situation, and execute a series of
appropriate responses finely adjusted in time and degree. The pilot's capacity
to process information may break down where input data do not agree with
expectations and are ambiguous or transitory. In this situation, the pilot may
fall victim to "tunnel" thinking and press ahead in an approach when
conditions actually call for a missed approach.
1.2.35 The above considerations suggest that it
is extremely important to ensure that the visual guidance system functions as a
system. Component elements must be in balance with regard to intensity and
spacing, ensuring that the pilot sees a pattern which is recognizable as the
expected standard system, rather than a disorderly collection of unco-ordinated
elements, with some overpowering the perception of others. Visual workload is
best moderated by standardization, balance and integrity of elements. A ragged
system with many missing lights can break the pattern from the pilot's eye
position, restricted as that position is by cockpit cut-off angles and possibly
by patchy fog or other conditions. It is possible when transitioning from the
instrument panel to become momentarily disorientated in a ragged or visually
unbalanced system.
Visual illusions during approach to landing
1.2.36 Pilots often are faced with complex
visual problems when approaching a runway which lacks visual or non-visual aid
guidance along the correct approach slope angle. Some of these problems are
commonly classified as visual illusions, although rather than false or
misleading cues, there is an actual absence or scarcity of visual cues upon
which to base height/distance judgements. In reviewing visual approach problems
below, an assumption has been made that visuallnon-visual aids are not
available (or not in use if available) to guide the pilot along the approach
slope to the runway.
Terrain-related problems
1.2.37 By day height/distance judgement
problems occur when approaching runways from over large bodies of water,
featureless terrain (including snow-covered terrain), and terrain that is
depressed below the horizontal plane of the runway in the form of deep valleys,
mountain top airports, steep slopes, etc. This is due to absence or reduction
of normal visual cues that complicate height/distance judgements. For this same
reason, height/distance judgements are difficult on dark nights where there is
an absence of sufficient extraneous lighting in and around the approach area.
However, extraneous lighting within deep valleys, along steep slopes, etc. can
complicate the decision-making process because pilots may think they are too
high when on the correct approach slope to the runway. Compensatory manoeuvring
is likely to place aircraft on a low approach slope angle to the runway.
1.2.38 Take‑offs over large bodies of
water or barren land during haze conditions, even during daylight, can prove
dangerous for pilots unable to operate aircraft with reference to flight
instruments. This problem is accentuated for such pilots if visual cues are not
visible following take‑off without requiring them to turn their heads
over large angles to establish ground visual reference. Inclining the head when
the aircraft is turning causes disorientation, known as vertigo, and is often
accompanied by nausea. Flight instrument discipline is necessary to overcome
vertigo; thus, if pilots are not instrument qualified, hazardous consequences
may develop.
1.2.39 Experienced pilots carry an
"ideal" perspective image of the runway in their minds; consequently,
runways sloping upward will tend to cause pilots to approach below the normal
approach slope angle and runways sloping downward will tend to cause pilots to
approach above the normal approach slope angle. Because the average
longitudinal slope of a runway should not exceed 2 per cent (1 per cent where
code number is 3 or 4) the error introduced would not normally create a serious
problem. It can be seen, however, that general area conditions can combine to
lessen or increase the total effect. For example, an approach to an upslope
runway from over a deep valley would increase the tendency for pilots to
approach below the normal approach slope angle to the runway.
1.2.40 Pilots unfamiliar with flying techniques
associated with mountainous terrain may initiate lower than normal approach
angles to runways when landing toward mountain ranges. This is due to the
apparent horizon being above the true horizon, causing erroneous judgement as
to the correct relationship of the aiming point on the runway below the true
horizon. If approaching over unlighted terrain on a dark night, the danger of
undershooting the runway is increased
.
Approach and
runway lighting ‑related problems
1.2.41 Because bright lights appear nearer than
less bright or dim lights, maintenance of a reasonably balanced intensity for
approach and runway lighting plays an important role in height/distance
judgements during approach. In considering the problems associated with
illusory perception. this factor is most important when visibility permits
pilots to view both the approach and runway lighting systems during approach.
Since approach lighting patterns afford weaker cues to height/distance
judgements than runway lighting patterns, bright approach lights combined with
dim runway lights are more of a piloting problem than the reverse situation.
Brighter approach lighting attracts pilot attention, and due to bright lights
dominating the scene and also screening‑off more distant runway lights,
height/distance cues are degraded
.
1.2.42 If one side of a runway contains bright
lights and the other side contains dim lights, pilots will tend to bank away
from the brighter side toward the weaker side in an effort to balance
intensities. Normally, both sides of runway lighting afford a good balance, but
one side of a runway can become dimmer than the other side when electrical
grounding occurs on one side or when snow ploughing or blowing operations (or
cross‑winds) deploy snow along one of the runway edges.
1.2.43 It is desirable that pilots operate
aircraft to runways having uniform spacing between rows of edge, touchdown zone
and centre line lights and between individual lights within the system. There
are indications that some pilots initiate flare when the rows of lights begin
to blend or take on a certain linear appearance.
1.2.44 Operating into a shallow ground fog can
prove quite dangerous because the approach and runway lighting systems, which
are visible through the fog as the approach descent is conducted, shorten
rapidly or disappear entirely when the aircraft approaches and enters the top
of the fog layer. In shallow fogs, lighting cues are lost at low heights and pilots
flying strictly visually during the transition from visual cues to loss of
visual cues can receive a false impression of the aircraft climbing rather than
descending. Reacting to the impression of aircraft climb, initiating an even
steeper rate of descent from a low height without visual cues, or at best very
limited visual cues, will cause the aircraft to impact terrain or the runway at
a high rate of descent.
Runway dimension
and contrast‑related problems
1.2.45 Runways of varying widths and lengths can
cause pilots to misjudge the approach slope angle because wide/long runways
will appear to be closer than will narrow/short runways. Pilots of large
aircraft normally fly in and out of airports affording reasonably uniform
perspective images. Pilots of small aircraft can operate into runways having
greatly varying widths and lengths; thus, the small aircraft pilot is normally
the one most often experiencing an approach and landing problem related to
runway configuration and will tend to operate at lowerthan‑normal
approach slope angles into large runways.
1.2.46 Operating aircraft toward the sun on
clear days during an approach can involve extremely difficult visual problems.
Under some conditions glare interferes with vision to the extent that the runway
is difficult to locate and when located, difficult to observe throughout the
approach. In addition to the glare problem, runway contrast is changed
(normally reduced) because the angle of the sunlight to the runway results in a
"backlight" on the texture surrounding the pavement and also lowers
contrast of runway markings.
1.2.47 Just as pilots approaching a runway are
most attracted by bright lights, similarly pilots are attracted by pavements
having greatest contrast with their surroundings. Thus, aircraft have been
landed on taxiways running parallel to runways due to the taxiway being sighted
first, followed by concentration on the taxiway throughout the approach and
landing. Such an incident occurred involving a pilot of a large transport at
his home base when heavy snow had been removed from both the runway and the
parallel taxiway, the black taxiway providing excellent contrast with the snow
while the concrete runway provided poor contrast with the snow.
Experience‑related
problems
1.2.48 Changes in experienced or accustomed
visual cues can cause illusory perception problems. Pilots accustomed to flight
over large trees can approach runways at lower than normal angles when flying
over "scrubby" trees appearing to be of the same variety as the larger
trees. Pilots who fly over basically flat land can experience difficulty in
judging an approach to a runway located in rolling or mountainous terrain.
Another example would be pilots, experienced in flight over heavily built‑up
areas, operating aircraft to runways located in open areas devoid of either
constructed or natural large vertical objects
.
Aircraft‑related
problems
1.2.49 Pilots of course will be able to make
the best possible use of ground visual cues and aids when aircraft windshields
are clean and free of precipitation. Rain‑swept windshields cause ripples
and blurs, which distort vision. Geometric patterns of ground visual aids can
be destroyed, making it difficult, if not impossible, to properly interpret the
design functioning of visual aids. There is evidence that with rain on
windshields, objects appear lower than they actually are ‑ a deception
which may lead pilots to fly lower than normal approach slope angles to
runways. It is apparent that pilots should make the best possible use of rain‑removal
systems (windshield wipers, pneumatic rain removal, chemical rain repellents)
when approaching to land in conditions of heavy rain.
1.3 OPERATING REQUIREMENTS
General
The operating requirements for visual aids vary
according to the type of aircraft being flown, the meteorological conditions at
the destination, the type of radio navigation aid used for the approach, the
physical characteristics of the runway or taxiway, and whether or not landing
information is available through radio communications.
Small airports
1.3.2 Airports designed for small single‑engine
and light‑twin aircraft below 5 700 kg are often not provided with
instrument approach aids or air traffic control facilities. Thus, at many small
airports, ground visual aids must satisfy all of the operating requirements of
pilots. Some of these airports may not be provided with paved runway surfaces ‑
a situation which adds to the problem of providing pilots with adequate visual
aids.
1.3.3 The operating requirements, listed in the
sequence normally used by pilots, are:
a)
airport location;
b)
airport identification;
c)
landing information:
1) wind direction and speed;
2) Runway designation;
3) runway status ‑ closed or usable;
4) preferential runway designation (normally for
noise abatement purposes, wind direction and speed permitting the runway to be
used);
d)
circling guidance;
e)
final approach guidance to touchdown:
1) runway edge and threshold delineation;
2) Approach slope guidance;
3) aiming point guidance;
4) runway centre line delineation;
Note.‑
Runway centre line delineation is not feasible for unpaved runways. Such
runways are normally used only in conditions of good visibility. Thus, centre
line delineation is not as important as it is at airports where low
visibilities are authorized in conjunction with an instrument approach aid.
f) roll‑out
guidance:
1) runway centre line delineation (See note
under e) 4) above);
2) runway edge delineation;
3) exit taxiway location;
4) Runway end indication;
g)
taxiing guidance:
1) taxiway edge and/or centre line delineation;
2) information signs (location and destination
signs) to parking and servicing areas;
3) information signs (location and destination
signs) to departure runway;
h)
departure information;
Note.‑
The information needed is the same as that listed in c) above; however pilots
normally obtain all such information prior to leaving the Operations Office
without reference to visual aids.
i) take‑off
guidance:
1) runway centre line delineation (See note under
e) 4) above);
2) runway edge delineation;
3) runway end indication.
Large airports
1.3.4 Large airports are normally provided with
radio navigation aids and air traffic control facilities requiring radio
communications. When used in Visual Meteorological Conditions (VMC) without
these aids, the requirements for ground visual aids are the same as those
stated for small airports. In addition, large airports are provided with
guidance systems to park aircraft on stands as well as visual docking guidance
systems at terminals equipped with passenger gangways (aerobridges). Effective
apron illumination also is needed to assist in parking aircraft and protect
passengers moving to and from aircraft at terminals.
1.3.5 When weather is below VMC, the role of ground
visual aids becomes more vital to flight operations safety. Flights conducted
in Instrument Meteorological Conditions (IMC) require visual aids in addition
to those listed above for small airports. The following additional operating
requirements concern the four categories of instrument runways.
Non‑precision
approach runway
Final approach guidance to touchdown:
‑Centre line alignment guidance for a
distance of at least 420 m before the threshold.
-An indication of distance 300 m before the
threshold.
Precision approach
runway ‑ category I
Final approach guidance to touchdown:
- Centre line alignment guidance for a distance
of 900 m before the threshold.
‑ An indication of distance 300 m before
the threshold.
- Touchdown zone guidance.
Precision approach
runway ‑ category I1
Final approach guidance to touchdown:
Centre line alignment guidance for a distance
of 900 m before the threshold.
‑ Indications of distance 300 m and 150 m
before the threshold.
‑ Touchdown zone alignment guidance for a
distance of 300 m before the threshold.
-
Touchdown zone
guidance.
Roll‑out guidance:
‑ Distance remaining information.
Taxiing guidance:
‑ Exit taxiway guidance.
- Taxiway centre line delineation with change
of direction coding.
Precision approach
runway ‑ category Ill
The operating requirements for visual aids in
category III meteorological conditions are, from the standpoint of
configuration for approach and landing, the same as those provided for category
II meteorological conditions. Photometric characteristics of lights adequate
for categories 1 and II operations need to be modified to provide increased
vertical coverage, especially for large ,,eye‑to‑wheel height '
aircraft.
1.3.6 Although pilots operating in category Ill
meteorological conditions are provided with the same visual aids used in
category II conditions, the opportunity to obtain visual guidance from the
system decreases in proportion to the lower meteorological conditions
encountered during the approach. Normally, visual guidance in the higher
category IIIA visibilities is established with the approach lighting system,
enabling the pilot to judge flight path with respect to alignment with the
centre line. It is not possible to judge the approach slope using visual aids
in such low visibilities.
1.3.7 When operating on the surface in category
III meteorological conditions at major airports, additional visual signals
designed to separate aircraft are needed. Two examples of these signals are
stop bars and clearance bars, as contained in Annex 14, Volume I, Chapter 5. This requirement also applies to major airports
in better visibilities, but the requirement is stated in this section because
the need is greatest when the visibility is lowest. Such a system is not a
visual guidance requirement but a requirement to prevent collision between
aircraft operating on the surface, with particular emphasis on separation of
aircraft movements on the landing and take‑off runways from other slow‑moving
taxiing aircraft.
1.4 How Visual Aids And Visual Cues Serve
Pilots
General
1.4.1 Establishing and maintaining dynamic
three-dimensional orientation with the runway during approach and landing are
complex, difficult piloting tasks, particularly during conditions of limited
visibility (IMC). Once on the ground, taxiing aircraft in poor visibilities
require continued reliance on visual aids up to and including the point of
docking. Section 1.3 lists operating requirements for small and large airports.
This section
concerns the interrelationship between a pilot,
the aircraft, and the visual and non‑visual aids provided for use,
particularly emphasizing how ground visual aids provide information and
guidance.
1.4.2 The frame of reference. Insight into the
importance of this pilot/machine relationship relative to visual flight can be
gained by observing a pilot seated at the controls of the aircraft; the
vertical seat adjustment used by the previous pilot is seldom satisfactory.
After moving the seat forward to a comfortable position at the flight controls,
the pilot usually releases the seat's vertical lock, looks out over the
aircraft nose with head and body erect, then moves the seat vertically until
the eye position seems "just right' relative to the bottom edge of the
windshield and the horizon, i.e. the frame of reference for visual flight. Some
pilots adjust the eye position to a fairly high point; others will prefer a
lower point. Each pilot senses where this eye position should be, based upon
past flight experience. This eye position is an aid in judging the angle of the
aircraft with visual aids while approaching the runway, the most important
angle being the intersection of the flight path of the aircraft with the ground
‑ the aiming point. The pilot's eye position also fixes the over the‑nose
vision angle, which is commonly called the cockpit cut‑off angle. The
bottom of the windshield also is used to establish and maintain visual level
flight and to assist in judging the angle of bank with the horizon or
transverse components of visual aid systems when the horizon is obscured. Thus,
it can be seen that the windshield of the aircraft plays an important role as a
pilot aid in visual flight.
1.4.3 Late generation transport aircraft are
equipped with alignment devices which assist pilots in positioning their eye
height so that the forward down‑vision (cockpit cut‑off) will
coincide with the design eye position for the aircraft being flown. These
alignment devices are quite simple, low in cost, and easy to use. They are
especially important for aircraft where the body angle is in a high (i.e. five
to ten degrees) nose‑up attitude during the approach and touchdown. An
example will serve to explain their construction and function. Three small
balls are mounted in a triangular configuration aft of the centre windshield
post that serve both the pilot and co‑pilot positions. Seats are adjusted
vertically, as well as fore and aft, so that one of the rearward balls is
aligned with the forward (central) ball ‑ pilots use the rearward ball
located on their side and co‑pilots use the rearward ball located on
their side. When the rearward ball obscures the forward ball, the eye coincides
with the design eye position for the aircraft.
Visual aids for visual meteorological
conditions (VMC)
1.4.4 The dynamics of the visual world as seen
by pilots should be understood. Ordinarily in speaking of perceived motion,
reference is made to the "movement of an object". In dealing with a
pilot's use of visual aids, however, it is apparent that what is involved is
the "movement of the observer" which is accompanied by expansion of
the visual scene as the pilot directs the aircraft toward the runway. The point
towards which the flight path is directed is the centre of expansion ‑
the point where visual cues are motionless. The speed of visual cues increases
outward from the centre, but decreases in speed and approaches zero speed at
the horizon.
1.4.7 The following visual aids, where provided
and where radio communication is not available, are viewed by the pilot usually
from a position close in and at a height well above the traffic pattern
altitude so as to avoid other aircraft operating in the pattern. (The colour of
these aids should provide maximum contrast .,,ith the surrounding terrain.) The
pilot then proceeds to join the appropriate traffic pattern in preparation for
landing.
Landing
information
1.4.8 Wind direction indicators are important
visual aids particularly at airports where landing information is not available
through radio communications. Landing direction indicators are seldom used due
to the necessity, and consequently responsibility, of changing their direction
as wind direction shifts. Visual ground signals for runway and taxiway
serviceability are contained in Annex 2. (See also Chapter 3 of this manual.)
Annex 14, Volume 1 contains specifications for runway designation markings.
1.4.9 A fabric wind cone is generally the type
preferred by pilots because it provides a general indication of wind speed.
Cones that extend fully at wind speeds of about 15 knots are most useful since
this is the maximum cross‑wind landing component for small aircraft. An
extended cone at 90 degrees to the landing runway would signal very useful
information to pilots.
Circling guidance
1.4.10 In VMC, most landing traffic patterns
require initial entry at a 45‑degree angle with the down‑wind leg
(Figure 1‑4). Pilots position their aircraft on the downwind leg by
judgement of distance from the runway and angle of the runway below the
horizon. Tracking the down‑wind leg is normally not a problem since the
crosswind component is usually quite low. Aircraft height during the down‑wind
leg is controlled with reference to the aircraft altimeter and the horizon
ahead of the aircraft.
1.4.11 The runway threshold is used as a
reference point for establishing the base leg. Pilots of small aircraft may
start the turn to base leg as the aircraft passes beyond the threshold; whereas
pilots of large aircraft extend the down‑wind leg to establish a longer
final approach leg. Pilots watch the runway angle decrease with respect to the
aircraft so that they can turn toward and intercept the final approach course
as the runway rotates to a point perpendicular with the horizon. The pilots of
all these aircraft have the same requirements: the need to fix their position
relative to the threshold and guidance to pick up and hold the extended runway
centre line on final approach.
Final approach,
flare and landing
1.4.12 This phase of piloting an aircraft is
quite difficult and involves complex estimates of distance, height, drift and
angle of flight with respect to the runway. Both novice and experienced pilots
see the same visual world from a cockpit, but experienced pilots, whether
consciously perceived or unconsciously incorporated into their reactions, are
able to make fine rather than gross visual assessments and are therefore able
to fly aircraft with greater precision.
1.4.13 When aircraft are operated in VMC, the
weather minima usually assure the pilot a horizon reference for piloting the
aircraft using outside visual cues. The horizon may be the real horizon or it
may be an apparent horizon an observed or imaginary horizontal plane reference
line presented by visual ground cues, cloud pattering, or sky/ground lighting
demarcation in the absence of a clear view of the true horizon. When the
landing runway is viewed in good visibility, aircraft location with respect to
the runway environment (unlike IMC) is not a problem. The final approach phase
is divided into a sequence of two parts: first, approaching the threshold and
second, after the runway threshold has been crossed, the landing.
1.4.14 On final approach, the path which the
pilot desires to follow may be regarded as the intersection of two planes -
one, the inclined plane of the optimum approach slope, and the other, the
vertical plane passing through the runway centre line.
1.4.15 To achieve this aim, the pilot m
continuously know six variables:
a) displacement relative to each reference
plane;
b) rate of closure with each reference plane,
i.e. rate information; and
iii)
rate of change of
rate of closure relative to each
reference plane, i.e. rate/rate information.
1.4.16 Pilots are continually matching
displacement and rate indications so as to achieve zero displacement and zero
rate of change of displacement as end conditions; or put another way, they must
know:
a) where they are at the moment;
b) where they are going at that moment; and
c) where they will be in a few moments' time.
The visual indications associated with these
two planes differ greatly and are considered in 1.4.17 and 1.4.18 below.
Azimuth guidance
1.4.17 Zero displacement relative to the
vertical plane (lateral displacement) is indicated by the perspective image of
the runway and of the approach lights, where provided, being perpendicular to
the horizon. Since the runway by itself has considerable length, the visual cue
to displacement (variable 1.4.15 a) above) is instantaneous. The track heading
and the rate of change of track heading (variables 1.4.15 b) and c) above) are
not instantaneous, but errors can be corrected in a manner resulting in minor
deviations from the desired track as the pilot proceeds inbound during final
approach. Thus, the runway, or the runway edge lights, can be considered as
visual cues enabling pilots to align aircraft quickly and maintain alignment
with small deviations from the extended centre line of the runway
.
Approach slope
information
1.4.18 This title does not include the word
"guidance" as used in 1.4.17 relative to azimuth guidance. Visual
approach slope indicator systems provide approach slope guidance, but other
visual aids currently associated with the runway can only provide cues to
approach slope angle. This part concerns the theory of flying an approach where
visual approach slope indicator systems are not provided or, if provided, may
be out of service. When pilots fly visual approach slope indicator systems,
they are relieved of a considerable workload in judging the correct approach
slope angle as seen in the following.
1.4.19 As the aircraft approaches the landing
runway prior to initiating descent for final approach, the pilot observes the
visual cues associated with the runway moving downward along the aircraft
windshield. When the point along the runway at which the aircraft will be aimed
during descent (the aiming point) is depressed below the horizon at the apparent
desired approach angle, the pilot initiates descent by aiming the aircraft at
the selected aiming point. The aiming point selected varies with aircraft size
and runway length available for landing. Small aircraft are normally aimed at
or just beyond the runway designation marking; large aircraft are normally
aimed at or in the near vicinity of the fixed distance markings that are
located 300 m beyond the threshold.
1.4.20 Displacement above or below the ideal
approach slope angle results in a vertical extension and compression of the
perspective image of the runway, accompanied by changes of runway edge angles
with the runway threshold and with the horizon (Figure 1‑5). Experienced
pilots can tell whether they are near this desired approach angle by comparing
the actual runway image with the "ideal" runway image carried in
their minds, an image impressed there by long years of training and practice.
As the aircraft descends, the runway edges converge. As aircraft height
increases, the runway edges spread.
1.4.21 As the aircraft descends through heights
of 45 m to 22.5 m above the runway (depending on approach slope angle and
speed), the pilot becomes more aware of the expanding runway scene as visual
cues are observed to move rapidly outward from the centre of expansion. This is
due to the speed of the "flowing visual field" increasing at a rate
inversely proportional to the distance of the pilot. Thus, it is at these
relatively low heights that the pilot becomes most aware of the precise direction
of the aircraft flight path, sensing the point of zero motion and, if required,
making final adjustments to the flight path to assure a safe landing within the
touchdown zone of the runway.
Flare and landing
1.4.22 Aircraft flare is a manoeuvre in which
the pilot or automatic pilot directs the aircraft flight path from final
approach to a path substantially parallel to the runway surface prior to
landing. The flare may be initiated well ahead of the threshold for large
aircraft and over the threshold for small aircraft.
1.4.23 The visual aids used in flare and
landing are those that mark the threshold, outline the edges of fullstrength
pavement and delineate the runway centre line. By day the edges are normally
seen due to the contrast of runway pavement with surrounding terrain, while
runway edge lights are needed at night. Runway threshold and centre line
markings are used during both day and night. The visual aids provide alignment
guidance. Texture of the pavement surface is the primary source of height appreciation
for both day and night (aircraft landing lights being used at night) unless, of
course, touchdown zone lights are available and used for VMC operations. Runway
lighting and, in particular, runway centre line and touchdown zone lighting
accentuate height appreciation due to blending of the lights into a linear
appearance and the nearer lights change to linear sources (streaking) because
of the high velocities involved.
Roll-out guidance
1.4.24 Rollout commences
immediately following contact of the main landing gear wheels with the runway
surface. Runway centre line markings or lights provide primary visual alignment
guidance during roll-out. Runway edge lighting is used at night to supplement
the centre line particularly where runway centre line lighting is not
available.
1.4.25 Where provided, the
colour coding of the runway centre line lighting assists the pilot in judging
the aircraft's position when decelerating during the roll-out. The coding
consists of alternating red/white lights within the zone 900 m to 300 m ahead
of the runway end and all-red lights within the zone 300 m on up to the runway
end. Touchdown zone markings, where provided for landing in the opposite
direction, also are useful in judging position within the final 900 m of the
roll‑out. The fixed distance markings indicate a position 300 m from the
runway end. Runway end lights mark the limit of the runway available for roll‑out.
Runway exit
guidance
1.4.26 As the pilot decelerates the aircraft to
exit speed, prompt exiting of the runway is important, especially at busy
airports. Where high‑speed exit taxiways are provided, prompt egress can
be realized. Pilots require prior notice of the exit point; because this is
lacking, all too often they are compelled to roll along searching for an exit
which frequently is seen too late for use. Taxiway centre line lighting
extending out to the runway centre line, as provided for in Annex 14, Volume 1,
for other than rapid exit taxiways, is a most useful aid at night. (Pilot head
and eye movement data show significantly reduced movements while taxiing
following landing when compared to movements while taxiing before take‑off.
This may account in part for the difficulties encountered immediately following
a landing, i.e. pilot fatigue and/or after‑effects of the "fixed
stare" adopted for flare and landing are indicated.)
1.4.27 Generally, taxiing guidance inbound to
the terminal or outbound to the runway for departure is not a major problem for
pilots who are familiar with the airport and operating in VMC. Pilots of long‑bodied
aircraft must carefully negotiate taxiway intersections, particularly at night.
The main problems concern the inadequacy of:
a)
location and destination information;
b) snow
removal where taxiway centre line lights are provided;
c)
surface traffic control particularly at intersections with runways; and
iv)
route delineation within large apron areas.
Take‑off
guidance
1.4.28 From a visual guidance standpoint, the
take off phase is not a problem. The pilot taxis into take‑off position,
using runway edge or centre line lights at night to centre the aircraft on the
runway. Alignment guidance is provided by the runway centre line markings
and/or lighting. The runway centre line light coding. Where provided, and the
runway end lights are of primary use when a pilot aborts the take‑off run
during night time.
Visual aids for instrument meteorological
conditions (IMC)
1.4.29 Paragraphs 1.4.4 through 1.4.28
discussed flight in VMC and analysed the design of ground visual aids for
assisting pilots. These same analyses apply to this part when, in conducting an
instrument approach, a pilot goes visual and completes the approach, flare and
landing solely by reference to outside visual cues.
1.4.30 Only experienced pilots, qualified in
instrument flying and in the use of radio, are permitted to operate aircraft in
IMC. However, approaches, landings and take‑offs made in IMC,
particularly in visibilities below 800 m, necessitate the use of more powerful
and more sophisticated visual aids than those needed in VMC
.
Airport location
.
Airport
identification
Landing
information
1.4.33 In order to prevent time‑wasting
and unnecessary missed approaches, it is essential that pilots obtain all
pertinent landing information (ceiling and visibility, wind direction and
speed, runway in use, etc.) prior to initiating an instrument approach
procedure. Those visual aids that provide landing information in VMC serve no
useful purpose in IMC.
The
non-precision approach runway
1.4.34 A straight-in, non-precision approach
procedure should not require a final approach heading change to the landing
runway in excess of 30 degrees (Figure 1-6). Non-precision approach procedures
normally authorize circling manoeuvres to other runways (if any) in addition to
the runway within 30 degrees of the final approach course. The piloting task is
less complicated, and thus safer, where the final approach is aligned with the
landing runway. The degree of difficulty can be considered to be in direct proportion
to the magnitude of heading change of the final approach course with the
runway.
1.4.35 Non-precision approach procedures are
developed so that the aircraft can descend to the minimum altitude established
for the procedure prior to the flight path intersecting the extended runway
centre line (Figure 1-6). Azimuth guidance is provided by the approach lighting
system (ALS) where available. If an ALS is not provided, higher visibility
minima must be applied to allow the pilot time to intercept the extended runway
centre line, using runway contrast with the surrounding terrain or runway edge
lights for visual guidance.
Circling guidance
1.4.36 Circling to land following a non‑precision
approach when the meteorological conditions are at or near the minima
established for the procedure is a piloting task requiring considerable skill.
Pilots must establish visual reference with the runway while flying their
aircraft as low as 90 m above obstacles. The visual cues are similar to those
required for VMC under 1.4.10 and 1.4.11; however, pilots make greater use of
aircraft instruments to assist in maintaining alignment and height. Apparent
size of known objects, apparent motion of objects, eclipsing of one object by
another and gradients of natural texture are important cues to height/distance
judgements during daylight.
Final approach,
flare and landing
1.4.37 As the aircraft is aligned with the
runway following a straight‑in or circling approach, ground visual aids
are used in ways quite similar to those set forth above for VMC operations,
with a few exceptions. Since the horizon is not visible, approach slope angle
(where visual approach slope indicator systems are not available) is obtained
by the height of the runway aiming point above the lower edge of the
windshield. As runway edges become sufficiently visible, convergence of the
angles assists the pilot in judging the approach slope angle to the aiming
point. Alignment guidance may not be instantaneous because a large part of the
runway is obscured during final approach.
1.4.38 Visual approach slope indicator systems
are most important visual aids because many visual cues are obscured by the
meteorological conditions. Pilots are severely handicapped at many locations in
conducting approaches without visual approach slope indicator systems,
especially where approaches are conducted over water or featureless terrain.
The precision
approach runway
1.4.39 The same type of non‑visual ground
aid (ILS) is used for all precision approach categories, the difference being
that greater accuracies are required of both ground and airborne equipment to
comply with certification requirements for operating in lower visibilities.
These greater accuracies are reflected in the flight path envelopes contained
in Annex 14, Volume 1, Attachment A, Figure A‑4.
1.4.40 From the pilot's point of view, the
major concern with operating into lower visibility categories is that, as the
instrument approach is continued to lower minima (and thus the pilot is on
instruments nearer the threshold), the instrument phase is lengthened and the
visual phase is shortened. For example, the normal minimum Decision Height (DH)
is 60 m for category 1 operations and 30 m for category 11 operations; no DH is
applicable to category 111 A and B operations; and, finally, there is no
reliance on visual aids for category Ill C operations. The actual decision
height at an airport will depend on local conditions.
1.4.41 As the instrument phase is flown, the
pilot seeks to know the aircraft's position laterally, vertically and
longitudinally and what the crab angle is likely to be when visual contact is
made with the lighting system. As the approach lighting system is sighted, the
pilot must quickly verify electronic guidance impressions and decide whether to
continue the approach below the DH, if applicable.
Final approach ‑
azimuth guidance
1.4.42 As a short section of the ALS centre
line comes into view, displacement with the centre line can be quickly
ascertained. Where side row barrettes are provided within the inner 300 m of
the system, pilots are provided with additional information concerning the
magnitude of the displacement. About three seconds of time are needed to decide
what the flight path is relative to the centre line (variable b), 1.4.15). If
the aircraft is aligned, the ALS centre line elements appear symmetrical. If
not aligned, the ALS centre line elements appear skewed, and the pilot must
decide whether the aircraft is tracking into the centre line, parallel to it,
or away from it. In either of the latter two cases, the magnitude of the
correction that can be executed safely depends not only on approach speed and
distance from the threshold but also on the aircraft's manoeuvrability and the
runway length available for landing. This vital decision involving many
variables has to be made in a few seconds of time.
1.4.43 Side row barrettes are especially
helpful in the lower visibilities. They accelerate decision‑making due to
their being located in line with the barrettes of the touchdown zone, thus
providing a positive fix relative to the zone on the runway within which the
aircraft should land. This inner zone of the ALS provides excellent cues for
judging the roll attitude of the aircraft ‑ cues that are essential to
maintaining alignment with the runway. When the aircraft arrives at the
category II DH of 30 m, the runway is less than about five seconds away; thus,
the decision to continue the approach is in a large measure based upon whether
the aircraft flight path will be within the side row barrettes.
Final approach -
height information
1.4.44 Approach slope guidance from visual aids
when a visual approach slope indicator system is not provided or is not visible
due to low visibility requires an aiming point to he visible. It is thus apparent
that operations in the lower visibilities of category II and below are
conducted without benefit of visual approach slope guidance (Figure 1-3). When
an aircraft descends below the glide path to heights of about 15 m above the
ALS, the transverse components define a linear-appearing plane where perception
of height is good provided the visibility enables the pilot to see and maintain
a visible segment equivalent to about three seconds of flight time. This is not
an approved or recommended procedure due to the possibility of encountering a
dense zone of fog resulting in a rapid loss or shortening of the visual
segment. Under this condition, pilots can receive an illusion of aircraft
pitch-up to which normal pilot response is to initiate a rate of descent with
the possibility of landing short of the runway threshold (Figure 1-7).
Flare and landing
1.4.45 Prior to the development of runway
centre line and touchdown zone lighting, pilots were faced with an extremely
difficult task when operating in visibilities equivalent to current category II,
and lower, meteorological conditions. The problem was most severe at night and
the condition was appropriately named the "Black Hole". Aircraft
landing lights were useless because the fog was illuminated rather than the
runway surface, creating even a more difficult visual environment. The
development and use of centre line and touchdown zone lighting provide pilots
with azimuth guidance and height information - the solution to the "Black
Hole" problem. The transverse components of touchdown zone lighting supply
roll guidance, the key to maintenance of alignment of the aircraft with the
runway. These lights also delineate the lateral (left/right) and longitudinal
limits of the touchdown zone, particularly for large aircraft.
1.4.46 During daylight hours, runway markings
within the touchdown zone provide azimuth guidance and height information for
category I operations. Markings are also important visual aids for category II
and II1 operations, especially during daylight when background brightness is at
high levels.
1.4.47 The individual lights that compose the
runway centre line and touchdown zone components are seen as point sources when
approaching the runway, but during flare at low heights the nearer point sources
change into linear (streaking) sources. The distance ahead of the aircraft at
which the change from point to linear sources occurs varies with aircraft speed
and cockpit height. The streaking effect is due to the high angular rate at
which the lights move across the retina of the eye; that is to say, they cannot
be fixated by eye pursuit movements. The effect is an increase in the pilot's
impression of height or any azimuth change that may develop. Thus, it can be
seen that when pilots of many aircraft (high landing speeds, low cockpit
heights) land in category 111 B conditions, they will see mostly streaks of
light at night since the point source stage of almost all lights will be
obscured by fog. The streaking effect is not as apparent by day because the
runway surface, composed of millions of textural cues, is visible and is also
streaking
Roll‑out
guidance
1.4.48 As the RVR decreases, pilot reliance
upon runway centre line lighting increases and reaches a point where the centre
line is nearly all that is seen in category 111 conditions. Centre line
lighting and marking are effective for ground steering in very low visual
ranges, especially where the pilot is over the signals. Displacement of 9 m
left or right is usually the maximum encountered, but displacement of this
magnitude significantly reduces the azimuth guidance in the lower visibilities.
Figure 1‑8 shows that these lights will move at a rather large angle with
the longitudinal axis of the aircraft. Pilots will normally steer their
aircraft toward and over (or nearer) the centre line to improve azimuth
guidance under such conditions.
Runway exit guidance
1.4.49 Exit taxiway location can be a major
problem when operating in RVRs below about 400 m unless the green taxiway
lights are extended into the runway according to Annex
14, Volume 1 specifications. Experience has shown that exiting runways
can be slow even in VMC unless lights extending to the runway centre line are
provided. High intensity lighting, halo effects about lights, high ambient
light levels associated with fog, rain on the windshield ‑ these factors,
combined with pilot fatigue following landing, create a firm operating
requirement for good exit lighting in low visibility operations.
Distance information
1.4.50 Approach and runway lighting incorporate
distance information in several stages throughout the full length of the
combined systems. These are outlined in Table 1‑1. Availability of visual
ground aids, which keep pilots informed of their position during low visibilities,
is a major safety feature of the system.
Taxiing guidance
1.4.51 Although taxiing in VMC is normally not
a major problem, taxiing in IMC (particularly at night) becomes progressively
more difficult as visibility decreases even for pilots thoroughly familiar with
the airport. Visual aids required for safe and expeditious movement of aircraft
on the surface are not yet fully developed. Pilots of long‑bodied
aircraft need signals to inform them that aircraft tails are clear of runways
and other taxiways where intersections are close together. Pilots require
advance notice when approaching a curve unless extremely slow taxiing speeds
are maintained for all taxiing. Upon entering the apron, delineation of apron
taxiways is as important as delineation of conventional taxiways. When
departing the apron in low visibilities, locating and identifying the taxiways
to be used can be a major task.
1.4.52 Taxiway centre line lighting, including
apron taxiways, that is switched to delineate the taxiing route, provides the
optimum visual aid solution. Where switching is not provided, well designed,
effective route identification signs are a most important visual aid to pilots
in low visibility conditions.
Docking parking
guidance
1.4.53 In the lower visibilities, centre line
guidance into the docking point is needed to prevent large heading changes when
the docking signals come into view. Docking signals providing left/right
guidance, an indication of rate of closure, and a stop command for the pilot
position that does not require head movement or marshalled assistance describe
the ideal visual docking guidance system. Where docking is not involved, visual
aids are needed to assist pilots in parking within open apron areas, with or
without marshalled assistance, as necessary to clear all other objects in the
parking area. General apron lighting should illuminate parking guidelines as
well as objects that might interfere with aircraft movements and should not
degrade docking or parking signals.
Take-off guidance
1.4.54 Take-guidance is provided by runway
centre line lighting and marking. Once over the system, alignment guidance is
excellent and operations can be safely executed in quite low visual ranges. The
coded centre line for the final 900 m is most valuable in the event of aborted
take-off, since the cues enable the pilot to use judgement in resorting to
emergency braking procedures to stop within the confines of the runway.
