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We take it pretty much for granted that almost 400 tonnes
of 747-400 screaming along at the best part of 1000km/h
manages to find and land at the right point on the right
runway, every time . . . but how do they do it so well?
Instrument Landing
Systems. . . how they work
I
t was a drizzly, miserable day with
a low, grey blanket of cloud, heavy
and oppressive.
Looking out the airport terminal
window, the world seemed to end
barely a kilometre away. Vague silhouettes of aircraft moved silently about
in the misty rain.
I looked up at the arrivals screen to
check my friend’s flight from Perth.
It had already arrived, five minutes
early, of course! Best get to the gate
quickly . . .
As I walked to the arrivals gate I
reflected on what I had just taken
for granted. Just a few minutes ago,
my friend’s plane had been hurtling
towards the ground at 250km/hr with
nothing but solid murky grey out all
of the windows – including those of
the cockpit.
In fact, the plane may have descended to as low as sixty metres above
runway height with little, perhaps no,
outside visibility.
We expect planes to land in all
8 Silicon Chip
sorts of weather. In Europe and North
America, a pilot can land an aeroplane
on some runways without seeing the
ground at all.
Automatic landing and taxying
systems are continually becoming
more capable. Today it is technically
possible to safely and reliably land,
slow down and taxi to the correct gate
with no outside visibility and hardly
any human intervention.
tral challenge is navigation.
In this article we will look at the
Instrument Landing System (ILS),
which still provides precision landing
guidance 65 years after its invention.
We will also see the impact of newer
technologies such as GPS, Microwave
Landing Systems and ever-increasing
on-board digital processing power. But
first, some background.
The issue is guidance
Most readers would be familiar with
basic radio direction finding using a
loop antenna. You rotate the loop until
you find a “null” in the signal. If you
point with your hand straight through
the loop, you are pointing either directly towards or directly away from
the transmitter. Direction finding is
the basis of radio navigation. Instead
of plotting a visual “fix” to a landmark
on a chart, you plot a radio “fix” to a
known transmitter.
Radio navigation today is quite sophisticated, even before you include
So how does all of this work? First,
let’s keep in mind what we’re looking
at. A modern aeroplane can fly equally
well through clear day or foggy night.
The challenge with all weather landings is to provide some form of guidance so that the pilot (or an autopilot)
can stay lined up with the runway and
descend on the correct path without
actually seeing the runway. The cen-
by Daniel Field
Radio Navigation
siliconchip.com.au
Somewhat stylised diagram of a typical runway with ILS. Not shown
here are the inner, middle and outer “markers”. A little trivia: one runway
number subtracted from its opposite end number will aways equal 18!
GPS. Apart from Instrument Landing Systems, three radio navigation
systems have been central to aircraft
navigation for several decades.
They are:
1. ADF (Automatic Direction Finder). Gives the bearing (direction) to the
transmitter relative to the nose of your
aircraft. Combined with “magnetic
heading” information from the aircraft
compass system, it gives the magnetic
bearing to the transmitter.
2. VOR (VHF Omnidirectional
Range). Tells you what your bearing
is from the transmitter, no matter
which way you are pointed at the time.
These fixed lines of bearing from the
transmitter are called “radials” and
are commonly followed as “roadways
in the sky”.
3. DME (Distance Measuring Equipment). Uses pulse timing techniques
to tell your distance in a straight line
from the ground equipment. It is usual
to co-locate DME and VOR ground
equipment to provide range and bearing from the same location.
Pilots use these systems to find
their position over the Earth’s surface
without any visual references. None of
them can determine height above the
ground: they are all lateral navigation
(LNAV) techniques.
The problem of landing and
approach
Using lateral navigation techniques
an aircraft can fly clear of obstacles and
line up with a runway. This procedure
is called a “Non Precision Approach”
(NPA).
During an NPA the pilot uses an
altimeter (which is based on air pressure) for height information. For a
variety of reasons, pressure altimeters
can theoretically be inaccurate by up
to 100 feet (30 metres).
So a published NPA must allow
plenty of margin for error if pilots of
varying skill levels in a wide variety
of aircraft are to follow it thousands
of times per year.
A typical NPA in an area clear of
obstacles can get the aircraft down
to about 400 feet (120 metres) above
the ground. If the pilot cannot clearly
see the runway then he can safely
“go around” – pulling out of the
landing approach and flying in a predetermined pattern to return to the
approach path and again line up with
the runway.
Therefore, without some form of
precise vertical guidance, an aeroplane
cannot land unless the cloud base is at
least 400 feet above the ground.
The Instrument Landing
System (ILS)
As early as 1928 (which, by the way,
is only a year after Lindbergh first
crossed the Atlantic), engineers and
scientists were giving careful attention
to the problem of vertical guidance for
landing. Teams in different countries
developed various solutions.
By 1940 a working “Instrument
Landing System” had been installed
at Indianapolis airport in the United
States.
The Instrument Landing System
(ILS) was a new radio navigation
system that provided precise vertical
guidance (referred to as the “Glide
The heart of the ILS system: at one end of the runway (far end from approach) is the antenna system for the “localiser”
beam. This gives the aircraft its left and right guidance signals to help it line up with the runway. Alongside the runway,
roughly at the touchdown point (ie the approach end), is the guideslope (or glidepath) antenna which helps the aircraft
approach the runway at the right angle and hit the tarmac at the right place.
siliconchip.com.au
June 2004 9
Path” or “Glide Slope”) as well as
precise lateral guidance (referred to
as the “Localiser”).
Both the glide slope and the localiser worked on the same technique:
a radio technique that is still in use
today at thousands of ILS-equipped
runways around the world.
The glide slope carrier signal is
in the range of 329-335MHz. The
localiser carrier is in the range of
108-112MHz. The glide slope and
localizer carriers are each directional
radio beams radiated in two parts: one
amplitude modulated at 90 Hz, and the
other at 150Hz.
In the case of the glide slope, a
directional antenna array radiates
the 90Hz signal just above the correct
approach path and the 150Hz signal
below it. Right on the approach path,
the modulation of both components
is 40%.
An aircraft anywhere along the
correct glide path will receive both
the 90Hz and 150Hz components
equally.
If the aircraft moves above the correct path it moves toward the centre
of the 90Hz beam and away from the
150Hz beam. Because they are both on
the same carrier, the detected depth
of modulation of the two signals is
no longer equal. The 90Hz signal will
seem to have deeper modulation than
the 150Hz signal. While every glide
path is adapted for each particular
airport, a typical path is at an angle
of about 2.5 to 3°.
The aircraft’s receiver detects the
90Hz and 150Hz components then
separates them using a simple filter
network. The two components are
full-wave rectified to produce two DC
signals: one representing the strength
Pilot’s-eye-view of a 747-400, lined up on runway 34R at Sydney International
Airport. This amazingly realistic view is actually taken from Microsoft Flight
Simulator IV, which we’ll have more to say about shortly . . .
of the 90Hz component, and the other
the 150Hz. The difference between
these DC signals drives a moving coil
meter. If the 90 and 150Hz components
are equal then there is no difference, so
the meter stays in its “at rest” position
in the centre of the indicator. If the
90Hz component is stronger, the meter
drives down to indicate “fly down”.
A deviation of half a degree above
or below the glide path gives full-scale
deflection of the meter. This corresponds to a difference in depth of
modulation (ddm) of 0.175, or 17.5%.
In the case of the localiser, a directional antenna array transmits the 90
Hz signal to the left of the runway cen-
tre line (from the point of view of the
approaching aircraft), and the 150Hz
signal to the right. The modulation of
both signals is 20% on the correct path.
Again, the receiver rectifies the two
components and drives a meter movement. Full-scale deflection indicates
about three degrees deviation from
the centre line, with a ddm of 0.155.
How far to go?
Assuming that the pilot has no outside visual cues, the Instrument Landing System that I have described still
relies heavily on the altimeter. Sure,
the pilot knows that he is approaching
the runway on the correct path.
The beam pattern set up by the glideslope (or glidepath) radio
signals. It really is quite simple – fly too high and the 90Hz
signal is received; fly below it and the 150Hz signal is received.
10 Silicon Chip
siliconchip.com.au
Again from Flight Simulator IV, compare the clean, modern instrumentation of
the 747-400 to the instrument panel of a Beechcraft King Air, here lined up on
runway 29C at Bankstown airport, Sydney (incidentally, the busiest airport in
Australia for aircraft movements).
But what is to stop him from staying
nicely on path until he crashes into the
runway? The fact that he crashed right
on the touch down point is unlikely
to be much consolation.
The designers of the Instrument
Landing System decided to place various “Markers” along the approach path
so that the pilot knows what stage of
the approach he is up to.
These markers are low power
transmitters that radiate in a narrow
beam straight up. The carrier is always
75MHz. The AM signal depends on the
function of the marker.
On a normal instrument approach,
the pilot initially uses his altimeter to
fly at a particular altitude (say, 2,500
feet above the ground) and various
radio navigation aids to intercept the
Localiser. The aircraft then flies along
the localiser toward the runway, maintaining a particular altitude (using the
altimeter). As the aircraft flies along,
it is actually below the plane of the
glide slope. If you were in the cockpit,
you could say that the glide slope is
in front of you, slanting down toward
the runway, and you are flying level
towards it.
Imagine for a minute how this works
in the cockpit: As the aircraft moves
into the lower part of the glide slope
signal the indicator shows “fly up”.
The pilot continues to hold the same
altitude. The glide slope indicator
starts to show that the aircraft is coming up to the centre of the glide path.
The pilot then initiates a descent to
capture the glide slope. As long as
the aircraft stays on the glide path, it
is safe to descend.
This is where the designers used the
first marker: The Outer Marker. Right
at the point where the pilot should
intercept the glide slope the aircraft
flies through the outer marker beam.
The pilot hears a 400Hz tone (a moderately low pitch) which also causes
a blue indicator light to illuminate in
the cockpit. The tone and light make
a continuous stream of Morse code
“dashes” at the slow rate of two dashes
per second.
If a pilot passes the outer marker
and still does not have a glide slope
signal then he knows that there is a
problem.
Australian approaches actually use
the Outer Marker at some point along
the descent rather than at the glide
slope intercept. The intercept point
may be directly over some other radio
beacon that does not normally form
part of an ILS.
Sometimes it is not marked at all but
can be anticipated a certain distance
from the airport using radio distance
measuring equipment. Whether or not
the outer marker coincides with the
glide slope intercept, it is an important
indication of the aircraft’s progress
along the ILS.
The next marker is the “Middle
Marker”. This is usually about a kilometre from the runway. The pilot hears
alternate dots and dashes at 1300Hz,
illuminating an amber light in the
cockpit. The middle marker normally
And here’s the way the middle and outer markers are set up. They are very narrow beams which
are received in a very specific location, telling the pilot the plane has passed through the marker.
siliconchip.com.au
June 2004 11
indicates that the aircraft is 200 feet
(60 metres) above the ground.
On basic Instrument Landing
Systems (including most systems
currently in use around Australia),
200 feet is the “decision height”. The
pilot may continue to descend beyond
the middle marker only if he sees the
runway.
Once again, there is some variation
from one approach to another. For
example, there is no middle marker
at Nowra, NSW. The pilot must use
DME to determine decision height (at
a distance of 0.8 nautical miles from
the runway). Perth’s ILS runway 03
has no markers at all. Three of Sydney’s six ILS approaches also have no
marker beacons (because of possible
confusion with markers for parallel
runways). They all use DME distances
instead.
On more advanced instrument landing systems the decision height can
be either 100 feet or zero feet. Those
systems can include an “Inner Marker”
which gives the sound of rapid dots at
3kHz (high pitch) and causes a white
light to flash in the cockpit. The inner
marker normally indicates a height of
100 feet above the ground. Note that at
this point, the aircraft altimeter could
possibly indicate anything from zero to
200 feet above the ground (though, in
12 Silicon Chip
siliconchip.com.au
reality, almost all altimeters in instrument rated aircraft
are likely to be within ten feet of the actual altitude).
Three levels of accuracy
Instrument Landing Systems are theoretically capable
of guiding an aircraft all the way to the ground. But the
very high accuracy and reliability required for this task
comes at a cost. Installations that can guide an aircraft
right down to the ground must be tested and proven over
a period of years, then continually monitored, tested and
maintained to exacting standards.
There is also a paradox that causes more accurate
systems to be less capable than less accurate systems: as
an aircraft travels through the directional localiser and
glide slope beams, it warps them. The signals received
by a following aircraft might not be accurate. The most
precise instrument landing systems depend on much
larger spaces between approaching aircraft than the
less precise systems. As a result, less precise systems
can handle more than double the number of landings
per hour.
None of the systems installed in Australia can guide
an aircraft all the way to the ground. The majority are
“Category One” Instrument Landing Systems (ILS Cat I),
with a decision height of 200 feet. A runway will only be
open for Cat I approaches if the “Runway Visual Range”
is at least 800 metres.
The next level of precision is a Category two ILS. In
a Cat II system the decision height and visibility (“Runway Visual Range”) requirements are half those of Cat
I. That is a decision height of 100 feet and a visibility
of 400 metres.
Category three systems are installed primarily in North
America and Europe. For example, there are 31 Cat III
systems installed at 15 airports in Germany. Nearly all of
these are “Cat IIIB” (see table). Europe is currently moving away from Instrument Landing Systems in favour of
the more capable “Microwave Landing Systems” (MLS).
Improving the system
The basic ILS with moving coil meters is still used
to-day in some private aircraft, older charter planes, and
many instrument-training planes. But these systems are
practically obsolete. Modern business aircraft and airliners only use mechanical instruments as back-ups, if at all.
LCD screens, modular digital computers and data-links
are standard fare in today’s new aircraft.
Autopilot coupling
One of the first refinements of the basic Instrument
Landing System was autopilot coupling. A traditional
ILS receiver puts out a DC analogue signal that drives a
meter movement. If you think of the signal as a command
to “fly up/fly down” or “fly left/fly right” then you can
use it as an input to an autopilot.
The human pilot may manually select “ILS” mode
siliconchip.com.au
June 2004 13
HF Radio
Antenna
“NAV” Antenna
“NAV” Antenna
Above: typical “rabbit ears” VOR/ILS antenna near on the fin of a Piper Navajo
Chieftain, with the longer wire HF antenna at top. The ILS receiver normally
has separate transmission line inputs, one for LOC (~110MHz) and one for
GS (~330MHz). Some aircraft have a separate GS antenna (typically dipole,
mounted inside the nose), while many have just one antenna with a splitter.
Right: a more aerodynamic nav antenna used on aircraft above about 250km/h.
on the autopilot, telling the autopilot
to follow the ILS output commands.
Future displays
Several companies are experimenting with new ways to display ILS
information. One major source of
inspiration is the video game industry.
Current prototypes display a 3D
graphic of the actual surrounding
landforms and hazards such as masts
and towers. The colours on the display indicate potential hazard, from
red (land at or above the level of the
aircraft) through yellow to green (land
far below). Contours are shaded to give
an easy to interpret depiction of the
actual surrounding area.
The display is based on a computer
model of the actual terrain (yes, every
aircraft might carry a detailed digital
model of the entire world in the near
future). Developers and promoters of
these systems often call them “Synthetic Vision Systems”.
During a landing in cloudy or foggy
Aircraft and video games designers might seem to be strange bedfellows but
they have a lot in common. One tries to make games simulate the real thing as
much as possible, the other is incorporating much of the graphics of the games
into the real thing, as this “Synthetic Vision” screen grab shows.
14 Silicon Chip
conditions, a Synthetic Vision System
can display the surrounding area as
if it was a clear day. With suitable
overlays (such as markers showing
the correct approach path, and an
aeroplane graphic displaying pitch
and roll attitude as well as actual position), an approach and landing in poor
weather could become very much like
a computer game.
On-board digital processing
power
The rise and rise of digital technology has hugely impacted the field of
aircraft avionics. One of the first tasks
given to digital processors was to process flight and navigation data using
algorithms designed to make the most
efficient use of resources.
In the area of navigation, this meant
keeping the aircraft right on the most
direct track, and manoeuvring through
standard terminal approach routes as
accurately as possible.
It wasn’t long before manufacturers
started to integrate avionics systems
that had previously been independent. As technology developed through
to the late 80s, various researchers
experimented with the idea of having
one central navigation computer.
By the mid 90s, the World’s major
avionics producers all offered some
variation on a central navigation and
performance computer: the “Flight
Management System” (FMS). Today,
practically all new jets and an increassiliconchip.com.au
Microwave Landing Systems
Europe is moving rapidly away from
the Instrument Landing System in favour of its newer rival, the Microwave
Landing System (MLS).
MLS is not simply an ILS using
different carrier frequencies; in fact
the operating principles of MLS are
completely different.
The purpose, though, is the same:
to give precise lateral and vertical
guidance, as well as distance from
the runway.
The basic technique used in the
Microwave Landing System is a “Time
Referenced Scanning Beam”.
Without going into too much detail, MLS transmits a narrow beam
at around 5GHz that sweeps across
the approach area in a set pattern.
The aircraft receiver measures the
time intervals between sweeps and
calculates its lateral position (azimuth)
and vertical position (elevation).
The “to and fro” azimuth and “up and
down” elevation beams both occupy
the same carrier frequency, although
they are transmitted from two different
antenna arrays located similarly to an
ILS.
The third, essential component of
MLS is a precision DME (distance
measuring equipment) which gives
range accurate to within 30 metres
(compared to 360 metres for regular
DME).
MLS also transmits data to the aircraft
by modulating the azimuth signal. Data
can include information about the approach, weather, runway condition, etc.
There are several advantages of
MLS over ILS. Perhaps the greatest
advantage is its flexibility – ILS has only
one correct path (where the difference
in depth of modulation is zero), so its
output must always be an error signal:
“fly right”, “fly up”, etc.
MLS is designed to tell the receiver
its precise angle from the runway centre
line (to about 40° either side) its elevation above the horizon, as seen from
the touch down point (to about 15°
up), and range from the runway. The
receiver’s output is a position rather
than an error.
The MLS computer in an aircraft
can be programmed with a desired
approach path and then guide the pilot
or autopilot along that path, comparing
the actual position with the desired
position to give the standard “fly right”,
“fly up” signals.
That means that the one MLS
installation can precisely guide many
different approaches at any glide path
angle as well as manoeuvres such as
dog-legs or curves around obstacles
out to a distance of about 35km.
It is reasonable to expect that MLS
will completely replace ILS in Europe
by about 2020, with only a few ILS
installations surviving beyond 2015.
The UK has purchased over 40 MLS
installations (including options) over
the past year alone.
However, the rest of the World
is likely to stick with ILS for several
decades more.
ing number of propeller planes come
with a Flight Management System as
part of an integrated, modular digital
system.
While different levels of integration are available, a fully functional
FMS will have inputs from all of the
on-board navigation and flight data
systems and outputs to the autopilot
computers, digital engine control computers, and various cockpit displays.
During an approach in low visibility
conditions the FMS can handle many
tasks like selecting the frequency on the
ILS receiver, continually monitoring
how well the aircraft is performing, and
commanding the autopilot and engines
so that the aircraft follows a pre-defined
“Standard Terminal Approach Route”
(that’s right: when an aircraft is within
about 30km of its destination it is usually “following a STAR”).
This centralisation of control and
monitoring functions has allowed
automation to move into the part
of flight that uses practically every
system on the aircraft: the approach
and landing.
system is not too hard to imagine. A
computer selects the right navigation
inputs and autopilot modes so that the
aircraft follows the ILS. A radar altimeter input (giving actual height above
the ground, potentially accurate to a
few feet), a precision DME (as in a Microwave Landing System), or a suitably
augmented GPS controls the timing of
the “flare” (the deliberate loss of lift as
the plane lands). Weight-on-wheels
switches detect the actual landing, and
the computer controls deployment of
spoilers, reverse thrust and brakes as
required to slow the aircraft. GPS combined with a database of the
airport layout provides for
the aircraft to automatically
taxi to its gate. (This ignores
taxi clearances and other
aircraft – a data link from
the airport surface move-
ment controllers could provide the
required information.)
However, technical possibility is not
the whole story: If planes full of people
are to routinely “autoland” in all sorts
of conditions then technically possible
is not enough. A reasonable margin of
safety must be a part of the system.
When a plane is landing itself, the
Autopilot system has control of the aircraft. All autoland-equipped aircraft
must have a “triplex” autopilot. That
means that there are actually three
separate autopilot systems installed
in the aircraft. There are various ways
Automatic landing
Technically, an automatic landing
siliconchip.com.au
This cockpit almost looks
like a video game –but it’s
not. It’s from the Eclipse
Aviation E500, a new mini
jet scheduled for release in
2006.
June 2004 15
GPS and all-weather landings...
“What about GPS?” I can hear
you asking. “Doesn’t GPS make the
whole instrument landing system
obsolete?”
The answer is a resounding “not
really”.
In Europe, ILS should be obsolete
by 2015. But that is due to a rival system (Microwave Landing System), not
GPS. Outside Europe, ILS will still be
around for a few more decades.
So why hasn’t GPS taken over the
precision approach scene? Since
selective availability was switched
off isn’t the accuracy down to millimetres?
There are several good reasons
why aviation has not relied on GPS
for precision approaches.
The first is political: the U.S. Department of Defence owns and operates
the GPS constellation. Five years
ago when the Clinton administration
announced that they would switch off
selective availability, they reserved
the right to switch it back on again at
any time.
Despite formal agreements between
the US Dept of Defense and the US
Federal Aviation Administration, there
has always been a tacit understanding
that civil aviation should never rely too
heavily on GPS without extra in-built
safety.
There are other issues with GPS.
Accuracy is excellent, but still variable.
For just a few minutes per day in any
location, the various random errors
combine to significantly degrade the
accuracy.
In the big picture it is hardly significant. But it does mean that you cannot
solely rely on GPS if you require very
high precision on demand. Another
issue is signal availability. The receiver
needs at least five satellites to verify
the integrity of its position solution.
Along the south coast of Australia,
for example, there may be fewer than
five satellites in view for a total of
around 30 minutes out of every 24
hours. Having said all of that, satellite
navigation systems are continually
improving.
Several developments are making
GPS more available and accurate. A
European consortium is developing
a rival system called “Galileo”, which
16 Silicon Chip
will have the political advantage of
civilian control. The International Civil
Aviation Organisation, which sees all
of the major Western countries jointly
determining policies, has decided that
satellite navigation systems will be the
basis for future aeronautical navigation
systems.
So does GPS have a role in all
weather landings?
Well, yes, it definitely does but not
by itself. GPS will be available for
precision approaches once suitable
methods of “Augmentation” have been
developed and tested.
One ground-based system, called
“Local Area Augmentation System”
(LAAS) is basically a GPS receiver
fixed to a precisely surveyed point on
the ground. A computer compares the
actual, known location of the receiver
with its “GPS location”. It instantly
detects any error. The system then
broadcasts information about the error over a data link to all aircraft within
a radius of, say, 50 kilometres. If the
LAAS site is located near an airport,
an aircraft can make a precision approach using GPS data corrected by
the LAAS data link.
Australia is likely to adopt this
system.
In the United States a satellitebased “Wide Area Augmentation System” (WAAS) is under development.
This is similar in principle to LAAS but
on a different scale:
In version one of the system there
are 25 surveyed “Reference Stations”
across the country and two “Master
Stations”. The data link to aircraft is
via communication satellites.
The advantage of covering the entire country is offset by the necessary
compromises in accuracy and integrity.
As a result, WAAS will only be good for
the equivalent of ILS Cat I approaches
(Decision Height of 200 feet).
Even that will only be under ideal
conditions. This limitation ensures the
continued use of ILS in North America,
at least for a few more decades.
Europe is set to adopt a combination of ground based augmentation
and microwave landing systems, with
the Galileo global navigation satellite
system likely to take over from GPS
for essentially political reasons.
of making a triplex system work:
Normally, all three work together.
They each gather their data (such
as airspeed, attitude, deviation from
intended path, etc) and then “vote”
on the action (for example, to roll left
at a certain rate). As long as all three
systems agree, the autopilot is working
in its full triplex mode.
International standards allow
automatic landings only when the
autopilot is working in triplex. If one
of the three systems fails or produces
an error, the aircraft can still fly under
the command of the other two autopilots but the safety of the triplex voting
system is lost. In that case, the pilot
must abandon the automatic landing,
but may continue with a regular instrument landing.
Instead of having three complete
autopilot systems, it is possible to have
a “pseudo-triplex” system. A computer
model that votes according to the aircraft’s expected movements replaces
one of the three autopilot systems.
Conclusion
Every year around the world,
aircraft of all sizes safely make millions of landings in conditions that
make a visual landing impossible.
The Instrument Landing System has
provided precise guidance for landing in these conditions for over sixty
years. According to some authorities,
ILS is likely to be in use for another
fifty years yet.
But in aircraft systems, like so many
other things, technology is continually advancing as individuals look for
better ways and companies look for
a competitive edge. Improvements
in GPS-related technologies, new
capabilities of Microwave Landing
Systems, and the almost limitless
memory and processing capabilities
of digital computers are turning our
heads toward the future.
Stanley Kubrik’s film “2001, A Space
Odyssey” may have proven to be a tad
optimistic in its setting. But anyone
from that sci-fi mad era transported to
the flight deck of a modern airliner as
it approaches and lands in cloud and
fog would surely think that he could
be in a space craft landing on another
planet. A crew of two, calmly watching
the large, clear, uncluttered displays
and checking altitude and system
parameters out aloud while the plane
lands itself: surely this was the stuff of
science fiction not so long ago.
siliconchip.com.au
Ever looked at an aircraft instrument panel?
What are all those meters and things for?
For the uninitiated (ie, non-pilots!) an aircraft instrument panel can be a pretty confusing place. To make matters worse,
every aircraft is different. But once you recognise what each is for and what it does, it’s not so daunting after all. . .
B
A
C
D
G
E
F
H
This is just a tiny section of what a pilot has to keep his/her eyes on. But apart
from radio systems, these are arguably the most important instruments as far
as the pilot is concerned.
(A) Clock (yep, to tell the time)
(B) Airspeed indicator – in knots
(C) Attitude Indicator (the plane’s, not the pilots!)
(D) Altimeter – how high you are above sea level
(E) Turn Co-ordinator (also called “turn & bank” or “turn & slip” indicators).
(F) Radio Magnetic Indicator – displays both magnetic and radio compass.
(G) Horizontal Situation Indicator – shows the localiser beam (the vertical
yellow line) and the glideslope (the two yellow triangles on the edges).
That’s the one that this article is most concerned with!
(H) Rate of Climb Indicator – tells you how fast you are going up or down.
And here’s how the loc/glideslope indicator helps you land...
If the receiver is not receiving a strong
enough signal, or if the signal is not
valid, then a red “NAV” flag on the
indicator warns the pilot not to follow
the indications.
Having used other radio navigation
aids for lateral guidance and the
altimeter for height, the aircraft is now
lined up with the runway centre line,
about 25km from the runway. The
localiser indicates “on localiser” but
there is no glideslope signal yet, so the
plane does not descend.
Having maintained altitude and
followed the LOC, the plane is
approaching the GS intercept. There
is now a GS signal, indicating that the
plane is below the glide path (ie, a “fly
up” indication).
“Fly Down” half scale ~
0.25° above glidepath (the
two yellow indicators
are below the horizontal
reference). “Fly Right” one
dot (on 5-dot scale) ~ 0.5°
left of path (the vertical
yellow line is to the right
of the vertical reference).
“Fly Up”,
“Fly Left”
“Fly Down”,
On LOC
Continuing to hold the same altitude
and follwing the localiser, the plane
is now only about 0.2° below the glide
path. The pilot (or autopilot) will start
initiating a descent soon.
“Fly Up”,
On LOC
The plane descends along the ILS
and simply follows any “fly up”, “fly
down”, “left” or “right” indications. By
doing so it is flying precisely along the
correct approach path.
On glidepath,
On LOC
SC
siliconchip.com.au
June 2004 17
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