Assured of administrative stability under United Air Lines, personnel at LAES pressed forward in the development of the Integrated Landing System which they had been formulating during the previous months. Despite difficulties in procuring suitable materials, in obtaining access to privately owned land in the approach zone, and in securing the necessary technical personnel, construction of a working version of the Integrated System was accomplished with remarkable speed, in time for a revealing series of tests during the 1946 fog season.
The Bartow Type D_1 Runway Maker Light was installed along the instrument runway. This required the laying of 12,000 feet of conduit and the pouring of 72 concrete light bases. An entirely new type of approach lighting installation was constructed in the approach zone, requiring some 7,000 feet of conduit, the erecting of 36 steel towers varying in height from 7 to 84 feet, set on concrete bases, and the installation of a 600 kw portable generator and 36 individual transformers. (This is the Funnel Approach Light System devised by W. T. Harding of the Air Materiel command at Wright Field. A special Wide-Angle High Candlepower approach light was Constructed for AMC by the American Gas Accumulator Company, Elizabeth, N.J., and mounted on the towers at LALES.)
I addition to new lighting equipment, major alteration to the high pressure FIDO system were carried out during the summer of 1946, and the low pressure systems were restored to good condition after a winter's neglect. Similar neglect had rendered the ISL system inoperative. Portions of the equipment corroded by salt spray were replaced, and marker beacons were installed in the approach zone. Various expedients were devised to protect the equipment from weathering and to assure stability of the radio beams. To complete the electronic landing aids system, a GCA unit was obtained from Navy stock and technicians were located to operate and maintain it.
Despite the tremendous
amount of new construction and repair work required, and the delays
occasioned by material shortages, lack of personnel and equipment,
and the difficulty of obtaining access to privately owned lands
upon which some of the installations must be located, approximation
of the Integrated landing system was achieved in time to conduct
a series of test during the months of September and October of
1946. These 14 flight test series, in which over 100 landings
were made during restricted visibility conditions, conclusively
demonstrated the feasibility of making aircraft landing a routine
procedure even in zero weather. Furthermore, it was shown that
there is no reason why this system should not be put in use at
once on commercial fields throughout the country.
II COMPONENTS OF THE INTEGRATED LANDING
SYSTEM
The Integrated Landing System, proposed
by the research staff of the Landing Aids Experiment station,
consists of the following basic units:
(1) Electronic low approach aids which will assure that the plane is brought safely down the approach zone to the runway at an altitude suitable for landing.
(2) A suitable method of marking the approach and runway areas so that the pilot will be certain of his location and have adequate visual reference points for completing a contact landing.
(3) A fog dispersal system for extreme
visibility conditions, which will nsure
that the final approach and runway areas are visible to the pilot
as he terminates his approach and sets his plane on the ground.
Figure l is an aerial view of LAES
looking down the instrument runway.
The backbone of the Integrated
Landing System is an electronic low approach aid. Tests at LAES
have indicated that either the Instrument Landing System (SCS-51)
employing radio beams, or the Ground Control Approach radar unit,
can be successfully applied.
A. Instrument Landing System
The ground equipment of
the ILS consists of two radio beam transmitters known as the Localizer
and the Glidepath and three marker beacon transmitters. In addition,
the aircraft must carry a special receiver tuned to the frequencies
emitted by the ground transmitters.
The Localizer, Fig. 2, is
installed beyond the terminal end of the instrument runway along
the extended centerline. It consists of a directional antenna
array mounted on an A-Frame, and a transmitter which may be housed
advantageously in a vault underneath the antenna. The transmitter
generates a carrier wave of 108.3 to 110.3 megacycles. A mechanical
modulator generates frequencies of 90 and 150 cycles which are
impressed upon the carrier wave. Bridge circuits and appropriate
phrasing result in the propagation of a 90 cycle pattern on the
left side of the runway and a 150 cycle pattern on the right (as
viewed from the approach zone).Over the centerline, the overlapping
of the two signals produces a narrow region within which the two
signals are of equal strength. The airborne receiver measures
the relative strength of the two signals and indicates the result
to the pilot by deflecting a vertical needle to the left or right
proportionally. Since the signal strengths vary directly with
the distance of the plane from centerline, the amount of needle
deflection shows an experienced pilot how much correction must
be applied to bring the plan
back to the centerline. For convenient interpretation, the circuit
is arrange to deflect the needle.
The glidepath transmitter,
fig. 3, located 1180 fet
up the runway from the threshold, operates in a similar fashion,
except that it generate a carrier wave of 332.6 to 335 megacycles.
The narrow zone of equal signal strength slants upward from ground
level at a small vertical angle which may be adjusted to clear
any obstruction in the approach. (The normal angle is 2-3/4.)
In the aircraft a horizontal needle is deflected up or down in
proportion to the relative signal strengths, in a fashion similar
to that employed for the Localizer needle.
The two needles are combined
into one instrument, Fig. 4. when they cross at right angles,
the aircraft is flying the true glidepath.
In addition to flying along
the proper slanting line down the approach, it is important that
the pilot landing in rain know the distance of his plane from
the touchdown point. For this purpose, Marker beacon transmitter,
fig. 5, are located along the approachway centerline at selected
distance from the runway threshold. These transmitters operate
at 75 megacycles and project a coded signal vertically. On the
aircraft panel a flashing light indicates that the plane is passing
over the maker
beacon.
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of the online publication.
GCA consists of two separate
radar devices. The first, a search radar unit, having a 360 deg.
scan, locates all planes flying within 30 miles of the field.
Using this set, a controller can pick up a plane at a distance
and give him appropriate course instructions to bring him into
the approach zone at the proper altitude and heading. In addition,
this unit is used for air traffic control, allowing a number of
planes to be flown safely in the same general area while lining
up for successive landing.
When the plane enters the
approach zone, it is followed on a second set of highly accurate
radar scopes, Fig. 7, which can scan an angular region over the
approach 20 deg. wide and 7 deg. high. These scopes show the
relationship of a plane to the field in azimuth, altitude, and
longitudinal distance. As the "pip" representing the
plane travels across these accurate scopes, operators track its
progress with "cursors", whose setting are relayed to
a pair of indicators located at the controller's desk. These
indicators continuously show the location of the plane relative
to the pre-select glidepath in terms of feet of deviation above
or below, and to left or right, of the ideal path. An accuracy
of ten feet can be obtain in measuring deviation with present
GCA equipment. The Controller reports these deviation to the
pilot, giving him corrections as he comes in.
Both the ILS and the GCA
instruments have been used in low visibility landing at LAES.
Both have proven dependable for usby well-trained pilots. At
the present time, a controversy is growing up over the relative
merits of the two devices. Pilots and technicians at LAES who
have had the opportunity to use both systems, feel that the real
question to be solved is not "which" should be used,
but rather how to obtain immediate installation of "both
and __" at airports throughout the nation. Information coming
from government and private research laboratories on NAVAR, TELERAN
, and LORAN indicates that radar, and television networks will,
in the foreseeable future, link all aircraft with airways control
centers. In the meantime, the air traffic control features of
GCA are indispensable to the successful control of present air
traffic over "socked in" airfields, while ILS provides
the most economical and speedy means now available for guiding
en aircraft to an invisible runway.
C. High Intensity, Wide Angle Approach
Lighting
As a plane approaches the
runway on an electronic approach, it becomes important that the
pilot be provided with adequate visual reference points from which
he can determine his location, altitude, attitude, and speed,
relative to the runway upon which he is to land. Although electronic
aids will bring the plane safely to the runway at an elevation
as low as 50 feet, the visual references should begin considerable
distance out in the approach zone, in order that the pilot will
have ample time to locate himself and ready his plane for the
touchdown. They must also be visible and identifiable with certainty
from points above the level of the ideal glide path, since many
pilots will be unwilling to reduce altitude before seeing what
lies below. In addition, since deviations will occur to the left
and right of centerline, the approach lights must be visible and
identifiable from points to left and right of the true glide path.
The length of an approach
lighting installation depends primarily on the landing speed of
the aircraft using it, and on the time deemed necessary for the
pilot to readjust from instrument flying to contact flying. Current
approach light design calls for an installation extending from
runway threshold to a point 3000 feet out in the approach zone.
A median figure for landing speed a plane will traverse a 3000-foot
approach zone in 17 seconds; flight tests indicate that such an
installation provides adequate time for the final adjustments
landing.
A study of deviations from
true glidepath was made by the Army Air Forces Equipment Laboratory
at Newark. The analyses of numerous approaches on electronic aids
showed that at the Middle Marker, (3500 feet from threshold),
all pilots should pass through a rectangular portal extending
from 100 to 500 feet above runway level and spreading 350 feet
to either side of the approach centerline. Accordingly , a funnel
shaped approach light pattern, Fig.
g, was designed, in which two rows of lights are placed symmetrically
with the approach centerline. At the approach end, the light
rows are 700 feet apart. Moving toward the runway, successive
lights are placed closer to the approachway centerline. At a point
700 feet from the converging light rows change direction and run
parallel with the runway for the remaining distance. This pattern
channels the landing at a safe altitude before coming over the
runway.
Several problems had to be
solved in designing a light suitable for the funnel installation.
In order to establish the ground plane, it is important that
the pilot see at least 3 of the lights in one row at all time
during his transit though the approach funnel. The lights, therefore,
must produce a beam of extremely high candlepower which will penetrate
the intervening fog. In order to be visible from any point within
the rectangular portal described about, they must have a wide
horizontal and vertical distribution. In addition, the light
source must be small in cross section, so that it will function
effectively as a point source and thus have approximately the
same apparent candlepower when viewed by the eye as when measured
by photometric methods. Finally, the lights must be so located
and aimed as not to be obscured by the aircraft structure.
The candlepower required to
penetrate a dense daylight fog (daylight object visibility 165
feet, or 1/32 mile), as calculated from the empirical formula
of Allard and Koschmieder1, becomes astronomical at a distance
of 990 feet. It is, however, practicable to build a light source
for the penetration of moderate fog (daylight object visibility
660 feet or 1\8 mile) for a distance of approximately 1000 feet.
For this penetration, in daylight fog, 24,000 candlepower is
required. (At night, it should be noted, 60 candlepower will
penetrate the same distance because of the greatly reduced background
brightness.) It can readily be seen that a 24,000 candlepower
light will b
more than adequate to cover the area between converting light
rows, whose maximum separation is 700 feet, up to an elevation
of 500 feet above the lights. In the left half of the approach
funnel, the pilot will pick up at least three lights on his side;
in the right half, the co-pilot will see the corresponding right
hand row. And as the plane
advances into the funnel along a slanting glide path, both rows
will come into view ahead of and alongside the aircraft, outlining
the correct path to the runway. Spacing of the lights is, of
course, important. At the approach end of the funnel, the lights
are located 100 feet apart, as measured along the funnel centerline,
for the first 600 feet. This spacing gives thoroughcoverage of the funnel at the
critical point where the pilot first makes contact. The spacing
over the remaining 2400 feet of the funnel is 200 feet, since
in this region, the decreased aircraft elevation, and the convergence
of the rows will give full coverage.
Within the funnel system, cockpit
visibility restrictions become a problem only when the pilot
flies high and almost over a row. In this position, the cockpit
structure will obscure the lights below the plane while the opposite
row may be attenuated below the pilot's visual threshold. Good
instrument pilots, however, do not deviate to this extent in both
azimuth and altitude from the indicated electronic glidepath.
Figure 9 shows the light used in the
funnel system. A wide angle beam emanating from a small source
was achieved by locating the monoplane filament of a spherical
reflector having a 10 - inch radius and subtending 120 degrees
horizontally by 80 degrees vertically. In front of the light
source is a
vertical condenser of clear glass, so ground as to produce a beam
35 degrees wide in the vertical plane. In front of this is a
horizontal condenser accurately moulded of aviation red lucite,
composed of vertical prisms which concentrate the light into a
uniform beam 45 degrees wide. Isocandle curves for the completed
unit show that it has a peak output of 25,000 candlepower, and
not less than 15,000 candlepower over a rectangular beam 45 degrees
wide and 35 degrees high.
The American Gas Accumulator Company
of Elizabeth, New Jersey, designed the optical system for the
High Intensity Approach Light and manufactured the units under
test at the Landing Aids Experiment Station.
The AGA lights are aimed so that the
outer edge of the rectangular beam is projected along the light
row. In addition, the outer lights are tilted upward to cover
the higher elevation of the glidepath at the approach end of the
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