FRANGIBILITY OF VISUAL AIDS
FIXTURES AND MOUNTING DEVICES
15.1 General
Many factors
must be considered in the selection of visual aids fixtures and mounting
devices to ensure that the reliability of the lighting system is maintained and
that the aids present the minimum hazard to aircraft whether in flight or
during ground maneuvering. It is therefore important that appropriate
frangibility characteristics of all visual aids be specified and published as
guidance material for design authorities. The following characteristics are
applicable regardless of whether devices are struck during takeoff or landing.
15.2 Elevated
Runway Edge, Threshold, End, Stop way And Taxiway Edge Lighting
15.2.1 The
height of these lights should be sufficiently low to ensure propeller and
engine pod clearance. Wing flex and strut compression under dynamic loads can
bring the engine pods of some aircraft to near ground level. Only a small
height can be tolerated an a maximum height of
15.2.2 These
aids should be mounted on frangible mounting devices. The impact load required
to cause failure at the break point should not exceed 5 kgm and a static load
required to cause failure should not exceed 230 kg applied horizontally 30 cm
above the break point of the mounting device. The desirable maximum height of
light units and frangible coupling is 36 cm above ground. Units exceeding this
height limitation may require higher breaking characteristics for the frangible
mounting device, but the frangibility should be such that, should a unit be hit
by an aircraft, the impact would result in minimum damage to the aircraft.
15.2.3 In
addition, all elevated lights installed on runways where the code number is 3 or 4 should be capable of withstanding a jet engine exhaust velocity of
300 knots; lights on runways where the code number is 1 or 2, a lower velocity
of 200 knots. Elevated taxiway edge lights should be able to withstand an
exhaust velocity of 200 knots.
15.3 Approach
Lighting System
15.3.1 Guidance on the
frangibility of approach lights is more difficult to develop as there is a
greater variation in their installation. Conditions surrounding installations
close to the threshold are different from those near the beginning of the
system; for example, lights within 90 m of the threshold or runway end are
required to withstand a 200-knot blast effect, whereas lights further out only
need to withstand a 100-knot blast or the natural environmental wind load. Also
the terrain close to the threshold can be expected to be near the same
elevation as the threshold, thus permitting the lights to be mounted on short
structures. Farther from the threshold, support structures of considerable
height may be required.
15.3.2 To minimize the hazard to aircraft, approach lights
should have a frangible device or their supports should be of a frangible
design.
15.3.3 Where
the terrain requires light fittings and their supporting structure to be taller
than approximately 1.8 m, thereby constituting the critical hazard, it is
considered that it is not practicable to require that the frangible mounting
device be at the base of the structure. The frangible portion may be limited to
the top 1.8 m of the structure except if the structure itself is frangible.
Though there is some question of the need to provide frangibility for approach
lights installed beyond 300 m before the threshold (as these lights are
required to be below the approach surface), it is recognized that protection
needs to be provided for aircraft that might descend below the approach or
take-off surfaces. A frangible top portion of 1.8 m is considered to be a
minimum specification and a longer frangible top portion should be provided
where possible.
15.3.4 In all cases, the unit and supports of the approach
lighting system should fail when an impact load of not more than 5 kgm and a
static load of not less than 230 kg is applied horizontally at 30 cm above the
break point of the structure.
15.3.5 Where it
is necessary for approach lights to be installed in stop ways, the lights
should be inset in the surface if the stop way is paved; if the stop way is not
paved, they should be either inset or elevated (in which case they should meet
the criteria for frangibility agreed for lights installed beyond the runway
end).
15.4 Other
Aids
15.4.1 These aids, for
example, VASIS, signs and markers, should be located as far as practicable from
the edges of runways, taxiways and aprons, as is compatible with their
function. Even effort should be made to ensure that the aids will
retain their structural integrity when subjected to the most severe
environmental conditions. However, when subjected to aircraft impact in excess
of the foregoing conditions, the aids will break or distort in a manner which
will cause minimum or no damage to the aircraft.
15.4.2 When installing
visual aids in the movement area, caution should be taken to ensure that the
light support base does not protrude above ground but rather terminates below
ground as required by environmental conditions so as to cause minimum or no
damage to the aircraft overrunning them. However, the frangible coupling should
always be above ground level.