Draft ICAO Circular on NLA.pdf - Airports Council International

<strong>ICAO</strong> Version 4.1 — 15 March 2004
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c) loss of visual taxiway guidance system (markings and lights covered by snow);
d) human factors (e.g. directional control, disorientation, pre-departure workload);
e) insufficient clearance between the outer wheels of the main gear and the edge of the
bridge;
f) position of the extremity of the escape slides; and
g) undercarriage design.
4.1.5.2.3 The main causes and accident factors for jet blast effect below the bridge are:
a) power plant characteristics (engine height, location and power);
b) bridge blast protection width; and
c) taxiway centre line deviation factors (see taxiway excursion risk).
4.1.5.2.4 These hazards would be classified as “major” to “catastrophic”.
4.1.5.3 Risk assessment and possible mitigation measures
4.1.5.3.1 Hazard prevention mechanisms could be based on the critical dimensions of the aeroplane
relative to the bridge width. However, before permitting use of taxiway bridges of narrower width than those
specified in Annex 14, Volume I, States should evaluate the operational feasibility of doing so.
4.1.5.3.2 For RFF intervention, it is necessary to ensure that the vehicles have access to both sides of
the aircraft, to fight any fire from the best position, and allowing for wind direction as necessary. A wingspan
of 80 m will in all cases exceed the width of a bridge. However, according to the experience of some
aerodromes, it may be feasible to use another bridge nearby for access to the “other” side of an aircraft rather
than an increased bridge width. This may be practicable only where bridges are paired (parallel taxiways)
or when there is a service road in the vicinity. The surface on the bypass routes would need to be stabilised
where it is unpaved.
4.1.5.3.3 For jet blast protection to vehicle traffic under/near the bridge, the requirement for protection
should be at least consistent with the overall width of the taxiway and its shoulders.
4.1.5.3.4 For weight capacity limitations, some alternative procedures may be required. Where an
existing bridge does not have the capability to support the weight of an NLA, either alternate routes may be
used, or a new bridge should be constructed.
4.1.5.3.5 In any case, the bridge width should be compatible with the deployment of escape slides.
If the width of the bridge do not meet this criterion, it should be ensured that the available blast protection
provides a safe and quick escape route.
4.1.5.3.6 The existing underground structures such as box culverts, bridges and pipe crossings all need
to be evaluated for their structural capability in view of the significant increase in the all-up mass of the given