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) TA
d h
h = vertical distance between the level being scanned and the
aircraft flight level (feet)
d = distance from the storm cell (NM)
TA = tilt angle (degrees)
2.2.4. For the case of a tilt angle setting of –1.75º; at a distance of 40 NM,
the centre of the radar beam would be scanning around 7,000 feet
below the cruising level; and at 80 NM, the radar would be
scanning at around 14,000 feet below. However, the above
formula is an approximation that does not take into consideration
the curvature of the earth, which could be a significant factor when
calculating the target level at long range. Based on information
provided by the radar manufacturer, with the earth’s curvature
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taken into account, at 40 NM the centre of the beam would be at
about 6000 feet below the cruising level, and at 80 NM about 9000
feet. Thus, while cruising at FL 410, with a tilt angle setting
of -1.75º at 40 NM distance, the centre of the radar beam would be
scanning for weather at around FL 350 and at 80 NM, at around
FL 320. Although the lower edge of the 3-degree radar beam could
be scanning the upper portion of the icing level band, the radar
return would be diminished because of the reduced power at the
periphery of the conical beam – approximately half of the power at
the centre of the beam. It was therefore very likely that,
with -1.75º tilt angle selected, the radar was ‘over-scanning’ i.e. the
centre of the beam was scanning above the most reflective part of
the weather cells, hence no significant weather (red) returns were
displayed on radar.
2.2.5. The Honeywell RDR-4B Forward Looking Windshear/Weather
Avoidance Radar System User’s Manual gives a very detailed
description of the operating procedures for the weather radar, with
particular emphasis on range selection and antenna tilt management.
Adopting the procedures in this document, or alternatively, those
described in FCOM 3.04.34, would have adequately established a
protection zone ahead of the aircraft. Both documents stress that it
is important for the antenna tilt angle to be lowered progressively
as the aircraft approaches weather to maintain a clear radar picture
of the weather ahead. This will help to ensure that a deviation will
clear any weather hazard by a safe margin. However, there was no
30
evidence that these procedures were adopted prior to the
occurrence. Although the Honeywell RDR-4B radar has a feature
that provide an automatic increase in gain above 25,000 feet to
compensate for the lower reflectivity of the ice crystals in the upper
levels of a storm cell, the radar remains limited by the reflectivity
characteristics of the target being scanned and the available gain. It
is for this reason that the RDR-4B Users Manual stated repeatedly
that effective tilt management is the single, most important key to
more informative weather radar displays.
2.3. Weather Avoidance
2.3.1. On approaching position NOBEN, weather returns were sighted on
the ND and the Co-pilot called Manila ATC to request permission
to deviate 10 NM right of track, upwind of the weather as indicated
on the ND. However, it was unlikely that, at the settings used, the
weather radar was scanning that portion of the convective storm
cell giving the strongest returns.
2.3.2. The use of weather radar for weather avoidance is contained in
FCOM 3.04.34 Pages 3–5 (Appendix 10) and tilt adjustment is
contained in FCOM 3.03.15 Page 2 (Appendix 11). FCOM
3.04.34 Page 5, under a section titled “Red/Magenta Areas:
Thunderstorm, Tornado, Hail”, recommends the use of the 160 NM
scale for the PNF (Pilot Not Flying) and the 80 NM scale for the
PF when scanning for thunderstorms. It also states that: -
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“Frequent tilt adjustments are recommended to monitor the
storm development and to provide the maximum cell echo”; and
“Do not forget that omission to periodically adjust tilt
downwards causes targets to disappear”.
2.3.3. Although this section is titled “Red/Magenta Area: Thunderstorm,
Tornado, Hail”, the above recommended procedures are obviously
also applicable to operation of the weather radar under
circumstances other than those when “red/magenta areas” are
present. The flight crew stated that they observed yellow and green
returns 10 NM north of NOBEN, which prompted their request for
deviation. It was likely that had they increased the negative tilt
angle at that time they would have seen red returns on the ND; and
possibly magenta, if the “Wx/Turb” Mode had been selected.
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