The ATSB recently released reports of the investigations into two occurrences that involved flight by high capacity air transport aircraft into severe convective weather. For further information readers are directed to ATSB occurrence investigations 200100213 and 200105157 and associated safety recommendations. One of the safety enhancement aspects addressed by these recommendations included initial and recurrent training in the use and interpretation of airborne weather radar.
CONCLUSION
- Due to the limitations of the airborne weather radar and possibly the radar antenna setting, the flight crew misinterpreted the depicted weather radar returns.
- The flight crew did not appear to understand the limitations of the airborne weather radar.
- The aircraft was inadvertently flown into an area of severe convective weather activity.
The weather encountered by the crew was as forecast. Action taken by preceding aircraft confirmed the crew's expectation that they would have to divert to the left of track when en route. After leaving controlled airspace, the crew had to rely, for operational decision-making, on their interpretation of information derived from the airborne weather radar. That information supported their decision to divert and attempt to track around the southern end of the thunderstorm line.
When the aircraft was overhead Batemans Bay, the crew interpreted the weather radar returns as indicating that they could safely track direct to Canberra from that position. However, a severe thunderstorm was located on that track, as depicted by the 1823 Bureau of Meteorology radar image. The crew incorrectly interpreted the radar display of green and yellow returns as being acceptable. However, the heavy precipitation and hail produced by the storm cell was likely to have resulted in significant attenuation of the radar beam. Attenuation would have reduced the ability of the weather radar to accurately depict the precipitation intensity.
Further, the radar antenna setting of 3 to 4 degrees up, as reported by the crew, would have resulted in the radar beam scanning above the level at which the aircraft was flying, and into an area that was above the freezing level. It is likely that above that level the hail was dry. As such, it would have provided a low reflectivity target for the weather radar, and may have contributed to the inability of the radar to provide the crew with an accurate picture of the precipitation intensity.
The degree to which radar attenuation and reduced reflectivity, either individually or in combination, influenced the circumstances of the occurrence could not be determined. However, with inadequate radar derived information, the crew did not recognise the significance of the convective weather, and the aircraft entered the storm cell.
The warning generated by the GPWS computer was correct for the parameters within which it was operating. The excessive rate of descent derived by the radar altimeter and GPWS was the result of reflected returns from heavy rainfall and hail. When the crew received the GPWS warning their response was immediate and positive despite the aircraft being at an altitude well above the surrounding terrain.
On 26 March 2002, VH-KEQ, a Saab 340A aircraft, was being
operated on a regular public transport flight from Sydney to
Canberra. During the pre-flight planning, the crew identified a
line of thunderstorms and associated weather moving through the
weather forecast area (ARFOR) 21, in which the flight was to be
conducted. Consequently, they anticipated that to avoid the adverse
weather, they would probably have to divert to the left of the
flight-planned track.
At the time, the weather in ARFOR 21 was under the influence of
a low pressure system that was situated over eastern New South
Wales (NSW), with troughs aligned to the north and south. An
upper-level disturbance was also present over central NSW. Because
of this complex weather system, showers and thunderstorms were
forecast over the eastern part of NSW during the afternoon and
evening. A significant meteorological forecast (SIGMET) for
thunderstorms was also current for ARFOR 21 at the time of the
flight.
The forecast wind at 10,000 ft and 14,000 ft was 330 degrees
true at 40 kts. The freezing level was forecast to be 11,500 ft to
12,000 ft.
The flight departed normally and, at 1750 ESuT, on first contact
with Melbourne Centre, at approximately 50 nm to the southwest of
Sydney, the air traffic controller advised the crew that there was
significant weather on track between Marulan and Canberra. The
controller told the crew that preceding aircraft diverted to the
left of track to avoid this weather. The crew indicated that they
also would divert left of track. The controller cleared the crew to
divert to the left of track and once clear of the weather, to track
direct to Canberra.
The crew of another aircraft that had diverted to the left of
track reported to the controller that they had experienced moderate
turbulence at flight level (FL) 140. The controller passed this
information to the crew of KEQ, which at the time was climbing to
FL120. At 1800, the controller advised the crew of KEQ that they
would soon be leaving controlled airspace. Soon after, the crew
reported that they would continue on their present heading as the
weather radar indicated that conditions were unsuitable for
tracking direct to Canberra. This heading resulted in the aircraft
continuing to diverge from the flight-planned track. At 1805, they
advised that they were descending from FL120 to 9,000 ft. The pilot
in command later reported that this was to remain clear of the
freezing level.
The pilot in command reported during a post occurrence interview
that when the aircraft became clear of cloud they were over the
coastline. Recorded radar information indicated that the aircraft
was over the Batemans Bay area on the NSW south coast. After
conducting a right orbit to confirm their position and to assess
the weather, the crew decided to track direct to Canberra.
The pilot in command also reported that the crew was using the
airborne weather radar in maximum gain mode, with the antenna tilt
set at approximately 3 to 4 degrees up, and with the range at
either the 25 or 50 NM setting. He reported that the radar
displayed mostly green returns with an occasional yellow return. He
also stated that there were no red returns on the radar. The crew
interpreted information provided by the weather radar as being
suitable to track direct to Canberra.
After setting course for Canberra, the conditions suddenly
became dark, associated with an increase in the turbulence level
and rain intensity. Shortly after, the ground proximity warning
system (GPWS) sounded a `TERRAIN' warning. The crew applied maximum
power and began to climb the aircraft, and at 1817, broadcast on
the area frequency that they were climbing due to a GPWS warning.
The pilot in command reported that during this event the outer pane
of the front left windscreen shattered.
The crew continued the climb to FL110 and shortly after
levelling the aircraft it became clear of cloud. They then climbed
to FL120 and tracked direct to Canberra for an uneventful landing.
Post flight inspection of the aircraft revealed that in addition to
the shattered windscreen, the strobe lights had been damaged and
several vortex generators were missing.
The pilot in command subsequently reported that he thought hail
had been falling at the time the GPWS warning activated. Analysis
of the recorded transmissions from the aircraft revealed background
noise consistent with hail impacting the airframe.
The Bureau of Meteorology (BoM) `Letterbox weather radar' was
situated about 27 NM south-south-west of Sydney, and about 90 NM to
the north-east of Batemans Bay. The 1803 image from that weather
radar was derived at a scan angle of 0.5 degrees. The returns from
that area would, therefore, have been from an altitude of about
5,000 ft. Those returns revealed that a thunderstorm cell with
reflected rainfall rates of over 100 mm per hour was located about
14 NM to the north-west of Batemans Bay. The 1823 image showed that
the thunderstorm was almost overhead Batemans Bay, and on the
direct track to Canberra.
BoM subsequently reported that the thunderstorm caused damaging
winds and a tornado about 5 NM east of Braidwood, and that the
storm was likely to have been a `supercell' thunderstorm.
Weather Radar
The aircraft was equipped with a WXR 200 colour weather radar.
The radar antenna transmitted microwave energy in the form of
pulses, which, if reflected off precipitation ahead of the
aircraft, would be returned to the antenna. The radar beam was a
narrow cone with a beam width of 8 degrees. The amount of energy
reflected back to the antenna depended on the intensity of the
precipitation, and was converted into a colour code for
presentation to the crew on their flight instruments. There were
four colour codes that were directly related to precipitation
intensity, ranging from black (no precipitation), green (minimum
detectable moisture), yellow (medium moisture level), to red
(strong to extreme moisture level).
The Saab 340A operations manual described the operation of the
airborne weather radar system. This information included the
expected radar displays equating to reflected precipitation
returns, and descriptions of how to avoid areas of severe weather
based on weather radar displays. Information was provided on the
reflectivity of various types of precipitation, with wet hail being
the most reflective, and dry hail and dry snow being the least
reflective.
The manual contained information in the use of the weather radar
variable gain control. By reducing the gain, red areas of
precipitation targets would eventually be displayed as yellow, and
yellow areas as green. The red area that was the last to change to
the next lowest level (yellow) would be the strongest part of the
precipitation target.
The manual also contained information on weather radar antenna
tilt setting, and the importance of precision tilt management for
detection, analysing, and avoiding hazardous convective weather.
The manual recommended that the antenna be set to the one degree
down position whenever tilt and range were not being used for
weather analysis. That setting provided pilots with a fast and
certain means to ensure that the radar was functioning, and ensured
the detection of weather returns ahead of the aircraft.
The manual also contained information that heavy rainfall could
reduce the ability of the weather radar to provide a complete
picture of the weather ahead. That phenomenon is termed `radar
attenuation'. If a radar beam is fully attenuated, the display will
indicate a radar shadow that appears to be the end of the
precipitation area, but which actually extends further than is
apparent from the display. Attenuation may reduce reflected
precipitation readings by as much as 20dBz, which the weather radar
interprets as an area of decreased rainfall. The effect of this is
a downward colour shift that the colour returns displayed to the
crew indicate a lower level of precipitation intensity than is
actually occurring. What would normally be displayed as a red
return (indicating strong to extreme rainfall rates, with the
possibility of associated hail) is displayed as a yellow
return.
Ground Proximity Warning System (GPWS)
The aircraft was equipped with a GPWS that provided 5 modes of
protection. One of those modes provided protection against
excessive closure rate to terrain. The GPWS processed radio height,
flap and gear position logic, vertical speed and indicated airspeed
to determine if a dangerous situation was developing with respect
to the aircraft's height above the ground. The mode had two warning
envelopes. Penetration of the first envelope resulted in a
`TERRAIN' aural warning with the corresponding warning lights. If
the second envelope was penetrated, the aural warning changed to a
`WHOOP WHOOP - PULL UP' that was repeated until the aircraft gained
300 ft radio altitude.
Analysis of recorded flight data indicated that at the time the
crew received the GPWS warning, the aircraft was at an altitude of
9,248 ft above mean sea level and flying at an indicated airspeed
of 231 knots. The radar altimeter was indicating 2,154 ft and the
derived rate of descent from the radar altimeter was 17,760 ft per
minute. These parameters were within the warning envelope for the
GPWS computer and resulted in a `TERRAIN' warning being provided to
the crew.