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.
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.
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.
- 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 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.
|Date:||26 March 2002||Investigation status:||Completed|
|Time:||1820 hours ESuT|
|Location:||83 km ESE Canberra, (VOR)|
|State:||New South Wales||Occurrence type:||Weather - Other|
|Release date:||19 December 2002||Occurrence class:||Environment|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||S.A.A.B. Aircraft Co|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Minor|
|Departure point||Sydney, NSW|
|Departure time||1730 hours ESuT|
|Role||Class of licence||Hours on type||Hours total|