The Bureau is examining the feasibility of a study into the phenomena of low-level windshear to be undertaken by a suitable research institution and involving the Bureau of Meteorology, Airservices Australia, Civil Aviation Safety Authority, Australian Transport Safety Bureau and industry.
At the time of the occurrence the environmental wind was strong, and the investigation concluded that it was likely that the downdrafts and associated surface outflows from the entrained convective activity were distorted in the direction of the prevailing airstream, and that this accounted for the gusting conditions that were present at the time of the occurrence.
The flight data recorder fitted to EBS was not equipped to record the aircraft's groundspeed, and the investigation was unable to determine the actual external winds that affected it during the approach and landing. However, from the meteorological data that was available, it was probable that the roll rate encountered by EBS as it commenced the landing flare resulted from an encounter with low-level windshear. It is likely that this was produced by a downdraft from one of the convective storm cells passing through the terminal area at the time.
Although the pilot in command responded in a timely manner with appropriate control input, under the dynamic conditions that were encountered, it is unlikely there was sufficient available aileron/spoiler authority to counteract the high rate of roll that had suddenly been experienced. This resulted in the number 1 engine pod momentarily striking the ground as the aircraft touched down.
Low-level windshear may occur as a result of thunderstorms, land/sea breezes, low-level jet streams, mountain waves and frontal systems. There have been accidents and incidents associated with low-level wind shear in Australia. Pilots should be aware that it is a phenomena that may occur at any location, is difficult to predict, and can present a hazard to aircraft on approach and departure.
History of the Flight
On arrival at Perth, the crew of a Boeing 747-238B, VH-EBS, were
cleared to conduct an instrument approach for landing on runway 03.
The pilot in command was the handling pilot for the sector, and the
crew subsequently reported that although they were in visual
meteorological conditions during the approach, turbulence was
encountered. Information "Whisky" was being broadcast on the
automatic terminal information service (ATIS), and provided
information to the crew that the wind speed and direction at the
aerodrome was 330 degrees magnetic at 20 knots. The ATIS included
information that the wind speed and direction at 200 ft above
ground level was 330 degrees magnetic at 30 knots, and advised
crews to expect moderate turbulence below 4,000 ft.
During the approach the co-pilot requested a wind check from the
aerodrome controller. The controller advised the crew that the
runway 03 threshold wind was 300 degrees at 12 kts, giving a
crosswind of 12 kts. The controller requested the crew to advise
the spot wind at 1,000 ft, and the co-pilot reported that the 1,000
ft spot wind was 280 degrees at 35 kts.
On short final, at approximately 500 ft above ground level, the
pilot in command discontinued the approach when the aircraft
experienced turbulence rendering the approach unstable. The
co-pilot notified air traffic control (ATC) that EBS was conducting
a missed approach, and the controller issued an instruction to the
crew to climb to 1,500 ft. The controller then issued further
instructions for EBS to climb to 3,000 ft and instructed the crew
to take up an easterly heading to intercept the 9 mile arc, from
the Perth distance measuring equipment beacon, to position the
aircraft for another approach onto runway 03.
As EBS proceeded towards the south to intercept the 9 mile arc
for the second approach to runway 03, the controller reassessed the
prevailing wind conditions. The wind had been steadily backing to a
more southerly direction, and the controller considered that the
wind had begun to favour operations on runway 24. The controller
notified the crew of EBS that runway 24 was available for landing,
and the crew advised that they would accept an approach for that
runway. The controller then issued radar vectors to the crew to
position EBS onto the approach for runway 24. As EBS was on final
approach the controller advised the crew that the threshold wind
for runway 24 was 290 degrees at 23 kts, and that the wind at 200
ft was 290 degrees at 35 kts.
The crew reported that the approach to runway 24 was conducted
normally and with the autopilot engaged. However, turbulence had
prevailed throughout the approach. Flaps 30 was the landing flap
setting, and as the aircraft flared for touchdown it suddenly
experienced an unexpected roll to the right and the pilot in
command applied a control wheel input to the left to counter the
roll. The aircraft then suddenly experienced a severe roll to the
left. Although the pilot in command applied an immediate control
wheel input to the right to arrest the roll, the aircraft touched
down in a left wing down attitude, and the number 1 engine pod
briefly struck the runway surface.
The crew reported that the touchdown was smooth, and appeared to
be on the centreline of runway 24. They also reported being unaware
that the number 1 engine pod had struck the ground during the
touchdown. As the aircraft taxied in to the international apron a
flight attendant advised the crew that a passenger had reported
seeing brown fluid leaking from the number 1 engine. After the
aircraft had parked the number 1 engine was inspected for damage.
The casing of the high speed external gearbox fitted to the engine
was fractured adjacent to the gearbox mount position, and the
number 1 engine thrust reverser was damaged. The pilot in command
then notified the controller that EBS had sustained a podstrike
during the landing on runway 24.
The subsequent inspection of runway 24 revealed a scrape mark on
the runway approximately 490 metres from the threshold of runway
24. The scrape mark was approximately 30 metres in length and was
located approximately 18 metres left of the runway centreline just
outside the outer edge of the runway touchdown zone markings.
Examination of the manufacturer's data for the B747-200 series
showed engine number 1 to be 21.2 metres outboard from the aircraft
centreline. This was consistent with position of the scrape mark on
runway 24.
Flight Data
Air traffic control radar plots and the flight path derived from
EBS's flight data recorder (FDR) were examined during the
investigation. They revealed that the pilot in command discontinued
the first approach onto runway 03 at 04:07 co-ordinated universal
time when EBS was at approximately 500 ft above ground level.
Following the discontinued approach onto runway 03, EBS was
vectored to the southeast of the airport, then back towards the
northeast when ATC reconfigured the terminal airspace for
operations onto runway 24. EBS commenced the approach onto runway
24 at 04:23, and the approach concluded at 04:28 when the aircraft
landed.
The FDR roll angle plot revealed that as EBS was 35 ft above
ground level it commenced an uncommanded roll to the right. The
pilot in command immediately applied 29.5 degrees of left control
wheel to counteract the roll. However, the roll continued to
increase, and EBS was in an 8.0 degrees right wing down attitude as
it reached 2 ft above ground level. The roll then suddenly
reversed, and within 2 seconds EBS was in an 8.4 degrees left wing
down attitude. Although the pilot in command immediately responded
with 40.7 degrees of right control wheel to counteract the roll,
the aircraft touched down still in an 8.4 degrees left wing down
attitude.
Groundspeed was not a recorded parameter on the Lockheed LAS209F
FDR that was fitted to EBS, and the investigation was therefore
unable to determine the actual wind conditions that it encountered
throughout the approach. However, variations in the FDR computed
airspeed plot throughout the occurrence sequence were consistent
with the reported turbulent conditions.
Aircraft Data
Roll control of the Boeing 747 aircraft is provided by inboard
and outboard ailerons and spoilers. The manufacturer advised that a
control wheel deflection of 40.7 degrees to roll the aircraft to
the right would result in outboard aileron deflections of left
outboard +13.8 degrees and right outboard -22.2 degrees. The
maximum outboard aileron deflection is +15 and -25 degrees, with
+ve signifying trailing edge down and -ve signifying trailing edge
up. With the same control wheel deflection of 40.7 degrees to roll
the aircraft to the right, the resultant inboard aileron
deflections would be left inboard +18.2 degrees and right inboard
-17.9 degrees. Normally the maximum inboard aileron deflection is
+/- 20 degrees.
The spoilers consist of 12 panels on both wings starting with no
1 on the left outboard wing and extending to no 12 on the right
outboard wing. For a control wheel deflection of 40.7 degrees to
roll the aircraft to the right, spoilers 1-7 would be deflected 0
degrees (faired with wing), spoiler 8 would be deflected 9.7
degrees, and spoilers 9-12 would be deflected 15.3 degrees. Aileron
and spoiler deflection would be reversed for a control wheel
deflection of 40.7 degrees to roll the aircraft to the left.
Data for Boeing 747-200 series aircraft fitted with Rolls Royce
RB211-524 engines showed that the ground clearance of the number
one engine pod was 188 cm at an operating empty weight of 164,610
kgs. This clearance was reduced to 158 cm when the aircraft was at
its maximum taxi weight of 352,894 kgs. The plan view of the Boeing
747-200 series aircraft showed the number 1 engine to be 21.2
metres outboard of the aircraft centreline, and 15.2 metres
outboard of the outboard wheel of the wing landing gear assembly.
Under static conditions and with 0 degrees nose pitch, a body roll
of 7.05 degrees at the operating empty weight would cause the
number one engine pod to contact the ground. A body roll of 5.93
degrees at the maximum taxi weight would also result in ground
contact of the number one engine pod.
The weight of EBS at the time of the occurrence was
approximately 230,000 kgs.
Meteorological Information
At the time of the occurrence, Perth was under the influence of
an unstable air flow as a result of a complex low pressure system
situated to the south of Western Australia. A series of fast moving
cold fronts were embedded in the strong to gale force westerly
airstream, and the unstable atmosphere resulted in widespread rain
showers, squalls and occasional thunderstorm activity.
The trend type forecasts (TTF's) for Perth from 00:33 leading up
to the time of the occurrence indicated that gusty wind conditions
could be expected in the terminal area. Additionally, the TTF's
from 01:31 indicated that thunderstorms were also likely to be
present in the area. The aerodrome forecast current for Perth at
the time of the occurrence also indicated the likely presence of
gusty conditions and rain showers. At 01:00 an airport warning was
issued for Perth containing information that a series of squall
lines were expected to cause wind gusts to 45 knots during the day,
and that thunderstorms were predicted. At 01:13 information
concerning en route weather phenomenon with the potential to affect
the safety of aircraft operations (SIGMET) was issued. The SIGMET,
valid from 02:00 until 08:00, forecast the presence of severe
turbulence below 4,000 ft for the Perth region, and was passed to
the operator by the Bureau of Meteorology.
The Perth aerodrome ATIS was changed to information "Whiskey" at
03:03. It provided information that the wind speed and direction
was 330 degrees magnetic at 20 kts, and warned pilots to expect
moderate turbulence below 4,000 ft. A windshear alert was also
provided, with the wind speed and direction at 200 ft being 330
degrees magnetic at 30 kts. The windshear alert was included on the
ATIS because there was a 10 kts difference between the wind speed
on the ground and the wind speed at 200 ft. The ATIS was amended to
information "X-Ray" at 04:27, providing information that the wind
speed and direction were 290 degrees magnetic at 25 kts. The
revised ATIS continued to provide a warning of moderate turbulence
below 4,000 ft and also a windshear alert.
Wind shear is defined as a sudden change in wind direction
and/or speed with height or horizontal distance. In most cases wind
shear does not present a hazard to aircraft and the majority of
pilots will be familiar with changes in wind direction and speed as
they ascend or descend. However, at low altitudes (below 1000 ft)
during critical stages of landing and takeoff, wind shear can
present a significant hazard to aircraft because there is a limited
ability to undertake a recovery manoeuvre if the aircraft
configuration changes. Low altitude windshear events are small
scale and short lived and only affect the approach / departure
flight path for a short period of time. The ability to identify
such events based on traditional airport observations is limited,
although systems which can detect wind shear and provide alerts in
a timely manner are available.
The Bureau of Meteorology
anemometer at Perth airport was the source of wind data
transmitted on the ATIS. The anemometer sampled the wind at
1-second intervals, and a display of the anemometer wind data was
located in the control tower. Controllers were able to select the
display for instantaneous, 2-minute average wind speed and
direction, or 10-minute peak wind speed. Data from the Bureau of
Meteorology anemometer was recorded and archived. The control tower
also had displays of threshold wind data obtained from anemometers
located adjacent to the threshold of each runway. A display also
provided wind data from an anemometer located on the control tower
cabin. The controllers could select the threshold anemometer and
tower cab displays to provide instantaneous, 2-minute average wind
speed and direction, or 10-minute peak wind speed. The controllers
reported that their usual practice was to leave the tower cab
displays selected to the instantaneous setting, with selection to
the 2-minute average wind speed and direction or the 10-minute peak
wind speed settings being made to determine the development of any
significant trends. Data from the threshold and tower cab
anemometers was not recorded and archived.
Radar imagery taken at 20 minute intervals during the occurrence
period showed a significant line of enhanced rain echoes passing
through Perth at 03:20 in a generally easterly direction at
approximately 35 to 40 kts. Scattered convective showers were
present behind the line of precipitation, with rain echoes being
randomly spaced and largely unorganised. The radar imagery also
revealed showers in the vicinity of Perth aerodrome at the time of
the occurrence.
The 1-minute data recorded by the Bureau of Meterorology
anemometer at Perth aerodrome showed that a fast moving front
passed across Perth aerodrome at 04:12, shortly after the pilot in
command discontinued EBS's approach onto runway 03. The front was
associated with a change in wind direction and a significant
increase in windspeed for a period of approximately 3 minutes. At
04:26 the 1-minute anemometer data revealed a marked increase in
wind speed which persisted until approximately 04:30. The wind
speeds increased to 23 - 25 kts during this period, with maximum
wind speeds being recorded at 27 - 35 kt. At 04:28, the time of the
occurrence, the maximum wind speed was approximately 29 kts.
However, during the period 04:26 to 04:30 there was little
variation in the recorded wind direction, and it remained
relatively constant from the west. The duration of the increase in
wind speed was considered characteristic of an outflow from a
convective rainshower.