AMOFSG/7-IP/11
2/9/08
AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
SEVENTH MEETING
Montréal, 9 to 12 September 2008
Agenda Item 5: Aerodrome observations
ATC REQUIREMENT FOR VISIBILITY REPORTING
(Presented by C.M. Cheng)
SUMMARY
This paper highlights a difference between ICAO requirement on visibility
reporting and expectation of ATC users and discusses possible options to
address this variance.
1.
INTRODUCTION
1.1
While the slant visual range (SVR) would be a good representation of the pilot’s visual
range from the cockpit of aircraft on approach down to the ground, there is currently no ICAO
requirement for SVR owing to the inherent difficulties in its measurement or assessment and the fact that
research into its assessment has been negligible in recent years. Furthermore, it is now widely accepted
that the use of Runway Visual Range (RVR) has ensured the safe conduct of low-visibility operations
over the last few decades (ICAO Doc 9328).
1.2
On the other hand, when the enabling provisions to use fully automatic observing systems
were introduced in 2004 in Amendment 73 to Annex 3, para. 4.6.2.2 was revised to recommend that the
visibility observations for the local routine and special reports (LRR/LSR) should be representative of
conditions along the runway and the touchdown zone, instead of the take-off/climb-out and
approach/landing areas. Subsequently, in Amendment No. 4 to PANS-ATM (Doc 4444) dated 24
November 2005, the words “if practicable, supplemented by slant visual range value(s), if provided” were
removed from item c) of para. 6.6.4, which currently reads:
“6.6.4 At the commencement of final approach, the following information shall be
transmitted to aircraft:
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a) …
b) …
c)
the current visibility representative of the direction of approach and landing or,
when provided, the current runway visual range value(s) and the trend.”
1.3
At the Hong Kong International Airport (HKIA), forward scatter visibility meters were
introduced along the runways for visibility reporting accordingly. However, there are sometimes
significant discrepancies between the reported visibility values in the LRR/LSR and the SVR estimated
by ATC, making reference to aircraft on approach, under inhomogeneous visibility conditions. This issue
has been brought up from time to time in the past few years both in the day-to-day liaison between the
MET and ATC personnel as well as in the regular coordination meetings between MET and ATM. This
paper presents possible options to address this gap.
1.4
This paper was presented at the CNS/MET SG/12 meeting held in July 2008. The
meeting recalled that currently SVR was not a MET element required by the SARPs and there were no
operational procedures relating to SVR. At the meeting, the expert from IFALPA welcomed the research
and development of techniques for observation of SVR but cautioned that there might be differences
between SVR observed from the ground and that observed from the aircraft cockpit. The meeting advised
that the AMOF SG could be approached if this issue was to be pursued.
2.
VISIBILITY DISCREPANCY
2.1
As an example, on 31 May 2008, visibility over HKIA began with a relatively low value
in the region of 3000 m. At that time, HKIA was largely under the influence of light northwesterly winds
which were climatologically quite frequently associated with hazy conditions. By 0440 UTC (1240 local
time) in the afternoon, a moderate southerly maritime air stream began to move into the eastern part of the
airport bringing significant improvements to the local visibility. The southerly air stream gradually
strengthened and extended its influence to the west and eventually brought good visibility to the entire
airport. The air mass replacement process took more than two hours to complete. The evolution of the
event was successfully captured by the six forward scatter visibility meters along the two runways (Figure
1). The variations in visibility at various locations of the runways are corroborated by the estimated
visibility based on aerosol backscatter measurements of a Light Detection And Ranging (LIDAR) system
(Figure 2) as well as the wind distribution measured by the anemometer network in and around HKIA
(Figure 3). The demarcation between the haze and the clear maritime air was sharp, and as such all
forward scatter meters recorded an abrupt change of visibility in a matter of minutes. At one stage, half
of the airport was reporting visibility of over 15 km while the remaining half was still reporting visibility
of 4 km or below.
2.2
The Hong Kong Observatory (HKO) issues visibility reports for the LRR/LSR by
professionally trained human observers aided by the forward scatter meters in accordance with para.
4.2.4.3 of Appendix 3 to Annex 3. On the other hand, ATM considers an assessment of SVR by ATC
personnel at the tower or as reported by the pilots on approach to be more representative of the actual
conditions. The tower/pilot observations are sometimes found to be rather different from the LRR/LSR
visibility reports. For example, in the case of 31 May 2008, the ATC personnel reported visual range in
excess of 9 km and 7 km respectively at 0637 and 0735 UTC, whereas the LRR reported, based on the
forward scatter meter readings, much lower values of 3200 m and 4600 m at the R2W location
(see Figure 1) at 0630 and 0730 respectively. From an ATM point of view, the current visibility reporting
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practice specified in ICAO Annex 3, at times when there are localized areas of lower visibility on the
airfield, may not provide a good representation of the visibility conditions of the approach/climb-out areas
which are of primary concern to the pilots. Previously, ATC personnel, who are competent human
observers, would sometimes amend the LRR/LSR visibility reports with their own estimates of SVR from
the tower or the pilot’s SVR report in the Automatic Terminal Information Service (ATIS) broadcast, but
with the recent commissioning of an automated ATIS system, the information is now downloaded directly
from the LRR and ATC cannot enter any manual amendment.
3.
PROPOSED SUPPLEMENTARY VISIBILITY
INFORMATION
3.1
To address the above apparent gap between LRR/LSR visibility report and
ATC/pilots’ expectation, the MET and ATM discussed and considered the following options:
a) reporting the prevailing visibility in LRR/LSR as supplementary information for
inclusion in ATIS as a near-term solution; and
b) exploring methods to estimate SVR over the take-off/climb-out areas for inclusion in
LRR/LSR and ATIS as a long-term solution.
3.2
For the near-term option (a), minimal extra efforts are required for implementation as the
prevailing visibility is readily available from the METAR/SPECI. Its inclusion is expected to alleviate
the discrepancy between the instrumented visibility reports and SVR estimates made by ATC and pilots
in inhomogeneous visibility conditions. A change of the LRR/LSR template (Table A3-1) in Annex 3
will however be required to enable the inclusion of the prevailing visibility as an optional supplementary
information.
3.3
For the long-term option, based on the experience of ATC personnel at HKIA, an
objective estimate of SVR by keeping track of landing aircraft visually at the tower might be feasible.
When an aircraft first becomes visible to the naked eye on the approach path, its distance from the tower
could be determined from the ATC radar, even though the line-of-sight between the aircraft and the tower
does not coincide with the approach path. ATC experience at HKIA suggests that estimates of SVR
based on this method agree well with the pilot’s assessment from the cockpit.
3.4
Regarding the use of aircraft on approach as an object for estimation of SVR, the WMO
Guide to Meteorological Instruments and Methods of Observation (WMO-No.8) recommends that, for
visibility observations to be representative, they should be made using objects subtending an angle of not
less than 0.5 degree at the observer’s eye. At the same time, such an object should not subtend an angle
of more than 5 degrees. An aircraft on the approach path may appear smaller than the WMO requirement
of 0.5 degree to the tower observer. However, this does not imply that the above method could not be
used to obtain meaningful results. The rationale is that, if an aircraft at such a distance with a subtending
angle less than 0.5 degree could clearly be seen by the observer, the true SVR can only be longer than the
estimated distance between the aircraft and the tower observer. In other words, the estimate made by the
tower observer in this way will likely provide the lower limit of the true SVR. When pilot reports of SVR
are available from ATC, the estimated SVR could be validated. On the practical and procedural aspects,
further deliberation on the period of observation (e.g. 10 minutes before observation time), availability of
aircraft on approach, condition for reporting (e.g. whether SVR should only be reported where there is
significant difference between the prevailing visibility and the runway instrumented reports) and the use
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of the aircraft landing light for aeronautical visibility reporting purpose would likely be required before
incorporating this method into the MET practices.
3.5
Apart from human observation of SVR from the tower, the feasibility of automatically
estimating SVR from the LIDAR (e.g. by integrating the visibility values over the take-off/climb-out and
approach/landing areas) or from web cameras equipped with artificial intelligence algorithm at the
touchdown zone to simulate the human eye for spotting aircraft along the approach path could be
explored. The available pilot reports of SVR could be used to validate/optimize these methods.
3.6
Based on the above discussions, the meeting might like to discuss the ATC requirement
for SVR reporting, and consider the merits of the following options to address the apparent gap between
the LRR/LSR visibility reports and ATC/pilots’ expectation:
a) the provision of the prevailing visibility in the local routine and special reports,
subject to local agreement with the appropriate ATS units and operators concerned.
If this option is pursued, the necessary SARPs and guidance material for
implementation will need to be developed; and
b) to study the observation and reporting of SVR in consultation with WMO, with a
view to its eventual provision in the local routine and special reports for relay to ATC
and pilot users.
4.
4.1
ACTION BY THE GROUP
The AMOFSG is invited to:
a) note the information in this paper; and
b) exchange views on the issue, discuss the ATC requirement for SVR, and the merits
of the options in para. 3.6 above.
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AMOFSG/7-IP/11
Appendix
APPENDIX
Figure 1. Visibility measured by the six forward scatter meters at HKIA (the arrows mark the time at
06:37 and 07:35 UTC).
Note: R1 = south runway; R2 = north runway; W = west; C = centre; E = east
A-2
UTC
0400
LIDAR Visibility Map
Fig. 2a
Wind Distribution
Fig. 3a
Winds were light everywhere. Winds converged
over the eastern edge of the airport island and
northern part of Lantau.
0430
Fig. 2b
Fig. 3b
Strengthening of southerlies observed over
mountain gaps (see pink dotted circles), e.g. Pak
Kung Au (PKA) and Tai Fung Au (TFA).
Improvement of visibility downwind of PKA
observed (green dashed ellipse).
A-3
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Appendix
UTC
0500
LIDAR Visibility Map
Fig. 2c
Wind Distribution
Fig. 3c
Convergent zone
pushing
northwestwards
0530
Fig. 2d
Visibility downwind of PKA and TFA continued
to improve (green dashed ellipses) and spread
northwards to cover the eastern end of the airport
together with the southerlies.
Fig. 3d
Organized southerlies pushed northwestwards to
cover half of the airport with improvement of
visibility following closely.
A-4
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Appendix
UTC
0600
LIDAR Visibility Map
Fig. 2e
Wind Distribution
Fig. 3e
Strengthening of the southerlies over the western
part of Lantau reaching two of the westernmost
weather buoys.
0630
Fig. 2f
Fig. 3f
Possibly blocked by Lantau Peak and Nei Lak
Shan upwind, the southerlies with good visibility
still could not reach a narrow corridor over the
western part of the airport.
A-5
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Appendix
UTC
0700
0730
LIDAR Visibility Map
Wind Distribution
Fig. 2g
Fig. 3g
Fig. 2h
Southerlies over the central part of the airport
strengthened. The light wind zone moved
westwards to just outside the airport. Visibility at
R2W improved rapidly.
Fig. 3h
With the strengthening of the southerlies over the
westernmost weather buoys area, the light wind
zone with low visibility moved back to cover
R2W temporarily.
A-6
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Appendix
UTC
0800
LIDAR Visibility Map
Fig. 2i
Wind Distribution
Fig. 3i
Organized southerlies prevailed over the entire
airport, so as the good visibility condition.
Figure 2. Visibility maps generated from Figure 3. Winds measured by anemometers near
LIDAR data.
HKIA.
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