ATC requirement for visibility reporting



[pic] | |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

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).

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:

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.”

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.

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

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.

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 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

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.

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.

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.

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 of the aircraft landing light for aeronautical visibility reporting purpose would likely be required before incorporating this method into the MET practices.

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.

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. 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.

— — — — — — — —

|AMOFSG/7-IP/11 |

|Appendix |

APPENDIX

[pic]

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

|UTC |LIDAR Visibility Map |Wind Distribution |

|0400 |Fig. 2a |Fig. 3a |

| |[pic] |[pic] |

| | |Winds were light everywhere. Winds converged over the eastern edge |

| | |of the airport island and northern part of Lantau. |

|0430 |Fig. 2b |Fig. 3b |

| |[pic] |[pic] |

| | |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). |

|UTC |LIDAR Visibility Map |Wind Distribution |

|0500 |Fig. 2c |Fig. 3c |

| |[pic] |[pic] |

| | |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. |

|0530 |Fig. 2d |Fig. 3d |

| |[pic] |[pic] |

| | |Organized southerlies pushed northwestwards to cover half of the |

| | |airport with improvement of visibility following closely. |

|UTC |LIDAR Visibility Map |Wind Distribution |

|0600 |Fig. 2e |Fig. 3e |

| |[pic] |[pic] |

| | |Strengthening of the southerlies over the western part of Lantau |

| | |reaching two of the westernmost weather buoys. |

|0630 |Fig. 2f |Fig. 3f |

| |[pic] |[pic] |

| | |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. |

|UTC |LIDAR Visibility Map |Wind Distribution |

|0700 |Fig. 2g |Fig. 3g |

| |[pic] |[pic] |

| | |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. |

|0730 |Fig. 2h |Fig. 3h |

| |[pic] |[pic] |

| | |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. |

|UTC |LIDAR Visibility Map |Wind Distribution |

|0800 |Fig. 2i |Fig. 3i |

| |[pic] |[pic] |

| | |Organized southerlies prevailed over the entire airport, so as the |

| | |good visibility condition. |

| |Figure 2. Visibility maps generated from LIDAR data. |Figure 3. Winds measured by anemometers near HKIA. |

— END —

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Convergent zone pushing northwestwards

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