Airbus A330-301, VH-QPC

1. Unforecast fog developed at Sydney aerodrome after the aircraft
   passed the flight planned DPA decision point, which had been based
   on a diversion to Melbourne.
   
   
   
2. By the time the crew became aware of the fog, the aircraft did
   not have sufficient fuel to proceed to an alternate aerodrome
   categorised by the operator as suitable for normal
   operations.
   
   
   
3. When the ATIS was amended at 0554 to include a runway visual
   range of 800 m, the actual visibility at the touchdown zone of the
   nominated landing runway, 34L, was greater than that figure.
   
   
   
4. The crew continued to track for an approach to runway 34L at
   Sydney after the nominated time for a diversion to Canberra.
   
   
   
5. While the crew was manoeuvring the aircraft for an approach to
   runway 34L at Sydney, the fog moved across the threshold of that
   runway reducing visibility to below the landing minima.

The aircraft landed at Sydney in weather conditions that were
below the specified landing minima. Although a landing in
conditions below the specified meteorological minima was permitted
under Civil Aviation Regulation 257(5) in an emergency situation, a
landing at Sydney only became a necessity after 0618, the latest
divert time to Canberra. This analysis examines the development of
the occurrence and discusses the factors that influenced the crew?s
decision to continue the approach to a landing at Sydney.

The aircraft departed Perth with sufficient fuel for the flight,
based on the current Sydney aerodrome forecast. There was no
requirement to divert to Melbourne at the flight planned Designated
Point All Engines Operating (DPA), because the forecast weather
conditions were not below the alternate minima for a landing at
Sydney. However, the aircraft passed the DPA at about the same time
as low level cloud was clearing at Sydney. The dissipation of the
cloud resulted in the subsequent unforecast movement of fog over
the aerodrome. At 0540 the crew was aware that a fogbank had formed
to the north-west of the aerodrome, but the other weather
information available to them at that time did not indicate that
fog would be a problem for their arrival at Sydney.

After commencing the descent, the crew used weather information
provided by controllers and from the onboard data-link when making
decisions in response to the deteriorating visibility at Sydney
aerodrome. However, during the latter stage of the flight the crew
were not provided with all of the weather information that was
available to ATC. This reduced the crew?s situational awareness of
the effects of the rapid progression of fog across the runway
complex. While information was available via the onboard data-link,
the crew was busy setting up for the approach into Sydney and would
have had an expectation that ATC would advise of any significant
deterioration in the weather conditions.

Although the crew considered that the Canberra 0530 special
automated weather observation indicated marginal weather
conditions, the reported conditions were just above the applicable
alternate minima for an ILS approach to runway 35. Had the crew
obtained or been provided with the Canberra 0600 trend type
forecast, which predicted that the weather conditions would not
significantly vary for the following three hours, they may have
been more confident about the weather trend at Canberra.

The declaration of ?minimum fuel? and reiteration of the
requirement to use 16R in the crew?s PAN broadcast at 0559
indicated their concern about having enough fuel to land at Sydney.
However, the crew?s advice to ATC five minutes later, that their
latest landing time was 0618, ?? and then we?d need to go straight
to Canberra? suggests that they had enough fuel to land at Sydney,
but were attempting to expedite their arrival to enable an
attempted landing while retaining Canberra as an alternate for as
long as possible.

This was supported by the pilot in command?s statement that the
PAN broadcast was made because there was insufficient fuel to
proceed to an operator approved suitable aerodrome, ATC were
vectoring the aircraft for a holding pattern and the weather
conditions at Sydney were deteriorating at an unknown rate. The
pilot in command also stated that, without the existence of the PAN
broadcast, the crew could not use Canberra because, in accordance
with the operator?s aerodrome categorisation, this aerodrome could
only be used in an emergency.

The crew?s decision to require an approach to runway 16R was
based on runway visual range (RVR) information that did not reflect
the actual conditions at the time. Had the crew been aware that the
RVR for runway 34L was greater than the landing minima for that
runway, they may not have advised ATC at 0556 that they required
runway 16R. Without the delay caused by commencing that approach,
the crew would probably have landed on runway 34L before the
weather conditions deteriorated below the specified landing minima
for that runway.

The report at 0614 from the crew of the Boeing 747, that they
were visual during the approach to runway 34L, was subsequently
supported by the advice from ATC that the RVR for that runway was
2,000 m, which was greater than the landing minima. This
information was received by the crew of QPC one minute before the
nominated last divert time and, given the pilot in command?s
statement that Canberra would be a preferred option only in the
event of weather conditions deteriorating to below the landing
minima at Sydney, probably influenced them to continue tracking to
runway 34L rather than diverting the aircraft to Canberra.

Although the crew could see the runway 34R threshold during the
ILS approach to 34L, an orbit to enable a change of runway to 34R
would have delayed the landing with the risk of a further
deterioration in weather conditions at both thresholds. Given that
the crew was committed to a landing at Sydney, the decision by the
pilot in command to continue the runway 34L approach reduced the
risk of conducting an approach at a later time when the visibility
was likely to be worse. Use of the autoland system by a trained
crew constituted the safest way to conduct a landing in conditions
below the specified meteorological minima.

Although the operator calculated that the amount of fuel
remaining on the aircraft after the landing was sufficient to
proceed to Canberra after an approach at Sydney, the crew?s fuel
calculations were conservative and were performed in a dynamic high
workload situation.

The occurrence highlights the significant safety issues that
unforecast weather conditions present to aircraft when these
conditions occur during the latter phase of flight. During this
phase, the remaining fuel on board may limit the options available
to flight crews to minimise the effect of the unforecast conditions
on the operation of the flight.

The occurrence also highlights the need for information sharing
between flight crews and air traffic controllers in a timely manner
so that the situational awareness of crews and controllers is
maintained at a high level and crew decision making is optimised in
dynamic weather situations.

Information sharing between flight crews and air traffic
controllers was also a factor in two other occurrences, involving
high capacity aircraft, which were investigated by the ATSB (ATSB
investigation reports
BO/200100213 and
BO/200304400).

Sequence of events

On 6 April 2004, at about 0625 Eastern Standard Time, an Airbus
A330-301 aircraft landed on runway 34L at Sydney aerodrome in
weather conditions that were below the applicable landing
minima¹. The
aircraft, registered VH-QPC, was being operated on an instrument
flight rules (IFR) scheduled passenger flight from Perth to Sydney.
During the latter stage of the flight unforecast fog developed at
Sydney aerodrome, which resulted in the deterioration of visibility
to below the landing minima.

The flight had been planned using a valid aerodrome forecast for
Sydney, which predicted rain showers and visibility greater than 10
km, with 1 to 2 oktas² of cloud at 1,200 ft and 5 to 7
oktas at 5,000 ft. Those weather conditions were above the Sydney
alternate minima³ of
1,479 ft cloud ceiling and 6 km visibility. As there were no
operational requirements due to the forecast weather conditions,
the flight departed Perth without fuel being specifically carried
for a diversion to an alternate aerodrome after an approach at
Sydney.

The flight planned ?Designated Point All Engines Operating?
(DPA)⁴, based on a
diversion to Melbourne, was the IFR waypoint TOBOB, located 314 NM
west of Sydney. Prior to TOBOB the crew obtained the 0400 and 0430
Sydney trend type forecasts (TTF). The TTFs were attached to the
Sydney aerodrome weather reports and detailed the weather
conditions expected to affect the aerodrome for the 3 hours
following the time of the weather report. TTFs were routinely
issued every 30 minutes for Sydney aerodrome and the 0400, 0430 and
0500 TTFs all indicated that, at the time of the aircraft?s
estimated time of arrival (ETA), the visibility and cloud ceiling
at Sydney would be above the alternate minima. The crew did not
obtain the 0500 TTF. The aircraft passed the TOBOB position at
0518.

The crew commenced descent for Sydney at 0540 and, at about the
same time, air traffic control (ATC) broadcast to aircraft that the
Sydney automatic terminal information service (ATIS) had changed to
include the remark that there was a fog bank to the north-west of
the aerodrome. The ATIS was also reporting visibility greater than
10 km, with 1 to 2 oktas of cloud at 1,000 ft and 3,000 ft. The
ATIS consisted of a continuous and repetitive broadcast of
pre-recorded information about the actual weather conditions at the
aerodrome.

The pilot in command reported that following receipt of the ATIS
information, the crew obtained weather information about three
aerodromes near Sydney; Williamtown, Richmond and Canberra. These
aerodromes were available for use during non-normal operations (see
Availability and selection of alternate aerodromes for
more information). The pilot in command stated that, at the time,
he considered the weather conditions at Williamtown might
deteriorate to below the landing minima and Richmond was
unavailable as the conditions were below the landing minima. He
assessed Canberra as presenting potential difficulties due to the
cloud ceiling being just below the alternate minima for runway 17
and just above the alternate minima for runway 35. In addition, the
crew estimated that the fuel on board was insufficient to allow
them to conduct a missed approach at Sydney and then divert the
aircraft to Canberra. The pilot in command also stated that
Canberra was a relatively unfamiliar aerodrome and he did not have
any Notice to Airmen (NOTAM) operational information about
Canberra.

The crew obtained the 0530 Sydney TTF soon after commencing
descent. It indicated that, at the time of the aircraft?s ETA,
there could be periods of up to 60 minutes when the visibility at
Sydney would be 7,000 m in rain showers, with 5 to 7 oktas of cloud
at 1,000 ft. These conditions were below the alternate minima, but
above the special alternate weather minima of 400 ft cloud ceiling
and 2 km visibility that applied to instrument landing system (ILS)
approaches at Sydney. The aircraft was fitted with navigation
equipment that allowed the use of the special alternate minima.

At 0555, as the aircraft descended through flight level 180
about 60 NM south-west of Sydney, ATC advised the crew that the
Sydney ATIS had changed to include a hazard alert. This was in
response to unforecast fog that had reduced the runway visual range
(RVR)⁵ to 800 m. The
ATIS also advised flight crew to expect ILS approaches to runways
34 Left (34L) and 34 Right (34R). One minute later, in response to
that broadcast, the crew advised ATC that they required runway 16
Right (16R) for landing. This was due to the runway 16R ILS
category one landing minima of 220 ft cloud ceiling and 800 m
visibility being less restrictive than the runway 34L ILS category
one landing minima of 270 ft and 1,500 m.

At 0558, ATC advised the crew that there would be a delay due to
their requirement for an approach to runway 16R and instructed them
to turn onto a heading of 230 degrees (taking the aircraft away
from the aerodrome). One minute later, the crew broadcast a
PAN⁶, declaring an
urgency condition due to ?minimum fuel, require runway 16R for
arrival?. The aircraft was then radar vectored for an ILS approach
to runway 16R. The pilot in command stated that the PAN call was
made because there was insufficient fuel to proceed to an alternate
aerodrome approved by the operator for normal operations, ATC were
vectoring the aircraft for a holding pattern and the weather
conditions at Sydney were deteriorating at an unknown rate.

At about the same time, the crew of a Boeing 737 on final
approach for runway 34L advised ATC that they could see the
threshold and the first 1,000 m of the runway. The crew of QPC,
being on a different radio frequency, did not hear that
information, nor was it subsequently provided to them by ATC.

At 0604, in response to a request by ATC for the crew?s latest
acceptable landing time at Sydney, they replied ?? time one eight
[0618] and then we?d need to go straight to Canberra?. The pilot in
command stated that a diversion to Canberra would have been the
selected option only in the event that weather conditions at Sydney
had deteriorated to below the landing minima. He also considered
that a diversion would only ensure a ?minimum fuel state, at best?,
upon arrival at Canberra.

At 0611, when the aircraft was about 17 NM north-north-west of
the airport and tracking to intercept the runway 16R ILS localiser,
the crew requested information about the visibility at the airport.
After receiving a report from an approved observer in a motor
vehicle positioned near the runway threshold, ATC advised the crew
that the visibility on the runway 16R threshold was 400 m. The crew
then requested the visibility on runway 34L. They were advised that
the crew of a Boeing 747 approaching runway 34L ?reported visual at
1000 ft and he?s been visual all the way to 34L?. The crew of QPC
then requested a vector ?for short final 34L? and were provided
with radar vectoring for an approach to runway 34L.

The Boeing 747 landed on runway 34L at about 0614 and its crew
reported to ATC that ??we were visual the whole way to touchdown,
but I?d say this vis [visibility] at midpoint is a thousand metres
or thereabouts and it?s rolling through to the south and
thickening. It won?t be long before it will be a bit worse at the
threshold at 34L?. The crew of QPC did not hear that information as
they using a different radio frequency, but ATC advised them at
0615 that ?? the fog is now moving around at the airfield. I can?t
guarantee you the same visibility as [the] company 747 of yours
[that] landed on 34L?. Soon after, at 0617, one minute before the
nominated divert time, ATC advised the crew of QPC that the RVR for
runway 34L from the threshold was 2,000 m, which was 500 m more
than the landing minima.

At 0622, the aircraft was established on the ILS approach for
runway 34L. At about the same time, the crew of a preceding
aircraft on approach to runway 34L conducted a missed approach
because they were unable to see the runway lights. The controller
passed that information to the crew of QPC. During the approach,
the pilot in command of QPC saw the threshold of the adjacent
parallel runway 34R and he advised ATC that he would need a right
orbit onto the ILS of the adjacent runway to enable a landing.
However, soon after, he decided to continue the approach to runway
34L as the aircraft was configured for an autoland. The pilot in
command reported that he was concerned that if a missed approach
was conducted and the aircraft was then manoeuvred for an approach
to the adjacent parallel runway 34R, the heavy fog to the north may
have moved across that runway during the next approach.

The aircraft was certified for autoland operations using the
auto-flight system to control the aircraft during the approach and
landing phase, and the crew had undergone the required training to
conduct an autoland. The instrument landing systems for the various
runways at Sydney were only certified to facility performance
category one criteria, which meant that pilots were not permitted
to continue approaches, including autolands, to runway 34L when the
weather conditions were less than the specified category one
landing minima.

The crew conducted a runway 34L ILS approach using the
aircraft?s autoland capability and landed at 0625 in weather
conditions that were less than the specified landing minima.

Civil Aviation Regulation (CAR) 257(5) specified that ?? if an
emergency arises that, in the interests of safety, makes it
necessary for an aircraft to land at an aerodrome where the
meteorological minima is less than that determined for that
aircraft operation at that aerodrome? then CAR 257(4) did not
apply. CAR 257(4) stipulated that if an element of the
meteorological minima for the landing of an aircraft at an
aerodrome was less than that determined for the aircraft operation
at the aerodrome, the aircraft was not to land at that
aerodrome.

Availability and selection of alternate
aerodromes

The aircraft operator provided information about aerodromes
approved for A330 operations in the Route Manual
Supplement (RMS), which was carried onboard the aircraft for
flight crew use. If an aerodrome was approved for normal operations
and the weather conditions were forecast to be better than the
alternate minima, the aerodrome was classified as ?suitable
aerodrome? and could be used as an alternate aerodrome for the
destination. Brisbane, Coolangatta, Melbourne and Adelaide were the
closest aerodromes that could be used as ?suitable aerodromes? for
operations to Sydney.

The RMS also listed other aerodromes that were categorised as
either emergency or adequate aerodromes that could be used during
non-normal operations. Emergency aerodromes had runways of a
sufficient length that could be used by A330 aircraft in an
emergency situation. Nowra and Canberra were the closest emergency
aerodromes to Sydney.

Adequate aerodromes met the requirements detailed in Civil
Aviation Order 82.0, Appendix 2. If the weather conditions were
forecast to be better than the specified ?adequate criteria?, an
adequate aerodrome was considered to be a ?usable adequate
aerodrome? and could be used by A330 aircraft following a critical
system failure. The closest adequate aerodromes to Sydney were
Williamtown and Richmond.

During the latter phase of the occurrence flight, the closest
?suitable aerodrome? was Melbourne and, once the aircraft flew past
the DPA at 0518, the fuel on board was insufficient for a diversion
to that aerodrome. When the crew became aware of the presence of
fog near Sydney at about 0540, they obtained information about two
adequate aerodromes, Williamtown and Richmond, and an emergency
aerodrome, Canberra (see Sequence of events for the crew?s
assessment of the weather conditions at these aerodromes).

The pilot in command stated that he considered that ATC had to
be advised of an emergency condition in order to use an emergency
aerodrome listed in the RMS. The operator also advised that the
decision to divert to Canberra would have required the broadcast of
PAN.

Aircraft fuel load

The aircraft departed Perth with 28,900 kg of fuel, which was
2,500 kg more than the minimum amount required by the operator?s
approved fuel policy for the flight. The planned flight time from
Perth to Sydney was 210 minutes and the fuel load gave the aircraft
an endurance of approximately 315 minutes. As noted earlier, the
aerodrome forecast did not require additional fuel, for either
holding or a diversion, to be carried on this flight to Sydney.

The operator?s Flight Administration Manual (FAM)
contained the fuel policy that specified the minimum mandatory fuel
requirements approved by the operator for the operation of its
aircraft. The FAM stated ?? it is recognised that there are
occasions when a flight may pass DPA with the required fuel on
board and a subsequent deterioration in forecast weather may then
result in the minimum mandatory requirements ? not being met?.

After the occurrence, the operator calculated that the total
amount of fuel required to divert the aircraft from a missed
approach at Sydney to a landing at Canberra with reserves intact
was 6,700 kg. Information from the aircraft?s flight data recorder
showed that the aircraft landed at Sydney with about 6,900 kg of
fuel remaining.

Meteorological information

The Bureau of Meteorology advised that fog at Sydney aerodrome
was difficult to forecast as it was an uncommon event with between
four and eight occurrences per year. Although the fog forecasting
models had been upgraded in 1999, those models did not predict the
formation of fog on the date of the occurrence. The classic fog
scenario occurs when radiative cooling in the north-west area of
the Sydney basin produces fog, which then moves to the aerodrome in
a north-westerly surface airflow. For this airflow to form, several
hours of clear sky is required to produce enough cooling so that
the surface airflow uncouples from the gradient level airflow,
which is 2,000 ft to 3,000 ft above ground level and may be moving
in a different direction.

As the cloud was expected to persist during the morning of the
occurrence, the duty operational meteorologist considered that the
airflow uncoupling would not occur and the surface airflow would
remain the same as the gradient airflow, which was from an east to
north-easterly direction. At 0230, the high resolution computer
forecasting models indicated precipitation and an east to
north-east surface airflow. Although fog had formed in some western
areas of the Sydney basin by 0330, the meteorologist considered
that the rain showers near Sydney aerodrome would assist in mixing
the slightly stronger east to north-easterly gradient airflow down
to the surface airflow and hence reduce the risk of fog drifting in
from the north-west.

The low-level cloud at the aerodrome cleared at about 0500,
which coincided with the surface airflow tending to be from the
north-west. About 40 minutes later fog was observed to the
north-west of the aerodrome and soon after it started to move over
the runways. The deterioration in visibility associated with the
movement of the fog was rapid. A reduction in visibility from 2,000
m to 400 m was reported during an 11 minute period from 0552 to
0603. The fog did not clear until 0830.

The Canberra 0530 special aerodrome weather report (SPECI)
obtained by the crew indicated that the wind was from the south at
3 to 4 kts, visibility was greater than 10 km, with 7 oktas of
cloud at 2,100 ft above the aerodrome. The Canberra 0600 TTF was
not obtained by the crew, but it did not vary significantly from
the 0530 SPECI. According to the applicable Jeppesen Airport
Chart, Canberra was available as an alternate aerodrome
provided the cloud base above the aerodrome was not below 2,084 ft
(2,144 ft for runway 17) and the visibility was not less than 6 km
(7 km for runway 17). That minima was applicable with an actual
QNH, which was available at Canberra, and was being broadcast on
116.7 MHz.

Provision of weather information to the flight
crew

Airservices Australia (Airservices) was the air traffic service
(ATS) provider. The procedures for the provision of weather
information to flight crew by air traffic controllers were detailed
by Airservices in the Manual of Air Traffic Services
(MATS). The Aeronautical Information Publication (AIP),
also published by Airservices, was an Australian operational
document used by pilots. The information in the AIP that related to
the provision of weather information was in accordance with the
MATS.

The MATS required air traffic controllers to monitor and assess
information from various reports. These included weather forecasts,
amended weather forecasts and observations and reports that
indicated the weather conditions at the destination aerodrome had
deteriorated below the alternate minima. If that information was
assessed to be of ?an unexpected and critical nature?, controllers
were required to broadcast a hazard alert to flight crews.

At 0554, soon after the fog was observed moving over the
aerodrome, the controllers amended the ATIS to include a hazard
alert due to unforecast fog. The ATIS also advised pilots to expect
an ILS approach to either runway 34L or 34R and that the RVR had
reduced to 800 m in fog. This information was also broadcast at
that time by the controller handling QPC.

A special aerodrome weather report (SPECI) was issued at 0603,
and this report indicated that the visibility had reduced to 400 m
in fog, which was below the landing minima of any runway at Sydney.
The TTF appended to the SPECI indicated that these conditions would
continue until 0800. The 0603 SPECI information was not passed to
the crew of QPC. However, they became aware of the deterioration in
the weather conditions at 0611 when ATC provided information about
the RVR on runway 16R.

The RVR was assessed by approved observers in motor vehicles
positioned near the runway thresholds. To determine the RVR, the
observers counted the number of visible runway lights or
identifiable features adjacent to the runway. This information was
then broadcast to the aerodrome controller in the control tower.
During the occurrence, there were observers in vehicles located
near the thresholds of runways 16R, 34L and 34R.

Landing information, including RVR, was provided to flight crews
either by controllers, or by the crews listening to the ATIS
broadcast. The MATS section 5.1.11 contained instructions regarding
the format of weather information provided in the ATIS. The MATS
specified that, when the visibility was between 1,500 m and 800 m,
the ATIS may report RVR information. It also stated that when
visibility was 800 m or less, the RVR ?shall be reported? in the
ATIS broadcast. The MATS and the AIP did not specify that the RVR
information in an ATIS broadcast was to be for the touchdown zone
of the runway in use.

When the ATIS was amended at 0554 to include the 800 m RVR
information, the actual visibility at the touchdown zone of runway
34L was greater than that figure.

The ICAO Annex 3 ? Meteorological Service for International
Air Navigation (Annex 3), contained the standards and
recommended practices regarding the provision of metrological
services in the aviation sector. Paragraph 4.7.4 of Annex 3
recommended that RVR observations should be made on precision
approach runways for category one instrument approach and landing
operations. Runway 16R and 34L at Sydney were category one
precision approach instrument runways. ICAO Annex 14 ?
Aerodromes defined a category one runway as ?? an instrument
runway served by ILS ? and visual aids intended for operations with
a decision height not lower than 60 m (200 ft) and either a
visibility not less than 800 m or a runway visual range not less
than 550 m?. Paragraph 4.7.2 of Annex 3 required that:

Runway visual range observations shall be representative of the
touchdown zone and, depending on the category of operation for
which the runway is intended and the length of the runway, of the
mid-point and stop-end of the runway.

Following the amendment of the Sydney ATIS at 0554, the weather
conditions continued to deteriorate with the visibility reducing to
400 m by about 0600. However, the next amendment of the ATIS was
not until 0633. Requirements for the revision of ATIS information
were covered in the MATS section 5.1.12. Paragraph 5.1.12.1
specified that when the current value of the visibility information
was less than 1,500 m, controllers were to amend the ATIS as
required if the value of the visibility was expected to vary and
remain that way for at least 15 minutes.

Paragraph 5.1.12.2 of the MATS stated that in situations where
controller workload precluded the amendment of the ATIS in
accordance with the MATS requirements:

Tower controllers shall ensure that aircraft under their control
are advised of sudden and perhaps unexpected changes to the
aerodrome information, pending an amended ATIS.

Notwithstanding these requirements, there was no provision in
the MATS for controllers, who were located remote from the control
tower and who were passing ATIS information to flight crews, to be
made aware of changes to the aerodrome information pending the
updating and broadcasting of an amended ATIS.

¹ The landing
minima are the Civil Aviation Safety Authority (CASA) specified
meteorological conditions of cloud ceiling and visibility. In order
for an aircraft to land at an aerodrome, the actual weather
conditions need to be at or above the landing minima.

² Okta is a
meteorological unit, equal to the area of one eighth of the
sky.

³ Alternate minima are
the cloud base and visibility values specified by CASA for a
particular aerodrome such that, if the conditions are forecast to
be worse than the alternate minima, the pilot in command must
provide for a suitable alternate aerodrome.

⁴ The term DPA was used
by the operator to indicate the point along the flight plan track
that equated to the last position furtherest removed from the
departure aerodrome from which an aircraft could divert to a
nominated off-track alternate aerodrome. The minimum fuel required
to be onboard at the DPA comprised the flight fuel to the alternate
aerodrome, a variable fuel reserve, approach fuel, a fixed fuel
reserve and holding fuel (if required).

⁵ The International
Civil Aviation Organization (ICAO) Annex 3 - Meteorological
Service for International Air Navigation defined runway visual
range as '? the range over which the pilot of an aircraft on the
centre line of a runway can see the runway surface markings or the
lights delineating the runway or identifying its centre
line'.

⁶ PAN is an expression,
spoken three times in succession, used in the case of an urgency
condition. ICAO Annex 10 - Aeronautical
Telecommunications, Volume II, defined an urgency condition as
'? a condition concerning the safety of an aircraft or other
vehicle, or of some person on board or within sight, but which does
not require immediate assistance'.