3.4                                   Forecasting requirements for aircraft operations

Arguably the most weather sensitive of all Antarctic based logistical operations are those concerned with aviation. Although often an inconvenience, and occasionally costly in money terms, most marine and land based activities have the benefit of being able to wait out adverse weather. Aircraft, on the other hand, have a relatively limited operating range before they must land. Even in the case of the long–range aircraft operations where weather is considered to be less of a risk to life, the purpose of the aircraft flight might be compromised by inclement weather. In the following sections some of the requirements of inter and intra‑continental aircraft operations are considered in the Antarctic context.

3.4.1                                Intercontinental operations

There are an increasing number of aircraft flights to, from and over Antarctica. For example, the great circle route from Australia to South America includes a flight path over the Antarctic sea ice. The US has been flying personnel into its station at McMurdo, from New Zealand, since the mid–1950s, while flights from South American countries to the Antarctic Peninsula are commonplace.

3.4.1.1                          Intercontinental operations at minimal weather risk

At the 1998 International Symposium on Operational Weather Forecasting in Antarctica, held in Hobart, Australia (Turner et al., 2000a), Captain J. Dennis described the tourist over–flights of the Antarctic that the Australian airline QANTAS facilitate. These flights start at Melbourne, Sydney, Adelaide, or Perth, with the routes passing over various parts of the continent, including Dumont d’Urville Station, the Transantarctic Mountains, Casey or Davis Stations and then return to Australia. The aircraft used are Boeing 747–400 series aircraft and the primary aim is to maximise time over the continent in clear sky conditions. Operating procedures dictate that the aircraft must, at all times, be able to return to an airport (in Australia, New Zealand or South Africa) if simultaneous failure of two engines and loss of cabin pressure occur.

Upper winds and the choice of route are critically important for these flights so as to maximise the tail wind for as much of the journey as possible. Meteorological analyses and forecasts are obviously very important in the pre–flight planning of these journeys and during the flights the pilots are in radio contact with meteorological personnel who are located in Australia and at the Antarctic research stations. In the future pilots hope to be able to receive, in flight, satellite imagery for themselves to aid the identification of cloud–free areas of the continent.

It will be inferred that this type of operation is virtually "weather" risk free provided that the aircraft always maintain a safe cruising altitude. The weather related penalties are mostly related to economic loss due to fuel burnt unnecessarily and to loss of viewing conditions when cloud obscures the Antarctic features of interest from fee–paying tourists.

3.4.1.2                          Intercontinental operations with some weather risk

In contrast to flights that only fly over the Antarctic, aircraft operations that have the prime purpose of ferrying people to and from the Antarctic face an increased risk by virtue of having to land in the Antarctic. Here we do not want to overstate the risk: for example the US has never lost a life during its many years of flying fixed–winged aircraft into McMurdo on an almost daily basis. However, generally speaking, the meteorological support/infrastructure for flights that must land in the Antarctic is substantial.

While each nation/agency will approach this task differently there are some common requirements. At the previously mentioned International Symposium on Operational Weather Forecasting in Antarctica, Mr. Jack Sayers (the then Executive Secretary of COMNAP) gave a summary (Turner et al. 2000a) of the requirements of inter and intra–continental flights and discussed the need for aviation forecasts for flights such as those undertaken by both national and commercial operators.

Mr. Sayers noted that COMNAP plays an important part in developing procedures for aircraft operations in Antarctica and has published an Antarctic Flight Information Manual plus an Antarctic Telecommunications Operators Manual. During his talk Mr. Sayers dealt in particular with the possible development of an East Antarctic Air Network that would air–link various countries with the Antarctic via appropriate embarkation/disembarkation airports/runways. Mr. Sayers indicated that such an enterprise would obviously require accurate weather forecasts including information such as:

·                         an outlook two to three days prior to flight;

·                         a further outlook including route winds and TAFs 24 hours prior to takeoff;

·                         detailed route forecast and diversion route forecasts, satellite pictures, TAFs and alternate TAFs two to five hours prior to flight;

·                         continual weather watch and phone contact with meteorological personnel during flight, plus hourly Meteorological Aerodrome Reports (METARs), aerodrome reports when special (adverse) criteria are met (SPECIs) and Trend Type Forecasts (TTFs) (or landing forecasts);

·                         updated METAR and TTF 30 minutes before the point of safe return (PSR).

3.4.2                                Aircraft requirements within the Antarctic

Aircraft operate within the Antarctic for a variety of reasons including: transport of supplies and personnel between stations, field camps and re–supply ships; sea ice reconnaissance flights from vessels; science purposes such as magnetometer and ground penetrating ice radar transverses; and for mapping. The actual weather limitations on aircraft operating parameters will be aircraft type dependent: rotary winged aircraft (helicopters) can, for example, land in very small spaces, and, can land in very strong surface winds that would otherwise prevent a fixed winged aircraft from safely touching down. On the other hand a fixed winged aircraft is somewhat safer to operate in white–out conditions in that such an aircraft can approach the landing site with a very shallow flight path. A helicopter, on the other hand, tends to blow up loose snow that exacerbates the surface definition problem and is prone to tipping on the slightest incline.

The following notes are an overview of how various elements might affect aircraft in a general sense. They are based on information supplied by V. Barkell for the Australian Antarctic Forecaster Handbook and have a bias towards helicopter operations. Nonetheless the information is of general interest to aviation activities in the Antarctic. Lied (1968) also provides useful information. A more detailed account of forecasting methods for these elements is given in Section 6.6, while site–specific information is given in Chapter 7.

3.4.2.1                          Visual Flight Rules

While aircraft operate under various jurisdictions in so far as operational rules are concerned, it is generally accepted that intra–Antarctic flights are conducted under "Visual Flight Rules" (VFR), that is under “Visual Meteorological Conditions” (VMC).

3.4.2.2                          White–out and cloud

White–out is the most potentially dangerous weather condition that a pilot in flight over unrelieved horizon to horizon might face. As a consequence the presence or likelihood of cloud developing in such areas must be forecast as accurately as possible: this requirement cannot be too heavily emphasised.

3.4.2.3                          Wind velocity

Surface

Surface wind speed and direction can affect aircraft operations in several ways including:

·                         on landing and take off: in common with other areas of the world, aircraft are operated most safely when landing and taking off into the wind. Moreover, aircraft will have specific cross–wind thresholds above which they cannot operate.

·                         Rotary winged aircraft (helicopters) blades generate lift at quite low rotational speeds during engine start or engine run–down: in strong winds the blades will tend to sail and there is a risk of damage to the tail boom as a result of a blade strike or damage to the rotor head. On the other hand helicopters can operate (hover) in quite high surface wind speeds as long as there is no attempt to switch off. Fixed winged aircraft would not generally be able to land at these high wind speeds (say 25 to 30 m s^–1 (~50–60 kt)).

·                         At high surface wind speeds both types of aircraft are susceptible to mechanical damage if parked. Fixed wing aircraft are likely to be affected at lower speeds.

·                         Strong winds can cause drift snow to rise to such a degree that take–off or landing becomes hazardous or impossible.

·                         Above about 25 m s^–1 (~50 kt) wind speeds will cause problems for sling–loading operations.

Upper

Turbulence aside, the main direct effects of upper winds on aircraft are that they are needed to be taken into account in navigation and they can affect aircraft speed relative to the ground and impact on fuel usage.

3.4.2.4                          Turbulence

Due to the general smoothness of the ice surface, mechanical turbulence is mostly confined to areas near mountains or nunataks. Clear Air Turbulence (CAT) is occasionally encountered. In some coastal areas after the fast ice has broken out, roll or rotor cloud has been frequently encountered together with associated very strong vertical air movements. Almost invariably lenticular cloud is associated with the roll or rotor cloud. Cloud level is often about 300 to 600 m (~1,000 to 2,000 ft) above the surface.

In general, fixed winged aircraft are more susceptible in flight to turbulence than are helicopters.

3.4.2.5                          Drift snow

Particles of drift snow are extremely fine and a wind speed of approximately 5–8 m s^–1 (~10‑15 kt) is enough to move them in a bouncing motion called "saltation". As the wind speed increases, drift snow travelling at wind speed, rises above the general surface. At about 10 m s^–1 (~20 kt) drift will rise to approximately 1 m while wind speeds of about 17 m s^–1 (~33 kt) causes the sky to be obscured to a standing human observer. During blizzards with wind speeds in excess of about 40 m s^–1 (~80 kt) drift snow may rise to 30 to 40 m (~100 to 150 ft).

Whilst in flight (under VMC) drift snow can be readily seen blowing over the snow surface. It is, however, somewhat harder to estimate the height to which the drift rises although the above rule of thumb may be used if the surface wind speed is known.

It is dangerous for aircraft to attempt to land in blowing drift snow conditions without some ground reference – a building; cane marker; fuel drum etc. (Without a reference a helicopter may easily come onto the surface with an appreciable rearward speed, so that the aircraft will inevitably tip backwards onto its tail rotor or perhaps roll if there is lateral travel in addition to rearward travel on surface contact.)

If blowing snow has risen to a height much in excess of 2 m (~6 to 7 ft) above the surface, a helicopter pilot attempting to land may lose all visibility before touch down and increasing the risk of an accident.

3.4.2.6                          Icing  

In the early part of each summer when fast ice may extend many kilometres seaward from the coast, the possibility of airframe icing in VMC over the continent is much reduced because the air is almost devoid of moisture. When operating inland from the coast in VMC the same generally applies regardless of the fast ice disposition. 

When the fast ice breaks out from the coast, moisture laden maritime air is present in coastal areas. When the dew–point and air temperature are within a couple of degrees of each other, rime icing occurs in clear air and may cover windscreens etc. As the airframes are at a temperature below freezing, the rime ice impacts and builds an opaque mass assuming that there is no de–icing equipment available.

Landing becomes hazardous, as the pilot has very restricted vision ahead. There is the possibility of rotor blades and engine air intake icing leading to severe airframe vibration on the one hand and possible engine failure on the other (again technological solutions notwithstanding).

3.4.2.7                          QNH  

The diurnal and semi–diurnal surface pressure variations over the Antarctic are small, the amplitudes of both these waves being about 0.2 hPa, and so most significant variations in air pressure will be due to weather systems. In calculating the QNH (the pressure value used on altimeters to give altitude) allowance needs to be made for the cold Antarctic environment.

Altimeters are normally calibrated according to a standard atmosphere with a fixed lapse rate of 0.65ºC per 100 m and a MSL temperature of 15ºC (288 K). To adjust for the real atmosphere's departure from 15ºC the following equation may be used (Meteorological Office (1971, p. 15)):

Z  =  Z_ind(1 + ( Ts – 15)/288)                                                                 Equation 3.4.2.7.1

It may be seen that when the surface air temperature (Ts (in ºC)) is below 15ºC the altimeter readings (Z_ind) will be higher than the estimated "true" height Z. For example, if the surface air temperature is –20ºC the indicated height (Z_ind) has to be multiplied by 0.88 to give a more accurate (lower) estimate of altitude. (See also the discussion on the Antarctic atmosphere in Appendix 3.)

3.4.3                                Volcanic Ash Advisory Centres

Air–borne volcanic ash is composed of fine pulverized rock and accompanied by a number of gases, which become converted into droplets of sulphuric acid and hydrochloric acid – a mixture potentially deadly to aircraft and their passengers. In 1982, for example, a British Airways airliner lost all of its engines on encountering ash from an Indonesian volcano, losing more than half its cruising altitude before the engines could be restarted and an emergency landing made in Jakarta.

The main problem is that the ash melts in the hot section of the engine and fuses into a glass-like coating on components further back in the engine, causing loss of thrust and possible flame out. The ash can also cause abrasion of engine and other parts and clogging of fuel and cooling systems.

Volcanic ash can circle the globe very quickly at mid to high latitudes and high‑temporal resolution satellite imagery are not always available over these area, making it hard to identify and track volcanic plumes. However, new communications technologies and increased international understanding are helping to address the problem. Figure 3.4.3.1 shows, for example, a Total Ozone Mapping Spectrometer (TOMS) instrument satellite image showing how ash from the Mount Hudson (Cerro Hudson, Chile) eruption in August 1991 circled the globe (aeroplanes, for example, encountered the ash over Australia). (A comprehensive archive of TOMS volcanic products at may be found at http://toms.umbc.edu/.)

There are nine Volcanic Ash Advisory Centres (VAACs) around the world advising aircraft about location and movement of ash clouds. The VAACs that are of relevance to the Antarctic are located in Buenos Aires (Argentina), Darwin (Australia), Toulouse (France), and Wellington (New Zealand). The Darwin VAAC, for example, covers Indonesia, Papua New Guinea and part of the Philippines, as well as the region to the South Pole between 75°E and 160°E (see http://www.bom.gov.au/info/vaac for more information). The Darwin centre combines satellite detection techniques with vulcanological ground reports, pilot reports, meteorological knowledge and numerical models to track and forecast movement of ash clouds.

From http://www.ees.nmt.edu/Geol/volcanology/erebus.html it may be seen that “Mount. Erebus (77°32'S, 167°10'E), Ross Island, Antarctica is the world’s southern-most active volcano (Figures 3.4.3.2 and 3.4.3.3). Discovered in 1841 by James Ross, it is one of only a very few volcanoes in the world with a long-lived (decades or more) lava lake”. However, volcanic ash eruptions from this volcano are rare and, being south of 60º S Mount Erebus does not come under any of the VAACs’ areas of responsibility.

Heard Island (see Section 7.2.8), just to the west of the Darwin VAAC's area of responsibility is in an area that is not officially monitored due to data sparsity. And the Balleny Island group, a 190 km long chain of three (main) volcanic islands (66° 25' to 67° 58' S, 162° 50' to 165° 00' E) is west of New Zealand’s VAAC area of responsibility. These are Antarctic–related examples of potential ash–problem areas for aircraft. Flights between Australia and South Africa might potentially be affected by volcanic activity on Heard Island.

Figure 3.4.3.3     A location map for Mount Erebus. (From http://www.ees.nmt.edu/Geop/mevo/erebus_info.html, courtesy of Philip Kyle.)

	Figure 3.4.3.4     A  faint plume extends  towards the right of  the image from the  summit of “Big  Ben”, Heard Island . The McDonald  Islands may be seen  near left center of  the image.  (NASA Space Shuttle  Photo ID: STS061A-49  -47, 3rd November 1985.  Courtesy of the NASA, Johnson Space Center, Houston, USA.)
	Figure 3.4.3.5  Although probably  taken on a day  different to that in  Figure 3.4.3.1  above, a small  plume also extends  towards the right of  this image, from the  summit of  “Big  Ben”.  (Photo by Bruce  Hull, courtesy of Paul  Carroll.)

In fact, both the “Big Ben” volcano on Heard Island (see Figure 3.4.3.4, 3.4.3.5 and 7.2.8.1.1) and the nearby McDonald Island are active, suggesting a need for aircraft flying in the vicinity to be aware of the possible dangers. The Antarctic tourist flights between Australia and the Transantarctic Mountains (see Section 3.4.1.1) fly may fly close to the Balleny Islands. An excellent paper about possible recent Balleny Islands activity is at http://www.volcano.si.edu/gvp/volcano/region19/  – in the paper several international experts discuss this event, with opinions divided on whether the data suggests volcanic activity, but there is agreement on the need for caution by aircraft operating in this area.