Volcanic ash

Volcanic ash emitted by active volcanoes can be extremely hazardous. Several examples have been recorded of jet engines shutting down as a result of ash belts found high in the jet stream.

Ash endangers aircraft causing engine thrust loss or flameout, erosion to leading edges, and clogging of pitot- ­static systems.

The presence of ash clouds is not always readily evident to a flight crew. The sky may appear to contain a typical cumulonimbus formation, but in fact it also contains an ash plume climbinging into the clouds.

Volcanic ash advisories help pilots to avoid ash encounters. They contain information on the eruption, location, height and amount of ash, current move­ment and other information. Volcanic ash advi­sories are updated at least every six hours.

Volcanic ash advisories are available to the aviation com­munity via a variety of communi­cations systems, including satellite broadcasts operated by the world area forecast centres and from the Global Telecommunications System.

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Wind shear and turbulence alerts at Hong Kong International Airport

Located in an area sensitive to wind shear and turbulence, the Hong Kong Observatory continues to find techniques for detecting these phenomena and providing timely alerts to aircraft approaching or departing Hong Kong International Airport.

Wind shear is a change (i.e. lasting more than a few seconds) in wind direction and speed resulting in a change in aircraft lift. A decrease in lift will cause the aircraft to descend below the intended flight path. A change of 15 knots or more in headwind or tailwind is considered significant wind shear which may require timely and appropriate corrective action by the pilot.

Turbulence is caused by rapid, irregu­lar motion of the air. It brings about bumps or jolts, but does not normally influence the intended flight path of an aircraft to a large extent. However, in severe turbulence, abrupt changes in the altitude and attitude of an aircraft may occur and the pilot may momentarily lose control of the aircraft. For reporting and alerting purposes, moderate or severe turbulence is considered significant.

There are a number of reasons for wind shear and turbulence at Hong Kong International Airport. The most common is windy conditions, where the hills, surrounding the airport, disrupt the flow of air. Thunderstorms are another cause because they can lead to gust fronts and microbursts. Sea breezes also cause wind shear and turbulence.

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Bird strike hazard

A particular hazard for jet engines is bird strike damage. Fan blades could snap if a large bird flies into them.

Airport authorities try many ways of scaring birds from runways. Falcons are used to chase them away, the grass is left long so they cannot land, blank shots are fired, the sounds of birds of prey are broadcast. At some airports collie dogs have been trained to prevent bird strikes.

When planes collide with birds, the results can be catastrophic - an event most likely to occur on lower-altitude flight paths near airports. Radar and infrared detectors can already spot birds but cannot identify a large bird that could cause serious damage from a small, low-risk one. For example, a large goose poses more of a threat that a smaller, lighter gull or hawk.

A new method in scaring birds is an “Audio telescope”. It is used to pick the sound of distant birds and very successful at discriminating between different birds.

An "audio telescope" that can identify different species of birds by their calls, could save planes from birds and protect them from crashing.

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