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WEATHER WISE

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Ike r Geoff Smith & Mal Wa

LTurbulence from mountain waves is a major threat for gen-eral aviation opera-

:1r •. tions, particularly for low level flying in low powered aircraft.

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Wind profile

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26 FLIGHT SAFETY AUSTRALIA AUTUMN 1997

W INTER IS THE TIME OF STRONG westerly winds over the southern half of Australia.

The subtropical ridge - a series of high pressure systems that lie east west around the world - has moved northwards, to lie between about 30 S to 25 S.

As a result, the southern half of the conti-nent experiences fronts embedded in the westerly airflow giving rise to vigorous winds in the late winter and early spring months.

Gale force winds are often found both ahead of and behind a frontal system. Winds ahead of a front are usually northwesterly; those behind the front are mostly southwesterly.

Strong wind can force you to abandon a flight, as the pilot of a Beechcraft Queenair on a regular public transport flight between Sydney and Nowra found.

This pilot encountered unusually strong downward vertical currents not far from Syd-ney airport. At about 1700EST when the air-craft was in the climb about 23km south of Sydney, it experienced a rate of descent of about 400ft per minute.

The pilot increased the power setting to the maximum climb power and the rate of descent reduced to about 1 20ft per minute at an elevation of about 3,800ft.

Abandoned flight Returning to Sydney, climbing attitude and power were initially maintained; however, the aircraft had descended to about 2,000ft at 16km from Sydney airport. The aircraft left Sydney for Nowra at 1800EST.

Turbulence and difficulty in maintaining height and roll attitude were again experi-enced at about 24km out from Sydney. The flight was again abandoned. A check of the aircraft indicated that all systems were oper-ating satisfactorily.

The atmosphere is like a fluid in motion. Just like water, when the air passes over obsta-cles the flow is disrupted and new and faster currents can be formed. The bigger the obsta-cle, the greater the effect.

If you operate in mountainous regions you need to be acutely aware of the potential for hills or mountains to induce severe wind events.

During take-off and landing in strong winds and mountainous terrain, airspeed fluctuations can result in an inadvertent stall.

On days when the localised wind gusts are exceeding SOkt, down draft winds can exceed

INDUCED WIND HAZARDS —

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1,000ft per minute. Greater downdrafts of 1,500ft per minute are not uncommon. A B-747 operating out of Anchorage, Alaska sev-eral years ago, experienced an engine separa-tion shortly after take-off as a result of a severe downdraft.

Examples of incidents closer to home involve a light helicopter as well as a DC3: • A Hughes 500 helicopter was approaching to land at a rig site atop a 9000ft ridge line. The approach was being conducted into a prevailing 30kt wind at a shallow angle at right angles to the ridge line.

As the helicopter approached within 1km of the rig site, it began to lose height, and despite application of full power, was unable to regain a satisfactory approach angle. The approach was aborted and another - suc-cessful - approach was conducted at a steep-er angle, with crosswind, along the ridge line. • A DC3 enroute from Canberra to Cooma at

600ft encountered standing wave conditions associated with a strong westerly wind. As the aircraft approached mountainous terrain, it began to lose altitude.

Despite the application of climb power, the aircraft was unable to climb clear of rising ground. Only with the application of maxi-mum continuous power was the aircraft able to climb and track clear of the standing wave.

Wave formation

Turbulence from mountain waves around hills or mountains is a major threat for gen-eral aviation operations, particularly for low level flying in low powered aircraft.

The effects increase with elevation as power margins decrease and wind speeds increase.

The conditions most favourable for the for-mation of mountain waves are: • Winds flowing at right angles or nearly right angles to the ranges. • Wind strengths of at least 20 knots at the ridge line. The stronger the wind the greater the turbulence and more likelihood of loss of control. If a localised gust were to reach 50 knots the downdraft effect is generally greater than 500ft per minute. • Wind speed increasing with height. • A relatively stable atmosphere.

If there is enough water in the atmosphere, lenticular clouds will provide a clue to moun-tain wave turbulence. If the atmosphere is dry, as it often is ahead of a front, there will be no visual indication of the presence of mountain waves.

Gusts can increase significantly as a front gets closer. Down draft winds may overcome the flying capabilities of the aircraft.

Beware of mountain waves on very hot days - the density altitude will be high and aircraft performance will be reduced.

The keys to avoiding the effects of standing waves are: • Select a cruising level at least 2,000-3,000ft above prominent ridge lines or mountain ranges. • Fly to the windward side of prominent mountain ranges and ridge lines. -Avoid flying in the vicinity of rotor clouds. • When operating in the vicinity of standing waves, reduce aircraft loading. • If forced to cross ridge lines at low level, do so at an oblique angle with a pre-determined escape path.

Geoff Smith is regional aviation and defence manager (NSW) for the Bureau of Meteorology. Mal Walker is a CASA flying operations inspector.

FLIGHT SAFETY AUSTRALIA AUTUMN 1997 27