FLYING THE SNOWIES IN STRONG WINDS
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General Description of Conditions
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High flight in calm air above the inversion
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Flights in turbulence beneath the inversion
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Inbound and outbound operations from Jindabyne Airstrip
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Special techniques for operating from Jindabyne airstrip
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Large Diagram explaining airflow over mountains
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Image showing patterns of lift and sink around the Snowies
FOREWORD
Flying around mountains in strong winds anywhere in the world can be conducted safely, but the secret
to safe operations is having local knowledge of the airflow and of the danger spots. The Snowies are no
less hazardous than any overseas sites, as they present a 6000ft barrier to westerly winds where the air
is forced to conform.
This guide is to assist visiting pilots to understand the conditions and to help promote safe, stress free flying
in this beautiful area, and from the Jindabyne Airstrip.
This information is condensed from a detailed study made during 28 years of flying around the Snowies
with Alpine Soaring, sailplanes, and from aircraft. Alpine Soaring, specialized in wave flying where the
best soaring conditions were during high winds, where the rotor systems were used to boost climbs to reach
the standing wave systems above. We conducted thousands of operations here, which included visiting
pilots, without any incidents or accidents, using the local knowledge gained.
Safe Flying
Barry Wrenford
FLYING THE SNOWIES IN STRONG WINDS
Jindabyne Airstrip is becoming increasingly popular during the winter for skiers flying to the mountains for
their holidays. As Australian pilots are not usually very familiar with conditions that occur in mountain
environments, problems have been encountered here. These can be broken down into not allowing for the
high altitude of the strip, not being aware of the local conditions around the strip in strong winds, and lack of
knowledge of the airflow over the mountains in strong winds. This article deals with the strong prevailing winds
from the westerly directions in winter.
FLYING OVER THE MOUNTAINS
Flying over the Snowy Mountains during the calm conditons of a high pressure system can be a safe and awe
inspiring flight, particularly over the west face of the range. In strong wind conditions without local knowledge,
it can be a fear inspiring experience with it “being better to be down there wishing to be up here, than up here
wishing you were down there”.
Anyone flying to Jindabyne should look at the forecast winds at 7000ft for area 21. The west face of the
mountains is a 6000ft high wall against the normal westerly winds, and air being forced against it is compressed
and accelerated over the top as a squeeze effect. The wind acceleration is very close to 1.5x for the wind at
7000ft, and this continues downwind from Jindabyne. So a 20 knot wind at altitude will become 30 knots, and
the pity of it is that forecasters don’t mention this.
The next thing is that during the cold conditions of winter, the inversions are often below or close to the crests
of the main range, and under and over the inversion are different worlds. An inversion is the surface of a lower
more dense layer of air, which behaves similarly to the surface of water except that the effects are hundreds of
times larger in size. The denser air beneath the inversion behaves like rapids in a river, where the surface is a
series of waveforms downstream from the obstacles beneath. The main range is the obstacle, with the air
beneath the inversion being chaotic with rotors, giving extreme turbulence combined with sink well beyond
the climb rates of most aircraft. The wave forms are fixed in relation to the terrain beneath, but do drift in
position during the day.
If forecasted winds at 7000ft are above 25 knots, the real winds at this height and above are approaching 40
knots. To be under this inversion and downwind and below the crest of the Snowy Range is not a good place
to be, so flights to land at Jindabyne are not advised for newcomers.
The surface of the inversion in the lee of the mountains usually undulates in these winds with an amplitude
of 500 to 1500 ft, but can go as high as 3000ft in some conditions. One needs to realize that at an average
inversion level, you can be in calm air over the trough of the wave and fly beneath the inversion into the rotor
at the crest of the wave. It is possible to maintain position in calm air in a wave trough, a thousand feet or more
below the rough air under the crest. Wavelengths are usually around 2 - 5 nm, but can be much shorter in the
valleys, and as long as 10 nm in high winds. Above the inversion there usually exists a lighter and stronger wind
which bounces off the windward side of the waves of the dense lower layer. This forces the upper air to follow
the undulations, and gives rise to mountain waves, which down here have been used by gliders to over 30,000ft.
The air above the inversion is usually glassy smooth, but only a few feet of the inversion surface separate it from
the violent rotors beneath. The wave form may be marked by a sequence of elongated rotor clouds at the crest
of the waves, downwind from the crest of the range. If there is no cloud, only the rate of climb indicator will
give you advance warning.
The smooth lift and sink above the inversion is roughly two thirds that of the maelstrom below. Typical lift
and sink above the inversion is 500 to 1000 ft/min. The equivalent below the inversion is 800 to 1600 fpm.
As a rule of thumb in normal wave conditions with forecasted winds between 25 to 30 knots at 7000ft, a safe
height above the inversion is usually 8000 ft. If stronger winds then 9000 ft. to allow for crests of the wave
which can peak in some conditions on the west side of Lake Jindabyne in a westerly, or in the Skitube Bullocks
Flat area in a northwest wind. These peaks in the wave are well above the normal crest heights and carry strong
rotors beneath.
FLIGHTS ABOVE THE INVERSION
If you wish to fly from Cooma to Kosciusko in say a forecasted 270/30 knots at 7000ft, then expect standing
wave conditions, and be prepared to fly high above the inversion in the calm air. As one approaches the mountains
the air above the inversion will be rising and falling as one flies across the standing waves. Our aircraft will climb
in the rising air, and we are taught to maintain altitude by reducing power or speeding up. When the rising air
dies away and becomes sinking air, we increase power and go to a climbing mode again to maintain altitude,
and so on. As long as we can maintain altitude this is OK, but the amplitude of the wave increases as we approach
the mountains, giving ever increasing lift and sink. If it appears that we may not be able to maintain altitude,
we must either change to another technique, abort the flight or be carried below the inversion into the rotors,
which has led to fatalities in the past.
Let’s be a little technical. The wind speed will be 45 kts. Let us say that you cruise speed is 120 knots and your
best rate of climb is 70 kts. If you are being carried down while in the climb mode trying to maintain height,
your ground speed will be 25 kts. If you flew across at cruise speed and ignored the descent your ground speed
would be 75 knots. Your dwell time in the descending part of the wave is 3x longer if you fly in the climb mode,
your height loss will be greater, and you may be carried into the rotors close to the high terrain. Gliders using
no engine power don’t have this problem, so why not use their technique! Glider pilots slow down in rising air
and gain height, then fly fast across the sink, and so on, and we have to do the same if a flight level cannot be
maintained.
The secret is to gain a height cushion in rising air, and minimise the time spent in the sinking
air by flying at fast cruise across it.
Once upwind of the west face of the range you will be free of the wave systems, and can fly normally.
Should you be carried under the inversion, the sink that you are encountering will be greatly increased, and will
be combined with severe turbulence. Do NOT attempt to continue into the wind, but turn 180° downwind,
where your ground speed will be trebled to get you back into the rising air. Ideally you could head back into
wind in the rising air to get you back up again, but the sensible course is to abort the trip and return back to
Cooma, above the inversion in calm air. When flying downwind through a wave system the speed of transiting
the areas of lift and sink greatly reduces their effects, and no special precautions are needed provided adequate
separation is kept from the inversion.
Rule. If you are being carried downs in strong sink, the only safe path is downwind to the rising
air. What goes down in wave conditions, must go up again, so turning downwind minimises
height loss, and gets you back into lift.
(SEE P6)
FLIGHTS UNDER THE INVERSION
Here we will need to refer to the diagram. Study this now for an understanding for approaches or departures
from the Jindabyne airstrip.
When strong winds are forecasted for the area, they start higher up and gradually descend until they are influenced
by the higher peaks on the range. At this time it is calm lower down. Over a few hours the strong winds descend
to lower levels, with associated rotors and turbulence. Rotors are spinning cylinders of turbulent air, but they
contain spinning cells revolving around inside the cylinder, and these in turn contain their own vortices, with
whirlwinds between. The cells are shown on the diagram, but note that they spin out below the rotors towards
lower levels.
When the strong upper winds start around the higher levels of the mountain, these random cells being spat out
from the rotors start to be flung down towards circuit heights at Jindabyne, and flying into one from calm air
is an experience not to be forgotten. They are a warning that conditions will soon get exciting as the high winds
get down to lake level.When this happens, local knowledge is the key to safe operations.
If the 7000ft wind forecast exceeds 20 knots then the following information will help. See the Google Earth
diagram for standing wave positions in west to northwest conditions. The lift and sink is shown as green for
rising air and red where it is descending. Note that the upwind end of the airstrip is placed in rising air and the
landing approaches are in descending air.
Climbing in Wave Conditions, and Through the Rotors Beneath. The lift and sink usually remains
fixed to the ground below. To climb, you cannot circle as you will be carried backwards into the descending
part of the wave. You won’t be able to fly continuously into wind as you will slowly fly forwards out of the lift.
You must crab along the wave adjusting the crab angle, using the terrain, or rotor cloud if present, as your
reference. Your rate of climb indicator tells you whether you are ahead of, or behind the zone of best lift. If you
make any correction, always turn more into wind first. It is easy to drop back from a forward position, but can
be impossible to fly forward if you drift too far back. For the same reason, if you need to reverse course along
a wave, then turn into wind then drift back afterwards. If you are travelling downwind and wish to climb in the
next band of lift, you must watch the VSI. As soon as it starts to rise, immediately start a fairly steep turn to
reverse direction, otherwise your turn will finish beyond the lift.
The Jindabyne strip is situated across the end of a ridge, with the terrain dropping away at all ends.
Expect sink on all approaches in windy conditions, particularly at low level.
INBOUND AND OUTBOUND OPERATIONS
For those wishing to depart Jindabyne for Canberra, Sydney or to the east during west to
northwest winds.
When crossing the end of 30, turn right at the end of the strip to gain hill lift, then proceed to the eastern side
of the lake. Because of sinking air across the lake you may not climb well until you approach the eastern side
of the lake. Here you will climb rapidly, and you should follow along the top of the escarpment northwards,
using the boost in the climb until you have sufficient height to proceed.
For those inbound from any direction in wave conditions the best method is to keep above the inversion in
calm air until overhead, and then to descend downwind of the strip in the sinking air to get down quickly.
When a low enough height is obtained go to the upwind side and conduct a circuit. In these conditions it is
advisable to make the final approach steeper and closer in than usual to allow for the sink present here.
Departing for the west or to the skifields, you must be able to get well above the inversion before
heading off, as the rotors along the Thredbo valley can be severe. Look at the image and the green areas. If
you take off on 30, proceed ahead slightly to the adjoining ridge, and turn left along it in the rising air. If you
muck it up by going too far to the east, the escape path is across the lake to the eastern side to climb well
above the inversion there.
For this ridge, go back and forth along it, always turning into wind, and climbing all the time until
you are a 1000ft or so above the inversion in calm air. You will see from the diagram that from here, (or from
the eastern side of the lake), you have to cross the down part of the wave to get to the Thredbo Valley. Fly
forwards at cruise speed to the next band of lift, then regain your lost height.
OPTION 1. If you want to go direct to Thredbo, you can crab along the valley to get there, but be
prepared to encounter turbulence and or sink over the Skitube terminal at Bullocks Flat. Beyond this the
wave re-establishes itself. Never let yourself be drifted downwind over the wave crest, especially in this area.
To penetrate forwards to Kosciusko from the Thredbo Valley, the air descending on the Kosciusko
side of the escarpment is a virtual waterfall, so gain lots of buffer height and go flatout across disregarding the
sink on the way. The vertical movement of the air over the Kosciusko part of the main range is much more
benign.
OPTION 2. Heading in the Albury - Melbourne direction. From the Thredbo Valley jump at right
angles across to the high ground near Sponars, regain height as needed, then proceed via Perisher and
Charlottes Pass on a track to Khancoban - Corryong airstrips. This is the shortest way to open country, and
beyond the west face of the Snowies you can reduce height if you so desire.
If you are coming from the west and overflying Perisher to Cooma, then one needs to be careful of
the possible jump up in the wave at the drop off to the Jindabyne Lake. I know of two aircraft inverted at this
point, as the normally invisible shape of the wave crest is more like a breaking wave facing into the wind.
Imagine a rapid in a river which is breaking upstream, a couple of thousand feet high. The trick is to keep
high and don’t skim the terrain no matter how smooth it may appear.
Southwest Winds. Wind directions from the southwest give similar effects, but the same techniques
apply. It is not unusual in winter for the airflow below the mountain plateau to be redirected to the
northwest, at right angles to the SW wind above. In this case approaches to the strip are the same as
for NW. Aircraft flying down the valleys may find the lift to be on the opposite side to that expected.
In any wind direction, if when flying along a valley and sink is found, then swap sides of the
valley.
SPECIAL TECHNIQUES FOR OPERATING FROM JINDABYNE AIRSTRIP
* Wind reversal on 30. Note from the Google image that the strip has rising air at the northern end and sinking
air at the other. This is because a revolving rotor sits above it. At ground level the reverse flow from the rotor
can damp down a 45 knot wind to 15 knots, and make landings relatively easy. However movements in the
wavelength can cause instantaneous reversal, with strong gusts coming from the opposite direction. Sometimes
the hill windsock can be pointing 180° to the airfield windsock. In either of these conditions it is best that visitors
don’t use the strip at all.
* Circuit directions are best conducted on the eastern side of the strip. In NW winds the 800ft Round Hill on
the western side of the strip has some nasty turbulence behind it.
* Always be aware of the density altitude and its effect on takeoff distances here. Heavily loaded aircraft have
problems here, both for takeoff, and outclimbing the areas of sink common in windy conditions.
* Taking off from 30, the wind is forced up by the hillside at the northen end to give lift there. However it often
descends again in the middle of the strip causing an airborne aircraft to descend. Continue takeoff as this height
will be regained at the end of the strip.
* Approaches to all runways will have marked sink during the flare at the threshold.
* Approaches to all runways should be steep and close in to allow for wind shear. Long flat approaches to 27
or 30 are dangerous as you may be carried below the strip altitude.
* For 27 and 30, approaches will be turbulent, but smooth out on the flare for the landing.
* A southwest wind comes down the hill slope on the right, making it very turbulent at ground level for the first
half of the runway when you are flaring and trying to touch down. The turbulence disappears in the second half
of the runway.
* Takeoff in these SW conditions is not a major problem.
Flying in Summer in Strong Winds.
I mention this because the hazards over the Snowies during the hot northwest winds are compounded by vicious
thermals intermixing with the rotors. Also that the rotor/thermal combination can extend to well above 10,000
ft, with the inversion being out of reach. The areas of lift and sink are similar to the winter patterns, but flights
need to be high and will not be comfortable.
Summer can also bring colder strong west or southwest winds, giving similar results to the winter pattern, but
with the inversion and calm air above being a bit higher.
Barry Wrenford