Sky surfer - how to fly the wave!
by Rick Agnew
If you are a regular reader of Australian Gliding, you will probably have noticed my articles on or about wave flying over the years. Yes, it is probably an obsession, flying the wave. However, this article has been requested by popular demand, and stems partly from the realisation by many that I am not alone in my obsession with sky surfing. Wave flying is challenging! It may at times (like many aspects of our sport) require advanced flying and decision-making skills and techniques as well as displays of raw courage.
In previous Wavelength articles, reprinted in Australian Gliding, I have described the preparation of both pilot and glider prior to a typical wave flight, oxygen equipment and usage, flight planning as well as some physiological effects of high altitude flying. These can be found in:
'Bunyan Lessons', Australian Gliding, Vol. 41, No. 8, August 1992, pp. 12-15
'Not so lonely heights... a hyperbaric chamber 'run', Australian Gliding, Vol. 44, No. 8, August 1995, pp. 16, 18-19
'An uplifting experience - when a plan comes together', Australian Gliding, Vol. 44, No. 11, November 1995, pp. 14-18
In this article I will attempt to describe how I fly wave. Why I say attempt, is because like most flying, after some time, some techniques become second nature, and nearly (dare I say) automatic, occurring without conscious effort. My experiences of late, in instructing ab initio students as well as some more advanced pilots, have demonstrated that this is not an across-the-board truism, but has allowed me to reconsider some of the basic techniques and practise flying by remote control. Getting someone to fly, or demonstrate, a manoeuvre can at times be challenging to say the least! After setting the 'student' into an area of lift, for example, then watching that person quickly lose it after your best efforts, can try the most placid. The desire at times to 'take-over' must be checked, and the old adage 'there goes me but by the grace of God' etc applied. Every flight should be a lesson for all concerned.
The indices of a wave system
A little theory before the practical...
As everyone will tell you, for wave flying you will need amongst other things a wind greater or equal to twenty knots or so, and blowing almost perpendicularly to a ridge (+/- 30 degrees), the edge of a plateau, or a deep valley, which may trigger a wave system. Like thermals, lift generated by a wave system is invisible unless it is marked by cloud. Characteristic clouds appear whenever the temperature and moisture content of the air are sufficient to cause cloud formation. Further, a moving air mass takes the path of least resistance, and when faced with a ridge, for example, it will move up the ridge's up-wind side and may become useable lift to the knowledgeable glider pilot.
As the air mass is forced upwards it cools (before condensation occurs, at about 3 degrees C/1,000ft, and when wet, at about 1.8 degrees C/1,000ft [the wet adiabatic lapse rate]) due to adiabatic expansion, until water vapour concentration reaches saturation. At this point the air mass begins to condense, forming cloud. Typically on the up-wind side of a ridge or mountain, a cloud wall forms with its base at the condensation level.
Depending on the moisture content of the air mass, the up-wind cloud reaches one to two times the height of the initial obstacle. The geographic relief triggering the wave system is often covered with this so-called cap cloud (stratocumulus or altocumulus) which appears to be immobile or stationary. In reality, each droplet of the cloud moves with the wind and evaporates after it crosses the ridge. Indeed, the air compresses and thus warms up when it goes down along the slope or lee of the feature after crossing the ridge. The droplets evaporate and the sky becomes clear. Brilliant! This phenomenon causes what is known as the Foehn effect. It is also responsible for the low rainfall in the Cooma region, in some parts of Switzerland, northern Italy, east of the American Rocky Mountains and southeastern France (all good wave flying sites).
Organisation of a wave system
Orographic wave or mountain lee wave will form to a greater or lesser extent if the following criteria are fulfilled:
if there is an obstruction to the prevailing wind (preferably a significant mountain or mountain range), wind blowing perpendicular or nearly so (+/- 30 degrees) to a mountain or significant ridge/range, a stable air flow, the air flow should have considerable horizontal velocity, and should remain so at least twice the height of the obstacle, increasing wind strength with height, and the wind direction remains or nearly remains constant with height.
Other factors that may influence mountain wave include air temperature as a function of height, and related to velocity, the height of the wind's obstruction, and local turbulence.
Amplitude and wavelength of mountain wave depends on the above criteria, however, vertical development in a wave system is often limited by the occurrence of another inversion level, which may affect wind velocity and or direction, thereby destroying the laminar flow of the air mass. Wind below such an inversion level characteristically forms turbulence known as rotor.
In a simplistic model of a wave system, the air mass can be split into two very different layers. In the lower layer, whose thickness can vary between a few hundredft to several kilometres, the air flow is turbulent: it is the turbulent under-laminar layer. In the higher layer, the air flow is laminar, ie. the air particles have regular and parallel trajectories. It is the laminar layer.
In the turbulent under-laminar air layer, more or less visible roll shaped clouds (cumulus or stratocumulus) mark the different waves. When the air moisture content is high, bands of clouds which vary in thickness and length form parallel to the relief. When the air moisture content is low, they disappear completely or are limited to very mobile and short-living light clouds. These clouds are known as rotor clouds or roll clouds. Below the roll clouds, pilots will more than likely experience strong turbulence and winds opposite to the wind gradient, which can make flying generally much more interesting and perhaps hazardous.
In the air mass exhibiting laminar flow, each wave is often marked by several layers of clouds vertically. These clouds (altocumulus and cirrus) are called lenticulars because of their 'lens' shape.
Roll clouds and lenticulars seem to be immobile above the ground. The leading edge of the clouds coincides with the lifting zone where the droplets continuously condense. The trailing edge coincides with the sinking zone where they in turn evaporate. The combination of these two effects gives the impression of immobility. This paragraph tells 'it all' to the budding wave pilot, ie where the lift should be and where the sink should be!
The vertical extension of rotor gives an idea of the strength of the wave system. If the lenticulars are less significant, then this may suggest that either the moisture content at that height is not sufficient to form such clouds or that, unfortunately for the wave flying glider pilot, the wind speeds at that height are not sufficient to generate wave, thereby not generating effective lift.
Lenticular clouds can appear in isolation or simultaneously. Again, their absence is not an indication or evidence that there is no wave, as they depend only on the temperature and water moisture of the moving air mass.
The wave system does not move by reference to the ground but extends past the obstacle to a distance depending on the stability of the air mass. The primary wave is usually the strongest of the system, however, this may not always be true, depending upon atmospheric conditions including temperature, inversion levels, and other geographic features. A given wave system can at times reach the stratosphere in the absence of unfavourable factors such as a rapid change in wind speed or direction with altitude.
The distance between two wave systems (known as the wavelength) can vary between 3 and 5km, depending on the wind speed and the stability of the air mass. The first wave system downwind of the wave-generating obstacle is known as the 'primary', the second wave system, the 'secondary' etc.
The amplitude of the wave determines the lift strength. Some pilots have experienced rates of climb above 20 knots (heaven), however, the average lift strengths are unfortunately much less, in the order of values between 4 and 8 knots. The rate of climb decreases more or less with altitude and distance from the relief. The world record for altitude, set by an American pilot in the Rocky Mountains, is over 49,000 ft. In Australia, altitudes above 20,000ft are usual near the Snowy Mountains (including Mount Kosciusko), in New South Wales, the Grampians in Victoria, and the Stirling Ranges in Western Australia. There are many other wave sites within Australia; however, it could be argued that these three are the best known.
Experience gained over many years has shown in Australia, at least, that the most predictable wave flying 'season' occurs from late June through to about late August. Having said that, we at the Canberra Gliding Club, based at the Snowy Mountain foothills at Bunyan, experience wave throughout the year. The reason for this can be covered elsewhere. Wally Wallington's superb book Meteorology for Glider Pilots, reprinted in 1989 (3rd ed.) by John Murray Publishers Ltd is a must for glider pilots interested in weather, and explains the weather patterns responsible for producing the suitable winds and wind strengths needed to produce any wave system.
The best time for contacting the wave system is early in the morning, or conversely, late in the afternoon. The wind strengths can be such that a lower launch is possible for the budding wave pilot to contact the smoother laminar level of the wave system. During the warmer parts of the day, convection can alter or even destroy a wave system, thus making it harder to contact the laminar level at a low height. As stated, a progressive increase in wind speed with altitude without significant change in direction is what is most desirable. On the other hand, a decrease in wind speed or a change in direction diminishes or suppresses wave formation.
The basics of wave flying
There are basically two different flying techniques that can be utilised to exploit the lift generated in a wave system depending whether you are flying in: the under-laminar layer air mass, or within the laminar layer or air stream.
Lift exploitation in the under-laminar layer air mass
There are several forms of under-laminar layer lift, but they all stay almost immobile above the ground.
To utilise this type of lift, it is advisable to fly upwind of the lightest clouds, even if they are tiny and short-living. I call these the scud cloud - a small rotor or roll cloud. These sometimes nearly invisible small bits of 'fluff' delineate the leading edge of the laminar-flowing air mass. It is important to the wave pilot to recognise these scud cloud and react appropriately. The success of the wave flight may depend upon both quick recognition and the decision to move to that area of the air mass.
By maintaining your air speed so as not to be forced backwards (i.e. wind speed plus) you can remain above the same location with reference to the ground. Choosing some distinguishing landmarks on the ground will assist you in your attempts at maintaining your position in the air mass. Frequent checking of your location in reference to your chosen land marks will assist you maintaining any lift. By manoeuvring either forward, back or laterally you may encounter stronger lift. GPS may also make this process easier for those who have such gadgets. Your position by reference to the rotor is also dependent on the development of the cloud.
You must get quickly a mental picture of the lift structure, using the variometer (quantitative but delayed information) and your sensations (qualitative but immediate information).
Tight lift, often rough, used with steep turns (at times very steep thermalling technique), flattening your turns in the upwind direction in areas of lift and conversely quickly steepening the turn and increasing your speed in the downwind sinking areas. By this method (seemingly two steps up, one step down!) you will gain height.
Usually you will have to use several techniques within a short time span, as flying the rotor lift changes quickly with time. Using a glider in such conditions requires good handling as well as a short time-response to the indications given by both the instruments and your sensations.
Avoid arriving low in this kind of lift. Climbing is nearly always difficult, and there is sometimes an altitude below which it is impossible to climb altogether.
Lift exploitation in the laminar layer
The atmosphere becomes calm when you reach the laminar layer. The strategy is then to stay in the area where the rate of climb is maximum. As in the under-laminar layer, using landmarks to stay immobile above ground is essential.
If the wind speed is less than the speed of minimum sink of the glider, you will need to fly parallel to the obstacle triggering the wave, compensating for the cross wind. The best location is usually below the leading edge of the lenticulars, or slightly ahead of it. When the lift is localised, use wide 8-shaped turns, like in front of a slope, turning always upwind.
If the wind is equal to greater than the speed of minimum sink of your glider, you will have to stay in front of the wind, immobile above the ground. To adjust your speed to wind speed, choose two landmarks as close as possible to the glider, and check your position often.
Do not forget that wind speed usually increases with altitude. You may have to increase your speed to stay immobile above ground. Only frequent checks of your position will prevent you from being pushed backwards into sink.
As the lift is not the same everywhere in the rising zone, you may have to move sightly upwind, downwind and/or laterally until you find the best rate of climb.
If you lose the lift, you are either downwind or upwind of the zone of best lift. For safety reasons, you will need to search for the lift upwind. Indeed, if you search downwind and find nothing but sink, you may lose too much height when you cross the zone of sink again, with the penalty of a strong head-wind.
An Example...
Let us consider that you fly at 40 knots and your variometer reading is 0 knots. You increase your speed to 60 knots, your variometer will go up slowly to reach a maximum value and will then go down if you fly on. You have just crossed the zone of best lift. Ah ha! So you must go back to 40 knots as soon as your variometer reading begins to drop. Do not get impatient, as flying into a head-wind, your speed, with respect to the wave system, is low. You may have to fly for several minutes in this manner. This example is typical whenever wind speed increases with height.
Let us now consider example 2, with the same speed of 40 knots and the same variometer reading of 0 knots. You increase your speed to 60 knots. The variometer shows the sink getting stronger and stronger: you therefore deduce that you are upwind of the rising area. So you turn back and look carefully at the variometer. Downwind, your speed, in respect with the wave system, is high and the changes in variometer readings are much faster than in the previous example. As soon as your readings become positive, turn 90°. The drift will take you to the zone of best lift. When the variometer tends to drop, turn up-wind, set a 40 knots speed and choose new landmarks for position references.
When you have reached a comfortable height, move a little laterally to find the best climb area.
As a general rule, the best rates of climb are upwind of clouds with high vertical extension and downwind of the highest obstacles.
Flying cross country in wave
For an experienced wave pilot, cross country flights of more than 500 km are usual in Europe, America and New Zealand. These flights normally require the use of several wave systems, triggered by different obstacles.
I will try and explain how to fly in a single system, but also jumping from one system to another.
Movement perpendicular to the wind direction
Once you are above your safety height (ie you are confident that you won't drop out of the bottom of the laminar air flow), you can move laterally along the wave, crabbing to compensate for the cross wind. This is usually rather easy when you have cloud to mark the wave. Surfing the cloud! In blue sky you must imagine on the ground an alignment parallel to the obstacle triggering the wave. If you lose the lift, you must search upwind.
I have flown many 300 km and over 600 km in the wave starting from Canberra Gliding Club's Bunyan site near Cooma, NSW.
Movements upwind and downwind
These movements imply a change of wave. Between the zones of lift, you will cross strong sink. Set your MacCready ring to the wind equivalent and follow its indications. You will have to fly pretty fast in sink. Be careful not to fly above the glider's VNE, remembering that VNE indicated alters as you gain altitude!
The height loss between waves varies, depending on the wave length, the rate of sink, and your trajectory. As an example, with a 40 knot wind and a wave length of 8 km, a change of wave upwind can cost up to 7,000ft for a 25:1 glider and 3,000 to 4,000ft for a 35:1 glider.
Flying downwind, the loss of height is lower, as the areas of sink can be crossed very quickly. With the previous values, the losses are 1,700 and 1,000ft, respectively.
To keep the height loss to a minimum, you can sometimes change wave at one edge of the system you are aiming to move to. When flying from one wave system to the other, always stay parallel to the wind direction, in order to follow the shortest trajectory in sink. You can jump to another wave system following a trajectory where the clouds are thinner or less organised. These clouds are indices that the wave system is weaker: the sink should be reduced, and therefore you should lose less height.
When flying upwind to another wave system, allow for a large safety height above the clouds. The glide ratio in sink can be deceptive and you will have the feeling of going down without going ahead. If you realise that you will arrive below the cloud summit of your target wave system, turn back to the previous wave. Top up your height before trying again. Avoid reaching the next wave system below the roll cloud, as you will probably find heavy sink and turbulence. Then you will have to attempt to 'thermal' the rotor to re-contact the laminar air flow again. Reaching the smooth lift may be difficult, if at all!
In wave outlandings are easily possible. We at Bunyan treat every wave flight as a fully fledged x-country flight. Your safety heights (altitude below which you will try not to go) will be chosen depending on the landscape and the make up of the wave system. Such a safety margin should be higher than in normal thermal conditions. Selection of an outlanding paddock downwind should only be used as a last resort.
Safety in wave
The ecstatic, and even hypnotic feeling associated with wave flying should not make you forget the safety rules associated with this type of flying.
On tow
Take off and towing most often occur in the under-laminar turbulent air mass. As such, they can be quite rough. Therefore before take off, check your harness and secure any loose objects in the cockpit (including that camera to capture the epic flight!). An experienced tug pilot can sometimes soften some of the roughness by avoiding the worst rotor. Be familiar with emergencies and potential landing areas in case either you have to release from the tug, or if the rope breaks - it may happen.
In flight
Throughout your flight you should have a contingency plan, ie where to land if you lose the wave, the cloud (Foehn gap) starts to close in etc. Maintaining a safe height, and being mindful of outlanding strips is probably the best insurance.
A headwind of 30 to 40 knots affects the average glider's glide ratio quite markedly. The glide ratio can be as low as 10:1 and quite often more like 5:1. Flying low downwind to a landing zone is dangerous. If you have to fly back to a landing area after missing the lift, do not fly across the wave system. Obviously, spending more time in sink reduces your chances of making it back. It is better to try and fly upwind of any roll clouds as long as possible.
Outlanding
Outlandings in normal conditions should present no problems. However, to the uninitiated, outlandings in wave conditions may include strong turbulence near the ground, wind shear giving you a tail-wind on final when you expected a strong head-wind, as well as poor visibility. Beware of these possibilities, and fly accordingly.
Last light
You may still be basking in the sun at some incredible height, but remember that it may take some time to descend - even with full dive brakes and in areas of strong sink. The light on the ground fades especially fast in winter time! Respect last light by making provision for the return flight.
Clouds
You are often near the clouds when you fly wave. If you are next to the leading edge, be careful not to be pushed downwind, as you may disappear into the cloud. If you see clouds forming in front of you this is where the lift is! If this happens, try to fly quickly upwind of these clouds to re-contact the area of lift.
Always watch out for changes in air mass humidity. With some winds, the humidity of the air can increase within minutes. The Foehn gap can shrink and even disappear altogether at an alarming rate. If this starts to occur make sure you have an access area clear enough to safely descend in clear air.
If you are stuck above the clouds after the Foehn gap has closed, radio your situation to your base, notifying your intentions. Normally there will still be some gaps downwind that will allow you a safe descent. However, this too might not be an option. Quick reaction and recalling where the gap was may get you down in the thinnest area of cloud.
Alternatively, if the situation gets worse, go down straight away, crossing the cloud where the Foehn gap was. Before being engulfed by the clouds, set the trim to 60 knots, deploy full airbrakes, and let the controls go loose. The glider will stabilise itself in its decent. As you are a trained pilot, well versed in unusual attitudes, you may need that skill to recover once below cloud base. Obviously this emergency procedure can only be attempted if you are certain that the clouds do not touch the ground. Otherwise I would probably recommend your last resort is to use your parachute. Really this scenario should never be encountered, and therefore, this is the reason why you must be very careful when flying above the clouds.
Radio
A (working) radio increases safety substantially. Tell other pilots your altitude, your position and your intentions. Giving a position at altitude to your ground crew assists in letting others know where you are, the best areas of lift, and allows them to monitor your progress vis a vis your levels of situational awareness.
This last point I can not stress too highly. High altitude flying is advanced flying and not totally without potential risks.
These risks can be minimised and controlled leading to some of the most memorable flights you will experience.
Know your (and the glider's) limitations and act appropriately!
-- Rick Agnew
