Briefing: Wave

Introduction

Aim: You will have a basic understanding of what wave is, and how to exploit it safely.

What do we know?
  • Has anyone here flown in wave?
  • How was it different?
  • Where did you find it?
  • How did you ‘keep it safe’?
What is Wave?

Appearance

  • Wave bars looks like the ripples on the surface of a shallow river.
  • Typically arranged in lines across the wind.
  • Upside-down saucers, often stretched.
  • Can be stacked.
Video

Brief Video illustrating wave formation.

Note:

  • The wave bar is stationary
  • The air is clearly moving through it
  • The bar is downwind of the ridge that triggered it
  • The bar is offset from the wind by 30 degrees or so, aligned with the ridge. Wind is blowing from bottom left to top right of the image.
  • Thanks to Chris Gill for the video.
Depth

  • Wave can set up in layers of air thousands, or tens of thousands, feet deep.
  • The rising and sinking air extends vertically for the full height of the air layer they occupy.
  • These layers can start close to ground level.
  • Their effects (Rotor) can be felt down to ground level.
Stationary

  • The wave patterns tend not to move much.
  • The air moves through the wave bar, sometimes forming cloud in the rising air, and dissipating in the sinking air, to form a cap along their length: lenticular clouds.
Triggers

  • Ground features: Ridges, Mountains.
  • Thermals protruding into the stable air layer above.

  • The descending air bounces to become the Primary Wave Bar.
  • Depending on conditions, more wave bars set up parallel to and downwind of the primary.

Associated with:

  • Wind speed increases with height
  • Constant wind direction with height
  • Lower layer of unstable air (to initiate the bounce), a stable layer above (to propagate the bounce), and an unstable layer above that (to allow the bounce).
  • Wind speed 15+ Kts over ridges
  • Parallel isobars over long distances (e.g. a High sitting over Germany sets up westerly wave in the UK).
Features
  • Wave is smooth.
  • Rotor is not.
  • Climb rates: Zero to 10+ knots.
  • High wind speeds
  • Sink and cloud downwind
How do we use Wave?

  • Lift: height gains of tens of thousands of feet.
  • Cross-country: fly along the wave bars for tens, or hundreds of Km.
  • Or simply to sustain our flight locally.
Accessing Wave

  • Aerotow to the wave bar.
  • Thermals can be wave enhanced:
    • climb to cloud base, beneath the wave bar,
    • push forward into wind, to penetrate into the wave.
    • This may involve climbing in rotor.
  • Ridge
    • climb on the ridge
    • push into wind in areas of strongest lift.
    • This may involve climbing in rotor.
    • Turn across the wind in the strongest lift.
Optimising the Climb

Vertical Movement, Up and Down

  • The strongest lift is upwind of the wave cap.
  • There is no vertical movement at the wave cap, nor in the trough.
  • The air is descending downwind of the wave cap.

Climbing

  • Find and stay in the strongest lift:
    • Upwind of wave bars
    • In the whispies, forming just ahead of the bar
    • GPS snail trail – set it to show climb rate or Netto
  • Better lift may be found in the next wave bar upwind. Transition into wind in the areas of least sink (at the ends of the bar).
Safe Flying

Gotcha’s

  • Turbulence, from ground level into the Rotor
    • Beware Max Rough Air Speed
  • Strong Sink
    • Escaping it may necessitate flying downwind, which will be rapid
  • High Wind Speeds
    • Can carry the glider downwind (into sink, airspace, or away from a safe landing area)
    • Crabbing may be extreme – progress along the bar may be very slow
    • Ground speed into wind will be very slow, and / or consume a lot of height
  • Low Temperatures
    • Comfort, concentration
    • Airframe cooling, near clouds – icing and gel coat damage
  • Oxygen Depletion
  • True Airspeed (TAS)
    • is Higher than Indicated Air Speed (IAS). Read the Flight Manual
  • Stall Speed increases
    • it may remain at or around the indicated value… TAS is rising
  • Vne occurs at lower IAS values
    • and may become the stall speed
  • Cloud Cover
    • Wave bars expand and contract, forming cloud sheets, above, below and around you.

These wave bars are merging in the distance:

Lookout

  • Track and Heading will diverge greatly
  • FLARM’s 12 o’clock is based on Track, not ahead
  • Gliders will tend to fly in the same space relative to a wave bar, increasing the risk of collision.
Situational Awareness

  • Disorientation: above cloud, difficulty in picking out detailed ground features
  • Distances travelled do not match expectations

Recap

  • Where and when does wave form?
  • How can you access it?
  • How is Indicated Air Speed affected by height?
  • When should you use Oxygen?
  • How is Lookout affected?
TEM
  • Collision: Lookout, FLARM
  • Loss of Situational Awareness: GPS helps