Spring Flying & Classic Days
Spring Flying | Classic Days
Spring Flying (Club Coach Notes, Feb 2001)
I have to admit to being pretty pleased with myself about my last rantings about Winter Weather. In the flying days we’ve had since then, we experienced every one of the features described - there were high wind gradients, turbulent shear layers, variable valley winds, horizontal rotor, rapidly developing orographic clouds. I don’t feel so pleased about having witnessed pilots being blown over the back, suffering large asymetric deflations close to the ground, and flying down in cloud using compass navigation !! Fortunately, there were no injuries and (I hope) a few lessons learned.
I’m working on the principle that if I rant on long enough someone will get some benefit out of it - so here we go again.
You’ll read the warnings every year in Skywings and elsewhere - The dangers of Pilots emerging from a long winter, desperate to fly, and launching into the sometimes radically turbulent Spring conditions. The warnings usually end with "Be Careful ! However, every year we read about, or hear about, or organise the helicopter for someone adding to the statistics.
 I often wonder exactly how does someone go about "being careful" on one of these days ? Lets look at a typical accident and work backwards whilst considering how we could have prevented it. We’ll leave out the messy bit with pilot lying in a crumpled heap, and the bit before with the glider flailing about uselessly as the pilot smacks into the deck with a combination of vertical and horizontal velocity - (although if he hadn‘t invested in some effective back protection and practiced his PLF he would have felt even worse !) More often that not, these accidents begin to happen close to the ground so the reserve parachute is useless. However, having the glider control skill, experience and calm nerve to keep the glider (or some of it !) flying and pointing away from the hill during a major collapse is usually enough to convert our accident into only a trouser problem. (If you haven’t watched Jocky’s SIV video recently, I suggest you give it yet another viewing)
The pilot is likely to have got into the major collapse in one of 3 ways:
- Failing to damp a surge whilst exiting a punchy thermal
- Flying too slowly into a thermal and stalling or spinning
- Not responding quickly enough to a bullet of strong lift on one side and allowing one side to collapse
To avoid getting into one of these situations, Active Flying is required. I wrote about that in detail a few months ago but as I said then you don’t learn it from a book - it needs practice because its all about feel, timing and anticipation - and they are not characteristics at which pilots excel if they are scared stiff!
The method of avoiding getting into that situation is where the judgement bit comes in. Firstly, if you have selected a glider which is truly matched to your experience, (rather than one you are happy with as long as its smooth ) you are likely to enjoy the whole experience much more. Secondly, taking a bit longer than usual over your sandwiches prior to launching, can be an excellent idea. Ask yourself a few questions before you go - If there are gliders flying, what is keeping them up ? (ie How much is ridge lift and how much is thermic ? - You’ll get a good idea within a few minutes - Are they all at the same height, or are they varying quite quickly ?) What does the sky look like ? How quickly are the clouds developing and dissipating ? What is the met wind speed ? If you don’t have a definite answer to these, ask yourself one more question - Do I really intend to launch into totally unknown conditions ???? (This might be good time to have a chat with one of the Club Coaches.)
So there you have it - 3 completely independent means of preventing our accident.
- Good judgement in not taking off into conditions unsuitable for you
- Adequate Active Flying skills to prevent the collapse
- Adequate Glider Control skills to recover from the collapse
All 3 of these need to fail in order to reach the end result of calling out the helicopter, and the hassle and grief that comes after that.
If you judge that you haven’t got the experience and skills to deal with the conditions when you arrive, or that you’ve flown so little over the winter that your skills are rusty., then its a much better idea to do some glider control practice (ground handling) at the bottom. Typical Spring days tend to mellow out later in the day, so its better to risk missing the strongest part of the day to avoid the risk of missing the whole season.
David Thomson
Classic Days (by Brendan Reid)
Spring 2006 has been one of the best ever springs for xc flying in scotland. At time of writing (mid may) scottish pilots are leading the uk xcleague with totals close to 2005 scottish xc league totals only 2 months into the season. Having enjoyed a number of classy days myself i was inspired to share a few thoughts on the ingredients that go to make up a classic Scottish spring day which offers simply some of the best flying anywhere.
Airmass
It needs to be an arctic airmass ie cold air coming down from the N. This often happens around the edge of a blocking high as below or on a smaller ridge of HP. You can see synoptic charts side by side with all the good distance flights by going to DAILY FLIGHTS on the XC league (http://www.pgcomps.org.uk/xcleague/xc/view.php)
Apr 4th 2006: 88km Open Distance to Dumfries
An arctic airmass brings with it good potential for xc flights for a number of reasons:
- Temperature
One straight forward aspect is that arctic air is well chilled. This means it will be much lower temperature than the land it is moving over. This is the definition of an unstable airmass so even a slightly warm patch on the ground should break away and form a nice thermal. In general, there should be lots of thermals and the convection layer (height of the thermals) should be deep. Typical temperatures the night before the flying day will be 0 to 5 deg C.
May 10th 2006: 57km Out and Return - Tarmachan to Ben Dorain
- Moisture Content
Another very important factor about air from an arctic airmass is that is very dry air. This is linked to the temperature in that cold air can hold much less water vapour gas than warm air. So in spite of the long track over the North Sea the air arrives over land with a low dewpoint and relative humidities of typically 40-50% (compare to warm south westerlies where the air arrives literally dripping wet).
The dryness of the air is very important for a number of reasons:
Firstly this lack of water vapour in the air means that the air is crystal clear – you can see for miles – and this is hugely important because it means that a much bigger proportion of the sun's energy gets through to the ground where it can start heating a nice thermal for you. Bring your camera because the views will be spectacular!
The second reason having a dry airmass is important (when it's unstable) is that with strong convection and an unstable airmass then if the air was moist it would quickly overdevelop and rain. With an arctic airmass, however, we get lots of puffy cumulus showing us the thermals early in the day. There may some significant shadowing of the ground as the afternoon goes on – the challenge is to stay in the air by using the clouds if you are high or sunny patches on nice looking hills if you are low.
The third reason having a dry airmass is important is the impact on cloud base. The dry air of a thermal leaving the ground will have to be chilled for a long time as it rises until it reaches the (dew) point at which it has too much moisture to hold as a gas (water vapour) and water condenses into a cloud. So cloud bases will be high, typically 5000+ ft by midday rising to 7000ft+ by later afternoon.
- Pressure
Pressure too high and it'll be blue, too low and it'll overdevelop.
- Airmass Stability with Height
The curse of good days in gliding sports in high pressure is spreadout. Imagine good, continuous convection rising and hitting a stable layer just ABOVE cloudbase. The cloud cannot grow vertically and grows horizontally instead shutting down the sun to the ground. This is sometimes resolved by the spreadout cycling or by cloudbase enough to limit the spreadout. On other days it's game over.
Two Thermals for the Price of One
The other thing that northerly winds and an arctic airmass bring is the potential for lots of thermals from both sides of the hill. This is something I feel i am still learning about and there's a lot to it! However with a N wind, sun is pointing directly at S side of hill so lee thermal potential is greatest.
Until recently you could only learn this slowly via trial and error but for those geeky enough to examine the track logs via SeeYou/Compegps/GoogleEarth there's a lot of info about what the experienced pilots do/have done via the xcleague.
Development of Leeside Thermals
Classic Leeside Thermal
This a complicated subject to deal. In simple terms S side of the hill will be sheltered and in full sun. Further more the cool air that is forced up by the hill and has no interest in rising in a thermal wants to dive back down again on the lee side into this warm pocket.
The combination of the two helps to force the warm air up and out into a lee thermal.
I don't recommend diddling about on the lee side below hill height unless you really know what you are doing or winds are very light (rotor potential).
Crossing sunny S facing bowls at good height definitely works on these special days. If you are in this position (well away from the hill) you are in good position to feel the thermal influence on your wing – so listen hard to your wing and if its yawing or drifting in a particular direction go with it.
Classic Leeside Thermal from Sunny Bowl on SW Side of Lowther hills (NE wind)
The weak thermal combines with the leeside thermal from the warm bowl (on S side of flat topped hill - too flat to show well in SeeYou) to generate lift off
Weak thermal leads you to strong one
One of the best ways of finding a strong lee thermal is to connect with something which leads you to it...a weak thermal drifting you gently over a sheltered sunny bowl is a really good bet. Thermals “like” to join together in lifty areas.
Tracklog of Weak Drifting Towards Stronger Thermal
Low Saves
Never give up on days like these – there may be a thermal that is very weak low down that leads you to a screamer!
Apr 10th 2006: New Scottish Record - 126km Open Distance to East Coast
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