Engineered Systems Ltd

Programme and Project Management by Ed Johnston

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The Need for Speed

How do you fly over 140kph day after day? Just find strong, consistent thermals under a readable sky then go like hell in a decent glider. Where do you get that? Uvalde Texas!

Texas Terrain

The airfield is about 1000ft above sea level, the ground level steadily reducing towards the Gulf coast. Towards the west the ground gets rockier and dryer while towards the South and especially East it gets quite damp and green.

The task area highlighting the major areas of interest and differing conditions

From Uvalde to the East and South there is quite a bit of agriculture and ranching with plenty of fields while further South East there are more oil fields in scrub country with fewer landing opportunities. Towards the Mexican boarder agriculture declines and with it the number of fields, though across the whole area there are plenty of airfields and strips.

Just north of the airfield the ground rises through a series of valleys into the ‘high country’, a rocky, rolling limestone plateau at 2000-3000ft above sea level. You can land on some roads if you have a small glider, or find an airfield, but there is nothing in between to land on safely.

Best and Worst Areas

One other lesson we learned the hard way was East of Uvalde, South of San Antonio was usually poor. During practice week the only time we went there it was really good, but we twice got caught out either taking or committing to longer AAT legs in this area.

On the other hand, the run up the Mexican border was usually very good. Unfortunately this was limited by the airspace enforced only during the week, reducing the task area West of Uvalde.

The High Country was generally very good; higher bases and stronger climbs. However if there were less cloud, generally that meant fewer thermals. Also the sun over wind late in the day kicked off thermals very effectively over the valleys between the plane and high country.

In any local situation, following the sun was always a good plan. Thermals were driven by ground heating and any cloud shadow had a swift effect, reducing climb rates.

Uvalde Airmass

Uvalde sits on the intersection of three major air masses; the relatively dryer continental air from the North, desert air from over the Mexican mountains and moist, warm, stable air from the Gulf in the South East.

Of these the Gulf air is dominant, providing regular supplies of cooler moist air from the cost about 300 miles away. The next day the sun gets to work on this intrinsically stable air and brews it up into strong thermals marked with plenty of cloud.

The classical pattern is for moist, warm air from the Gluf to come through the region over night. In the morning you get a layer of stratus at about 700ft which the sun quickly gets to work on. By mid day, this 80% humidity has been reduced to 40% or so, temperatures rising to the break through triggers of 36-38C and you get nicely arranged cumulus at about 5-6000ft. As the day progresses and temperatures rise, thermals strengthen to 5-7kt and cloud bases to 6-8000, occasionally up to 9000. Often conditions are further enhanced by the prevailing 12-15kt South East wind at (about every runway is built on 150), arranging these clouds into neatly arranged streets.

However all this vertical development and prevailing SE breeze will bring in the sea breeze. This often progresses about 150 miles inland to Cotulla by 6-7pm, in a crescent shape following the line of the coast, but it can come right through Uvalde or stay away near the coast. On the best days it gives a strong line of increased lift with occasional showers that can be used for very high speeds. On other days, it is just the end of the clouds or a solid wall of thunder storms. Either way, south of the sea breeze is dead air.

The Reality of Weather

Even at Uvalde you don’t get classical conditions every day. However every single day we were at Uvalde was flyable except for the closing ceremony.

The continental air to the North provides a barrier to stop Gulf air coming in too far and fast, but if dominant can reduce cloud amounts and lift rates if it dominates through increasing pressure. This happened during Practice Week.

In other circumstances, the boundary produces a ‘Moisture Discontinuity Feature’ (or cold front to you and me) and kicks off thunder storms. Knowing where and how vigorously this happens is critical to good tasks as we found out during days 3-6 when the line progressively moved toward the airfield. An added feature is when the thunder storms collapse they form gust fronts with winds up to 60kt from the North and at least a change of landing direction and an interesting final glide.

Usually the Mexican air stays over the mountains, but we felt it’s influence on the last day, when old thunder storm blow off covered Uvalde from the West and threatened to lose the day’s flying. In fact the Texas sun was able to just combat it enough and we got a day after a hasty re-set away from the trouble.

What we Got

Very early in Practice we got some classical Uvalde weather which slowly become dominated by a ridge from the North, moving to weaker thermals and lower bases.

The first day had a wave in the Gulf which pushed the sea breeze in much earlier and further than forecast, but the next day showed no problems at all, just a line of thundery development that marched slowly South until it caused proper excitement on day 6.

We never had substantial influence from the Sea Breeze but we did get lots of varied shut off times. For reasons I could never explain, some days went on well beyond 7pm while others started to decline from 6pm. We never really spotted this feature properly and Matt and I were the wrong side of these decisions on 2 or 3 days.

We got genuine classical conditions on our cancelled Day 12, achieving 150kph over all having a relatively slow first half of the flight over the High Country.

The final day was heavily influenced by old thunder storm blow off from Mexico but despite looking like we would not go, the best start times were later in the day as the leading edge continued to burn off.