Flying under a thermal cloud street the lift gets so strong that I have to fly the glider just below the maximum permissible airspeed to prevent the aircraft from climbing up into the clouds.  At sea level, the maximum speed of the glider is 154 kts (177 mph / 285 kph).  However, at the altitude where I’m flying (between 16,000 and 17,000 feet) the air is so thin that the true air speed is much higher than the indicated air speed.  This means, that the maximum permissible indicated air speed (i.e. the speed shown on the airspeed indicator) is much lower than the « red line », in the case of my Ventus 2 it is 122 kts (140 mph = 226 kph) above 16,000 feet.

You may wonder what I would have done if the strength of the lift would have increased even further (which is entirely possible).  In this case I would have slowed down to 80 kts, which would have caused the glider to climb by several hundred feet, and at that point I would have extended the spoilers (also known as air brakes).  These are big metal bars that pop up on top of the wings.  Their effect is to decrease lift and to increase drag.  As a result the glider would have started to descend even in extreme lift.  Executing this plan would have not been dangerous because I kept a safe distance below the clouds.

When flying gliders in strong thermal conditions it is therefore imperative to always maintain a safe cloud clearance (more than the legal minimum) because this enables the pilot to slow down to a speed when it is perfectly safe to extend the air brakes. Glider pilots must always have a « Plan B » in case « Plan A » does not work out.  In most cases « Plan B » involves landing at a safe landing site if the glider runs out of lift.  This cloud street example shows, however, that a Plan B may also be needed if there is no shortage of lift and therefore no risk of having to land away from an airport.

The flight was out of Boulder, Colorado.  The glider is a Ventus 2cxT high performance sailplane with a wingspan of 18 meters.