Climbing and flying in wave is wonderfully quiet and gives the chance to see the landscape from a great height. However, there have been several accidents in which gliders have been destroyed in strong wave. Naturally, this occurs in rotor which sometimes can throw a glider back and forth with unbelievable force.
I have had the pleasure of being blown about like a dry leaf in an autumn storm. This happened in the south of France near the Pic de Bure causing my video camera to try to punch a hole in the canopy or my head because I had not tied it down adequately.
Is such a situation dangerous for a glider? In principle, No, as long as the pilot holds the speed under the maneuvering speed, that is, in the green ark of the ASI. At or below this speed, the worst flight forces can not break the glider because that is what it is built and tested for. In these conditions it is much more likely that the pilot, not the glider, will fail.
The situation at higher speeds is quite different; that you already know. Gliders have come apart several times in wave. We would like to pass on the theory of an American pilot about this.
Imagine the following situation:
A pilot flies with a tail wind in a wave system and intentionally stays under the maneuvering speed. The air is wave-smooth and he could fly much faster but he is being careful! He is about to fly over a mountain ridge and expects the next lee wave behind it and wants to do a 180 in it. Suddenly the ASI jumps up well into the yellow arc. The pilot pulls back carefully on the stick to reduce speed. He doesn’t want to pull the stick back too quickly at this high speed. Then he hits hard turbulence and loses a wing. At this high speed and not so great an altitude, the surprised pilot has only a few seconds to get out (without a rescue system such as the NOAH) or it is too late.
What happened? As the pilot flew over the ridge, he fell out of the wave without knowing it. Because of the strong tail wind, his ground speed was perhaps over 165 knots. Because of the suddenly lost tail wind, the ASI jumped then the glider hit the rotor turbulence that was stationary behind the ridge. The aircraft came apart completely legally.
Normally you wouldnt fly unnecessarily fast with such a tail wind in order to make use of it as long as possible. In record and competition flying, this situation could be quite realistic and when the pilot flies over the ridge he must begin to slow way down – perhaps to the minimum sink speed.
This logical but theoretically possible situation has a basis in an experience described to us from the USA. Maybe you have already had such an experience? Do you remember a situation in which you were approaching a rotor and the airspeed suddenly climbed and the turbulence was unexpectedly strong? Perhaps you can give us a report of such an experience. It would also be interesting to hear the opinion of a meteorologist about this.
translated by David Noyes, Ohio
I believe my student was trying to explain that waves are stationary to the ground yet when flying you are watching indicated speed for Va. Va in this case I feel should be referenced by ground speed or speed relative to the rotor. If you have a 20 kts tail wind and hit the rotor at 100 indicated at 20,000 ft your true ground speed or speed relative to the rotor might be as high as 140kt + 20 kts of tailwind = 160. Va is an indicated speed limit. When you hit the rotor you do so at a relative speed to the rotor which is indicated + true tail wind.
In the wave in Boulder we are very cautious to stay well below Va.