Around my twentieth flight I was gliding along happily when it was time to join the circuit to prepare for landing. The instructor asked what circuit I would fly. Looking at the windsock the wind was a little north of west so I decided to do a left hander south of the airfield. I arrived at the high key area a bit too high so I circled once to get down to about 850 feet. The instructor asked what approach and landing speed I should set, and I hesitated a bit. To date it had always been 55 knots. Today he recommended 60 knots as there was a decent headwind down the runway. So far my circuit planning hadn't been too brilliant - a bit high and a bit slow when left to my own devices. The instructor emphasised that it is a good idea to settle the aircraft at the correct approach speed early on the circuit, right back at the high key area if possible. That sets a speed safety margin before you get too low and makes it easier to take care of other business such as fine tuning the glider's position (height and distance) down to landing.
So as I passed the high key area into the downwind leg I lowered the nose to increase our speed from the 45 knot cruise to the approach speed of 60 knots. I picked a reference point a couple of hundred feet beyond the yellow threshold marker As we had some spare height I moved further along the base leg than normal before turning onto the diagonal leg. That put us at a better angle to the runway, a little higher and further back than normal. A lot better than previous circuits when I had been too high and too close. Turning onto the base leg I reckoned we were still OK for height and distance.
As I turned quickly again onto final approach for the north runway, we were starting to look a little high so I unlocked and pulled the airbrake handle to open the airbrakes a little less than halfway. I checked the speed was steady at 60 knots and continued to monitor the position of the reference point. It was sitting still in the view, indicating we were headed straight for it.
At what I thought was about 30 feet I reckoned I should start to round out and I gently eased back on the stick. As the nose raised I noticed we were still sinking more than expected. "Speed" called out the instructor. Another glance at the instruments revealed that suddenly the speed was decaying below 45 knots ! I held the nose steady, not wanting to dive onto the rapidly approaching runway, and suddenly with a bang the tail hit the tarmac, followed by the rest of us and we were down. In a faintly familiar way I kind of forgot to keep us under control and the instructor took over to complete the ground roll.
During the debriefing the instructor explained what had happened and gave me some tips to improve things. The problem with the speed was a classic case of wind gradient. On a windy day the wind-speed decays rapidly close to the ground - so the airspeed of the glider also decays which loses lift or increases sink. That is why we set an approach speed at 60 knots rather than the more standard 55 knots for a calm day. On a very windy day the approach speed might need to be as high as 70 knots. As the speed decays nearer the ground you might need to dive a little steeper for a moment then round out faster than normal for the float. I had started the round out a little late and at the same time the airspeed had decayed in the wind gradient, reducing the lift. So I was sinking too fast during the roundout. Like most beginners I had then raised the nose reflexively to compensate - which lost more speed and height rather than stretching the glide. I should have closed the airbrakes. So with insufficient height left, rather than rounding out parallel to the ground for a float onto the runway, just as I levelled out we hit the ground, with the tail hitting first due to the nose-high attitude. Fortunately we weren't descending too rapidly.
This was a good lesson. It is critical to set the correct approach speed and maintain it. Always keep glancing at the airspeed indicator right down to the float just above the ground and quickly take any corrective action.