Convergence

Friday 7th May, 2004

There was a good forecast so I went flying. However, the extensive top cover didn't burn off, so I ended up helping out on the ground all day. I'd decided not to fly at all, but we were operating at the far end of the airfield from the hangars and I was eventually persuaded to fly HQM, a club Discus, back to the hangar at the end of the day. So, at about 17:30 I found myself getting strapped in and running pre-flight checks. I was expecting a 5 min sled ride because we were operating off 34, our shortest runway, and the sky was solid overcast. However, there was a dark street of cumulus almost above the winch, so my game plan was to see if it might be working.

I got a good winch launch to 1400 feet which was the first surprise: 1000 feet is considered not too bad for this run. I popped off the cable straight into lift alongside the clouds. I quickly found the lift was too narrow to circle in and, as most of the cumulus were upwind I extended my explorations to the north under the street and was soon up to 2000 ft. Here I discovered that I was dealing with a convergence line: a wall of cloud was hanging from the main overcast and running, with the darker clouds, to the north-west. There was a narrow lift band on its west side so I ran along this, cruising at just under 60 kts while climbing at between 2 and 6 kts. The wall sloped up to the east from its base at about 2000 ft, so by the time I was half way up it I was over the lower clouds: the lift band was about two wing spans away from the wall and too narrow to circle in, so I was following its contours exactly as if I was soaring a ridge.

Eventually I got to the base of the overlying cloud at 3800 ft and found that I often had to run at up to 100 kts to avoid getting sucked up into the cloudbase. The view was fabulous: you don't get too many chances to look down on clouds from a glider. That run took me well past St. Neots and I could see Bedford airfield coming up on the left. I could have gone on but as the only GPS on board was a GPS 55, which I hate, and I was already a bit disoriented by the lack of landmarks anywhere to the right, I shot through a gap to the east side of the cloud to get a looksee. The east side of the wall was mainly sink, so I glided over to the southern shore of Grafham Water, which is 16 km from our field, before re-crossing back to the west of the cloud wall at 2000 ft in moderate rain and cruising back the way I'd come. Again there was steady lift so I was soon back over 3000 ft again. Meanwhile the whole convergence line had been drifting steadily south-west, so by the time I was back to the Gransdens our airfield was on the wrong side of the cloud wall. Once again I turned east and dived back through heavy rain under the wall, the wet Discus emerging into clear air just north of GRL and still at 3000 ft.

The air mass near the field was dry and pretty neutral, so I floated round a bit while the wings dried and one of the ASK-21s, which was being towed back to the hangar, cleared the landing area. By now I was down to 1000 feet so I set up a large circuit and came in along 34 to touch down just short of the diamond and roll to a stop 100 m from the hangar. The whole flight took 45 minutes.