On Mon, 19 Oct 2020 12:22:08 -0700, john firth wrote:

Maybe the last flight of the year, but memorable.

Atmosphere 10C, very unstable , with cloudbase 5000ft and tops above
10K; showers, wind W 15kt @ 1000ft.

climbed to 4000 (airspace limit) over the airfield ; flew upwind along
a cloud street, holding 4000ft. Street ended after 10 miles , then
strong sink for 5 miles ending under a large cu.; at 1200 AGL found zero
sink but little more. Started circling but losing slowly, expecting to
find real lift,
but probing found nothing better. Considered starting the engine, but
that is admitting defeat too early, with good fields below; and so I
circled in smooth air and drifted, gaining 100 and losing it. Clouds
4000 ft above drifted past.
Have faith! Unstable air has to get it together and accelerate; the
fields drifted steadily by underneath, in the weak October sunshine,
interspersed with large woods, .
This went on for a long time, round and round, airfield unreachable
even downwind.
( would I ever contemplate a 15 kt downwind landing? only as a life
saver)
and so after 40 mins, still at about 1000 AGL, I see a dual lane
highway, the airfield a mile beyond.
surprise! I can reach the airfield; after another couple of circles, I
abandon the "thermal" and immediately hit 2 kts. End of suspense. I
had drifted 14 miles at 1000ft in 50 mins doing 120 circles.

Now for the question: how does a parcel of air over 600ft diameter
maintain a steady ascent of 150 fpm for nearly an hour without either
breaking away or dissipating. This was not a donut as there was no
central core. It must have been replenished from below.

Any similar experiences?

Yep, but not recently.

This was definitely convergence over the front of sea-air incursion. A
winch launch at Gransden Lodge, 10 miles west of Cambridge UK, put me in
zero sink with a cloud line, which turned out to be a convergence line,
running N-S along the road about 1 km to the east. I cruised over to it
with no height loss, turned north along it cruising at 50 kts and
climbing slowly but steadily to cloud base at 3500/4000 ft. It ended
around 5miles/8km I glided out to the motel on the A1 at Peterborough and
back to the cloud line for the loss of around 1500ft, drove back along it
to GRL and landed. That was my first XC in my 201 Libelle - a 100km out
and return in almost exactly an hour.

I haven't seen convergences like that in the last few years, but they
used to be frequent enough not to be unusual in Eastern UK and were
almost always in calm, high pressure conditions when an E or NE breeze
brought sea in from the North Sea.


--
Martin | martin at
Gregorie | gregorie dot org