Centring a thermal: the 270-degree rule and the correction

You entered the thermal, you are turning, the vario is beeping, but you get 2 m/s on one half of the circle and zero on the other. The thermal is narrower than your circle and you are on its edge. Centring is the job of moving the circle onto the core, and good pilots do it by rule, not by feel.

First: wait one full circle

Do not correct on the first turn. One full circle is the only reliable data on which side the thermal is strong. Through the circle, remember the two points where the vario was highest and lowest; the line between them points at the thermal's centre.

Rule 1: tighten on the good side, open on the bad side

This is the simplest correction and usually enough:

• When the vario starts increasing, tighten the turn: a little more inside brake, weight in. You keep that part of the circle inside the thermal longer.
• When the vario starts decreasing, open the turn: fly straight for a second or two. The circle moves away from the bad side.

A small step every circle. Within three or four circles the centre of the circle settles on the core and the vario stops swinging.

Rule 2: 270 degrees

Tighten-and-open is reactive; it works with a lag. The 270-degree rule does the same job by anticipating.

Note the point where the vario peaks. From that point keep turning a full 270 degrees, then straighten for a second or two, then resume the turn.

Why 270? At the moment you feel the strongest lift the core is right beside you, on the inside of the turn. Another 90 degrees would point you at the core, but vario lag and your own reaction time take about another quarter circle. In practice, when you straighten after 270 degrees your nose points at the core and the two straight seconds carry you into it.

One correction per circle is enough. Pick a new "strongest point" every time; the core may not be where you first guessed.

Rule 3: do not reverse the turn

Reversing the direction of the turn while centring is almost always a loss: during the change you fly straight for half a circle and leave the core. Change direction only if another pilot is in the thermal and turning the other way, and do it on entry, not in the middle.

How much bank?

Steep in a narrow thermal, gentle in a wide one. Steep bank increases sink, but it is cheaper than staying outside a narrow core. If the climb rate does not fall as you add bank, the core is narrow and the bank is right; if it falls, the thermal is wide, open up. How much your wing sinks in a turn is covered in the net vario article.

Seeing centring in the log

After the flight, the thermals table shows each thermal's average radius and average climb. If, among the thermals of one day, climb drops as radius grows, you are holding the circle too wide. A large gap between peak climb and average climb in one thermal is the sign that you never settled on the core: you found the best moment but could not keep the circle there. The three criteria that define a thermal explains where those numbers come from.

Summary

1. Wait one full circle, mark the best and worst points.

2. Tighten on the good side, open on the bad side.

3. Straighten for a second or two 270 degrees after the strongest point.

4. Do not reverse the turn.

5. Check yourself in the log with radius and the climb gap.