4. LESSON 26 - FLYING IN THERMALS

Key points of attention:
  • Always plan ahead
  • Enter the thermal with care – look out!
  • Turn in the same direction as the glider(s) already flying in the thermal
  • Avoid collisions, keep sufficient distance to others
  • Stay within sight of other gliders – take the blind spots into account
  • Leave before a dangerous situation occurs
  • Do not overtake on the inside of the circle
  • Take drifting off by the wind into account

Flying in thermals is a lot of fun. We need to climb in thermals to perform all our exercises over and over again. Try to have a careful look which direction other pilots set off in after they have released from the winch or aerotow and you will better understand where to find them. Your instructor will explain the things you need to pay attention to when flying thermals. Flying thermals is a learning process and the goal is that you can make your own decisions in a safe and responsible manner. It requires a high level of concentration. “Centering” a thermal is a continuous process. Most thermal are not round and the thermal core may shift all the time. In the meantime you have to look out for other gliders and stay aware of your position in relation to the airfield.

 

Like most new things, thermal flying will become easier after practise. During your training, thermal flying is practised continuously. Here, we will concentrate on the safety aspects of flying in thermals only. If you want to improve and perfect your thermal flying technique, you can read at least twenty books on the subject, or spend some time eavesdropping at the bar of a random gliding club. Gliders in a thermal seem to attract each other like magnets. Very often several gliders end up flying in the same thermal at the same time, which is called flying in a “gaggle”. Here are five important safety rules:

1 TURN IN THE SAME DIRECTION
The first pilot to enter a thermal determines the direction of circling. All others who arrive later adopt the same direction of circling. Sometimes during circling you will see another glider circling nearby – at the same altitude but in a different thermal core. As you climb further you will then notice that the two thermal cores get closer; be careful to keep sufficient distance from the pilot in the other core or change your circle to join him or her. In general, the rule is that you try not to hinder a glider that is already circling when you join the same circle. Try to stay on the opposite side of their circle and always maintain visual contact. There’s a simple way to let another pilot know that you see him or her: wave! If the other pilot waves back, you know they’ve seen you as well. Try not to stare at your instruments when thermalling, but keep looking outside of the cockpit for most of the time. Electronic vertical speed indicators indicate whether you are climbing or descending by high or low beeping noises. There is no need to keep staring at the needles, simply pay attention to the beeps.

 

2 KEEP YOUR DISTANCE
It is important that you always keep as much distance as possible from all other circling gliders. Especially vertically: the higher pilot cannot see you if you are flying directly below or behind them. And if the lower glider would climb faster than the higher glider, a very dangerous situation could arise. Always take position in a thermal in such a way that all gliders have sufficient space to circle and correct, and that everyone can see everyone. If you want to leave the thermal core you clearly level your wings and fly away in a straight line. By doing this you show the others that you are leaving. Remember that if you feel uncomfortable or it gets too crowded, it might be better to find a new thermal.

3 DO NOT OVERTAKE ON THE INSIDE OF THE CIRCLE
Efficient thermalling is not easy in the beginning. To achieve the highest climb rate, you will need to adapt your circle continuously. When you are alone, you can do this more easily than when more gliders arrive. In that case, you will have to adjust your bank angle and rate of turn to the other pilots. Don’t overtake someone within the circle, even if you suspect that you’ll find a better climb over there. Very experienced glider pilots have no problem flying with several planes in the same thermal. But as long as you don’t have this experience or when you notice other pilots keep insufficient distance, it is better to leave and find another thermal.

If you cross a thermal where a glider is already circling, pass on the outside of the circle and never in the middle. Also, do not pull up hard below other gliders to gain height while crossing thermals.

4 THERMAL DRIFT – STAY WITHIN RANGE OF THE CIRCUIT
When a thermal is still being “fueled” by its hot air reserve, it will travel slower than the speed of the surrounding wind. However, thermals do move downwind! After releasing from the winch or aerotow, try to find thermals in the direction where the wind is coming from and monitor your position regularly as you fly. Do not circle above the winch and keep your distance from other gliders doing a winch launch. Calculate the required height above the ground at a certain distance from the airfield in such a way that you can always reach the circuit safely. At first, your instructor will help you to do this. How much height you need depends on:
  • the performance of your glider
  • the wind direction and strength
  • areas with possible increased sink
  • your experience

5 RETURN TO THE AIRFIELD IN AN EFFICIENT MANNER
On your way back to the airfield, you will pass through areas with rising and sinking airmasses. If you fly through sinking airmasses at a higher airspeed, you will pass through them faster and lose less height. The other way around, when you encounter a climb, you can stay in rising air a bit longer and gain some height. In the beginning, the rotatable McCready ring on your vertical speed indicator will help you a lot. This is a ring that indicates the best airspeed to fly in sinking and rising air. In modern flight computers this “ring” is integrated in the software and when set correctly, it gives you a very accurate optimal speed to fly.

 
                             Situation 1                                                               Situation 2
 
McCready ring in meters per second
 
 
McCready ring in feet per minute
 

SITUATION 1: MINIMUM ALTITUDE LOSS IN CALM WIND CONDITIONS
On the McCready ring you will see an engraved triangle mark. In this case, you will turn it to 0. When you pass an area with rising or sinking air, the needle of the vertical speed indicator will show you the optimal speed to fly. Follow this indication roughly; do not make airspeed alterations every second, but stay close to the airspeed the needle is suggesting. This is the best method of arriving back at your airfield at the greatest height possible.

SITUATION 2: MINIMUM ALTITUDE LOSS IN A HEADWIND
If you want to arrive at your airfield in a strong headwind or make it to the next thermal you will need to adjust the McCready setting a little bit (turn the triangle mark up to 0,5 m/s – 100 fpm). The McCready ring will now advise a slightly higher airspeed, which will result in a greater height on arrival. The full explanation of how this works is quite complicated at this stage, but keep in mind that when flying into a headwind we need to fly a little faster in order to travel further. The altitude penalty (the amount of height we lose) for flying too slow in a headwind is much bigger than the penalty for flying slightly too fast.

“McCready theory” is mainly used for cross-country flying. You will learn a lot more about it at a later stage in your flight training. For now, you might like to know that it has been developed to achieve the highest possible average speed during a distance or competition task. The contestants set the engraved triangle to the expected rate of climb of the next thermal and the ring tells them at what airspeed they should be racing there.