Circuit Planning

by Chris Rollings (Senior BGA Instructor) 

From the earliest formalizations of glider circuit planning the “ideal” circuit was visualized as being rectangular with the downwind leg parallel to the approach direction and base leg at right angles to these two. Heights and distance varied with glider types and instructors perceptions of required safety margins. Generally, nowadays we work around (Fig 1).

• A Start of circuit/high key point 300m, 1km to 1½ km from the landing area.

• B Abeam landing area low key point 225m 0.8 km from the landing area.

• C Turn onto base leg 150m rather more than 0.65 km from the landing area.

• D Final turn 100m 0.5 km from the landing area.

• E Reference points for approach/landing area. These figures for a K 13 and a light wind.

In the early days of instructing, correct positioning in the circuit was taught by reference to local landmarks (secondary references) e.g. “Start downwind leg over the red barn”, “Turn on to base leg at the crossroads in the village” etc.

It was realized fairly quickly that this only worked well for a limited range of conditions and a particular take-off direction.

Change the wind direction and/or strength and the pupil needed retraining. More important it was quite useless for a field landing and cross-country flying was no longer limited to just a few pilots.

The answer, introduced mainly in the 1960’s was to judge the glider’s correct position in the circuit solely by its position in relation to the intended landing area (primary reference) referred to as the aiming point at GGC - ed.

Correctly applied, the method requires the pilot to assess his distance from a primary reference angle below horizontal to primary reference and height above primary reference.

This method worked better, but still left me with a growing feeling that all was not well. In particular, a sense of unease was often present (on instructional flights) between the low key point (B) and the turn onto base leg (C): this sense of unease appeared to occur in most if not all instructors, from time to time.

The reason took some time to puzzle out but is actually quite simple, being in two parts. First. As the glider flies the first part of the downwind leg from A to B, although the height is reducing the distance from the landing area E is reducing quicker. Height loss from A to B is about 25% of initial height. Total distance from B to E is 33% to 50% less than from A to E so the angle to the landing area is continuously improving.

After the glider passes B on its way to C the distance to the landing area E starts to increase and the further, downwind, the glider goes the more rapidly this distance increases. Meanwhile, height

continues to be lost (often at a greater rate, the low key point is usually about where speed is increased to the approach speed) so the angle to the landing area is worsening, and at a rapid rate.

On a moderately windy day the difference between a correctly positioned turn on to base leg C and a glider out of gliding range of the landing area (and quite likely to have a serious accident trying to get back) is just a few seconds. No wonder instructors get aggravated. Once the glider is on the base leg flying from C to D, the angle starts to improve again and on the final turn D is about the same as it was at the low key point B.

Fig. 2 shows the changing angles throughout the circuit.

The rapidly worsening angle as the glider flies downwind from the low key point towards the turn onto base leg is one part of the weakness of this method of circuit planning.

The second weakness is more profound. It is that once you are past the low key point in most gliders the position of the wing is such that you cannot see the intended landing area. Since the training concentrates on judging your position in the circuit relative to this spot a certain amount of swearing results.

Now comes the blinding flash of the obvious. If the part of the circuit from B to C is potentially fraught and makes you uncomfortable because you cant see the landing area, don’t fly that part. As you pass abeam the landing area and it starts to disappear behind the wing turn through about 45° and aim to join a “normal” base leg about half way along.

Two things happen, one is the landing area stays in sight and you can continue to monitor the all important angle, the other is that the distance to the landing area is constantly reducing more or less in step with the height and the angle remains about constant.

Putting this idea to groups of experienced instructors generally, gets responses like “That’s what I do when I’m flying solo.

Experiments last year, on instructors courses, found no problems with this new style circuit and at the BGA instructors Committee meeting in November, it was agreed to adopt this method as the normal method of teaching circuit planning.

Perceptive readers will note that since the total distance round the circuit is now less, either the final turn will be a little higher than before on the downwind leg will need to start a little lower or a little further away. The last is the preferred option but the differences are generally small so the choice is not critical.

What I have written here makes no mention of variations for strong winds, crosswinds, being below, too far away, too high or too close, and in these events this circuit needs to be modified in the same ways as its predecessor. Detailing all the possibilities here would result in an article far too long for S & G but you will find all the information in the new instructors Manual which will soon be available from the BGA.

Finally it would not be fair if I did not share the credit for this new approach with Julie Angell, Graham McAndrew, Chris Pullen and Terry Slater who helped to formulate it; and also the many trainee instructors in 1992 and 1993 who were unwilling guinea pigs for the developing area.

Once in the circuit, or overhead the airfield, if you find yourself low remember your training, don’t panic. Make a decision and stick with it and land in a safe place. Remember “Low and Slow Don’t Go.” If you think you can do a turn when you are very low, forget it because you will lose more precious height. A good circuit usually means a good landing, there is nothing funny or clever about just scraping home.

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High Altitude Physiology or

High on Oxygen

Among the problems encountered with flying high, such as extreme cold, reference to ground features, true vs. indicated airspeed etc, the most significant risk to the pilot is posed by hypoxia, a cellular deficiency of oxygen. In a healthy individual the brain is unfortunately the organ most sensitive to hypoxia. (I know some women believe that men don’t think with their brains but I still think that a brain in good working order is useful).

At sea level the atmospheric pressure is roughly 760mm Hg (mm mercury). The same pressure as in our lungs, otherwise we would explode. However the composition of the gasses in our lungs is different to the composition of air. In our lung air sacks (known as alveoli) water vapour and carbon dioxide are added as metabolic waste. The comparison of the composition of gasses is as follows:

You will notice that the pressure of oxygen in the lungs is already lower than that of atmospheric oxygen. This is due to the increased pressure of water vapour and carbon dioxide. By the time this oxygen has diffused across the alveolar wall into the blood, the pressure of the oxygen (at sea level) will have dropped to 95mm Hg, this is remarkable efficiency. Unfortunately with age and disease this efficiency is quickly reduced.

Our cells can function quite well down to a blood oxygen pressure of 60mm Hg. Below this pressure things start to go wrong very quickly. Unfortunately there is not obvious way of detecting when we have reached this level of hypoxia. As an anaesthetist I am required by law to use sophisticated monitors to detect hypoxia in my patients. Evan as a trained expert I would be grossly negligent if I relied on physical signs as an indication of hypoxia. By the time you notice anything, it is too late. I know its your own life, but having a wrecked glider is no fun, I know, I’ve been there.

The Effect of Altitude.

With increasing altitude the atmospheric pressure drops exponentially according to some fancy equation. The composition of the gasses however remains constant, thus oxygen will always make up to 21% of the air we breathe in.

In the lungs the pressure is the same as ambient (unless you kept your breath all the way up, then you would explode!). In the lung alveoli the oxygen pressure is lower than that of the inspired air due to the additional of carbon dioxide and water vapour. However the pressure of these two gasses does not decrease with altitude, as our bodies produce a set amount of these gasses as waste. The effect of this is that the higher we go, the more the oxygen pressure is diluted by these waste gasses. From the simple equation: Oxygen pressure in lungs = 21 % x (atmospheric pressure 47)40.

Where             :           47 = pressure of water vapour (See Table 1).

And                 :           40 = pressure of carbon dioxide.

From this we can work out the pressure of oxygen in our alveoli that can cross into our

bloodstream.

From these values we can see that somewhere between 3,000 m and 4,000 m we start operating on the edge of our design limits (i.e blood oxygen pressure less than 60mm Hg).

What to Do

Increasing the concentration of oxygen you breathe in will increase the concentration in the lungs. It will never equal 100%due to the dilution by water vapour and carbon dioxide (unless you are dry and produce no metabolic waste, a condition commonly known as death). The glider pilot flying with 100 % oxygen at altitude will give the following values (in the previous equation replace 21 % with 100 %).

Note that at lower altitude 100 % oxygen is an overkill as it gives unnecessarily high oxygen pressures, and above 10,000 m even breathing 100 % oxygen the oxygen pressure in the lungs is already lower than when breathing air at sea level. All glider oxygen systems therefore have some way in which the correct oxygen concentration for a given height can be selected.

The above values assume that there is no leak in the mask, diluting the oxygen being breathed in. Even a very small leak causes a pronounced reduction in the inspired concentration of oxygen. The effect is similar to a home loan, we all know how a small change in the interest rate is compounded over time into a heck of a lot of money.

Using this knowledge the following rules (not guidelines) should be applied when flying high:-

• Always use oxygen when flying above 4,000 m (13,000 feet), ASL.

• If you are older or a smoker, use oxygen sooner.

• Select the correct oxygen concentration for a given altitude. It is clearly marked on the MH equipment.

• Above 27,000 feet use 100 % oxygen.

• Check that your oxygen system works. (Have the instrumentation tested).

• Use only tight-fitting masks.

• Never wait for symptoms of hypoxia, by then it is always too late as your judgment is the first function to be impaired.

Hopefully this thesis is understandable and you now grasp the science behind the rules. If you don’t believe me I will happily show you patients (not mine of course) that suffered from hypoxia, the results are devastating. Fly high but don’t get high (poxic), something to think about for the “real men” who go to 17000’ (just for a while, of course) without oxygen.

The author of this great article is unknown.