Soon after I had started to get the hang of coordinated turns, the instructor introduced the big subject of circuit planning.
The route taken down to a landing follows a standard pattern called a circuit. A circuit can be left-hand or right-hand, and be long or short subject to the wind & weather. In reality every circuit is different but should follow the standard method as far as possible. The key to a good landing is good circuit planning. The reason for flying a circuit is to make your approach to a landing predictable for other traffic while making sure you reach a point in the sky for the final turn at a safe height, distance and speed - with an alternative landing area in mind.
Click on the diagram for a larger view:
By around 1000 feet you should be close to the airfield. You should be able to see the airfield's windsock(s) in order to decide the landing direction and the side on which to perform the circuit so that you are flying into the wind as much as possible in the latter stages and the landing. A simple memory aid is that the windsock points towards the circuit (diagonal/base legs). If the wind is exactly aligned with the runway or there are other operational aspects like a parallel runway (as at Deeside), there will normally be a standard circuit direction. This is usually left-hand - the traditional direction as in most aircraft the pilot sits on the left, so visibility is better. As you approach the circuit, before you start it and the pace of life quickens, is a good time to perform the pre-landing checks.
Usually when flying above 1000 feet you maintain the best glide speed, which is about 45 knots on the Puchacz. If it is a windy day, when you descend below 1000 feet you should speed up a little to be on the safe side, typically 50 knots. This reduces the risk of a stall and spin if you encounter turbulence or wind shear when the airspeed can suddenly decrease. The same applies if there is any rain or ice on the wings. If conditions are calm you can stay at the best glide speed to the end of the downwind leg.
To start the circuit you aim for a chunk of sky called the high key area. It is at a height of around 800 feet, well to one side and well upwind of the airfield. It is an area because at the start of the circuit your exact position is not critical.
Ideally you should enter the high key area at an angle of roughly 45 degrees to the runway or landing area, from the upwind side, i.e. the opposite end to where the landing will take place. As the landing is normally into the wind to reduce speed, this means that your initial approach to the circuit is with the wind behind you and your ground speed may increase somewhat as you are carried along. The important thing is to monitor the airspeed and not be distracted by the groundspeed. A common fault early in training is to slow down as the glider descends due to the sensation of accelerating over the ground as it gets nearer. As above, that can lead to a stall and spin. For safe flying monitor airspeed not groundspeed, and correct your position as necessary.
Around the high key area you should choose a reference point upon the runway. The reference point is where the glider would touch down if it were flown straight down onto the runway without levelling off (bump). In reality the glider is eased out of the decent (see the round-out below) so it actually touches down further down the runway. The reference point is normally about a third of the way along the runway, although this depends upon the runway length. You may be able to use a mark on the runway or simply have to use your judgement. In high winds the reference point moves further into the landing area, perhaps half-way in a strong headwind, to reduce the risk of the glider under-shooting. The circuit is flown with constant reference to the reference point. The idea is to make sure that the angle down to the reference point from the glider remains adequate to reach it - avoiding a landing before the runway (an undershoot) or after the runway (an overshoot). This is all based on judgement that comes with practice.
When you execute a circuit, it is important not to use landmarks as references for performing the various manoeuvers required. Otherwise if you end up landing out (of the airfield) one day, you will have extra trouble as there will be no familiar landmarks in the right places. In essence, you should fly the circuit as if the airfield were in the middle of a featureless plain. All height and distance calculations are based upon the reference point.
From the high key area you turn as appropriate to fly parallel to the runway, but in the opposite direction to the intended landing, along the downwind leg. If you haven't already performed the pre-landing checks and radio call, make sure you do them now.
As you glide along the downwind leg parallel to the runway, you should monitor the reference point and move closer to or further from the the runway to adjust for any lift or sink you encounter. At Aboyne the close ridges on both sides of the airfield mean it is safer to lose excess height with the airbrakes rather than moving further out, especially on a windy day. But if you have to descend a great deal, you probably started the circuit too high - so be very careful that there isn't someone in a better circuit below you. Along the downwind leg you soon reach the low key point at about 500 feet, roughly opposite the reference point near the landing end of the runway.
The approach speed is faster than the best glide speed to provide a safeguard against any turbulence or sink low down in the circuit which could stall the glider (maybe leading to a spin) when there is insufficient height for a recovery... The approach speed should be increased on a windy day if there is a chance of a wind gradient or wind shear. Turbulence or wind shear can make the headwind drop or even reverse causing a sudden loss of airspeed. A wind gradient is a quick loss of wind-speed near the ground due to surface friction. In any glider it is essential in stronger winds to avoid drifting downwind of the airfield during the circuit or you might not reach it. Always turn in early from the downwind leg with the reference point well into the landing area. Old / slow gliders with poor penetration should alter the circuit accordingly by turning in even closer to the airfield: in extreme cases the final turn might be above the airfield.
How much should you increase the approach speed?
One rule of thumb is as follows: If there is no risk of wind gradient or wind shear, fly a normal approach speed (55 knots in most gliders). When there is a fairly low risk, add 5 knots. For a medium risk add 10 knots. Add 15 knots for a high risk (usually only in strong winds).
A more mathematical formula for the approach speed is (1.5 * Vs) + (Vw/3). This means 1.5 times the stall speed, plus one third of the windspeed (if the wind is above about 5 knots). For the Puchacz flying dual (two people), the usual stall speed is around 37 knots. So on a calm day the recommended approach speed is 1.5 * 37 knots = 55 knots. With a 15 knot wind the speed would be increased by another 5 knots to 60 knots. Simplified variations of this are: 1) Fly at 50 knots plus half the wind speed (with a minimum of 55 knots); or better still 2) Fly at 55 knots plus a third of the windspeed if the wind is above 5 knots.
But increasing the approach speed based only upon the indicated wind speed can result in approaching too fast if there is no wind gradient or wind shear! So how do you know if there is a risk and how much is it? Each gliding site has its own susceptibilities usually dependent upon local terrain, trees/buildings, and wind direction. Experience helps. Asking pilots who have just landed is a good idea.
This can all be a bit of a handful. The advice I was given during training, shared by fellow pilots I have spoken to, is to play safe and keep it simple with the approach speed being as follows: Still or light breeze = 55 knots, moderate wind = 60 knots, fresh wind = 65 knots, strong wind = 70 knots (shouldn't be flying!).
Whatever your approach speed you should always pick an exact speed and stick to it, tolerating an increase of up to 5 knots but not permitting any drop lower than the selected speed. As described in approach and landing, you control the rate of descent with the airbrakes (and perhaps spoilers / flaps on more advanced or unusual gliders).
Soon after the low key point, and certainly before you start to lose sight of the runway - you turn about 45 degrees towards it to enter the diagonal leg. This modification to a standard powered-flying circuit keeps the runway in sight and avoids the glider getting too far away and losing sight of the runway. Then it would be easy to turn too late onto the base leg - and if there were any sink around you might not reach the airfield.
By the low key area, turning onto the diagonal leg, you should have chosen and established a safe approach speed as described above, depending on the glider's stalling speed and the wind strength.
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Still flying at an angle of elevation relative to the reference point you turn again another 45 degrees from the diagonal leg onto the base leg so that the runway is now 90 degrees to one side. On base leg looking at the runway through the left side of the canopy |
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When you are almost opposite the end of the runway you perform the final turn of about 90 degrees to line up with the imaginary extended runway centre-line, onto the approach. If the circuit has been well planned you should be well lined-up in plenty of time for the landing. It is useful to line-up early so that you can then concentrate mainly on speed and altitude down to the touchdown. The purpose of the circuit is essentially to get the glider to the position of the final turn at a safe position and a safe height at a safe speed - with alternatives available in case the anticipated landing area is suddenly blocked or you encounter unexpected lift or sink.
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It is normal to deploy about half or two-thirds airbrake on the base leg and/or approach to steepen the descent angle while maintaining a safe speed. Then if you start to lose airspeed or lift you can close the airbrake as appropriate to remedy the situation. If you approach without airbrakes the gliding angle is very flat, the landing point is harder to judge, and you are too close to the ground in the final stages with no margin for error. This is like opening the throttle on a powered aircraft - but once again you only have one chance in a glider. |
There is no such thing as a standard circuit in a glider as you don't have the luxury of an engine to pull you along the ideal path irrespective of the flying conditions. In many cases it is necessary to modify the circuit to adapt to the conditions.
After the final turn you deal with the approach and landing.
During early flights I soon realised how hard it can be to work out where you are relative to the airfield - or even to remember where the airfield is while manoeuvering around the sky.
After a meandering tow it was easy to lose the sense of direction - until the local scenery became more familiar it was hard to work out where the airfield was.It is obviously important to make sure where you are at all times so that as you lose height you can be sure to make it back to the airfield, in a decent position to start a proper circuit. Most bad landings can be traced back to problems in the circuit, even as far back as a poor join at the start of the circuit.
Even when I was starting to develop this "situational awareness", it was still surprising how easy it was to quickly become disorientated. For example, when moving between clouds it was easy to lose sight of the airfield completely. Or after exercises like spinning, the landscape before and after could seem very different due to the loss of height and change of direction.
A good understanding of the local scenery is a major advantage so that you can relate your position relative to the airfield by recognising landmarks directly below. Then you can make sure you get back to the airfield at a height suitable to start a circuit and landing.
One day as we approached the airfield the instructor asked me what circuit we should fly. Before now I had always been told our route...
| Looking down at the airfield windsock, I saw that the wind was almost southerly, i.e. a complete crosswind. This was a puzzle: normally the landing is made into the available wind to reduce the groundspeed and assist braking. But this time it seemed that an approach from either end of the runway would have the same (lack of) headwind. |
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As I swithered, we continued to sink earthwards.
Eventually the penny dropped - if there was little to choose from in terms of which end to make the approach, then the next best thing was to fly into the wind on the base leg. (The base leg is the part of the circuit just before the approach, when you are flying at 90 degrees to the runway, about to turn towards it for landing). Since we were on the north side of the field and opposite the east end of the two parallel runways, I decided that we were effectively at the start of our downwind leg on a left-hand circuit. So I turned us around to fly parallel to the runway on the downwind leg (actually crosswind due to the conditions), set the attitude for the correct approach speed of 55 knots and re-trimmed .
Then I noticed that meantime our height had decayed to well below normal for this stage. As I flew us onwards to the diagonal leg, the instructor suggested I turn early to keep us closer to the landing site as we continued to sink. Then as I entered the base leg it was clear we were way too low and it dawned on me that we might not even reach the airfield.
"I have control" called the instructor, and he immediately made a fast and steep turn like a fighter jet, diagonally onto a short final and lining us up nicely for a landing almost immediately.
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During the debriefing the instructor stressed the need to make an early decision about what circuit to fly and to get into position to join it early. We had lost valuable height while the cogs ground around slowly in my head. He also emphasised the need to be able to modify the circuit immediately if conditions changed - like wind direction or encountering excess sink. As he had ably demonstrated. He had originally planned for me to make the landing, but we had ended up so low in the circuit that he had no option but to take over at the end (such final manoeuvers are too extreme for a student at an early stage). |
I found out later that if the wind is essentially parallel to the runway then the standard circuit is left-handed.
One day we were still on the south east side of the airfield and approaching 1000 feet when I realised from peering down at the distant wind-socks that the wind had turned to the south west. So our best approach circuit would now be a right-hander from the north. That was unfortunate as it meant I had to start the circuit on completely the opposite side of the airfield. I quickly aimed over there and increased our speed to preserve more height as we continued to descend in sinking air.
By the time we reached the high-key area at the start of the downwind leg, the altimeter was indicating about 800 feet, which I reckoned was OK. But the instructor pointed out that below 1000 feet the altimeter usually lags or sticks, and we were in fact already nearer 600 feet - too low ! The situation wasn't helped by the fact that I had approached the start of the circuit from the inside (the wrong side) - so I lost more height as I turned us around to the east to fly along the downwind leg parallel to the runways. Turning can lose a lot of height, perhaps 200 feet or more if done slowly.
Now acutely aware that we were too low, I moved us in closer to the airfield and started the diagonal leg earlier as we passed the landing point. Then I turned us a bit hastily onto a very short base leg and quickly around onto the final approach - and now we were too high ! I grabbed the airbrake handle and yanked out over half airbrake to increase our rate of descent. We were still a bit high and we started to drift in the strong crosswind to the right of the extended runway centre-line.
I felt I was losing it. The instructor clearly agreed - as the ground loomed up he took control and after some hurried yet impressive manouevers he landed us nicely. During the debriefing, he cheered me up by remarking that my general flying had greatly improved since the last time we had flown (seven flights previously). But as I was all too aware, he remarked that I had made a meal of the circuit and approach. My loss of position shortly before landing was a direct consequence of being set up on the approach too high and too close. I had turned too tightly onto the final approach and had deployed the airbrake too late in an attempt to compensate for being too high. Looking further back, the start of the circuit had been in the wrong place, so the trouble had started a long time before the landing. "Null points" I thought, in a suitably Eurovision accent.
I realised that I hadn't seen an ideal circuit being executed for many flights, because I had been flying so many of the previous circuits (and not ideally on some occasions!). So I was having trouble visualising the idea height and distance relationship throughout the circuit. The point the instructor had made about the altimeter reading too high below 1000 feet was particularly interesting. I remembered the advice that once you start the circuit you should forget all about the altimeter and instead use external visual references to estimate the height. On the circuit and the final approach the key instrument is the airspeed indicator.
