When you are flying IFR (i.e. in clouds) in a busy, radar-controlled environment you normally receive radar vectors to intercept the approach. The controller tells you what heading to fly and you keep following her instructions until you have intercepted an electronic line in the sky, and you can fly straight in and land. But not all airports have radar coverage, and some approaches are not appropriate for radar vectors. You have to turn around and line yourself up to land.
An instrument approach normally has a radio beacon of some sort (NDB or VOR) either right at the end of the runway, or pretty much lined up with the straight in path to the runway. Lets say that you want to fly an approach to runway 18, so that to be lined up perfectly you have to be flying on a heading of 180, or due south. You have to work out some way to be exactly north of the beacon, pointing south. If you were looking out the window to plan this, you'd just fly to a point a few miles north of the beacon, turn around, and line up, like widening out and planning a turn with your car into a parking stall. But the point is that IFR you're in cloud, so you can't see to line up. And there is not usually another beacon ten miles north and exactly aligned with the first, so you can't fly there to turn around and line up. You have to fly to the beacon that is there.
Reaching the beacon, you're either directly overhead the runway or within a few miles. Either way, if you try to turn around and point at the runway to land, it's not going to work. So you when you reach the beacon you turn away from the runway and then later turn around and come back: a procedure turn.
There are various flavours of procedure turn, and in Canada you can choose any one you want for an approach, or make up your own, provided that you remain within protected airspace. Approach designers typically protect airspace 10 miles back from the beacon (it wil say on the plate exactly how far), 5 nm on the procedure turn side, and a mile and half on the non-turning side. Protection implies that if you fly at the published altitude you will be one thousand feet above the highest obstacle, and that ATC won't allow others to fly there while you're using it. Take a piece of scrap paper and draw a vertical line near the bottom, then a small circle further up the page in line with your vertical line. That's your runway, with the beacon to the north. Now draw a big rectangle, such that the beacon is near the bottom right hand corner of the rectangle. That rectangle represents the protected airspace for a left procedure turn. (A turn might be left or right, depending on what mountains, airspace or enormous TV antennae the designers had to contend with).
The standard procedure turn is called the hockey stick. Imagine a hockey stick (the Canadian kind, for real hockey, played on ice, not the little curly kind for grass hockey) with the butt end at the beacon and the blade turned off to the left, all inside the protected airspace. The airplane will follow a path down the shaft, left around the top of the blade, right along the bottom of the blade (where it would be resting on the ice) and back down the shaft to the beacon, and hence the runway.
So you fly directly to the beacon, from whereever you are, descending to the higher of the altitudes specified by the approach plate and ATC. When you reach the beacon, you turn, left or right, whichever is the shortest turn to the outbound heading. And then you turn a little bit more. Why? Because you were moving forward while you turned around, so if you turned due north you would be paralleling the shaft of the hockey stick and not flying along it. The outbound heading is written right on the approach plate, so that pilots don't have to worry about difficult math like adding or subtracting 180 degrees from the inbound heading.
You continue flying that little-bit-more heading until you are almost on the shaft of the hockey stick (correctly set and interpreted instruments will let you know when this happens). I say "almost on" it, because if you wait until you are exactly on it, you will overshoot it while you are making the turn. You don't want to be zig-zagging back and forth like a drunk driver trying to stay on the road. You want to establish yourself, with the minimum amount of zig-zagging, on an outbound track along the shaft of that hockey stick.
You fly along the hockey stick for a minute or two, doesn't really matter exactly how long, so long as you remain within protected airspace and have a plan. If my co-pilot has a thing about flying one minute and thirty seconds exactly, that's okay with me. We just want to get far enough out that turning around and lining up is easy, and so we have enough time to descend. After that agreed upon amount of time, you make a 45 degree turn to the left (or to the right, if the plate specifies a right PT, but my example is to the left). You're following the blade of the hockey stick. The heading to fly to get a 45 degree turn is also printed on the plate. After one minute (more or less if you know there's a strong wind affecting your progress towarads the tip of the blade) you turn right one hundred eighty degrees and head back towards the shaft of the hockey stick. Approaching the shaft (needles tell you when) you turn to the inbound course, timing it perfectly so that you roll out wings level, exactly aligned with the inbound course. Failing that, you make an appropriate correction to get on track, and start descending, according to the limitations on the plate. When you get back to the beacon at the butt of the hockey stick, you descend further, in the hopes of actually seeing the runway.
I could probably get an account that allowed me to put pictures up, but then I'd have to draw them, and scan them. Describing things in words that really ought to have pictures is kind of fun. I'll describe more procedure turns later. For those of you who know what it means when I write about holds: no this isn't a "holds" blog entry. I'm glad hockey is back. I'll describe other procedure turns later.