Monday, 18 May 2009

Off Airport


The two main reasons for using a helicopter, rather than an airplane, are (a) to land where there is no runway and (b) to hover in place. So, naturally enough, these form part of helicopter training. Tonight we did a bit of both. Both are harder than they look. We went out to a remote area and landed on hilltops and on gravel tracks. After circling round to look for animals, power lines, fences, people, houses and so on, you try to land on a hilltop. Compared to an airport (a) you don't know the altitude, so you have to judge everything by eye (b) it's surrounded by hills and other inconvenient obstacles (c) typically there are trees whose distance from the target you have to judge from afar (d) surfaces look deceptively smooth until you get up close. So when we finally got to a landing hover above our first chosen location, we realised that the grass was eighteen inches high. Probably you COULD land there if someone's life was at stake, but it's too risky for a training exercise - you have no idea what is under the grass until you get the skids on the ground, and there's always the risk of tail-rotor damage.

Off to the next place. We find a gravel track winding around the contours of some hills, and make a couple of approaches to it. But the chosen landing spot looks awful small until you get very close in. It's hard to believe you can get this big helicopter (relatively speaking) into that tiny postage stamp of gravel, surrounded by trees. I confess that as my instructor flew the short final I had visions of our rotor blades thwacking into the adjacent bank (forgot to mention that part). In fact there was plenty of room, or adequate anyway. But then you have to get off again, with said bank directly in front and trees on either side. No problem (if, like my instructor, you have 10,000 hours of heli time) - lift off into a 5 foot hover, 180 degree pedal turn, and swoop back off the way you come. But it takes nerve and experience - which of course is what the training is all about.

We survived all that, and went off to practice high-altitude hovering - OGE is the technical term, short for "out of ground effect", i.e. far enough away from the ground that it doesn't interfere with the down-draft from the rotor, which provides extra lift when close to the ground. It's surprisingly hard to enter a hover smoothly. For a fixed-wing pilot the idea of stopping just doesn't come naturally, so as the airspeed gets down to 40 knots or so it already feels like you're standing still. It takes very careful control of the cyclic and collective to get to zero airspeed without losing control of altitude - either climbing or descending. We were doing this just under a Class B shelf, which made it more important not to gain altitude - as my instructor pointed out rather forcefully when I got to within 100 feet of it.

Then you have to stand still. The ground is a long way away when it comes to judging position to within a few yards, and a 25 knot headwind didn't make it easier. The GPS helps a lot, although you have to remember that a ground speed of 10 knots can be in any direction - the GPS doesn't know which way the heli is pointing.

So after a bit of practice at that, we took advantage of our hover to practice "settling with power" also called "vortex ring state". This is one of the peculiarities of helicopters. If you descend too fast with very low airspeed, the rotor falls into the downward-rushing column of air that it is creating itself - and stops providing any lift. Recovery, as long as you have enough altitude, is similar to stall recovery in an airplane - push the nose down and accelerate out of it. If you don't have enough altitude, you will hit the ground very hard - which is why it's best to practice at altitude (also like stalls).

Then back home. If all goes well, my next flight will be another solo.

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