Mid-air collisions

Do you know that pilots need a total of 12.5 seconds to avoid a collision? This is the time an American military study concluded was necessary to avoid a collision. That’s an awful lot of time! In 12.5 seconds two gliders on a head on collision course traverse 250-700 metres depending on their combined airspeeds. Sailplanes may cover twice this distance. The researchers determined that the 12.5 seconds are broken down as follows:

  • 0.1 seconds to see the threat
  • 1.0 seconds to recognise the threat as an aircraft
  • 5.0 seconds to perceive a collision course
  • 4.0 seconds to decide how to avoid the collision
  • 0.4 seconds for the pilot to move the controls
  • 2.0 seconds for the aircraft to respond

I will not bore you by reciting the rules of the air here for they are both well known and unfortunately somewhat poorly adhered to. Rather I will offer some practical advice on avoiding mid-airs in the real world.

Detecting other aircraft at a distance is usually dependent on detecting relative movement between the conflicting aircraft and the background. Unfortunately if a pinprick of an object is detected moving against the background, this will be an aircraft that is not a collision hazard. The problem with aircraft that are collision hazards is that they do not change in their relative position to your field of view. In fact they appear stationary. This makes them much harder to spot as it is usually relative movement that first catches our attention.

The relative angular position of the hang glider to the paraglider and vice versa does not change. The only changing element will be the size of the other aircraft which will get bigger as it gets closer. To make matters worse the other aircraft will only start to get significantly bigger in your field of view when you are very close to impact. This visual problem is shown in the diagram where we can see it is only in the last 5 seconds that an approaching paraglider starts to get much bigger in our field of view. You can obviously only collide with aircraft at the same altitude as you. To further complicate matters these aircraft will generally appear superimposed on the horizon or the regions just above and below where they are often hardest to see.

So how do we counter all this?

  • Know and follow the rules of the air.
  • Do not expect other pilots to know or follow the rules of the air!
  • Actively scan for other aircraft. Move your head to counter the blind spot behind helmet and sunglasses.
  • Pay close attention to the region of the horizon for this is where the threats will usually hide.
  • You are on a collision course with any object which appears remain in the same relative position.
  • A small course deviation early is easier than a large deviation late.
  • Look before you leap – do not turn into the path of other gliders.

Wirestrikes

All pilots should be aware of the danger that powerlines represent. Difficult to see, easy to hit, powerlines represent one of the real dangers in our flying. Although always a hazard when low, it is when coming in for a landing we must be especially on the lookout for wires. Because they are such a ubiquitous part of the landscape over which we fly, somewhere, sometime, each one of us is going to have a close call with a wire. The pilot almost always comes out second best in such encounters so how can we minimise the risk?

While it is usually easy to see a wire silhouetted against the sky it can be almost impossible to pick out a wire when looking down when it is hidden against the ground features. What we can see easily are those ground features. Wherever there is human settlement there are usually power lines. We should assume that all houses, buildings and roads have wires associated with them and look for the tell tale power poles and their shadows. We must use the scanning technique described before to effectively search for them. Once one pole is spotted we must look for the others. A valuable clue is the insulators which are visible on the top of the poles. These give us a guide to the direction of the wire and help us to spot other poles. Poles are often more easily spotted from the side when they appear as a vertical line rather than from on top when they appear as a dot. This is a very good argument for doing an aircraft type circuit landing approach which lets us look at our chosen landing field from the side.

The shadows cast by power poles can be very useful but we must remember that the sun must be shining and that around midday shadows are very short because the sun is directly overhead. Wires can often be easily seen it we are “up sun” i.e. between the sun and the wire. In this position the wire reflects sunlight back to us often making them quite obvious. They are however quite invisible from a “down sun” position. This applies regardless of the wires thickness.

One useful game to play on those long boring trips through the countryside is play spot the power pole. Look at all the different power poles and wires. After some time you will become familiar with all the various configurations and the way the insulators can tell you which way the wires are going. The tell tale bracing wires running from the side of the pole to the ground indicate a side branch from the main line. You will probably see a lot of SWER (single wire earth return) powerlines. These SWER lines means you are often looking for a single wire instead of the multiple ones seen in the suburbs. You will also notice the disturbing tendency of electrical engineers to place poles on the top of two hills and string wires between them. Sometimes these wires are marked with coloured balls to warn pilots but often they are not. They are probably the hardest wires to spot and may be hundreds of feet above the valley floor. Because of this problem you should scan not only your chosen landing field for tell tale power poles but also adjacent areas in case a wire runs across the field but there are not actually any poles within that field.

Firebreaks in forests may look like possible emergency landing zones from the air but a closer look often reveals high tension power lines.Once we have spotted the poles and insulators we must then draw imaginary lines to give an idea of the likely position of the wires if we can not actually see them. We should fly accordingly so as to avoid them. Once we have spotted a wire we should not just relax and forget it is there. It will still be there whether we remember it or not. If despite all our precautions we do find ourselves low on final approach towards a wire we have several options: pick another field, try to lose height to land short, land crosswind or try to fly under (hang gliders only ;-)) or over the wire. These options are in pretty much in order of desirability. Flying low over wires is generally always the most dangerous option.

If you do hit a wire and happen to survive DON’T panic but DO think before you move. You obviously have not been fatally electrocuted but you’re not safe yet! If hung up in the wires you are safe unless current can be conducted through you to the ground. If you are on the ground look around and see if the wire is down and touching the ground. When a high voltage wire touches the ground it sets up a voltage field around it which basically means anyone who comes too close to where it goes to ground is likely to be electrocuted. Don’t touch anything you are not already touching. Don’t let anyone touch you, or even come near you, until the power has been cut off. Reassure any good Samaritans that you are OK and could they please phone the emergency services and get the power cut off. Pilots have died unnecessarily by not following this advice. So here are the basic tips for avoiding wires:

  • Always suspect wires in the region of roads, houses, and buildings. You will rarely be wrong. Whenever you are low actively look for wires by using your scanning technique. Use the sequence settlement, pole, insulator, then finally wire. If you can’t see the wire play join the dots with the poles.If flying at a new site inspect the bomb out for wires from the ground before flying if possible. Ask local pilots about any wires in the area that may be hard to see. Don’t just look for wires but try to prove to yourself they are not there by leaving enough height and time to adequately assess any potential landing site.
  • Know thine enemy – become familiar with the power poles and wires in your flying area when on the ground so you can make quicker, more accurate judgements in the air.

Thoughts for the day: A mid-air collision seriously erodes climb performance. Gravity never loses, the best we can hope for is a draw.

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