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Light aircraft flown by the majority of PPL holders cruise at speeds in the range of 90 knots to 140 knots. Faced with the extreme situation, therefore, of another light aircraft approaching head on, a possible closing speed could be as high as 280kts, (i.e. the sum of the speeds of the individual aircraft). (See Figure 5.13.)

The problem of closing speeds of this magnitude is how to take avoiding action sufficiently rapidly to be able to prevent a collision. If a collision is to be avoided, the reaction time of the pilots must be less than the time it would take for the aircraft to come together.

The total reaction time in a pilot is a function of the time taken for all the separate recognition and reaction processes to take place. These processes may be summarised as:

• Visual input.

• Brain reaction.

• Recognition.

• Perception.

• Evaluation.

• Decision.

• Response.

These processes are illustrated in Figure 5.14.

In perfect conditions, all these processes take approximately 5-7 seconds to complete.

The first four processes take approximately 1 second to complete.

Times may be extended by factors such as:

• Workload.

• Fatigue.

Figure 5.13 Closing Speeds.

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CHAPTER 5: THE EYE

• Poor atmospheric conditions.

• Darkness.

• Size and contrast of object.

• Angular approach.

Time-to-Impact Calculations.

In any event, as a practical consideration, it is extremely useful for a pilot to be able to estimate how long it would take for aircraft on a collision course to close to impact. The important factor is the closing speed; that is, for two aircraft on a head-on collisihead-on course, the sum of their individual airspeeds. We can already appreciate that two light aircraft might be approaching collision at anything between 180 knots and 280 knots. If one of those aircraft is a jet, closing speeds will be much higher.

Low-flying military fast jets may be flying at a speed of 450 knots. So, if the aircraft approaching a light aircraft, head-on, is a fast jet, closing speeds may be in the order of 600 knots. We will carry out a few calculations to illustrate the “time-to-collision”

of various aircraft approaching each other head-on.

Of course, visual acuity matters enormously when a collision hazard is present. Early detection gives the pilot more time to take avoiding action.

The calculation we need to make to work out “time-to-collision” is simple. Speed is measured in knots, miles per hour, kilometres per hour, metres per second, feet per minute and so on. In every case, though, speed is measured as distance divided by time.

distance time

By simple mathematical transposition, this basic equation can be expressed as time equals distance divided by speed:

distance speed

Figure 5.14 Reaction Times.

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All we have to do to calculate time, then, is to divide distance by speed. Care must be taken, though, to ensure the correct units for distance, speed and time are used.

Have a look at the situation illustrated in Figure 5.15. If the closing speed is given in miles per hour and the distance in miles, as in the diagram, the formula we have obtained will give us a time to impact, in hours. In this case, we have 0.013 hours.

But 0.013 hours is a meaningless figure for us. If we wanted the closing time in minutes we would have to multiply the answer in hours by 60. If we wanted the closing time in seconds, which is much more sensible, we would have to multiply the answer in hours by 60 × 60. We finally arrive, then, at the sensible answer of a time-to-impact of 48 seconds.

In our example then, the closing time was in hours, because the speed was in miles per hour and the distance was in miles. Had the speed been given in knots (nautical miles per hour), the distance would have to have been in nautical miles for the formula to give us the time in hours.

Thus, if you are given mixed units such as speed in miles per hour and distance in, say, kilometres, you would either have to convert the speed into kilometres per hour or the distance into miles.

Time-to-Impact Examples.

Example 1. (See Figure 5.16 overleaf)

Two light aircraft are on a head-on collision course with a closing speed of 200 knots.

Flight visibility is 4 kilometres. What would be the time available to either pilot to take avoiding action, if visual contact were made at maximum visual range?

Step One. We note that the closing speed is in knots (nautical miles per hour), but that the distance at which visual contact is made is in kilometres.

Step Two. We must, therefore, convert one of the units so that speed and distance units match. Thus, we either have to change speed into kilometres/hour or the distance into nautical miles. Here, we convert kilometres into nautical miles. You can do this using the flight navigation computer.

4 kilometres = 2.16 nautical miles

Figure 5.15 Time/Speed/Distance Calculations.

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Step Three. We now insert the figures into the formula, Time = = = 0.0108 hours = 39 seconds

The formula gives us 0.0108. Remember that this answer is in hours. To convert this answer, we multiply by 60 to give minutes and by 60 again to give seconds. The answer, thus, becomes 38.88 seconds, which we can round up to 39 seconds.

Example 2. (See Figure 5.17)

With an in-flight visibility of 5 kilometres, a light aircraft flying at 120 knots sees a fast jet at maximum visual range. The light aircraft pilot assumes that the fast jet is flying at 450 knots. What time would be left to the pilot of either aircraft to take avoiding action?

Step One. Calculate closing speed. This would be 120 knots plus 450 knots, giving 570 knots or, otherwise expressed, 570 nautical miles per hour.

Step Two. Convert the 5 kilometres in-flight visibility to nautical miles. This gives us 2.7 nautical miles

Figure 5.16.

Appreciate how little time is available

for collision avoidance, when aircraft are approaching head-on.

Figure 5.17.

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Step Three. Enter the figures in the formula to calculate that the time available for reaction is 0.0047 hours, or, just over 17 seconds.

Example 3. (See Figure 5.18)

With an in-flight visibility of 3 kilometres, a light aircraft flying at 90 knots sees a fast jet at maximum visual range. The light aircraft pilot assumes that the fast jet is flying at 450 knots. What time would be left to the pilot of either aircraft to take avoiding action?

Example 4. (See Figure 5.19)

Two light aircraft are on a head-on collision course with a closing speed of 280 knots.

Flight visibility is 5 kilometres. What would be the time available to either pilot to take avoiding action, if visual contact were made at maximum visual range?

A Point to Ponder.

It is worth noting that for the pilot of an aircraft on a head-on collision course with a fast-moving jet, the image of the approaching jet will remain small, increasing in size only slowly at first, until just before impact when the image would grow in size very rapidly.

Figure 5.19.

Figure 5.18.

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Representative PPL - type questions to test your theoretical