[emseaaytea]

If the other aircraft is aft of you (behind you) going faster, then you should subtract your speeds, not add them, because your speed gets taken off their speed (you travelling at 80, negates some of their speed). So the closing speed is 200-80=120kts (or 120 nautical miles per hour)

If you were 120nm apart, it would take an hour until he/she would catch up, but you are only 6nm apart

120/60= 2nm per minute, so 6nm would be covered in 3 minutes, so that’s how long until impact


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[Fellsteruk]

May be of no help at all so disregard if needed.

From all the exam questions I’ve looked at and I’ve read a load doing my airlaw and HP prep I’ve only ever see collision time questions with head on collision situation.

Just thought I’d share in case that’s what your thinking off as I say ignore if not

[tcc1000]

To help your understanding:

A nautical mile is a measure of distance. A knot is a measure of speed - how far you travel in a set period of time.
1 knot = 1nm per hour = 1/60 nm per minute.

[David Wood]

Like a lot of these questions, the one you mention has absolutely no application in the real world. In the real world of GA you are never ever going to know the distance, relative speed and direction of other traffic in circumstances that make such a calculation remotely useful to you.

Its only value is in engendering an understanding of the basic speed/distance/time calcs which ARE useful. But in realilty, for practical purposes a few basic yard-sticks are what you want in your mind. So:

At a ground speed of 60kts (ie a slow GA aircraft, or one heading into a stiff wind) as mentioned above you will cover 1 nm per minute. In other words, in X minutes you will have covered 1 x X in nautical miles. Or, to turn it around, to work out how long it would take you to cover Y nm the answer is 1 x Y in minutes.

At a ground speed of 90kts (ie most GA aircraft) you cover 1.5 nm per minute. In other words, in X minutes you will have covered 1.5 x X nautical miles. Or, to turn it around, to work out how long it would take you to cover Y nm the answer is 2/3 of Y in minutes (or x 0.66).

At a ground speed of 120kts (ie, a slightly more meaty GA aircraft or a favourable wind) you cover 2nm per minute. For every minute flown you cover 2 nautical miles, so to work out how far you go in X minutes it's twice X in miles. And to work out how long it will take you to travel Y miles it's half Y (ie, y divided by 2) in minutes.

Since most of the ground-speeds that you will be flying at will be in the range of around 60-150 kts you can approximate quite accurately and it's worth getting into the habit of mentally double-checking your maths (or your skydemon ) to make sure that you haven't come up with an anwer that's just nonsense. You'd be surprised how often that happens. I always recommend doing what I call an 'idiot check' on all your time/distance/drift calculations before you set off. In other words, review each leg and say to yourself, "I'm doing 90kts IAS but the wind is more or less behind me at 20kts so my GS will be around 110ks. Check your calculation: were you right or wrong? The leg is 30 miles long so if I'm doing 110kts over the ground then that's very nearly 120kts or nearly 2 miles a minute. So I'd expect it to take around 16 minutes (a bit more than 15 because I'm going a tad slower than 120kts). Check your calculations; were you right or wrong. Finally, the wind's not directly behind me, it's blowing a bit from the left as well, so I'll be heading a bit to the left to compensate; so my Heading will be a number that's a bit LESS than my Track. Check your calculation; right or wrong? If the mental answer matches the calculated number more or less then you've a good chance that you've done it right. But if there is a discrepancy of more than a few degrees, a few knots or a minute or so then go back and find out why because it's likely that you've done something wrong somewhere down the line and you'll need to re-work that bit.

Just make sure that you're working in the same units. Knots and Nautical Miles; Miles and Miles per Hour; Kilometers and Kilometers per hour.