View Poll Results: On a fine day VFR flight, what offset from cruise height do you use?
Voters: 65. This poll is closed
Most popular cruising altitudes
Thread Starter
OK... I'm sorry for the delay in posting results. Sometimes life gets busy. I quite agree with the frustration some people feel when confronted with such an odd question. However, I did have a halfway sensible rationale behind it. It relates to a trick many pilots use to reduce the risk of a mid-air collision, which is to fly at a 'random' altitude, the assumption being that most other pilots will be flying at 'round' altitudes such as 2000 feet, or 1500 feet.
There's a children's game where if you ask someone to think of a vegetable under time pressure, almost everybody thinks of a carrot. If you ask people to think of a flower, most people think of a rose. But what if you ask people to think of a random number - for example a random altitude?
There's unfortunately surprisingly little research on this, though what little I've seen tends to suggest that we also have prototypical 'random' numbers that we prefer. And intuitively 2300 feet somehow seems more random than 2000 feet or 2500 feet, doesn't it? It does to me, though of course I know that 'randomness' occurs in the process of selecting an altitude, not in the individual result.
In my polls, I randomised the order of presentation of the altitudes to try and avoid any bias due to page position, and posted different versions of the same quizes on several fora. I'm not particularly serious about this question and I'm sure I could have carried out the questionnaires more rigorously, but it's a start. And the results were significant, both in practical and statistical terms.
This first graph shows the altitudes pilots select if they consciously choose to pick a 'random' cruising altitude.
"Many pilots try to pick 'random' cruising altitudes in order to reduce the risk of mid-air collisions.
Ignoring all other considerations, if you were trying to use this strategy on a VFR flight, which altitude would you be most likely to pick?"
182 responses - risk of collision 150% greater than the theoretical optimum.
The result is clearly biased towards '3' and '7', which I had predicted, though '2' and '8' surprised me in their popularity. '2' was only really popular on one forum - and this one, which made me wonder whether someone was possibly trying to fiddle the results. However '3' was popular on every forum where I posted the quiz, whatever order the options were presented in.
Incidentally, and without meaning to be churlish, I did not count results from the poll that got posted on this page for a number of reasons (missing entries, change in question, ability to see and discuss results prior to their final compilation). However, thanks to everyone who filled in both this poll and the one that I initially posted. I am also grateful to the mod who tried to recreate the poll on this forum - PPRUNE initially pulled it due to a breach of terms (no links to external polls) of which I was unaware, and have no quarrel with - their site, their rules.
However, not every pilot knows the 'trick'. What altitudes do people actually fly at? The best way to do this would be to sit watching mode-C returns over Wales or Scotland - somewhere where people choose altitudes on the basis of preference rather than compulsion - we often don't have much choice. However, that's not something I currently have the time to do so I posted a separate poll elsewhere asking people to pick an altitude.
"It's a fine day for a VFR flight, and you can choose to fly at whatever level you want. Pick your favourite cruising altitude, and select the whole number of hundreds-of-feet on the poll below. Thanks!"
112 responses altogether. Risk of collision 152% greater than the theoretical optimum.
The result - to my eye - shows a mix of 'random' cruising altitudes, with the big round numbers of thousands of feet, or five-hundred foot increments. Perhaps surprisingly here, people have tended towards lower numbers rather than higher numbers. i.e. x200 feet seems much more popular than x800 feet.
~~~~
Working from the reasoning that the risk of collision increases with the square of traffic density, it's possible to calculate a theoretical optimum if traffic were evenly spread throughout ten possible cruising levels offered as options... which begs the question 'why only ten options'. It's a good question to which I don't have a ready answer, except to point out that most of us are limited in our altitude-keeping ability. I realistically keep within about +/- 100 feet most of the time. Perhaps some of you do much better. However, there's no advantage to aiming to fly at 10 foot increments if we're straying so far from our allotted altitude. Really I picked hundreds of feet simply because that's how we normally report our altitude. I never heard anybody say they were currently at '1775 feet'.
Whenever traffic is spread unevenly, risk of collision increases above the optimum. e.g. if pilots fly at whole 1000s of feet, the risk is 10 times greater than the optimum (assuming 100 foot increments).
Someone on another forum quite reasonably pointed out that there are advantages to flying according to quadrantal rules - though under EASA I understand that we're moving towards the semicircular rules in line with the rest of the world. If we restricted ourselves to 500 foot increments, our theoretical risk of collision would be 5 times greater than the 'optimal' risk. Of course it's mitigated by the fact that we are now less likely to meet another pilot on a reciprocal heading (though closing speeds may remain quite substantial). Does this mitigation outweigh the relatively large risk increase by confining ourselves to just two levels per 1000 feet? Quite possibly. I don't know.
Perhaps the ideal would be to combine the two techniques:
Take the last digit of the day of the month in your birthday - e.g. 16 february. Add or subtract 5 - whichever leaves you with a positive, single digit answer. You now have two single digits, 1 and 6. Fly quadrantal rules, but instead of flying at x000 feet, fly at x100 feet. Instead of flying at x500 feet, fly at x600 feet. If everybody did this you would be relatively unlikely to meet anybody going the opposite direction, and you would also spread out the traffic almost evenly over the available airspace.
~~~~~
Another point is simply that en-route mid-air collisions are vanishingly rare and not really something to worry about unduly.
So arguably fairly pointless, but I had fun anyway. Thanks to everyone who filled in the polls.
There's a children's game where if you ask someone to think of a vegetable under time pressure, almost everybody thinks of a carrot. If you ask people to think of a flower, most people think of a rose. But what if you ask people to think of a random number - for example a random altitude?
There's unfortunately surprisingly little research on this, though what little I've seen tends to suggest that we also have prototypical 'random' numbers that we prefer. And intuitively 2300 feet somehow seems more random than 2000 feet or 2500 feet, doesn't it? It does to me, though of course I know that 'randomness' occurs in the process of selecting an altitude, not in the individual result.
In my polls, I randomised the order of presentation of the altitudes to try and avoid any bias due to page position, and posted different versions of the same quizes on several fora. I'm not particularly serious about this question and I'm sure I could have carried out the questionnaires more rigorously, but it's a start. And the results were significant, both in practical and statistical terms.
This first graph shows the altitudes pilots select if they consciously choose to pick a 'random' cruising altitude.
"Many pilots try to pick 'random' cruising altitudes in order to reduce the risk of mid-air collisions.
Ignoring all other considerations, if you were trying to use this strategy on a VFR flight, which altitude would you be most likely to pick?"
182 responses - risk of collision 150% greater than the theoretical optimum.
The result is clearly biased towards '3' and '7', which I had predicted, though '2' and '8' surprised me in their popularity. '2' was only really popular on one forum - and this one, which made me wonder whether someone was possibly trying to fiddle the results. However '3' was popular on every forum where I posted the quiz, whatever order the options were presented in.
Incidentally, and without meaning to be churlish, I did not count results from the poll that got posted on this page for a number of reasons (missing entries, change in question, ability to see and discuss results prior to their final compilation). However, thanks to everyone who filled in both this poll and the one that I initially posted. I am also grateful to the mod who tried to recreate the poll on this forum - PPRUNE initially pulled it due to a breach of terms (no links to external polls) of which I was unaware, and have no quarrel with - their site, their rules.
However, not every pilot knows the 'trick'. What altitudes do people actually fly at? The best way to do this would be to sit watching mode-C returns over Wales or Scotland - somewhere where people choose altitudes on the basis of preference rather than compulsion - we often don't have much choice. However, that's not something I currently have the time to do so I posted a separate poll elsewhere asking people to pick an altitude.
"It's a fine day for a VFR flight, and you can choose to fly at whatever level you want. Pick your favourite cruising altitude, and select the whole number of hundreds-of-feet on the poll below. Thanks!"
112 responses altogether. Risk of collision 152% greater than the theoretical optimum.
The result - to my eye - shows a mix of 'random' cruising altitudes, with the big round numbers of thousands of feet, or five-hundred foot increments. Perhaps surprisingly here, people have tended towards lower numbers rather than higher numbers. i.e. x200 feet seems much more popular than x800 feet.
~~~~
Working from the reasoning that the risk of collision increases with the square of traffic density, it's possible to calculate a theoretical optimum if traffic were evenly spread throughout ten possible cruising levels offered as options... which begs the question 'why only ten options'. It's a good question to which I don't have a ready answer, except to point out that most of us are limited in our altitude-keeping ability. I realistically keep within about +/- 100 feet most of the time. Perhaps some of you do much better. However, there's no advantage to aiming to fly at 10 foot increments if we're straying so far from our allotted altitude. Really I picked hundreds of feet simply because that's how we normally report our altitude. I never heard anybody say they were currently at '1775 feet'.
Whenever traffic is spread unevenly, risk of collision increases above the optimum. e.g. if pilots fly at whole 1000s of feet, the risk is 10 times greater than the optimum (assuming 100 foot increments).
Someone on another forum quite reasonably pointed out that there are advantages to flying according to quadrantal rules - though under EASA I understand that we're moving towards the semicircular rules in line with the rest of the world. If we restricted ourselves to 500 foot increments, our theoretical risk of collision would be 5 times greater than the 'optimal' risk. Of course it's mitigated by the fact that we are now less likely to meet another pilot on a reciprocal heading (though closing speeds may remain quite substantial). Does this mitigation outweigh the relatively large risk increase by confining ourselves to just two levels per 1000 feet? Quite possibly. I don't know.
Perhaps the ideal would be to combine the two techniques:
Take the last digit of the day of the month in your birthday - e.g. 16 february. Add or subtract 5 - whichever leaves you with a positive, single digit answer. You now have two single digits, 1 and 6. Fly quadrantal rules, but instead of flying at x000 feet, fly at x100 feet. Instead of flying at x500 feet, fly at x600 feet. If everybody did this you would be relatively unlikely to meet anybody going the opposite direction, and you would also spread out the traffic almost evenly over the available airspace.
~~~~~
Another point is simply that en-route mid-air collisions are vanishingly rare and not really something to worry about unduly.
So arguably fairly pointless, but I had fun anyway. Thanks to everyone who filled in the polls.
Last edited by abgd; 25th Jun 2013 at 01:27.
