Loneliest cruising altitude

[abgd]

People often try to avoid flying at 'standard' altitudes, and a little while ago there was a discussion about the best cruising altitude if you don't want to meet people coming the other way. What altitude would you be most likely to choose? Please don't over-think the question - just pick the number that seems best to you.

Poll - EasyPolls.net (http://www.easypolls.net/poll.html?p=516f85c1e4b08064e284b0f6)

[abgd]

p.s. please don't discuss the poll until results are in to avoid biasing other respondents. I'll report them on this thread in due course.

[phiggsbroadband]

Hi abgd... You have missed out the thousands levels in the poll. So I did not vote.

[darkroomsource]

X000 is there.

[abgd]

@phiggsbroadband -

Do you mean I missed out the whole numbers of thousands? e.g. xn00, (x+1)n00, (x+2)n00?

I did so intentionally - it would have made the poll too big if I had included them. Also, we all have different aircraft with different capabilities, and we may fly with very different goals. Including the thousands would just muddy the waters.

[phiggsbroadband]

Hi abgd, it's just that my favourite level is somewhere between 7000ft and 9000ft... just above the tops if I can get there and maintain vfr. It is also the most efficient height for my normally aspirated plane. Vis is usually 50+nm and I have yet to see any other aircraft heading towards me.

By choosing just the hundreds, you can have a dozen or more planes stacked separately by 1000ft each, and yet they would never bump into each other.

There is also a factor of the wind gradient to take into account; There is little point, on some days, of climbing into very strong headwinds, when the lower winds are backed and calmer.

But where in GB can you put a straight line of over 100nm on your Chart at 9000ft, without bumping into Controlled Air Space?

[astir 8]

Don't forget on a day with good thermals, in class G there will be a raft of gliders as high as they can get, and definitely not operating at set altitude intervals. But they'll be below cloudbase and not above the tops.

I always felt that the Grob Tutor/glider midair near Didcot the other year may have partly resulted from the Grob pilot thinking that he was at a "safe" height for some aerobatics

[sevenstrokeroll]

in the US there are high altitude ATC sectors (enroute) and low altitude sectors....ifyou fly shorter routes (say less than 500 miles) the secret to jet direct is to fly at the highest altitude in the low altitude sectors (22-23) and getting direct!~

[wrecker]

Many years ago I remember being in London Mil when a rather exotic callsign was being worked. He did not have mode charlie on but said he would select mode C when descending through FL600 which he duly did. I dont think he was worried about conflicting traffic in the cruise!

[snapper1]

Astir 8.
And don't forget that on a day with good wave, gliders will be happily cruising at 9,000-10,000ft and well above the tops.

[Silvaire1]

And don't forget that on a day with good wave, gliders will be happily cruising at 9,000-10,000ft and well above the tops

Can you imagine how cool it was when the very first time I'd ever been in a glider, we found ourselves at 17,800 ft, watching CAT passing under us perhaps a mile lower, on their approach? This was in a Duo Discuss belonging to a friend....

OBTW and on-topic the 17,800 ft altitude was completely deserted.

[phiggsbroadband]

Hi, just as a bit of an exercise I tried to find some routes on the Southern UK Chart where you could Cruise VFR for 100nm at FL090. There seem to be just two...

Coal Aston to Ipswich/Monewden, 119nm.
and
Lands End to Swansea, 109nm.

These would require you to reach and descend from FL090 in 19 and 9nm resp.

There may be longer routes from Lands End into Caernarfon, but this requires a lot of Danger Zone and Water Crossing.

[abgd]

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?"


http://farm8.staticflickr.com/7348/9110412361_939b91f686_z.jpg

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, 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.

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!"


http://farm4.staticflickr.com/3773/9110412241_c519f866fc_z.jpg

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.

[Gertrude the Wombat]

I never heard anybody say they were currently at '1775 feet'.

I did once - I was asked to "report your altitude" by the controller of the airspace I was flying beneath. Whoops! - I'd crept up a bit ... but was still 25' below the controlled airspace. So I read out exactly what the altimeter said, 2475' or whatever it was.

[AdamFrisch]

Now this example of is valid in the the US mostly, but anything above oxygen level (12500ft), but below the flight levels (FL180) is pretty much empty. I went back at 14500ft for close to 5hrs the other day and didn't have a single traffic call from ATC. I assume it's similar in Europe and I seem to recall Peter337 always mentioning that's the case on his Euro travels.

[darkroomsource]

Actually, I believe your assumptions are incorrect.
If we are all flying VFR, then there's no issue, and you may be correct, however when you have planes travelling IFR then you have issues, because your idea of picking some random flight level means that you could be choosing one that is head-on with another plane. Head-on is much harder to see and avoid than parallel.

[fisbangwollop]

Most of the traffic I talk to in my day job at "Scottish Information" sits in the not above 3500ft bracket...and on nice days there is an awful lot of traffic around at those levels......very rare indeed to talk to much above those levels!......so the moral of the story is keep a VERY good look out and if your not talking to ATC why not consider giving us a call as I am only able to advise traffic of other traffic if you have told me your there in the first place! :cool::cool::cool:

[BabyBear]

Ironically, of course, your study will no longer be valid as the results will change the altitudes chosen. Me I'm off for the 900s from now on!;)

Interesting study abgd, thanks for sharing.:ok:

BB

[On Track]

(Shaking my head in utter disbelief)

What exactly do you people think is the purpose of standard cruising altitudes, if not to separate traffic on reciprocal tracks?

[BabyBear]

standard cruising altitudes

What's that then?

BB

[Capot]

Am I missing something? I read this whole thread expecting to see the words "quadrantal rule" for the UK, or what I recall as "semi-circular rule" for Europe/USA, I think, or something like that.

NOSE was the mnemonic; North Odd South Even...

Wasn't/isn't that supposed to contribute to separation under VFR? Is that what this thread is about?

Perhaps I've got it all wrong, not for the first time.

[abgd]

Quadrantal rules are about to be replaced by the semicircular rule in the uk, and aren't obligatory for vfr.

A lot of UK airspace is so busy that it's not practical to comply with them whilst remaining out of controlled airspace and cloud, retaining the ability to glide clear of built-up areas, and whilst making the frequent heading changes required to pick your way through the airspace. On another forum the consensus appeared to be that few people even attempt to follow them here.

[mm_flynn]

(Shaking my head in utter disbelief)What exactly do you people think is the purpose of standard cruising altitudes, if not to separate traffic on reciprocal tracks?I think the standard cruising levels are supposed to provide the illusion of separation. ;-)

While it is certainly true a direct head on is very difficult to see in the limited time available, a 90 degree crossing only provides 40% more time to capture the traffic, is just as intrinsically difficult to spot and it is more likely to be obscured by a window pillar believe there is robust analysis that show the odds of collision are significantly reduced if all aircraft adopted random altitudes rather than standard cruising levels. Of course flying the 'wrong way' at a standard level substantially increases the odds of collision.

[On Track]

Babybear, in most of the world it works this way:


IFR:

Odd thousands on easterly tracks
Even thousands on westerly tracks

VFR:

Odd thousands plus 500 on easterly tracks
Even thousands plus 500 on westerly tracks

I know there are variations in some countries, e.g. New Zealand uses northerly/southerly rather than easterly/westerly.

In Australia and NZ standard altitudes or flight levels are mandatory for all aircraft above a specified altitude or height AGL.

[abgd]

@mm_flynn

If I understand correctly, there is robust analysis to show that random cruising altitudes are safer than the quadrantal/semicircular rule? I'd be really interested to read more.

[mm_flynn]

It was a piece of work undetaken I believe by bookworm (or someone he was engaged with). I will see if I can get a copy.

If you think about it, it is very logical with a couple of assumptions

1 - seeing a motionless target is no easier off to the side (and maybe harder) than in front of you.
2 - cruising levels reduce the possible altitude separations to 2 or 4 per thousand feet, vs random altitudes which have at least 10 levels that would be a miss

Therefore cruising levels reduce significantly the odds of two aircraft approaching head to head at high relative speed, but due to compressing all aircraft into either 2 or 4 levels per thousand feet significantly increase the odds of you being at the level of the intersecting traffic.

[BabyBear]

Thanks On Track, I agree that is as written, however to be standard altitudes the quadrantal 'rule' would have to be adhered to. The consequence of it not being, I would argue, is that there is no such thing as standard altitudes that are flown.

Other than during doing PPL and coming across it on forums I don't recall anyone ever discussing it during flight, or planning.

If your post was demonstrating frustration at the fact the suggested standards are not adhered to that is another argument.

BB

[On Track]

Well it appears that in Britain there's no requirement to fly standard altitudes if VFR.

In any event quadrantal is not an international standard, just the British way of doing things. I know that in Australia it was discarded a long time ago and replaced by the ICAO system.

I don't want to seem jingoistic or offensive in any way, but the more time I spend on this forum the more convinced I become that British aviation is a bizarre little realm of its own, quite remote from the rest of the world.

I can assure you that in other countries there are standard altitudes, and they are flown.

[Agaricus bisporus]

It is certainly true that Britain has its idiosyncracies rules wise, but the oddest and most disturbing thing about the British mentality is the way rank amateurs in all fields seem to think, as soon as they've reached a certain level, that they know more about the subject than everyone who've gone before them (especially the experts) and proceed to reinvent rules to suit themselves and then have the gall to justify this anarchy on the basis the "it seems more sensible to me", despite no more scientific knowledge or experience than the absolute minimum in the field.

It is not an impressive commendable attribute.

:ugh:

[abgd]

mm_flynn:

I agree qualitatively with your points, but would love to read a quantitative analysis. If you're spread out over 2 levels per 1000 feet your chances of collision are 5 times greater than if you're spread randomly out over 10. However, as we're probably not spread out randomly they may be only about 3 times greater.

Papers I've read seem to suggest that we see such a small proportion of the traffic out there, that keeping a good lookout is almost irrelevant to collision risk whilst en-route (flying the pattern, thermaling, ridge soaring it's a different matter). If you see (for the sake of argument) 10% of potentially conflicting traffic when flying at a 'random' altitude but 20% when flying a quadrantal level, this in itself is not going to materially reduce your chances of avoiding a collision.

However, if everybody's flying more or less the same direction albeit at differing speeds, then this will reduce your chances of colliding quite substantially - maybe even by a factor of 3. It seems to me that this question should be 'answerable' by analysis or simulation except you would also need to characterise the spread of airspeeds and feed it in to the model, which would be hard to do.

[On Track]

Thank you Agaricus -- an interesting observation.

[abgd]

I can assure you that in other countries there are standard altitudes, and they are flown.

I see you're from New Zealand/Australia. I confess I haven't seen an airspace map of Australia, but I suspect it looks rather different from an UK one.

I don't know if you'd be able to get hold of an UK VFR map for Southern England, but if you can then I suggest you try to plot a route from Redhill to Redhill, circumnavigating London, remaining clear of cloud and airspace, and staying high enough to satisfy the 'glide clear' rule. Then you'll appreciate why so few people here try to use them. Things are a little better away from London, but even then I rarely find myself able to follow the quadrantal rules for one reason or another.

the oddest and most disturbing thing about the British mentality is the way rank amateurs in all fields seem to think, as soon as they've reached a certain level, that they know more about the subject than everyone who've gone before them (especially the experts) and proceed to reinvent rules to suit themselves and then have the gall to justify this anarchy on the basis the "it seems more sensible to me", despite no more scientific knowledge or experience than the absolute minimum in the field.

I'm uncertain whether that's aimed at me. This is, it's true an American/UK attribute. There's an effect called the Dunning-Kruger effect that states that incompetent people are generally unaware of their incompetence:

http://roland.pri.ee/doktor/papers/Unskilled%20and%20Unaware%20of%20It%20-%20psp7761121.pdf

However, when people have tried to replicate it in other more heirarchical societies, such as in Germany or Korea IIRC, they fail to find the effect or find that it is much weaker. The corollary of course is that it's a big problem if received wisdom from authority figures is never questioned.

I did some reading around before doing this 'study' and found that whilst there was unsurprisingly a lot of literature relating to preventing collisions between airliners, there wasn't a great deal on preventing collisions between GA flying VFR. That may well be simply because I couldn't find it because it was swamped in the commercial stuff, or possibly because it dated back to WWII and hasn't been digitised yet.

Incidentally, I have a PhD, albeit not in a field obviously related to aviation. I am aware that I'm outside of home turf, but I as far as I'm aware my finding (that asked to pick a random altitude within a given range, over 25% of people choose the same one) is both original and pertinent to discussion about whether standard or randomly chosen cruising levels are effective. I did it for fun, and within the constraints of the resources available to me I think I managed to be reasonably diligent. I don't take my result too seriously and I'm open to suggestions as to how my results might be interpreted differently, or how my methodology might have been improved.

You, on the other hand, haven't contributed anything constructive, merely pointing out that unspecified authority figures might differ, presumably on the basis of unreferenced evidence.

[Agaricus bisporus]

Then of course there is the self-evident fact that there is no significant collision problem in UK VFR airspace - at least, the statistics don't indicate one, but one suspects that the majority of VFR pilots are at least subliminally aware that they keep a grossly inadequate lookout and this is a means of compensating.

What it results in is that no one - least of all those who do fly responsibly, have any idea what direction a conflict is likely to appear from which actually makes the situation less safe, not more. But as chaos is the inevitable result of anarchy why would anyone be surprised?

A similar thing is happening in the boating world where some idiots are replacing their steady, all round anchor light with a strobe apparently on the spurious reasoning, if I may so misuse that word, that it "makes them more visible". Spurious because there is no evidence of yachts at anchor being run into by anything, let alone anything that might do them any harm (yachts very, very seldom anchor in other than shallow and secluded water where big ships simply can't go) and the added problem that a strobe turns them, in the eyes of a proper navigator, into a Northerly Cardinal, a rather important buoy marking a dangerous obstruction and is this highly misleading. The fact that some show this strobe while alongside a jetty or on a mooring buoy where no such lights are appropriate suggets that this is at least partly a bizarre form of showing off/attention getting by demonstrating a wilful disregard of well understood rules adhered to by the majority.
If so it seems to indicate character traits that are incompatible with the activity in question.

[abgd]

Then of course there is the self-evident fact that there is no significant collision problem in UK VFR airspace - at least, the statistics don't indicate one, but one suspects that the majority of VFR pilots are at least subliminally aware that they keep a grossly inadequate lookout and this is a means of compensating. I'm not quite sure what you're advocating here.

The evidence is that pilots simply aren't physiologically capable of maintaining a good lookout, including professional pilots. e.g. www.atsb.gov.au/media/32918/limit_see_avoid.pdf (http://www.atsb.gov.au/media/32918/limit_see_avoid.pdf). The current low rate of collisions probably has more to do with the low chances of a collision occurring, and there's only limited scope for further reducing accident rates by improving pilot lookout.

That isn't to say that pilots shouldn't be looking out of the window, or even work on improving their skills, but I put more faith in strategies such as random cruising altitudes or for that matter the quadrantal/semicircular rules.

What it results in is that no one - least of all those who do fly responsibly, have any idea what direction a conflict is likely to appear from which actually makes the situation less safe, not more. But as chaos is the inevitable result of anarchy why would anyone be surprised?The AIP only suggests (and does not mandate) that quadrantal rules be adhered to. And this only above the transition altitude. In practice this means that in much of the busiest airspace (London TMA, Manchester and Luton/Stanstead corridors) you're not allowed to fly at an altitude that would enable you follow the quadrantal rules.

So how to cope with this government-mandated 'anarchy'? If random cruising levels are one way to reduce the risk of collisions, then it's perfectly legitimate to discuss them.

It's also quite reasonable to discuss whether random cruising levels may be safer than quadrantal cruising levels. One other poster has mentioned work showing that they may actually be less safe. I remained on the fence. But matters are serious if we live in a country where we may not even question the justification of rules (or in this case, a recommendation).

[mm_flynn]

Then of course there is the self-evident fact that there is no significant collision problem in UK VFR airspace ....


What it results in is that no one - least of all those who do fly responsibly, have any idea what direction a conflict is likely to appear from which actually makes the situation less safe, not more. But as chaos is the inevitable result of anarchy why would anyone be surprised?

I think the first point is a bit mixed. I believe the UK trend is that 10% of all GA fatalities are a result of mid air collisions, which I would have thought is significant. On the other hand, I believe almost every incident is below 3000 feet so quite rightly there is no significant risk above 3000 feet where either the low average traffic density or standard cruising levels (or something else) mitigates the risk.

Of the limited collisions that occur, most are either a faster aircraft running over, decending onto or climbing into another aircraft or intercepts of roughly 90 degrees (probably associated with some being in or near the traffic pattern).

At higher levels, the quadrantial rule certainly has the advantage of restricting the areas a slow aircaft needs to look for avoiding action (assuming you can't do much to avoid being run down). High relative speed aircraft (i.e. 2x the conflicting traffic) can't really hit anything that isn't pretty much +/- 30 degrees of the track projection regardless of the other aircraft's heading. In that sense quadrantial gives 1/2 the risk density as semicircular and about 2.5 times random 100 foot levels.

On the other hand, in traditional IFR, or VFR beacon to beacon. You have a very limited number of likely tracks and many of them will be opposite direction. In this case the odds of A to B and B to A traffic being on exactly opposite tracks is high and standard crusing levels provide an invaluable separation technique (which is why they were developed).

I suspect the case for random levels is very specific to UK OCAS where random routing is the norm and the number of available standard levels is very restricted. It also ackowledges a fact (which class E and VFR IFR class D separation rules do not), that the human eye can not really see an object on a steady collision vector unless we have a good long time to identify it (and good long time is more about 60 knot relative speed vs the potential 300-400 knot closing speeds of higher performance machines).