Semicircular level scheme OCAS -- safety case
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Semicircular level scheme OCAS -- safety case
In aiding collision avoidance, the semicircular rule outside controlled airspace (and the quadrantal rule in the UK) are a trade off. On the one hand, mean closing speeds are in principle reduced by the directionality, on the other, compelling aircraft to fly at multiples of 500 ft reduces the vertical separation that would be afforded by random selection of levels.
Has anyone seen a safety case, or research (e.g. mathematical modelling) conducted, to test the safety value of cruising level schemes?
Has anyone seen a safety case, or research (e.g. mathematical modelling) conducted, to test the safety value of cruising level schemes?
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<<compelling aircraft to fly at multiples of 500 ft reduces the vertical separation that would be afforded by random selection of levels.>>
Unless, perhaps, two pilots randomly choose the same level whilst flying towards each other.. The military routinely used 500 ft separation in civil CAS, but that was when I was working and they've probably changed it now?
Unless, perhaps, two pilots randomly choose the same level whilst flying towards each other.. The military routinely used 500 ft separation in civil CAS, but that was when I was working and they've probably changed it now?
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Semicircular rule is simply a method for organising traffic in a logical manner. While in general west even, east odd applies, it is possible in controlled airspace to use a north - south organisation.
There can never be a safety case for a system that can have two aircraft which comply with the system being in the same place at the same level at the same time.
The semicircular system only provides limited (500ft) separation between VFR flights and IFR flights while they are in the cruise because the IFR flights fly at whole levels and the VFR ones a those levels +500ft.
Pick a point A and imagine how many ways that aircraft can comply with the semicircular rule but both end up over A at the same time and the same level and you will see that it is a very long list.
Same track different speed.
Converging tracks
Crossing tracks on same hemisphere i.e. 170 track and 080 track
opposite direction tracks - 179 track and 000 Track
All of which can be fully compliant with semicircular rule but arive aqt the same point at the same level at the same time = not safe.
The UK quadrantal rule does reduce the probability of IFR-IFR collision slightly by further sectorisation but at the same time introduces the VFR flight at the same level at the IFR flight or even (since the quadrantal levels are optional for VFR) head on.
Therefore I would find it hard to say that making converging, opposite and crossing traffic fly at the same level in IMC would make it more safe than a natural random scatter if pilots could pick random odd levels.
There can never be a safety case for a system that can have two aircraft which comply with the system being in the same place at the same level at the same time.
The semicircular system only provides limited (500ft) separation between VFR flights and IFR flights while they are in the cruise because the IFR flights fly at whole levels and the VFR ones a those levels +500ft.
Pick a point A and imagine how many ways that aircraft can comply with the semicircular rule but both end up over A at the same time and the same level and you will see that it is a very long list.
Same track different speed.
Converging tracks
Crossing tracks on same hemisphere i.e. 170 track and 080 track
opposite direction tracks - 179 track and 000 Track
All of which can be fully compliant with semicircular rule but arive aqt the same point at the same level at the same time = not safe.
The UK quadrantal rule does reduce the probability of IFR-IFR collision slightly by further sectorisation but at the same time introduces the VFR flight at the same level at the IFR flight or even (since the quadrantal levels are optional for VFR) head on.
Therefore I would find it hard to say that making converging, opposite and crossing traffic fly at the same level in IMC would make it more safe than a natural random scatter if pilots could pick random odd levels.
The semicircular (in UK, quadrantal) system outside CAS has been in use since way before anybody dreamed up the concept of a safety case.
Bearing in mind that the subject is uncontrolled airspace, surely you don't need a safety case to demonstrate that it is a Good Idea to have opposite direction traffic in level flight at a different level?
2 s
Bearing in mind that the subject is uncontrolled airspace, surely you don't need a safety case to demonstrate that it is a Good Idea to have opposite direction traffic in level flight at a different level?
2 s
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There is also traffic climbing and descending through the levels to add to the mix 
I would guess cruising level schemes is only slightly safer than a random selection of levels, and has no advantage once traffic density reaches a certain point.
I would guess cruising level schemes is only slightly safer than a random selection of levels, and has no advantage once traffic density reaches a certain point.
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surely you don't need a safety case to demonstrate that it is a Good Idea to have opposite direction traffic in level flight at a different level?
The semi-circular rule will only provide "automatic" separation on an airway / ATS route which is relatively short and thus by definition within controlled airsapce.
ie your airway is 179 degrees one way and 359 degrees the other and with no other crossing routes, all traffic travelling west will be at even levels and all travelling east will be at odd levels and thus no head to head at the same level.
Take that semi circular rule outside controlled airspace where aircraft have no requirement to fly on fixed routes and straight away it makes no difference since while the aircraft at FL80 track 359 will not hit the opposite direction track 179 FL90, it will hit all the others at FL80 on the track 180, 200, 270 300 and 359 (but 300 knots slower aircraft) that will all pass over head ABC at the exact same time with GPS precision.
Bookworm is asking us to come up with a safety case for a system that can cause more than 1 aircraft to occupy the same 10m squared box of airspace with no possible intervention to pre-warn or prevent such a situation!!
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There can never be a safety case for a system that can have two aircraft which comply with the system being in the same place at the same level at the same time.
Therefore I would find it hard to say that making converging, opposite and crossing traffic fly at the same level in IMC would make it more safe than a natural random scatter if pilots could pick random odd levels.
On the Use of Height Rules in Off-route Airspace R L Ford 1983 (RSRE, now QinetiQ). I'm too mean to buy the article. The abstract says:
The standard height rules applied in off-route airspace are examined to assess the degree of intrinsic safety they provide, i.e. the reduction of conflicts without action being taken by pilots or ATC. The yardstick used is the conflict rate which would obtain if the aircraft were uniformly randomly distributed in the height dimension and flying straight and level on uniformly randomly distributed tracks. It is shown that the application of the standard rules can lead to a reduction in intrinsic safety unless significant height-keeping errors are present. An alternative height rule apparently having more desirable characteristics is examined on the same basis.
It seems that Robert Ford is of a similar opinion. It makes me wonder why ICAO continues to use the semicircular rule, when simply deleting it would apparently make us safer.
Thanks for the suggestion Spitoon.
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It makes me wonder why ICAO continues to use the semicircular rule, when simply deleting it would apparently make us safer.
That is why I say it is a system of organising traffic which does work but it is not designed to make it safer. The only natural separation in the ICAO system is VFR-IFR during the cruise (500ft).
Finally you have to remember that as far as ICAO is concerned, in class F and G airspace, separation is not provided to all IFR flights and as such they will never attempt to try and say that anything one does in IMC in such airspace is by definition safe.
I suppose what you have to ask is the reduction in head to head conflicts between traffic on 090/270 increasing the safety level to a greather degree than the introduction of crossing conflicts 179/090.
ICAO will probably point out that in the crossing conflict the aircraft on the right was right of way and the other shall avoid it. Which is 100% correct but very hard to do in IMC whereupon ICAO says, if you are worried about that then start will controlled airspac class E.
There are so many interlaced rules that rely on each other without one realising theri significance.
eg Airspace Base should be a VFR level. Therefore, if there is IFR flight cruising in class G below the base they will be no closer than 500ft below so traffic in the system 500ft above is 1000ft above the other IFR traffic below. VFR flights can cruise at the base (in VMC) so that IFR flight 500ft above gets the standard ICAO 500ft from VFRs. Thus the systems are complimentary.
For an example outside controlled airspace let's look at the ICAO system. IFR flights above 3000ft must cruise at whole levels, VFR flights at those levels +500ft.
In the same airspace VMC minima require VFR flights to be a minimum of 1500ft above or below cloud.
You may not have realised it previously but when one combines those two rules, you will find that an IFR flight in cloud will be a minimum of 1500ft above or below any VFR traffic so they can climb or descend 1000ft as they change direction in IMC without having to think about VFR traffic.
So if you want to delete one rule there could be many consequences you did not expect.
DFC
I take your point entirely. The semicircular rule reduces the potential tracks that a conflicting aircraft could be on in level flight at the same level, but still leaves a lot of possibilities as you describe. The UK quadrantal rule reduces that potential further, but there is probably considerable merit in the concept of random selection of levels. However, just wait until the first airprox or worse with opposite direction aircraft at the same "randomly selected" level - what price the mathematical theory, then? The bottom line, of course, is that all this only relates to aircraft in level flight.
2 s
I take your point entirely. The semicircular rule reduces the potential tracks that a conflicting aircraft could be on in level flight at the same level, but still leaves a lot of possibilities as you describe. The UK quadrantal rule reduces that potential further, but there is probably considerable merit in the concept of random selection of levels. However, just wait until the first airprox or worse with opposite direction aircraft at the same "randomly selected" level - what price the mathematical theory, then? The bottom line, of course, is that all this only relates to aircraft in level flight.
2 s
The Standardised European Rules of the Air (SERA) drafted by Eurocontrol, for which the consultation period ended recently, will - if implemented as proposed - make the UK quadrant system illegal and put all Europe semicircular.
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The Standardised European Rules of the Air (SERA) drafted by Eurocontrol, for which the consultation period ended recently, will - if implemented as proposed - make the UK quadrant system illegal and put all Europe semicircular.
I did get hold of the Ford paper. As you might expect, the degree to which cruising level rules increase the rate of potential conflicts depends on both how close you consider a "conflict" to be and the height-keeping accuracy of the aircraft using them (if everyone is able to maintain level to +/- 10 ft, the semicircular rule looks very bad indeed!).
