Primarily for general aviation discussion, but other aviation topics are also welcome.
By Ibra
#1894182
David Wood wrote:I don't think I've ever flown by the semi-circular rule when inside controlled airspace. Why would one do that when being, by definition, controlled?


Maybe he meant in the swaths of Echo (controlled) airspace bellow FL100? in Germany, Spain, France…where VFR flies uncontrolled semi-circular on FLX5 by choice while IFR flies controlled levels assigned by ATC to FLX0, just let’s hope it’s not a layered stratus day with IMC layer at FLX5 and VMC between layers at FLX0 otherwise that allocation will not work (I had to please French Echo TMA controller by climbing +2000ft into clouds, after I heard something like “negative, we are between the layers” :lol: )

It’s the only case where I think VFR semi-circular makes sense? but hey, I never saw any Echo in UK (maybe once or twice in Scotland while gliding),

PS: the topic reminds me of “IR pilot in IFR aircraft getting caught VFR in IMC OCAS”, how that could ever happen? supposedly some IFR pilots claim you need ATC clearance to switch VFR/IFR and fly level/heading in clouds while being uncontrolled in Golf, heck where this comes from? it’s only in Echo where VFR/IFR switch require positive ATC clearance…in Golf it will be daft to sneak between clouds or avoid them by going down to terrain (if one can fly in clouds), besides we all know how a cloud look like? it’s part of MET exams and we all know it’s legal and safe to fly in Golf if one is IFR rated/equipped: this is not just UK Golf, it’s also the case in ICAO Golf, SERA Golf and even US Golf as per 14 CFR § 91.173

If the pilot can’t distinguish Echo from Golf in the map, they better stay away !
Last edited by Ibra on Tue Jan 18, 2022 2:22 pm, edited 24 times in total.
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By lobstaboy
#1894184
Aerials wrote:A bit of a thread diversion, this! I use QRM as the local electrical interference and QRN as atmospheric noise . Those are better defined here. https://en.wikipedia.org/wiki/Q_code


Hmmmm? Not sure about that in terms of common usage in amateur radio. QRN is any random electric interference, QRM is specifically interference from another transmitter on or close to your frequency.
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By BoeingBoy
#1894195
Sometimes I read the replies to some of my posts and realise that in the presence of such high intelligence I can only bow in sheer reverence and consider myself lucky to be allowed on these hallowed pages! :roll: :lol:
Last edited by BoeingBoy on Tue Jan 18, 2022 6:11 pm, edited 1 time in total.
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By David Wood
FLYER Club Member  FLYER Club Member
#1894215
I'm not sure if that was aimed at me @BoeingBoy and if I've offended you then I'm truly sorry. I'm just puzzled by the underpinning logic behind the semi-circular rule and genuinely interested to find out why it exists. It seems to me to have an effect that is the inverse of that desired.
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By kanga
#1894253
<nostalgia :oops: >

One of the UK holiday airlines for which I worked in university Long Vacations (how I paid for my PPL :) ) in the '60s had a lot of business taking UK holidaymakers on 'package' holidays in Spain. These could be massively disrupted by short notice ATCO strikes in France. Our airline (with insurer's permission, I gathered) used sometimes to circumvent these by flying to Cornwall and then flying 'procedurally' to Northern Spain outside French 'controlled' airspace, but in contact with French military radar controllers (who were, apparently, very helpful), and analogously back; presumably on semicircular FLs. Some other UK airlines did the same. It took a bit longer and more fuel, but passengers (and their holiday companies) liked it. One of our Captains, formerly on FAA Buccaneers, wondered what the problem was :)

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By riverrock
FLYER Club Member  FLYER Club Member
#1894256
David Wood wrote:I'm just puzzled by the underpinning logic behind the semi-circular rule and genuinely interested to find out why it exists. It seems to me to have an effect that is the inverse of that desired.

Semi-circular rule stops two aircraft, travelling accurately between same two waypoints in opposite directions, having a head on collision.
If you are within controlled airspace, ATC shouldn't let that happen anyway.

If you are travelling in the same direction as someone else, the closing speed will be much slower, so collision risk is less.
If you are travelling to a different waypoint, the risk of crossing is lower.

For max efficiency, the same type of aircraft is likely to want to fly at the same height, so the risk would be increased if you didn't have that separation.
Last edited by riverrock on Tue Jan 18, 2022 9:16 pm, edited 1 time in total.
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By David Wood
FLYER Club Member  FLYER Club Member
#1894258
riverrock wrote:
David Wood wrote:I'm just puzzled by the underpinning logic behind the semi-circular rule and genuinely interested to find out why it exists. It seems to me to have an effect that is the inverse of that desired.

Semi-circular rule stops two aircraft, travelling accurately between two waypoints, having a head on collision.


Errr... Aeroplane 1 is tracking 355, Aeroplane 2 is tracking 185. They are (correctly) on the same IFR cruising levels (FL60, 80 etc). Yet they are potentially 10 degrees from head-on.
By Ibra
#1894307
riverrock wrote:I shall edit the above to say "same two waypoints in opposite directions"


Yes the whole thing of semi-circular rule only make sense on A to B routes with prescribed tracks: the likelihood of collision on Airway is 1/100 with VOR navigation, 1/10 on 2×DME navigation and probably 1/2 with GPS/SBAS navigation (say 50m airframe), you need that +/-1000ft on opposite tracks from semi-circular outside controlled airspace (ATC don't give clearances or separation in Golf but they may control Golf Airways on in/out, you need to be bloody sure ATC or thr other PIC is not cooking it, this has happened dozens of times already, all of them were human errors: clearance of two aircraft on same route & level or one pilot flying the wrong level), 10m tiny airframe may help but it's not what will save the day !

Off-Airway, if routes are random, levels has to be random and vice versa, I can't dig the maths behind but something seems to work that way...