By Johan Grauers
This has changed, and this post is no longer accurate. Since I find removing something several years old slightly strange I will let the original post remain below, but it is no longer correct!
There has been a trend recently where a poor understanding of VMC minimas is shown. This is posted as a general reminder to all controllers of what the VMC minimas are and how to apply them. This post is based on MATS part 1,sec 1, chap 2, §4.
The basic VMC minimas are affected by two things:
The time, they are different at Day and Night
The class of Airspace.
Further there are a few exceptions when the minimas can be reduced further, these depend on both the above and also on the aircraft speed and type (fixed wing or helicopter).
At and above FL100 there is only one minima which applies at day and night in all UK airspace.
Minimum flight visibility 8KM, minimum distance from cloud 1500 meters horizontally and 1000 feet vertically.
Below FL100 there is a slightly different basic minima (that apply to all aircraft), it applies day and night, in all UK airspace:
Minimim flight visbility, 5KM, minimum distance from cloud 1500 meters horizontally and 1000 feet vertically.
The above two are basic minimas, they can be used by any aircraft type at any speed. Then there are a few special cases where you can reduce the minimas further, these are as follows:
Controlled airspace (UK class C, D and E)
For aircraft other than helicopters, flying 140 knots IAS or less, 5KM flight visibility, Clear of Cloud with Surface in Sight (CCSIS).
For helictopers, Flight vis 1500 meters, CCSIS
Uncontrolled airspace (class F and G)
At or below 3000 feet AMSL, CCSIS, 5km flight visibility
For aircraft other than helicopters, flying 140 knots IAS or less, 1500m flight visibility, CCSIS
Helicopters, 1500m flight visibility, CCSIS, with a speed which having regard to the visibility, is reasonable.
Inside CAS at night there is no reduction of VMC minima, you have to fulfill the basic minima.
In class F and G airspace, at or below 3000 feet AMSL
For aircraft other than helicopters, CCSIS, 5km visibility
For helicopters, CCSIS, with a speed which having regard to the visibility, is reasonable.
The above are all the options for VMC minimas. Sometimes you can easily tell if it's VMC (ie, CAVOK is always going to be VMC), but when the weather gets marginal it can get more complicated. I suggest that you always refer to the table in the MATS part 1 if you are at all unsure.
In aviation night is defined as half an hour after sunset to half an hour before sunrise, both times inclusive. Ie if sunset is 2010 and sunrise is 0542 night is from 2040 (2010+30 minutes) to 0512 (0542-30 minutes).
You will notice that all VMC minimas refer to "flight visibility", this is the visibility from the flight deck. It is assessed by the commander. However the MATS pt 1 says:
"For fixed- wing and helicopter flights wishing to operate under VFR to or from an aerodrome, or enter the aerodrome traffic zone, or aerodrome traffic circuit in Class D airspace the ground visibility shall be used "
This means that any traffic coming close to an airport inside class D will have their VMC minima determined by using the metreport visibility (the number in the METAR). This means that regardless of their own assesment the ATC reported visibility overrides them, but only when operating inside the ATZ. Traffic outside of this area may self assess visibility even though they are inside CAS (for example, a zone transit passing the corner of a CTR).
If a METAR gives two visibility values the lowest one should always be used.
As controllers then, our responsibility lies with knowing if we may legally issue a VFR clearance or not.
We need to do this in two cases, traffic departing inside of CAS, or traffic entering CAS (for whatever reason).
We still start looking at a departure, and again begin with the time of day.
If it is daytime we look at the visibility. If it is 5km or more, the visibility is fine.
We then look at the cloud.
When we look at the cloud what we need to determine is if the pilot can stay at least 1000 feet away from cloud, while also complying with the low flying rules. This is a set of rules which I won't go into here but the one we're interested in is the 500 foot rule. This says that an aircraft has to be at least 500 feet away from obstacles, vessles, vehicles, persons, and structures.
Since the METAR gives cloud as a figure above aerodrome level the magic number is 1500, as this allows the pilot to fly at 500 above ground, and thereby obey both the 500 foot rule and staying VMC.
Therefore if the cloud ceiling (at least half the sky, ie BKN or OVC) is 1500 or more, it is VMC and we can issue the clearance.
If the cloud ceiling is lower than 1400 feet, we have to ask the pilot to ensure he can fly VFR.
"GCD, visibility 9 kilometers, clouds broken 1400 feet, what clearance do you require?"
It is up to the pilot to ensure that he can accept VFR (in this case if he will be able to fly 140 IAS or less).
If we conclude it is IMC (for example, visibility 4500 meters in haze, fixed wing wanting VFR) we do the same thing:
"GCD visibility 4500 meters in haze, no significant cloud, what clearance do you require?"
If the pilot requests a VFR clearance we need to inform him that we can't issue one
"GCD, I am unable to issue a VFR clearance due to the visibility, what clearance do you require?"
At this point you hope he will decide to go back to the flying club, but it is up to the pilot to request an alternative SVFR or IFR clearance if they want to and can accept it.
I will get back to the issue of pilots with their own weather further down.
The other version is for inbounds, this is the same thing again. Check the weather and determine if it is VMC, IMC or marginal (ie depends on aircraft type, day or night etc).
Then it's the same thing again, if need be we inform the pilot of the weather and ask them what clearance they need.
The slightly more complicated thing is that for traffic entering controlled airspace, but not the ATZ, the pilots actual flight conditions will be used. This means that if the pilot determines it is VMC. This means that they can assess the conditions and decide that they can accept VFR, if so ATC will issue the clearance.
So what happens if we issue a clearance and the conditions change? Well it depends on the circumstances.
If an aircraft has either departed and is airborne, or has already entered CAS, then the original clearance remains valid. ATC will inform the pilot of the changes and ask them if they wish to remain VFR; if they answer yes then they may continue.
If traffic has not departed or entered CAS the clearance is void and ATC have to cancel it. Then the above procedure restarts with ATC giving the pilot conditions and asking what clearance they need.
Finally on a related topic, I want to mention SVFR.
SVFR is a hybrid between IFR and VFR. It allows a pilot to fly VFR even though it is IMC. There are limits even for SVFR which can be found in the mats part 1 sec 1 chap 2 §8 (some units also publish more stringent criteria in their part 2).
The main difference is that SVFR although it is VFR can not be deemed to have the same option of separating against other traffic through see and avoid. Therefore we have to separate SVFR from all SVFR and IFR flights.
This catches a lot of controllers out as they are unfamiliar with what consitutes separation in these matters. Basically, this can be:
Vertical separation (ie 1000 feet)
Radar separation (3 or 5 miles depending on the local instructions)
Geographical separation (which always has to be specified in the vMATS)
In rare cases, longitudinal separation (ie MATS pt 1 or part 2 speed tables), this will only be used if you have SVFR departures on set routes.
Reduced separation in the vicinity of an aerodrome (difficult to apply in SVFR conditions)
Most commonly you will use a combination of vertical and radar separation. This means that approach radar have to ensure at least 1000 feet vertical or radar separation.
For example, if you have IFR inbounds and an inbound SVFR you have to hold the SVFR at least 3 (or 5) miles away from the ILS and any go around tracks to ensure radar separation. When you bring it in to land you still have to ensure radar separation until the aircraft ahead has landed.
Also remember that SVFR is flying under very challenging conditions, they may have to otice dodge clouds or manoeuvre to remain legal with late or no notice to ATC. Therefore always allow leeway to SVFR traffic to give a buffer for the pilot.
As always I would be happy to answer any questions on the above or provide clarfication as needed.
It is also worth noting that on vatsim we can't force anyone to use a particular time or weather setting, therefore you may get pilots requesting VFR because they have set their pilot client to CAVOK. In this case the suggestion is that you issue a VFR clearance but attempt to treat them as SVFR for anyone else, as the other pilots can't see them and therefore can't see and avoid. However that is up to the individual controllers to decide. We can however not refuse a VFR clearance if a pilot chooses to simulate conditions which permit VFR flight.
By Stephen Fullam
Hello, I have completed all my theory training on Moodle but I can not sit the exam. I believe this is because I have not attended the OBS_PT1 group session. Do I have to attend this session before I can start my mentoring/training? Thank you
By Rhys Warner-Smith
Hello everybody, I hope you’re all well.
Ive had a little look through the forums and I can’t quite find the answer to this specific question;
Many years ago I was an active S2 on the network, but due to the cumulative effects of a new job, new house and life in general I was pulled away from my hobby. Now I’m back in a position where I wish to dedicate my time to resuming my studies and getting back into the community. My question is this: where do I start? It’s all changed significantly since I was last controlling and I’m a little lost. I was wondering if someone could help me back into it all?
Thank you all, it is much appreciated. ?.