View Full Version : Does VMC define VFR?
VMC is a necessary condition for visual flight but is it a sufficient condition? Day VFR minima below 3000ft include visibility of 5km and sight of the surface. Visibility means the air is sufficiently transparent to see something prominent: sufficient size and contrast, that is, conspicuous. What if there is little or no contrast? In sight of the surface below or 20nm away?
Is not the essence of safe visual flying below MSA seeing where you are and something where you are going: a positive visual ground reference? Is this assumed basic airmanship? It is not in CAR.
What are the implications in the mountains and polar regions? Can problems arise elsewhere?
FGD135
5th May 2012, 01:41
Ornis,
The existing VMC definition, and relationship to VFR works really well for the 99.999% of flying that occurs around the world.
The 0.001% encompasses situations such as Erebus - where, although in VMC, the pilots could not see the mountain they were about to hit.
prospector
5th May 2012, 01:52
The rulemakers obviously never envisaged a DC10 at 260kts+ below 3,000ft attempting to navigate with 5km vis.
30 seconds from sighting to impacting, in a 250 ton aircraft is not a pleasant thought.
Oktas8
5th May 2012, 05:09
Yes, VMC is obligatory for VFR and optional for IFR. But as you've said it's inadequate in mountains or unfamiliar terrain.
You can be in VMC and completely unable to see anything useful, and you can be in IMC and yet still flying visually (circling approach). VMC has nothing to do with being able to see a useful horizon, although it's highly recommended if you're VFR!
The important thing is airmanship, over and above obeying the law. That's what all pilots have in common, even though laws change between states. NZ law regarding night VFR is particularly lax, although it seems to work because very few are silly enough to push the limits.
I actually like laws that are "lax" in this sense for private flying, because it allows the pilot to do what is right in the given situation. Sometimes it's safe to be VFR in absolute bare minimum VMC - e.g. joining the circuit at your home airfield. If they tried to write law for every situation, the laws would have to be very much more restrictive in every situation, to allow for lack of currency, lack of familiarity, etc.
High capacity public transport is a different story of course.
LeadSled
5th May 2012, 05:38
The important thing is airmanship, over and above obeying the law.
Oktas8,
Unfortunately, "the authorities" don't see it that way, a defense of "airmanship" cuts no ice, and even the defense of the regulated authority
of the pilot in command is very hard to sustain, unless you have deep pockets.
Tootle pip!!
4Greens
5th May 2012, 05:40
Check out 'Night VFR'. A contradiction in terms.
Frank Arouet
5th May 2012, 06:09
'Night VFR'. A contradiction in terms
How so 4 Greens?
UnderneathTheRadar
5th May 2012, 06:47
VMC is only one element of VFR. You must also be able to navigate with reference to ground or water or via a navigation system (aids or GPS/AINS etc).
What is contradictory (might be the allusion to NVFR?) is that being VFR and navigating by instruments doesnt - so far as I can recall seeing - require you to be at LSALT. The logic being that if you're VMC, you'll see the mountain I guess. I think that VFR rules should require you to be at LSALT if not in contact with ground or water (and able to navigate with reference to).
After much reading and consideration, I think Captain Collins was doing what he thought was right & legal - being VFR and navigating by AINS - but not blameless - for not following SOPs, not cross-checking his map plot from the night before against the actual coordinates in his nav system and for making an apparently spur-of-the-minute decision. I think AirNZ and the CAA were far more culpable for allowing the situation to develop whereby it was possible for Captain Collins to do these things without a) understanding of the risks of sector whiteout, b) without fear of ramifications of operating outside of the SOPs and c) from having the goalposts (literally) shifted around him - all of which removed most layers of the swiss cheese from around him.
UTR
Day VFR above 3000ft demand 2km separation from cloud laterally, visibility 5km. It is implied that is 5km ahead of the aircraft.
Scenario. An aircraft descends from FL160 to 2000ft through a hole in the cloud layer, that is essentially a clear vertical shaft >4km deep.
Is the aircraft required to remain 5km from the cloud at all times, so the pilot can see 5km ahead?
If the SOP requires 20km visibility, does that require seeing 20km ahead as the aircraft orbits down this "mine" shaft? That is, to maintain 20km separation from cloud?
NOTE: CAR give minima, not guidelines or suggestions. If you break the rules you must notify the CAA and explain why. Really.
The original post from Ornis stated that VMC is a necessary condition for visual flight. This is not correct. Take these two examples:
1) Special VFR does not require VMC. It uses reduced minima to allow visual flight without the VMC minima.
2) An IFR flight may make a visual approach when able to continue with continuous visual reference to the ground or water but doesn't require VMC.
VMC is required for VFR but not for visual flight.
fujii. Thank you for your response. Perhaps I could ask you to explain or define the term VMC. NZ CAR uses the term VFR minima, which is 1500ft* vis uncontrolled AD or SVFR. That is, the VMC requirements change. Not that I want to get side-tracked in semantics; hitherto I have qualified any statement with the word cruise. Even so, I would have thought visual flight in IMC was an oxymoron, and it's either VMC or IMC surely. (IFR in VMC, yes.)
Anyway, would you care to tackle the last question? What does 20km visibility mean in the context of an orbiting descent through a cloud layer >4km deep.
*As Tarq57 has pointed out below, it's 1500m. Sorry.
Oktas8
6th May 2012, 03:10
Visibility is defined in meteorology.
5000m visibility does not imply that the pilot can always see 5000m directly ahead. For example, there might be a mountain or a cloud that hides the next patch of sky. But the clear air around the aircraft is such that, if there weren't obstacles in the way, you could see 5000m.
Two examples illustrate:
- you are flying up a narrow valley (never mind the airmanship aspect). You can only see 2km, because there's a corner in the valley. It's still legal because meteorological visibility is 50km.
- you are flying night VFR. You can't actually see anything much, because the country side is dark. But the air is clear enough to see a light 100km away, if the light existed.
Does NZ law allow you to descend through a hole in the cloud 4km across? I would say yes, but I would hesitate to do so, for airmanship reasons. (Unless the cloud was only a thin layer - in which case I've done so several times.) But it's an interpretation, and others may disagree.
That said, I once had a near miss with a Harvard out of Ardmore. I was IFR in scattered cloud, he was VFR in the same scattered cloud. I think he never even saw me as we appeared around opposite sides of the same patch of cloud. I'm sure he was legally VMC, but I'm not sure it was a good idea for him to be there.
4greens - NZ does not have the same law as the EU. Perhaps you didn't notice that the OP was posting from NZ?
Leadsled - airmanship is over and above the law. So exercising good airmanship presupposes that the law is obeyed, as far as preserving life permits. Don't you think?
I think that VFR rules should require you to be at LSALT if not in contact with ground or water (and able to navigate with reference to).
The rules do, in one sense. If you're not in sight of ground or water, you have to be at least 1000' agl (some countries) or 3000' agl (other countries). Which means, ignoring Australia's bizarre definition of LSALT as 1400' above terrain or 1500' above the sea, you are actually at MSA. Again, I'm not sure it would be wise, but you would be legally ok.
Tarq57
6th May 2012, 03:29
Day VFR above 3000ft demand 2km separation from cloud laterally, visibility 5km. It is implied that is 5km ahead of the aircraft.Scenario. An aircraft descends from FL160 to 2000ft through a hole in the cloud layer, that is essentially a clear vertical shaft >4km deep.This would be unusual, but I expect you're using the example to illustrate a point. In actual operation, I've heard of such a thing happening once, and the vertical extent was about 4 or 5 thousand feet. (See Fate is the Hunter, by E.K.Gann)
Is the aircraft required to remain 5km from the cloud at all times, so the pilot can see 5km ahead? Visibility relates to the transparency of the air to determine how far away an object can be seen, rather than to "stuff" that might be in the way, be it a cloud, or a mountain. Although you may only be able to see 2km, technically, the visibility within the hole could be considerably more. (I would hope so, otherwise you wouldn't see the ground at the bottom of the hole until descending through ~12,000ft.)
If the SOP requires 20km visibility, does that require seeing 20km ahead as the aircraft orbits down this "mine" shaft? That is, to maintain 20km separation from cloud?Rather abstract question. As indicated above, can't see a likely application for it. An operator with an SOP of 20k vis for visual manoeuvering is just not going to be doing this, even if such a hole could be found.
NOTE: CAR give minima, not guidelines or suggestions. If you break the rules you must notify the CAA and explain why. Really. True. Best not break the rules, then.
...~Perhaps I could ask you to explain or define the term VMC. NZ CAR uses the term VFR minima, which is 1500ft vis uncontrolled AD or SVFR. That is, the VMC requirements change. Not that I want to get side-tracked in semantics; hitherto I have qualified any statement with the word cruise. Even so, I would have thought visual flight in IMC was an oxymoron, and it's either VMC or IMC surely. VMC, as you can see in the table in the NZAIP, ENR section,figure ENR 1.4-1, varies depending on the category of airspace, and the level flown. It seems rather well defined to me. Link Here: AIP New Zealand (http://www.aip.net.nz/NavWalk.aspx?section=ENR&tree=GENERAL+RULES+AND+PROCEDURES) and select 1.4.
The vis is never 1500ft. I suspect that's a typo. The minimum vis is 1500M.
An example of VFR in not-VMC conditions exists when you are special VFR. In a control zone (C), but outside the circuit, and unable to maintain 1000ft or greater vertically from the cloud. You would need a clearance from ATC to do this.
Anyway, would you care to tackle the last question? What does 20km visibility mean in the context of an orbiting descent through a cloud layer >4km deep.Yeah, good luck with that, fujii.
prospector
6th May 2012, 05:32
Rather abstract question. As indicated above, can't see a likely application for it. An operator with an SOP of 20k vis for visual manoeuvering is just not going to be doing this, even if such a hole could be found.
That was the company (Air New Zealand) requirement for VMC flight during the Antarctic sight seeing operation.
I agree with your statement regarding such a hole.
remoak
6th May 2012, 06:27
Regarding rules around VMC/VFR - Douglas Bader said it best:
"Rules are for the obedience of fools and the guidance of wise men".
He could fly a bit, too.
The NZ CAR do not use the term VMC, as far as I can see(!) The terms are VFR meteorological or weather minima.
Therefore it seems to me that it is the definition of VMC that changes for SVFR, not that one is given a clearance to fly visually in IMC, Tarq57.
A descent from an altitude above 3000ft amsl or 1000ft agl does not require sight of the surface until that level is reached. There could be cloud above fog.
Visibility of 5km means ahead of the aircraft. If there is a corner 2km ahead, the pilot assumes it is 5km around the corner until proved otherwise. Which could happen. I have flown through Haast Pass with the cloud base lowering, landed at Haast to watch that cloud become fog.
Oktas8, I don't think you could legally orbit through a deep hole 4km across above 3000ft. If obeying the law (to remain 2km clear of cloud) outweighed all else, one could try a stall...
The descent in a break in the cloud cover from ~FL160 to <2000ft, ie an open shaft 4km deep, was performed by Captain Collins in the Antarctic. You're onto it, prospector.
I cannot get a mental picture of what this descent would have been like, or how one would judge the visibility within this space, or how it related to the visibility outside it. Collins certainly seems to have come within cooee of Mt Bird without realising it; SOP required 20km visibility.
Seems under close examination, there is a problem with definitions. Take a chair. If I say a chair is a piece of furniture with four legs people sit on, what would an alien make of a chair that had one leg broken off?
Yes, remoak, there is limit to what you can do to save pilots with rules. In another thread, ampan argues Collins was not VMC under SOP, I am trying to analyse the initial descent.