interesting cloud-clearance puzzle
Tom McDougal
it is relevant to fixed-wing aircraft as well.
Assume scattered clouds at 800 AGL, clear skies above.
Suppose you are flying a helicopter at 600 AGL. In uncontrolled
airspace that's no problem because the only requirement for helicopters
is clear of clouds, see-and-avoid. But in class E-and-above, the
VFR cloud-clearance requirement is the same as other aircraft,
which soon presents a problem.
Now what happens if you want to enter class E-or-above airspace?
The conditions are VFR and other aircraft can safely cross
over the top VFR. The helicopter cannot maintain 500' vertical cloud
clearence w/out descending to 300 AGL, which may not be safe.
[Here is the part that is relevant to fixed-wing:] Nor may it be
possible for the helicopter to ascend or for other aircraft to
descend through the cloud layer while keeping 2000' horizontal
clearance. This could be a problem to an airplane which wishes
to land.
So, the question: Do you request SVFR? Would such a request make
sense, even though the conditions are not, strictly speaking, IFR?
Go to it, master aviators!
--
Tom McDougal University of Chicago Artificial Intelligence
mailto:mcdo...@cs.uchicago.edu
http://cs-www.uchicago.edu/~mcdougal
PP-RH
William LeFebvre
Tom McDougal <mcdo...@cs.uchicago.edu> wrote:
>Assume scattered clouds at 800 AGL, clear skies above.
>Now what happens if you want to enter class E-or-above airspace?
>The conditions are VFR and other aircraft can safely cross
>over the top VFR. The helicopter cannot maintain 500' vertical cloud
>clearence w/out descending to 300 AGL, which may not be safe.
Basically correct.
>[Here is the part that is relevant to fixed-wing:] Nor may it be
>possible for the helicopter to ascend or for other aircraft to
>descend through the cloud layer while keeping 2000' horizontal
>clearance. This could be a problem to an airplane which wishes
>to land.
I would argue that if you can't find a hole big enough in the layer
then it is probably pretty close to qualifying as a broken layer.
If broken, it's a ceiling, hence the class E airspace is technically
IFR and requesting an SVFR is reasonable.
>So, the question: Do you request SVFR? Would such a request make
>sense, even though the conditions are not, strictly speaking, IFR?
Unless the control zone (oh, sorry, class E) is IFR, then requesting
SVFR will sound real funny. Can you go around the class E airspace?
Or is it Meigs? :-)
William LeFebvre
Decision and Information Sciences
Argonne National Laboratory
lefe...@dis.anl.gov
patterson,george r
>Go to it, master aviators!
Well, that leaves *me* out. :-)
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Tom McDougal
>I would argue that if you can't find a hole big enough in the layer
>then it is probably pretty close to qualifying as a broken layer.
>If broken, it's a ceiling, hence the class E airspace is technically
>IFR and requesting an SVFR is reasonable.
This happened to me yesterday, and I can assure you that (a) the
clouds were SCT not BKN, and (b) the airport (Meigs) was VFR.
(Answering your second question. :-))
Think about it. You have to have 2000' horizontal clearance.
If the clouds themselves are <2000' across, then they can be
spaced between 2000' and 4000' apart, qualifying as SCT but
not allowing a hole big enough EVEN IF YOU COULD ASCEND
(or descend) VERTICALLY. Any reasonable aircraft *I* expect
to fly will require some horizontal component, which makes
threading the holes that much more difficult.
It sounds like I might have violated the cloud clearance regs
and I'm trying to figure out what I should have done short of
flying around the area.
Steve Gallagher
>
>Assume scattered clouds at 800 AGL, clear skies above.
>
>sense, even though the conditions are not, strictly speaking, IFR?
How do you figure "800 scattered" is IFR?
Tom McDougal
>>Assume scattered clouds at 800 AGL, clear skies above.
>>
>>So, the question: Do you request SVFR? Would such a request make
>>sense, even though the conditions are not, strictly speaking, IFR?
>
>How do you figure "800 scattered" is IFR?
Of course it isn't, see the line just above yours. The problem is
that a SCT layer may nonetheless be impossible to penetrate
legally under VFR. If the clouds are less than 4000'
apart you won't have the required 2000' horiz. clearance.
Does this mean that you could be stuck above the clouds because
(a) you can't descend while maintaining VFR, and
(b) you can't request SVFR because conditions aren't IFR?
In my case the problem was, I'm in a helicopter at 600 AGL, the
clouds are SCT at 800 AGL. I can't safely fly low enough to have
500' vertical clearance. So I can't legally enter class D
airspace under VFR, nor can I request SVFR because conditions
aren't IFR. Ergo, I'm stuck!
It seems silly! If conditions were lots worse I could obtain
SVFR clearance and just remain clear of clouds, yet here it is
a beautiful day with a few (low) scattered clouds and I
can't get through.
I'm really very interested in knowing how one could resolve
this dilemma.
Jetson
>Does this mean that you could be stuck above the clouds because
>(a) you can't descend while maintaining VFR, and
>(b) you can't request SVFR because conditions aren't IFR?
>In my case the problem was, I'm in a helicopter at 600 AGL, the
>clouds are SCT at 800 AGL. I can't safely fly low enough to have
>500' vertical clearance. So I can't legally enter class D
>airspace under VFR, nor can I request SVFR because conditions
>aren't IFR. Ergo, I'm stuck!
>It seems silly! If conditions were lots worse I could obtain
>SVFR clearance and just remain clear of clouds, yet here it is
>a beautiful day with a few (low) scattered clouds and I
>can't get through.
>I'm really very interested in knowing how one could resolve
>this dilemma.
You could always call up the tower/FSS and tell them that your *flight
conditions* are below VFR and that you are requesting special. Everyone
knows that flight conditions don't always agree with the issued weather
sequence, and your pirep will generally take precedence over everything
"official" except an RVR reading.
The weather sequence for an airport is based on the total sky coverage and
the minimum visibility over 50% or more of the horizon as viewed from the
reporting station. The fact that an airport zone is reported as VFR does
not in any way imply that the *entire* zone is VFR.
In any case, there is nothing in the rules that prevents the center from
approving SVFR for a VFR airport. If you request special and the
tower/FSS tells you that the zone is VFR, they're just trying to avoid a
phone call. Do whatever you think is safest.
--
Jetson
Moncton, New Brunswick, Canada.