Can GPS be *too* accurate? Do I need some XTE??

"Bill Denton" wrote in
:


"Peter R." wrote in message
...
Bill Denton ) wrote:

And I don't know if this is a trick question, but if you are at 6000
(no +500) wouldn't you be on an IFR flight plan, talking to ATC, and
receiving traffic advisories?

In the US and outside of class B airspace, it is the pilot, not ATC,
who is ultimately responsible for IFR/VFR traffic separation. A VFR
traffic advisory to an IFR aircraft is a courtesy offered by ATC; it
is not a guarantee.

--
Peter

You might want to rethink your reply.

A pilot in clouds or other IMC cannot provide separation to any traffic
he cannot see.



Neither can ATC - for example, when there is no Radar Coverage.

The system is designed to work even in those conditions, and the rules
follow suit...

In VMC, ALL pilots are required to "see and avoid", VFR or IFR.

VFR pilots are supposed to stay out of IMC to prevent getting hit by IFR
pilots on IFR flight plans in the IMC.

When there is no Radar, IFR separation is done using spacing and reporting
points. There are no traffic advisories, IFR or VFR.

VFR-to-IFR separation is a courtesy, as Peter said. If ATC calls out a
target to an IFR flight, and they are not talking to the VFR target too,
they can't even provide instructions that guarantee safe avoidance...


Don't take your advisories for granted. They are a favor.

"Bill Denton" wrote in message
...
The question mark "implied" nothing; it directly indicated that I was
asking
a question.

Fine. As hard as it is to imagine, let's grant your claim regarding the
lack of an implication. Then the answer to your question is "yes, assuming
you're also above 3000' AGL, but so what?" Emphasis on "so what?"

Given that there was no implication intended, what in the world was the
point of your original reply?

And if the conditions in my question were "true", you would not need to
fly
an offset on a GPS course, despite GPS improved accuracy, any more than
you
would need to fly an offset on a VOR course; traffic avoidance would be
handled by ATC.

In other words, you WOULD "need" to fly an offset, in order to ensure no
conflict with other traffic. There is a very real risk of a collision when
flying an airway, whether you're using GPS or VOR navigation. In practice,
we as pilots generally choose looking out the window over offsetting our
course, but the "need" is there nevertheless.

You continue to claim (just as your first post implied) that ATC handles
traffic avoidance for IFR flights, but that's simply not true. ATC only
separates IFR flights from other IFR flights (except in particular kinds of
airspace where VFR flights are also given traffic separation...a very small
portion of the national airspace system).

Pete

"Bill Denton" wrote in message
...
[snip]

That's the section I posted. The third line down references VFR-on-top.

So, now you are agreeing that it was you that brought up VFR-on-top? As
irrelevant as it is to this thread, you are the person who introduced it.
By your own admission.

This thread has gotten so long, I including my original reply to ensure that
we are on the same page:

I noticed the later posts referenced a set of "rules" for setting up the
"error", but absent those, you are back to the same old game of chance.
What's to prevent another pilot from picking a corresponding "error" that
would still maintain the head-on courses?

And I don't know if this is a trick question, but if you are at 6000 (no
+500) wouldn't you be on an IFR flight plan, talking to ATC, and receiving
traffic advisories?

My comments are in the text...4


"Peter Duniho" wrote in message
...
"Bill Denton" wrote in message
...
The question mark "implied" nothing; it directly indicated that I was
asking
a question.

Fine. As hard as it is to imagine, let's grant your claim regarding the
lack of an implication. Then the answer to your question is "yes,
assuming
you're also above 3000' AGL, but so what?" Emphasis on "so what?"

Given that there was no implication intended, what in the world was the
point of your original reply?

And if the conditions in my question were "true", you would not need to
fly
an offset on a GPS course, despite GPS improved accuracy, any more than
you
would need to fly an offset on a VOR course; traffic avoidance would be
handled by ATC.

In other words, you WOULD "need" to fly an offset, in order to ensure no
conflict with other traffic. There is a very real risk of a collision
when
flying an airway, whether you're using GPS or VOR navigation. In
practice,
we as pilots generally choose looking out the window over offsetting our
course, but the "need" is there nevertheless.

As you will note from my original reply, the point was that if everyone is
randomly choosing their own offset, you aren't really improving your odds of
avoiding a head-on very much. You would probably be better off just flying
the centerline; at least you would know where to look for other traffic.



You continue to claim (just as your first post implied) that ATC handles
traffic avoidance for IFR flights, but that's simply not true. ATC only
separates IFR flights from other IFR flights (except in particular kinds
of
airspace where VFR flights are also given traffic separation...a very
small
portion of the national airspace system).

Aircraft operate under the same traffic principles as automobiles: There is
a set of rules. If everyone follows the rules, no problem. But is someone
breaks the rules, you have a strong potential for a problem.

If you are IFR in IMC you probably will not be able to use "see and avoid".
You simply cannot see through the clouds/rain/whatever. You have to look to
ATC for separation, which they will provide for all aircraft on IFR flight
plans. Aircraft flying under VFR are not supposed to be in IMC. If they are,
that's a violation of the rules. And VFR and IFR aircraft are supposed to
maintain a 500 foot vertical separtaion above 3000 feet AGL. If the
separtion is not maintained, that's a violation of the rules. And when you
have a violation of the rules, the accident risk increases.

FAR 91.113 (c) mandates that when weather condtions permit, all aircraft,
whether flying VFR or IFR, must observe "see and avoid".

To sum it up, if you are IFR in IMC, you have to rely on ATC to separate you
from other IFR traffic; there should not be any VFR traffic there. If you
are in VMC, whether VFR or IFR, you must observe "see and avoid". And all
aircraft must maintain the 500 foot separation between VFR and IFR aircraft.

Obviously, when aircraft are ascending or descending, the risk of collision
increases. But you can only rely on ATC in IMC to reduce this risk, or use
"see and avoid" in VMC.

And I am aware that there are other IFR separation methods such as takeoff
sequencing and timing, maintaining separation via speed, flying your flight
plan exactly if something goes wrong, and similar methods, but they aren't
really germane to this limited discussion.





Pete

No, I am not agreeing to anything. I will state that I was the first to
introduce the words VFR-on-top into the thread, while quoting a section of
the AIM.

The AIM states that ATC will provide IFR/IFR separation. But, a VFR-on-fop
flight is an IFR flight, and ATC does not provide separation to VFR-on-top
aircraft. We were discussing IFR separation and this is a part of them, so
it is relevant to the thread.



"Peter Duniho" wrote in message
...
"Bill Denton" wrote in message
...
[snip]

That's the section I posted. The third line down references VFR-on-top.

So, now you are agreeing that it was you that brought up VFR-on-top? As
irrelevant as it is to this thread, you are the person who introduced it.
By your own admission.

Y'All,
This entire thread seems to be totally entranced with the possible conflict
of aircraft on a heading/course. Whereas, the most likely conflict is in
altitude between IFR and VFR supposedly flying with 500 feet of FAA
separation.

Some time ago I was told that ATC figures a + - error 300 feet.
The altimeter is likewise allowed a 75 foot + - error. Not knowing for
certain but assuming it is so. Look at the following senario.

If we have an IFR and a VFR fllying in opposite hemisphereic directions in
VFR conditions we have several possible extreme conditions. Take the first
aircraft indicating 6000 feet west bound. The second aircraft indicating
5500 feet east bound. If both the transponders and altimeters have errors
to the extreme in the opposite directions, they could still miss each other.

If the first aircraft is flying 250 lower than indicated due to accumulated
instrment error, while the other is actually flying 250 feet higher than
indicated we have only see and be seen to save the situation.

To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. I believe this
because the distances are matters of feet rather than miles. It takes both
to actually cause the midair.so the total emphasis on course/heading is only
a part of the equation.

I haven't even mentioned GPS altitude as a factor.
Mud wrestling anyone?

Gene

"Gene Whitt" wrote in message
link.net...
Y'All,
This entire thread seems to be totally entranced with the possible
conflict of aircraft on a heading/course. Whereas, the most likely
conflict is in altitude between IFR and VFR supposedly flying with 500
feet of FAA separation.
...snip...
To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. .


Actually, if the two of them are not on the exact same location
horizontally, the odds of a conflict because of altitude error is zero.

While there is some remote possibility of being in the same location
horizontally at the same time while crossing tracks, the potential to be in
the same location horizontally is much greater when they are navigating
reciprocal tracks between the same two waypoints.

"Peter Duniho" wrote in message
...
"Icebound" wrote in message
...
[...]
Never having flown a VOR course myself... I still doubt very much that
any two pilots (OR auto-pilots), flying reciprocal headings between two
VORs, would both be able to *simultaneously* hold a course to within 10
feet of the centre-line for the whole course, considering the receiver
errors and that the VOR radial-signal *itself* probably varies more than
that.

I could be wrong.

You are wrong.

For two pilots to *intentionally* stay exactly on course center on a VOR
airway would be challenging, granted. But the airway provides an
"attractor" for airplanes, and inasmuch as the airplanes average toward
the center of the airway, eventually a couple will come along flying the
exact same distance from the actual airway (whether that's 0.0 miles
off-center or 3.9 miles off-center).

Accepted and agreed. "eventually".

But in the GPS case, it is pretty much in "every" case that two aircraft
using those two waypoints will be pretty much in the center.

"Bill Denton" wrote in message
...
[...] We were discussing IFR separation

No, we weren't. YOU turned it into that discussion, but nothing about the
original post had anything to do with IFR separation.

That is, in fact, the entire point of this monster subthread you've
inspired.

"Icebound" wrote in message
...
But in the GPS case, it is pretty much in "every" case that two aircraft
using those two waypoints will be pretty much in the center.

But "pretty much" still covers quite a bit of ground. There is only an
increased risk of a collision, not a virtual certainty.