End of Season Sunset Warning for SSA-OLC Participants

"GC flight logs contain a GPS time stamp which is the most exact time
standard readily available. So, violations of this requirement are
quite plain to see in these logs."

Unfortunately, sunset doesn't come with a GPS timestamp, so I'm afraid
this isn't as easy as you make it sound.

Yes, I know there are standard tables of sunset times, but these make
assumptions that may not be exactly correct and at a given location
actual sunset may differ from the calculated one by many minutes.
(Ask any local physicist./astronomer).

Of course, gross violations are pretty obvious, but I have operated
from a field where landing to the west is best done after the sun has
descended behind mountains, which usually occurs close to the time of
"official" sunset and is standard practice by those flying late.

Methinks all this scrutiny of flight logs is getting a bit too
intrusive and nit-picky.

Mike

Mike the Strike wrote:
"IGC flight logs contain a GPS time stamp which is the most exact time
standard readily available. So, violations of this requirement are
quite plain to see in these logs."

Unfortunately, sunset doesn't come with a GPS timestamp, so I'm afraid
this isn't as easy as you make it sound.

Yes, I know there are standard tables of sunset times, but these make
assumptions that may not be exactly correct and at a given location
actual sunset may differ from the calculated one by many minutes.
(Ask any local physicist./astronomer).

Of course, gross violations are pretty obvious, but I have operated
from a field where landing to the west is best done after the sun has
descended behind mountains, which usually occurs close to the time of
"official" sunset and is standard practice by those flying late.

Methinks all this scrutiny of flight logs is getting a bit too
intrusive and nit-picky.

Mike

The apparent movement of the sun in the sky is known very precisely (we
have been observing the sun for centuries). The only variable is the
refraction of the atmosphere, which changes the precise angle at which
the sun appears to pass below the local horizon. Various sunset
calculations use different refraction calculations, so they may differ
by a few minutes, but not "many minutes". The time of actual sunset may
also vary by a few minutes from the calculated value. The US Naval
Observatory has a sunrise/sunset calculator available online, and this
is probably the most reliable source for sunset times.

Unfortunately there is widespread ignorance of the sunset rule for
aircraft lighting, despite the fact that it has remained unchanged for
many years--longer than most pilots have been flying. So, we have seen
a few gross violations posted to OLC.

The point of all this is to get pilots to self-police, and not post
these flights in the first place. They should not be making flights
that continue after sunset witout lights anyway, but we certainly don't
want to see them on the OLC in any case.

P.S. The sun will appear to set below mountains before official sunset
when at an altitude lower than the mountains, becase the mountains will
appear to project above the local horizon.

P.P.S. Note that the sun will appear to set later than official sunset
when flying for the opposite reason--the local horizon appears to be
depressed. So you will need to enter the pattern to land well before
the sun appears to be setting in flight.

Mike the Strike wrote:
"GC flight logs contain a GPS time stamp which is the most exact time
standard readily available. So, violations of this requirement are
quite plain to see in these logs."

Unfortunately, sunset doesn't come with a GPS timestamp, so I'm afraid
this isn't as easy as you make it sound.

Yes, I know there are standard tables of sunset times, but these make
assumptions that may not be exactly correct and at a given location
actual sunset may differ from the calculated one by many minutes.
(Ask any local physicist./astronomer).

I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

You may have difficulty observing the position of the sun. Like,
it might be cloudy, there might be a mountain in the way, or
you might be indoors. But that is not relevent.

Any regulatory requirement should (and I daresay will) be based
on that well-defined time, or the equally well-defined times of civil,
nautical, or astronomical twilight.

http://en.wikipedia.org/wiki/Astronomical_twilight
http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Obviously you may be safer flying a few minutes after sunset in
a clear sky than a few minutes before sunset in a cloudy sky.
But that is a practical consideration.

You are correct that these definitions are based on certain
assumptions, but the point is that a rule that says you are
supposed to be on the ground by sunset, the end of civil
twilight, or whatever, should be a rule that uses the defined
time, not some local observation. That gives you a clear,
unambiguous requirement.

--

FF

A minor clarification:

wrote:
I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

Well, not exactly to the second. Even the USNO only reports sunset time
to the nearest minute (see the "Accuracy of rise/set computations"
section in the link below). And their calculator only accepts Lat/Lon
to the nearest 1/10th of a dagree. So the error in the calculations are
+/- a minute or more. This is a technical point, but it is important to
remember that all measurements have some error.

You may have difficulty observing the position of the sun. Like,
it might be cloudy, there might be a mountain in the way, or
you might be indoors. But that is not relevent.

You are correct that the time of actual sunset is moot, since nobody
regularly observes and records this. The only relevant time is the
official prediction. This prediction is normally in the pilot's favor,
since the sun sets earlier than predicted at high surface elevation and
high temperature due to recuced refraction in the less dense air.

Any regulatory requirement should (and I daresay will) be based
on that well-defined time, or the equally well-defined times of civil,
nautical, or astronomical twilight.

http://en.wikipedia.org/wiki/Astronomical_twilight
http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Obviously you may be safer flying a few minutes after sunset in
a clear sky than a few minutes before sunset in a cloudy sky.
But that is a practical consideration.

You are correct that these definitions are based on certain
assumptions, but the point is that a rule that says you are
supposed to be on the ground by sunset, the end of civil
twilight, or whatever, should be a rule that uses the defined
time, not some local observation. That gives you a clear,
unambiguous requirement.

--

FF

Doug Haluza wrote:
A minor clarification:

wrote:
I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

Well, not exactly to the second. Even the USNO only reports sunset time
to the nearest minute (see the "Accuracy of rise/set computations"
section in the link below). And their calculator only accepts Lat/Lon
to the nearest 1/10th of a dagree. So the error in the calculations are
+/- a minute or more. This is a technical point, but it is important to
remember that all measurements have some error.

True it is not possible to predict when the sun would be
observed tangent below the horizon, with an accuracy of
better than a minute or so. But that's a silly way to define
sunset in the first place. If the atmospheric conditions
(e.g. clouds) made make the sun unobservable, you wouldn't
say there was no sunset that day. (Though the sun would
still be observable outside of the visible spectrum)

If the "time of sunset" is _defined_ based on nominal atmospheric
conditions that moots the issue, just like defining the horizon
to be 90.8333 degrees from zenith moots the issue of the local
topography.

Those conditions can be defined as accurately as one wants.

Similarly, uncertainty in your lattitute, longitude and elevation
may also be mooted. You may be uncertain as to where
YOU are, but the estimation of the time of sunset for an arbitrary
location (which therefor you can define with arbitrary accuracy)
is uncertain only due to the variablity in the motions of the earth,
uncertainty and variablility in the orbital parameters of the Earth,
and uncertainty and variability in the apparent size of the sun.

Actually, since Universal Time is _defined_ by the orientation
of the Earth and not by atomic time, the time of sunset is
only affected by variabilty in the motions of the Earth because
they affect the place on the horizon where the sun is tangent.
That is why we have leap seconds from time to time, to
keep international atomic time and universal time in agreement
(coordinated) to within one second, though there has been
debate about discontinuing that practice.

Those leap-seconds are pretty important in orbit determination,
which gets us back to something of potential interest to glider
pilots. A GPS satellite moves moves more than a mile in one
second.

--

FF

Fred:

You should really check your facts before posting twaddle!

Clearly I should have said to ask any physicist or astronomer who is
conversant with atmospheric refraction, which you clearly are not.
Because of variations in refraction, sunset (the time when the limb of
the sun becomes invisible to an observer) may vary by several minutes
from that calculated by the accepted formula. This formula assumes a
constant refraction that is unlikely to be exact for any specific place
or time.


Mike

(also a physicist and astronomer - and who knows a sunset when I see
one!)



I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

You may have difficulty observing the position of the sun. Like,
it might be cloudy, there might be a mountain in the way, or
you might be indoors. But that is not relevent.

Any regulatory requirement should (and I daresay will) be based
on that well-defined time, or the equally well-defined times of civil,
nautical, or astronomical twilight.

http://en.wikipedia.org/wiki/Astronomical_twilight
http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Obviously you may be safer flying a few minutes after sunset in
a clear sky than a few minutes before sunset in a cloudy sky.
But that is a practical consideration.

You are correct that these definitions are based on certain
assumptions, but the point is that a rule that says you are
supposed to be on the ground by sunset, the end of civil
twilight, or whatever, should be a rule that uses the defined
time, not some local observation. That gives you a clear,
unambiguous requirement.

--

FF

Mike the Strike wrote:
Fred:

You should really check your facts before posting twaddle!

Clearly I should have said to ask any physicist or astronomer who is
conversant with atmospheric refraction, which you clearly are not.
Because of variations in refraction, sunset (the time when the limb of
the sun becomes invisible to an observer) may vary by several minutes
from that calculated by the accepted formula. This formula assumes a
constant refraction that is unlikely to be exact for any specific place
or time.


Mike

This is like the advice from the old joke about the guys who get lost
in a balloon, then ask someone on the ground where they are, and the
person on the ground tells them they are up in the air in a balloon.
The joke is that the person on the ground must be a lawyer, beacuse his
answer was technicaly correct, but totally useless.

Same goes for actual sunset. Unless you are on a ship at sea, you won't
have a clear level horizon to observe actual sunset. So the refraction
issue is moot. And if you wait until you observe actual sunset in
flight, the sun will have already set on the ground.

The predicted sunset time is the only thing relevant for flight
planning.

Doug Haluza wrote:
The predicted sunset time is the only thing relevant for flight
planning.

It is interesting that part 61 and part 91 define regulations that
refer to sunrise and sunset but that the FAA provides no definition of
what those terms mean. At least I couldn't find those definitions. I
expected to find them in part 1.

I used to fly a lot in an Aerona Chief that did not meet night lighting
requirements. A sunrise/sunset time table was always available for
flight planning, but I don't know if it agreed with whatever time table
FAA was using. I once landed a minute before sunset (according to my
table) after an xc flight and was still in motion on the airport after
sunset. Neither tower nor ground control expressed any concern.

I know I can call flight service and ask for sunset time for my local
airport. Their reply is "I can give you sunset for Phoenix, is that
close enough?" If I say yes, does that become the offical sunset time
for my local airport?

Given the uncertainty of the actual official sunset time I repeat my
request that SAA's sunset time be used only to determine the "end of
soaring flight" and not used to determine the validity of the entire
flight.

Andy

Andy wrote:

Given the uncertainty of the actual official sunset time I repeat my
request that SAA's sunset time be used only to determine the "end of
soaring flight" and not used to determine the validity of the entire
flight.

I like that suggestion. What about it Doug?


Jack

Hi Andy,

"Andy" wrote in message
oups.com...
Doug Haluza wrote:
The predicted sunset time is the only thing relevant for flight
planning.

It is interesting that part 61 and part 91 define regulations that
refer to sunrise and sunset but that the FAA provides no definition of
what those terms mean. At least I couldn't find those definitions. I
expected to find them in part 1.

I used to fly a lot in an Aerona Chief that did not meet night lighting
requirements. A sunrise/sunset time table was always available for
flight planning, but I don't know if it agreed with whatever time table
FAA was using. I once landed a minute before sunset (according to my
table) after an xc flight and was still in motion on the airport after
sunset. Neither tower nor ground control expressed any concern.

I know I can call flight service and ask for sunset time for my local
airport. Their reply is "I can give you sunset for Phoenix, is that
close enough?" If I say yes, does that become the offical sunset time
for my local airport?

Given the uncertainty of the actual official sunset time I repeat my
request that SAA's sunset time be used only to determine the "end of
soaring flight" and not used to determine the validity of the entire
flight.

Andy


I must respectfully disagree. If we just use sunset as the end of soaring
flight that gives an unfair advantage to a pilot that is far from home or at
altitude when the sun sets. He/she should have planned ahead and landed on
time.

Paul Remde