Article on glide strategy

[John Cochrane[_2_]]

I wrote a new article on how to use computers to help judge glides.

http://faculty.chicagobooth.edu/john.cochrane/soaring/docs/safety_glides.pdf

or the first item here

http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#maccready

This will probably end up in Soaring sooner or later, but I always get
a lot of help from early readers. If it's not clear or you see
problems etc. let me know. (john dot cochrane at chicagobooth dot
edu0

John Cochrane

[Fred[_5_]]

Good article, John. It deserves wider distribution. Thanks for the
analysis.

[Mike the Strike]

On Feb 27, 9:47*pm, Fred > wrote:
> Good article, John. *It deserves wider distribution. Thanks for the
> analysis.

Anyone who flies out west knows about "rivers of sink". However, many
can't decouple the MacCready of the flight director from the computer
(SN-10 users, for example). I have typically used a height reserve
above my chosen MacCready setting (something like 1,000 feet for 25
miles). Flying dry in weaker winter conditions, I usually use a
MacCready of 3 to 4 and up that to 5 to 6 in summer conditions flying
ballasted.

I have also been experimenting with forecasting these "rivers of
sink". Even in the blue, there tends to be some minor wave activity.
If you have a local RASP, you can see these on the Boundary Layer Up/
Down Motion plot or on the HRRR plots of average vertical velocity.
We have seen quite good agreement between these forecast wave/
convergence lines and both lift and sink. For example, last Sunday we
had a line of cumulus form exactly where we had a forecast wave line,
running from southwest to northeast about 20 miles south of our field.

In the blue you can't see these lines, but it is useful to know which
way they are aligned. If you end up flying in a "river of sink",
chances are you are running down one of these wave lines and you need
to turn at right angles to it to get back in lift or zero sink. The
forecasts might not predict them in the right place, but usually get
the alignment right. Check the plots before you fly. Better yet,
print a copy and carry it with you.

Mike

[Tony[_5_]]

I've been on a few of those sink streets too. Although some people
will say that I bring my own sink street with me on each flight.

From looking at your graph of altitude required to minimize the chance
of landing out it seems that the old rule I've heard of programming in
a 800 or 1000 foot get home altitude would allow you to program in a
slightly shallower Macready setting and still stay above the curve.
Then when you get into that last few miles where the curve starts to
drop you can speed up and finish at a lower altitude if you wish.

One nice thing in the Cherokee is that if you can see it you possibly
can't glide to it. I had several single digit glides at the Region 10
Contest. In fact John if you're looking for a river sink look at my
trace from Day 2 at Region 10 where I landed out 4 miles from the
finish. I don't think it was just the built in sink in the Cherokee
either as Dave Coggins reported seeing L/D's in the sub-10 range in
his Nimbus on the way back too.

When in Marfa i drew 10:1 circles which for the Cherokee was close
enough for easy math to the 1/2 your L/D method to make sure you can
make it home. Best I can figure that is about a Mc 9 or 10. I only
thought I was going to land out once during that week, see the old
adage of where there is big lift there is big sink.

[Dave Nadler]

Rivers of sink are pretty common.

Example: Uvalde typically has SW streeting and
task returns from SW. If bases are high its not
too hard (for me anyway) to get into the sink street
when descending on final glide.

You may want to stay way above glideslope (stay
with the clouds) longer than "MC reasonable".

Ask Ron how he almost landed 10 miles out on
last day of Open Nats last year...

Thanks John !
See ya, Dave "YO electric"

[akiley]

On Feb 27, 10:15*pm, John Cochrane >
wrote:
> I wrote a new article on how to use computers to help judge glides.
>
> http://faculty.chicagobooth.edu/john.cochrane/soaring/docs/safety_gli...
>
> or the first item here
>
> http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#maccr...
>
> This will probably end up in Soaring sooner or later, but I always get
> a lot of help from early readers. If it's not clear or you see
> problems etc. let me know. (john dot cochrane at chicagobooth dot
> edu0
>
> John Cochrane

Great article. Makes total sense. If we are using flight computers
to do the work of calculating arrival based an many factors such as
wind, polar, bugs, ballast and such, It doesn't make sense to me to
use something like L/D when L/D skips the wind and the polar info.
Arrival height takes these factors into account, but it seems MC
covers so many bases. It's the theory that describes how to maximize
lift/speed to fly, but can also describe reserve energy for safety
glides. I always think in terms of required MC to make the nearest
airport as I'm just starting to get into cross country soaring.

When I bought SeeYou, It took me a long time to sort out what critical
items (winds, reserve altitude, polar, bugs, ballast) were included in
calculations I saw in the various info boxes. When I discovered MC
included all of these items, plus the lift and speed to fly theory, it
seemed a no brainier to use required MC as my safety glide. With my
home airport as the goto, I would start thinking of heading home when
MC dropped below 8 based on conditions and distance from the field.
10 miles out with minimal lift, and MC 4 to target would make me
nervous.

I've only used SeeYou and XCSoar and they are not connected to our
club gliders.. I think these apps should have a Required MC to target
info box, and an option to replace arrival altitudes on airport tags
with required MC. Having a separate info box for ReqMC allows for
using the MC manual setting separately.

I hit a sink street last summer. The airport I was trying make it to
was aligned with the sink street based on the winds aloft. My options
were to continue in sink toward the airport, or turn away and hope fot
lift. I had something like MC 10 to make the airport with 800' safety
reserve, so I went direct and just made it. ... Aaron

[Ramy]

On Feb 27, 7:15*pm, John Cochrane >
wrote:
> I wrote a new article on how to use computers to help judge glides.
>
> http://faculty.chicagobooth.edu/john.cochrane/soaring/docs/safety_gli...
>
> or the first item here
>
> http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#maccr...
>
> This will probably end up in Soaring sooner or later, but I always get
> a lot of help from early readers. If it's not clear or you see
> problems etc. let me know. (john dot cochrane at chicagobooth dot
> edu0
>
> John Cochrane

Finally! I've been arguing this with flight computer developers
(Winpilot, XCSoar) for years. STF MC, and safety MC are 2 different
things that needs to be decoupled. Use your STF Vario MC setting for
speed to fly, and keep a constant MC for safety (I use 4 which seem to
work for almost any situation) in your glide computer (i.e. PDA).
Problem is, that if you connet your 302 to winpilot/xcsoar (and
probably others) you could not decouple the two. The good news, is
that XCSoar 6.3 will give the option to decouple the two different MC
settings.

There is another solution though. Instead of setting a high safety MC
in your glide computer, you can degrade the polar using the bug factor
to achieve the same results. (typically 33% -50% degradation, depend
how aggressive you want to be). Probelm is, that some flight
computers, such as XCSoar, did not store this value, which means you
had to remember to set it before every flight. The good news is that
this is also addressed in 6.3, which will have persistent polar
degradation.

And last, this excellent article also demonstrate why just keeping a
safety altitude does not work, as it will be appropriate only for one
distance. The further you are the highest it would need to be. This is
why Safety MC, or polar degradation are better solutions, since they
are not depending on distance.

Ramy

[Free Flight 107[_2_]]

On Feb 28, 9:52*am, akiley > wrote:
> On Feb 27, 10:15*pm, John Cochrane >
> wrote:
>
> > I wrote a new article on how to use computers to help judge glides.
>
> >http://faculty.chicagobooth.edu/john.cochrane/soaring/docs/safety_gli...
>
> > or the first item here
>
> >http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#maccr...
>
> > This will probably end up in Soaring sooner or later, but I always get
> > a lot of help from early readers. If it's not clear or you see
> > problems etc. let me know. (john dot cochrane at chicagobooth dot
> > edu0
>
> > John Cochrane
>
> Great article. *Makes total sense. *If we are using flight computers
> to do the work of calculating arrival based an many factors such as
> wind, polar, bugs, ballast and such, It doesn't make sense to me to
> use something like L/D when L/D skips the wind and the polar info.
> Arrival height takes these factors into account, but it seems MC
> covers so many bases. *It's the theory that describes how to maximize
> lift/speed to fly, but can also describe reserve energy for safety
> glides. *I always think in terms of required MC to make the nearest
> airport as I'm just starting to get into cross country soaring.
>
> When I bought SeeYou, It took me a long time to sort out what critical
> items (winds, reserve altitude, polar, bugs, ballast) were included in
> calculations I saw in the various info boxes. *When I discovered MC
> included all of these items, plus the lift and speed to fly theory, it
> seemed a no brainier to use required MC as my safety glide. *With my
> home airport as the goto, I would start thinking of heading home when
> MC dropped below 8 based on conditions and distance from the field.
> 10 miles out with minimal lift, and MC 4 to target would make me
> nervous.
>
> I've only used SeeYou and XCSoar and they are not connected to our
> club gliders.. *I think these apps should have a Required MC to target
> info box, and an option to replace arrival altitudes on airport tags
> with required MC. *Having a separate info box for ReqMC allows for
> using the MC manual setting separately.
>
> I hit a sink street last summer. *The airport I was trying make it to
> was aligned with the sink street based on the winds aloft. *My options
> were to continue in sink toward the airport, or turn away and hope fot
> lift. *I had something like MC 10 to make the airport with 800' safety
> reserve, so I went direct and just made it. * ... Aaron

[2KA]

As a devoted SN-10 user, I would love to have a way of decoupling the
MC setting used to calculate alternates from the flight director
function. And of course also it would be great to be able to set the
reserve altitude for alternates as well. Maybe Easter Bunny Dave is
listening. Please, oh please, Easter Bunny!

Lynn

[Max Kellermann]

Ramy > wrote:
> This is why Safety MC, or polar degradation are better solutions,
> since they are not depending on distance.

Safety MC is poor man's polar degradation. It doesn't make a lot of
sense, and will eventually be removed from XCSoar. We should have
implemented persistent polar degradation from the start.

Max

[Ramy]

On Feb 28, 4:13*pm, Max Kellermann > wrote:
> Ramy > wrote:
> > This is why Safety MC, or polar degradation are better solutions,
> > since they are not depending on distance.
>
> Safety MC is poor man's polar degradation. *It doesn't make a lot of
> sense, and will eventually be removed from XCSoar. *We should have
> implemented persistent polar degradation from the start.
>
> Max

Max, first thanks for addressing those issues in 6.3.
I tried both options (polar degradation and safety MC) recently, with
exact same results, so am not completely sure why Safety MC is poor
man's polar degradation. Since MC is now decoupled, I don't see a
problem using it for polar degradation, from the reasons John
described in his article. I think many still prefer to use MC to
degrade the polar instead of polar degradation. I also noticed at
least one advantage in safety MC: when clicking on a waypoint, you can
see you arrival altitude both with safety MC and with MC=0, so you can
estimate your chances to make it in still air. Polar degradation does
not provide such best case scenario an option since MC=0 is already
degraded.
Please explain why safety MC doesn't make a lot of sense.
In any case, I would recommend against removing safety MC, as pilots
may be using it for various reasons.

Ramy

[Andy[_1_]]

On Feb 27, 8:15*pm, John Cochrane >
wrote:
> This will probably end up in Soaring sooner or later, but I always get
> a lot of help from early readers. If it's not clear or you see
> problems etc. let me know. (john dot cochrane at chicagobooth dot
> edu0
>
> John Cochrane

John,

You asked for feedback.

The lift to drag ratio of a glider is not impacted by wind. I know
you know that so why use the term L/D out of context? Can you please
use a term such as (achieved) glide angle instead of L/D. Others have
coined the term "efficiency" but I have not taken to that yet.

Other that that nice article. I have been flying decoupled MC glides
for a long time. Even if don't split the speed director and glide
computer I alternate between the two MC values to sample "what glide
do I have" and "what speed should I fly right now". I suppose a
classic example of this is to climb in the last thermal to the
equivalent MC required glide altitude and then fly a lower MC until
clear of hostile terrain. I like to think of my final glides as x
feet over a y MC setting where how good x and y feel depends directly
on the terrain between myself and the goal. Where I fly having a bit
extra 5 miles out is worth quite a few contest points.

The biggest gotcha when believing a glide computer may be when there
is a strong tail wind at altitude. Easy to get suckered into a final
glide that doesn't work out when the tail wind decreases.

Andy (GY)

[Max Kellermann]

Ramy > wrote:
> Please explain why safety MC doesn't make a lot of sense.

Safety MC was implemented to degrade the polar, nothing else. It has
no effect on speed suggestions or anything else related to MacCready's
theory.

So what you probably want is a third polar degradation setting:

1. "polar degradation" adjusts the configured polar to match your
real-world glider (persistent, new XCSoar 6.3 feature)

2. "bugs" adjusts the polar to match the dirty glider (not persistent,
has always been there)

3. "safety degradation" to replace "safety MC"; basically the same as
"safety MC" just with a name and definition that makes more sense.

I'm not 100% sure about this; these are just my unfinished thoughts
after weeks of reading XCSoar's glide solver code. Maybe safety MC is
a setting to account for long distances of sink lacking any lift, and
the setting should be the expected sink value (and not the expected
lift value in the next thermal!).

Max

[T8]

On Feb 28, 9:22*pm, Max Kellermann > wrote:
> Maybe safety MC is
> a setting to account for long distances of sink lacking any lift, and
> the setting should be the expected sink value (and not the expected
> lift value in the next thermal!).
>
> Max

That's a really insightful thought.

-Evan Ludeman / T8

[Ramy]

On Feb 28, 6:22*pm, Max Kellermann > wrote:
> Ramy > wrote:
> > Please explain why safety MC doesn't make a lot of sense.
>
> Safety MC was implemented to degrade the polar, nothing else. *It has
> no effect on speed suggestions or anything else related to MacCready's
> theory.
>
> So what you probably want is a third polar degradation setting:
>
> 1. "polar degradation" adjusts the configured polar to match your
> * *real-world glider (persistent, new XCSoar 6.3 feature)
>
> 2. "bugs" adjusts the polar to match the dirty glider (not persistent,
> * *has always been there)
>
> 3. "safety degradation" to replace "safety MC"; basically the same as
> * *"safety MC" just with a name and definition that makes more sense.
>
> I'm not 100% sure about this; these are just my unfinished thoughts
> after weeks of reading XCSoar's glide solver code. *Maybe safety MC is
> a setting to account for long distances of sink lacking any lift, and
> the setting should be the expected sink value (and not the expected
> lift value in the next thermal!).
>
> Max

Max, I understand your points from pure theoretical perspective, but I
believe in practical most pilots use STF varios for speed, not PDA
flight computers, since STF varios are already doing this very well
and have audio signal as well. This is why the decoupling was
important. I also believe most pilots prefer to use MC for safety
degradation instead of polar degradation, or a combination of both.
But I guess one can just use the MC in XCSoar instead of safety MC.
Not sure about expected sink value, since this should be average sink,
and as such hard to predict or estimate (sink always seem stronger
than it really is, since we fly faster and see the combined result of
our increased sink rate and the neto sink).
Anyway, I think the most important thing here is the ability to
decouple the vario MC setting from the flight computer, or
alternatively use polar degradation. One thing is clear, using the
same MC for STF and glide calculation is a sure way to arrive short
without additional polar degradation.

Ramy

[[email protected]]

On Tuesday, February 28, 2012 2:45:58 PM UTC-8, Ramy wrote:
> On Feb 27, 7:15*pm, John Cochrane >
> wrote:
> > I wrote a new article on how to use computers to help judge glides.
> >
> > http://faculty.chicagobooth.edu/john.cochrane/soaring/docs/safety_gli....
> >
> > or the first item here
> >
> > http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#maccr....
> >
> > This will probably end up in Soaring sooner or later, but I always get
> > a lot of help from early readers. If it's not clear or you see
> > problems etc. let me know. (john dot cochrane at chicagobooth dot
> > edu0
> >
> > John Cochrane
>
> Finally! I've been arguing this with flight computer developers
> (Winpilot, XCSoar) for years. STF MC, and safety MC are 2 different
> things that needs to be decoupled. Use your STF Vario MC setting for
> speed to fly, and keep a constant MC for safety (I use 4 which seem to
> work for almost any situation) in your glide computer (i.e. PDA).
> Problem is, that if you connet your 302 to winpilot/xcsoar (and
> probably others) you could not decouple the two. The good news, is
> that XCSoar 6.3 will give the option to decouple the two different MC
> settings.
>
> There is another solution though. Instead of setting a high safety MC
> in your glide computer, you can degrade the polar using the bug factor
> to achieve the same results. (typically 33% -50% degradation, depend
> how aggressive you want to be). Probelm is, that some flight
> computers, such as XCSoar, did not store this value, which means you
> had to remember to set it before every flight. The good news is that
> this is also addressed in 6.3, which will have persistent polar
> degradation.
>
> And last, this excellent article also demonstrate why just keeping a
> safety altitude does not work, as it will be appropriate only for one
> distance. The further you are the highest it would need to be. This is
> why Safety MC, or polar degradation are better solutions, since they
> are not depending on distance.
>
> Ramy

So, assuming an undegraded polar, if the last thermal is 4 knots and your Safety MC is 4, at what speed do you go home and where is the safety?

David

[Nine Bravo]

Thanks John.

I tend to think about it slightly differently. On the one hand I think about worst-case, inescapable sink for the conditions as measured by minutes times sink rate (result - a fixed number of feet that I am at risk of losing - I don't generally consider a string of sink occurrences - I assume one low probability sink patch is worse than multiple, higher probability sink patches and that multiple low probability events aren't likely enough to worry about). On the other hand I think about the probability of being able to find lift to recover after hitting a sink patch - which is a function of altitude above ground.

On very short final glides the constraint is the rate of sink (not much glide time left), on longer final glides the constraint is the probability of finding decent lift to get back up. The pinch point for me tends to be around 25 miles out - chances of finding lift are declining, odds of a long of stretch of sink still decent. This seems roughly consistent with your square root rule, though the math is different.

Your square root rule breaks down for me on very long final glides where I tend to optimize more around trying to transition away from climb and glide to cruise-climb in an effort to avoid thermal centering losses. This translates to an altitude buffer of maybe -1000 feet on a 50 mile "final glide" that you are trying to "bump-up" to +1000 feet by the time you get to 25 miles out. Not sure if/how that figures in your analysis.

9B

[lanebush]

This article of John's ranks in my top ten for competition pilots.
Written with enough technical mumbo jumbo to be believable yet enough
plain talk to be understandable. I don't have a speed to fly Vario.
I am one if the cheepies that uses the PNA for speed to fly. This
article makes me want a speed to fly vario so I can decouple the PNA
from elevator.

Thanks John,
Lane
XF

[Jim White[_3_]]

Thought provoking article but I feel that Nine Bravo's comments are most
interesting too. It is my belief that final gliding is far more of an art
than a science. So many variables that theory is only useful to inform some
rules of thumb. Most of the decision making has to based upon circumstance
and experience.

Jim


At 15:36 01 March 2012, Nine Bravo wrote:
>Thanks John.
>
>I tend to think about it slightly differently. On the one hand I think
>abou=
>t worst-case, inescapable sink for the conditions as measured by minutes
>ti=
>mes sink rate (result - a fixed number of feet that I am at risk of
losing
>=
>- I don't generally consider a string of sink occurrences - I assume one
>lo=
>w probability sink patch is worse than multiple, higher probability sink
>pa=
>tches and that multiple low probability events aren't likely enough to
>worr=
>y about). On the other hand I think about the probability of being able
to
>=
>find lift to recover after hitting a sink patch - which is a function of
>al=
>titude above ground.
>
>On very short final glides the constraint is the rate of sink (not much
>gli=
>de time left), on longer final glides the constraint is the probability
of
>=
>finding decent lift to get back up. The pinch point for me tends to be
>arou=
>nd 25 miles out - chances of finding lift are declining, odds of a long
of
>=
>stretch of sink still decent. This seems roughly consistent with your
>squar=
>e root rule, though the math is different.
>
>Your square root rule breaks down for me on very long final glides where
I
>=
>tend to optimize more around trying to transition away from climb and
>glide=
> to cruise-climb in an effort to avoid thermal centering losses. This
>trans=
>lates to an altitude buffer of maybe -1000 feet on a 50 mile "final
glide"
>=
>that you are trying to "bump-up" to +1000 feet by the time you get to 25
>mi=
>les out. Not sure if/how that figures in your analysis.
>
>9B
>

[John Cochrane[_2_]]

On Mar 1, 9:36*am, Nine Bravo > wrote:
> Thanks John.
>
> I tend to think about it slightly differently. On the one hand I think about worst-case, inescapable sink for the conditions as measured by minutes times sink rate (result - a fixed number of feet that I am at risk of losing - I don't generally consider a string of sink occurrences - I assume one low probability sink patch is worse than multiple, higher probability sink patches and that multiple low probability events aren't likely enough to worry about). On the other hand I think about the probability of being able to find lift to recover after hitting a sink patch - which is a function of altitude above ground.
>
> On very short final glides the constraint is the rate of sink (not much glide time left), on longer final glides the constraint is the probability of finding decent lift to get back up. The pinch point for me tends to be around 25 miles out - chances of finding lift are declining, odds of a long of stretch of sink still decent. This seems roughly consistent with your square root rule, though the math is different.
>
> Your square root rule breaks down for me on very long final glides where I tend to optimize more around trying to transition away from climb and glide to cruise-climb in an effort to avoid thermal centering losses. This translates to an altitude buffer of maybe -1000 feet on a 50 mile "final glide" that you are trying to "bump-up" to +1000 feet by the time you get to 25 miles out. Not sure if/how that figures in your analysis.
>
> 9B

Good thoughts. But just to clarify, the article is not about final
glides -- how to do it efficiently. The article is about safety
margins -- how to do it with x percent chance of landing out.

John Cochrane

[Bert TW]

Interesting approach.
I for myself use the MC for optimizing cruising, and I work with
required L/D for safety - exclusively.
The good thing about required L/D is that there are no assumptions
whatsoever to it, it's plain geometry.
In Alpine soaring (which I've been doing for the last couple of
thousand hours), with my 47:1 ship I feel safe with a required L/D
somewhere between 20 and 25, and unsafe above 30.
That still depends on meterological conditions (end of day vs high
winds...). I once had 20:1 in a 40:1 ship and I didn't make it :-(

[Cliff Hilty[_2_]]

At 15:23 07 March 2012, John Cochrane wrote:
>On Mar 1, 9:36=A0am, Nine Bravo wrote:
>> Thanks John.
>>
>> I tend to think about it slightly differently. On the one hand I think
>ab=
>out worst-case, inescapable sink for the conditions as measured by
minutes
>=
>times sink rate (result - a fixed number of feet that I am at risk of
>losin=
>g - I don't generally consider a string of sink occurrences - I assume
one
>=
>low probability sink patch is worse than multiple, higher probability
sink
>=
>patches and that multiple low probability events aren't likely enough to
>wo=
>rry about). On the other hand I think about the probability of being able
>t=
>o find lift to recover after hitting a sink patch - which is a function
of
>=
>altitude above ground.
>>
>> On very short final glides the constraint is the rate of sink (not much
>g=
>lide time left), on longer final glides the constraint is the probability
>o=
>f finding decent lift to get back up. The pinch point for me tends to be
>ar=
>ound 25 miles out - chances of finding lift are declining, odds of a long
>o=
>f stretch of sink still decent. This seems roughly consistent with your
>squ=
>are root rule, though the math is different.
>>
>> Your square root rule breaks down for me on very long final glides
where
>=
>I tend to optimize more around trying to transition away from climb and
>gli=
>de to cruise-climb in an effort to avoid thermal centering losses. This
>tra=
>nslates to an altitude buffer of maybe -1000 feet on a 50 mile "final
>glide=
>" that you are trying to "bump-up" to +1000 feet by the time you get to
25
>=
>miles out. Not sure if/how that figures in your analysis.
>>
>> 9B
>
>Good thoughts. But just to clarify, the article is not about final
>glides -- how to do it efficiently. The article is about safety
>margins -- how to do it with x percent chance of landing out.
>
>John Cochrane
>

Very good thoughs both John and Andy, I tend to think of it more like a
funnel. The further out the closer I can be to my actual glide ratio and
the closer in i need way bigger margin. The funnel idea makes me put the
"art of final glide" into a logrithmic equation in my head and is instantly
scalable and movable to alternate landing sites. Especially flying out west
where most of the airports that I fly at do not have any safe landing spots
close by and the last few miles need to have extra high safety margins.
Thanks for sharing :)

CH

[Mike the Strike]

In addition to considering the probability of a landout, you have to take into consideration the consequences of one. If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.

I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!

Mike

[John Cochrane[_2_]]

On Mar 7, 12:08*pm, Mike the Strike > wrote:
> In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.
>
> I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!
>
> Mike

Exactly. In the paper analysis, you adjust the "probability of not
making it" parameter according to the consequences of a landout. The
profile to follow over mowed fields, where the costs are inconvenience
or contest points, is very different than the profile to follow over
Zion. They're both square roots but the Zion profile is much higher up
-- in the range of MacCready settings you may never have used before.

That's one of the big points. We get used to Mc 3 or so glides and
that they almost always work out. Over Zion, that experience is not
good enough. To an earlier comment that experience trumps analysis,
well, you don't want to be the guy that learns about how often Mc 1
glides work out by experience!

John

[Ramy]

On Wednesday, March 7, 2012 11:34:23 AM UTC-8, John Cochrane wrote:
> On Mar 7, 12:08*pm, Mike the Strike > wrote:
> > In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.
> >
> > I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!
> >
> > Mike
>
> Exactly. In the paper analysis, you adjust the "probability of not
> making it" parameter according to the consequences of a landout. The
> profile to follow over mowed fields, where the costs are inconvenience
> or contest points, is very different than the profile to follow over
> Zion. They're both square roots but the Zion profile is much higher up
> -- in the range of MacCready settings you may never have used before.
>
> That's one of the big points. We get used to Mc 3 or so glides and
> that they almost always work out. Over Zion, that experience is not
> good enough. To an earlier comment that experience trumps analysis,
> well, you don't want to be the guy that learns about how often Mc 1
> glides work out by experience!
>
> John

Excellent article and excellent thread.
John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition.

Ramy

[John Cochrane[_2_]]

>
> Excellent article and excellent thread.
> John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition.
>
> Ramy

I don't think using the bugs setting to calculate glides in real time
is that helpful. I never know what percent bugs means.

I do think it would be useful if our instrument makers could allow us
to input lift or sink. I'd like to input, "MacCready 3, 500 foot
reserve, and 100 fpm sink." Or when doing a final glide in Uvalde,
"MacCready 3, 500 foot reserve, and 100 fpm lift." (Clearnav has a few
emails from me on this!)

This simply shifts the polar curve up and down by the given lift and
sink, and would be easy for them to program.

For contest final glides, by keeping track of average netto in the
last few legs you could have an idea of lift/sink to be expected on
final glide, and then bump up / be cautious accordingly.

For safety reasons this would be very educational. You'd see directly
just how disastrous small bits of extra sink can be on your
glideslope. I also think many pilots would find it easier to take the
advice "assume 200 fpm sink all the way to your safest landing" than
they would to take the advice "input Mc 10 into your glide computer."
The former sounds reasonable, the latter outlandish based on cross-
country experience, yet they are the same thing.

Good point -- I'll add this to the article.

John

[Sean Fidler]

Super interesting. Thankyou!

[Mike the Strike]

Still important, but perhaps for a separate thread, is a strategy for escaping from a line of sink rather than just dealing with the mathematical consequences.

As I noted earlier, lift and sink lines are frequently aligned and knowledge of the relative heading of the glider along or across these lines would give the pilot useful information for an avoidance strategy.

You can see the forecast predictions of any such lift/sink lines on the RASP Boundary Layer Up/Down Motion or the equivalent HRRR field "Mean Layer Vertical Velocity".

Changing your final glide heading from a line of sink into a line of lift might have more benefit than accepting the inevitable and dialing up doom on your flight computer!

Mike

[John Cochrane[_2_]]

On Mar 8, 9:55*am, Mike the Strike > wrote:
> Still important, but perhaps for a separate thread, is a strategy for escaping from a line of sink rather than just dealing with the mathematical consequences.
>
> As I noted earlier, lift and sink lines are frequently aligned and knowledge of the relative heading of the glider along or across these lines would give the pilot useful information for an avoidance strategy.
>
> You can see the forecast predictions of any such lift/sink lines on the RASP Boundary Layer Up/Down Motion or the equivalent HRRR field "Mean Layer Vertical Velocity".
>
> Changing your final glide heading from a line of sink into a line of lift might have more benefit than accepting the inevitable and dialing up doom on your flight computer!
>
> Mike

Yes. In rivers of blue sink, I often just head 90 degrees off course
and wait.

Put another way, though, you have to use a much higher glide slope
(MacCready value + reserve) for safety spots that are upwind/downwind
or aligned with lift/sink streets than for safety spots that are
crosswind or not so aligned. The airports are where they are, so going
crosswind isn't always an option.

John Cochrane

[Chip Bearden[_2_]]

Great article, John, and another valuable service (as usual)! I like
to think it analyzes and explains what many pilots have been doing
somewhat intuitively (based on experience) but I suspect we all have
our own style.

Often I will leave the MC set as speed to fly but keep climbing in
what may be the last thermal. If the climb rate is above MC, I just
keep climbing, of course. As the thermal weakens and they converge and
I'm now above glide path, I bump the MC setting up enough to push the
final glide margin down to zero (though I've still got my 300 or 500
goal margin set) to see how "fat" I am. I'm frequently surprised at
how quickly the MC settings increase to what seem to be absurd levels
in the East, which is another way of saying that using very high MC
settings doesn't give you nearly the unassailable safety margin (in
feet) you might think. I usually don't fly that absurdly high MC
setting until I'm fairly close in (and/or over landable terrain or
with good clouds and/or markers ahead to pick up a little extra). In
fact, I'll switch back and forth and to see what the MC setting is
that would take me all the way home and whether it's increasing or
decreasing vs. what is my margin over final glide at cruise MC and
whether that's increasing or decreasing. No wonder final glides are so
confusing. :)

This argues for decoupling final glide MC from speed to fly MC, as you
suggest.

Chip Bearden
ASW 24 "JB"
U.S.A.