Safety Altitude

[Jonathan St. Cloud]

Does anyone use a safety altitude in their flight computers for final glide? If so why or why not?

[Steve Leonard[_2_]]

On Monday, October 3, 2016 at 4:00:50 PM UTC-5, Jonathan St. Cloud wrote:
> Does anyone use a safety altitude in their flight computers for final glide? If so why or why not?

I put in 1000 feet for my arrival altitude. Stick to it pretty religiously.. Also generally set a higher than actual MC to give a bit more pad for my poor (but improving) final glide air reading skills.

Why? Because I want to be sure I can get there with enough altitude to safely fly a pattern, even if I do encounter a bit more sink than lift on the way home. And I tend to set the MC higher because you probably remember how flat the glide is in something like your old Nimbus 4 with a 15-20 knot tailwind. Even with a 1000 foot arrival and MC=2, it seems awful low when still 10 miles out!

Steve Leonard

[John Cochrane[_3_]]

I wrote a Soaring article about this a while back. Some key points: The theory says you want an altitude minimum that is a quadratic function of distance to go. Basically, thermals are random. The chance of getting 3 tails in a row, 3 miles out, is higher than the chance of getting 30 tails in a row, 30 miles out. A McReady setting plus safety altitude is a good approximation.

Now, do you put that safety in the flight computer, so it says "0" when you really have 1000 feet margin? I used to, but turned them all off. I couldn't remember which margin applied to task end point, glides to turnpoints, glides to selected airports, and the glide amoeba. It's much easier to set them all to zero and then mentally say "I wont go unless I have 1000 over Mc 4" than it is to remember just what padding you put in the computer.

Also use a substantially higher McCready for safety than you do for speed. Work out your glide angle for Mc 2. You'll never do a Mc 2 glide over unlandable terrain after that!

The risk of not making it is actually more under strong conditions than under weak conditions. No lift = no sink! The safety margin is really about how much unexpected sink could you find.

John Cochrane

[[email protected]]

I agree. Also consider making a final glide over a long string of freshly cut fields all the way to the airport vs. trees, rocks and cactus for the last 20 miles. You could safely fly to zero in the first case and maybe give yourself a couple thousand feet in the latter.

By using zero and then deciding on a per glide basis it's no longer a "no brainer", you need to put a little thought into what you want this time.

-Tom

On Monday, October 3, 2016 at 2:24:22 PM UTC-7, John Cochrane wrote:
> It's much easier to set them all to zero and then mentally
> say "I wont go unless I have 1000 over Mc 4" than it is to
> remember just what padding you put in the computer.

[Tango Eight]

On Monday, October 3, 2016 at 5:00:50 PM UTC-4, Jonathan St. Cloud wrote:
> Does anyone use a safety altitude in their flight computers for final glide? If so why or why not?

No.

There is no one safe arrival height.

The correct course of action is to let the computer report your estimated arrival height at your destination (f(MC, wind, ballast, bugs) and then do that PIC thing.

Evan Ludeman / T8

[Jonathan St. Cloud]

The below is what I do, but I had not actually thought of putting in a higher MC than flying with at the time of final glide. Just curious about using a higher MC, when do you input this value, before beginning final glide or while I glide. If the day is still active I began final glide before I have arrival altitude.

On Monday, October 3, 2016 at 3:14:32 PM UTC-7, Tango Eight wrote:

> There is no one safe arrival height.
>
> The correct course of action is to let the computer report your estimated arrival height at your destination (f(MC, wind, ballast, bugs) and then do that PIC thing.
>
> Evan Ludeman / T8

[Tango Eight]

There's an infinity of scenarios. In the mountain sites I mostly fly, there are final glides that are predictable winners and others that are almost guaranteed to be losers. One common one for me into my home club (at day's end) has a last likely thermal source 20-some miles out, with a glide over a river valley that is normally a net loser, followed by four or five miles of completely unlandable terrain, followed by tall trees, then our airport, which is pretty much a bowl scooped out of higher surrounding terrain and is famous for obstructions on the runway (some with legs, some with wheels, some with large fabric envelopes filled with hot air) that you won't see until you are right there. I usually set that final glide up with MC=2.0 plus 1400. I pretty much always show up high enough to assess landing options, then "check the compass" against the appropriate runway heading, and that's the way I like it.

1400 is silly high for many sites, especially out in the flat lands.

On a wave day at Sugarbush, 1500 may not be enough.

If the wind is blowing, MC goes up because uncertainty goes up (another alternative is enter a manual wind, but that's more work). A stiff tailwind usually means wind shear which means your fat final glide may go "poof" when 25 kts at altitude runs into something less down low. Or you may run into wave sink, or whatever.

If wind is blowing hard across mountains or ridges, then most often I'm not flying a final glide per se... I'm "soaring home" on ridges or wave and the objective is to get there at sufficiently high altitude that I can deal with any conceivable nonsense (wave sink, rotor, what have you) when I get there.

I normally don't set MC higher than 3.5 for final glide. About the time I feel like I might need MC 4.0 (for safety) I'm thinking a final glide in the usual sense isn't such a hot idea.

best,
Evan Ludeman / T8

On Monday, October 3, 2016 at 6:28:12 PM UTC-4, Jonathan St. Cloud wrote:
> The below is what I do, but I had not actually thought of putting in a higher MC than flying with at the time of final glide. Just curious about using a higher MC, when do you input this value, before beginning final glide or while I glide. If the day is still active I began final glide before I have arrival altitude.
>
> On Monday, October 3, 2016 at 3:14:32 PM UTC-7, Tango Eight wrote:
>
> > There is no one safe arrival height.
> >
> > The correct course of action is to let the computer report your estimated arrival height at your destination (f(MC, wind, ballast, bugs) and then do that PIC thing.
> >
> > Evan Ludeman / T8

[waremark]

On Monday, 3 October 2016 22:00:50 UTC+1, Jonathan St. Cloud wrote:
> Does anyone use a safety altitude in their flight computers for final glide? If so why or why not?

I have reachable landpoints on the moving map identified in green. In order for the green to be at all meaningful I do set a safety altitude. That said, I am mindful of what safety altitude I have set and would take into account the factors mentioned by others in deciding whether to set off on final glide above or below that figure. (I also do a gross error check in terms of kms per thousand feet, and look at the glide ratio required which the computer shows taking into account the set safety altitude).

The basic rule for speed is to set MC for final glide to the actual climb rate achieved after centering in what is expected to be the last thermal. If in a good last thermal I do wind the MC up while climbing, to inform my decision on when to leave. It would not make sense to have that MC set at any time when you expect to have to find and center another thermal.

[Craig Funston]

On Monday, October 3, 2016 at 3:28:12 PM UTC-7, Jonathan St. Cloud wrote:
> The below is what I do, but I had not actually thought of putting in a higher MC than flying with at the time of final glide. Just curious about using a higher MC, when do you input this value, before beginning final glide or while I glide. If the day is still active I began final glide before I have arrival altitude.
>
> On Monday, October 3, 2016 at 3:14:32 PM UTC-7, Tango Eight wrote:
>
> > There is no one safe arrival height.
> >
> > The correct course of action is to let the computer report your estimated arrival height at your destination (f(MC, wind, ballast, bugs) and then do that PIC thing.
> >
> > Evan Ludeman / T8
John Cochrane's written a very good article on the subject. Well worth the read and it should answer many of your questions.

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

Click on the "Safety Glides" button.

Cheers,
7Q

[[email protected]]

On Monday, October 3, 2016 at 5:24:22 PM UTC-4, John Cochrane wrote:
> I wrote a Soaring article about this a while back. Some key points: The theory says you want an altitude minimum that is a quadratic function of distance to go. Basically, thermals are random. The chance of getting 3 tails in a row, 3 miles out, is higher than the chance of getting 30 tails in a row, 30 miles out. A McReady setting plus safety altitude is a good approximation.
>
> Now, do you put that safety in the flight computer, so it says "0" when you really have 1000 feet margin? I used to, but turned them all off. I couldn't remember which margin applied to task end point, glides to turnpoints, glides to selected airports, and the glide amoeba. It's much easier to set them all to zero and then mentally say "I wont go unless I have 1000 over Mc 4" than it is to remember just what padding you put in the computer.
>
> Also use a substantially higher McCready for safety than you do for speed.. Work out your glide angle for Mc 2. You'll never do a Mc 2 glide over unlandable terrain after that!
>
> The risk of not making it is actually more under strong conditions than under weak conditions. No lift = no sink! The safety margin is really about how much unexpected sink could you find.
>
> John Cochrane

I use the same methodology as John. McCready setting is raised to "final glide number" during the last climb. I may fly at 2 all day, but the final glide won't be at 2.
UH

[Per Carlin]

I always flying with 0m in the computer. Safety margin has to be adopted by local and weather conditions. Headwind, tailwind, terrain etc has to be taken into account every final glide.

In typical flatland conditions with evenly distributed Cu and no convergences is my tactics to have about +200-250m on actual Mc-value about 30-50km out. The closer I get to the airfield the lower margin can I accept as there is a smaller chance of getting in trouble, ideally do I have +50m 3km out to make straight in landing if applicable. As I’m flying an unloaded club class glider do I always have the option to burn out the extra energy by push the stick forward if I get to high 
On the other hand, flying in mountains with a ridge could easily get you home when you are -200m 20-30km out, the same goes for strong convergences.
I also tend to have more margin in tailwind than headwind situations, more than once have the tailwind decreased rapidly with height. A typical scenario is that you are +200m with strong tailwind at 1500m, after a few minutes glide are you at 1000m and the tailwind is now Zero. When the computer recalculates the final glide is all your margin gone, you are low and its late in the afternoon. An close to home outlanding is inevitable…

Therefore is it pointless to have an preset safety margin, it has to be set according to current situation.

[Tango Whisky]

I have set 150 m (500 ft) safety altitude, and I monitor required L/D versus actual L/D.
I exclusively fly in the Alps with a 47:1 ship. A required L/D of 20-25 is safe in almost all conditions. Anything between 30-40 might work, but you need a plan B. Did a 50:1 over 90 km this summer to return to Fayence after thermals died (same flight as in Philipp's video Fayence-Furka-Fayence), but only in almost still air with 10 km/h of tailwind.

Bert
Ventus cM "TW"

[Tango Eight]

Worth noting here that some guys are talking about contest flying and some are talking about wring-out-the-day XC flying. Those are very different scenarios with very different tactics.

If your last thermal is much over 2 knots... then you didn't really wring out the day :-).

-Evan Ludeman / T8

[krasw]

During last WGC at least club class leaders consistently took 200+ meters safety altitude even close to home, and over not very hostile terrain. So it should not be accepted as a fact that there is different set of safety margins during competitions, no matter what the level.

[Tango Eight]

On Tuesday, October 4, 2016 at 6:49:51 AM UTC-4, krasw wrote:
> During last WGC at least club class leaders consistently took 200+ meters safety altitude even close to home, and over not very hostile terrain. So it should not be accepted as a fact that there is different set of safety margins during competitions, no matter what the level.

It's completely different when your "destination" is a 1 mi radius circle 800' higher than the hosting airport, which is typical of US comps.

Glad some WGC guys are (finally) wising up. Hasn't always been that way.

-Evan

[Pat Russell[_2_]]

Programming an arrival height is like setting your watch ahead so that you'll get to your appointments early. It works, but only as long as you promise yourself that you'll never take the margin into account.

Why bother? Why not just let the watch tell you the truth? You can handle the truth.

Regardless of how bold or shy you are, there is no reason to program the instrument with an arrival height other than zero.

[RR]

I used to fly with a built in margin, I have now set it to 0. I think it was set to 800ft, so a normal pattern could be flown when I got home. Then on day my final glide got washed out (literally in the rain) and I really needed to know the correct arrival height. Was it 700ft reserve, 1000ft, what did 300 under glide realy mean? Now when I most needed the correct info, I was doing mental math. I switched that day.

Now on my kobo, I have set my ground clearance height to 1000ft, so my landing amobea shows a ring with reserve, but the arrival is shown correctly.

Lastly, everyone I know that uses a reserve (including me when I used it) would think something like this "I have a thousand over a thousand to the airport".

It seems that a feature that was supposed to reduce pilot workload, was only increasing it.

RR

[Bruce Hoult]

On Tuesday, October 4, 2016 at 12:24:22 AM UTC+3, John Cochrane wrote:
> I wrote a Soaring article about this a while back. Some key points: The theory says you want an altitude minimum that is a quadratic function of distance to go. Basically, thermals are random. The chance of getting 3 tails in a row, 3 miles out, is higher than the chance of getting 30 tails in a row, 30 miles out. A McReady setting plus safety altitude is a good approximation.
>
> Now, do you put that safety in the flight computer, so it says "0" when you really have 1000 feet margin? I used to, but turned them all off. I couldn't remember which margin applied to task end point, glides to turnpoints, glides to selected airports, and the glide amoeba. It's much easier to set them all to zero and then mentally say "I wont go unless I have 1000 over Mc 4" than it is to remember just what padding you put in the computer.
>
> Also use a substantially higher McCready for safety than you do for speed.. Work out your glide angle for Mc 2. You'll never do a Mc 2 glide over unlandable terrain after that!
>
> The risk of not making it is actually more under strong conditions than under weak conditions. No lift = no sink! The safety margin is really about how much unexpected sink could you find.

Is the "arrival height" the actual physical height of the aircraft at arrival at final glide speed, or the height after slowing down to approach speed?

Or maybe it's physical arrival height at 80 - 100 knots for most of the final glider, but then 5 km out you wind it up to cross the line a few seconds earlier with less than arrival height (but you can get it back).

[Dan Marotta]

I set mine at 1,000' above the center of the airport so when I see
1,500' above glide slope I just ignore it because I know that I'll hit
sink enroute and it really means nothing. I seem to get palm-sweatingly
low during the final glide but there's always a bunch of lift closer
in. It always works that way... Except when it doesn't.

Bottom line for me is that, though I jump through all the computer hoops
to get that great feeling of knowing exactly where I stand energy-wise,
deep down I know that it's not a perfect airmass and so I trust my eyes,
not the computer.

On 10/4/2016 5:32 AM, RR wrote:
> I used to fly with a built in margin, I have now set it to 0. I think it was set to 800ft, so a normal pattern could be flown when I got home. Then on day my final glide got washed out (literally in the rain) and I really needed to know the correct arrival height. Was it 700ft reserve, 1000ft, what did 300 under glide realy mean? Now when I most needed the correct info, I was doing mental math. I switched that day.
>
> Now on my kobo, I have set my ground clearance height to 1000ft, so my landing amobea shows a ring with reserve, but the arrival is shown correctly.
>
> Lastly, everyone I know that uses a reserve (including me when I used it) would think something like this "I have a thousand over a thousand to the airport".
>
> It seems that a feature that was supposed to reduce pilot workload, was only increasing it.
>
> RR

--
Dan, 5J

[Tim Taylor]

On Monday, October 3, 2016 at 3:00:50 PM UTC-6, Jonathan St. Cloud wrote:
> Does anyone use a safety altitude in their flight computers for final glide? If so why or why not?

Jonathan, you have many good responses so far. But I think you need to clarify the situation you are thinking of to understand the answers you have received. Most final glides are about risk/reward. Here are a few types of flights I can think of.
1. Fun glider flight with plenty of fields near the airport.
2. Fun glider flight with unlandable terrain near the airport.
3. Racing flight.

I set mine for 1000 feet above the desired finish height for most flights, but as John Cochrane stated less margin is required the closer you get to home. It is a sliding scale that should be more farther out and less as you get close. The 1000 feet gives me a reasonable number for most flights to provided a margin of error from 30 to 70 miles out. As I get closer I trade the extra height for speed. I fly with the MacCready setting for the thermal strength I am willing to take or the last one of the day that put me over final glide. If I gain more than my safety margin I can just turn the MC up to keep the margin I wish to the finish.

Why set a safety height rather than 0? It is easy for me to see, even at 80 or more miles out that when the computer says zero I can likely make it home. In racing the penalty for landing short is high verses the reward so it good to have the extra margin.

[Per Carlin]

On Tuesday, October 4, 2016 at 2:12:36 PM UTC+2, Bruce Hoult wrote:
> On Tuesday, October 4, 2016 at 12:24:22 AM UTC+3, John Cochrane wrote:
> > I wrote a Soaring article about this a while back. Some key points: The theory says you want an altitude minimum that is a quadratic function of distance to go. Basically, thermals are random. The chance of getting 3 tails in a row, 3 miles out, is higher than the chance of getting 30 tails in a row, 30 miles out. A McReady setting plus safety altitude is a good approximation.
> >
> > Now, do you put that safety in the flight computer, so it says "0" when you really have 1000 feet margin? I used to, but turned them all off. I couldn't remember which margin applied to task end point, glides to turnpoints, glides to selected airports, and the glide amoeba. It's much easier to set them all to zero and then mentally say "I wont go unless I have 1000 over Mc 4" than it is to remember just what padding you put in the computer.
> >
> > Also use a substantially higher McCready for safety than you do for speed. Work out your glide angle for Mc 2. You'll never do a Mc 2 glide over unlandable terrain after that!
> >
> > The risk of not making it is actually more under strong conditions than under weak conditions. No lift = no sink! The safety margin is really about how much unexpected sink could you find.
>
> Is the "arrival height" the actual physical height of the aircraft at arrival at final glide speed, or the height after slowing down to approach speed?
>

It is usually energy compensated. That means if you plan to arrive at Vne at GND will you hit the ground a few km out.
Nothing I as a clubbclass pilot cares about, but the energy compensation is with 100% in pull up yield. In an teroretcal pull up will you reach the arrival hight, in reality slightly below.

[Tango Eight]

On Tuesday, October 4, 2016 at 6:29:53 PM UTC-4, Tim Taylor wrote:

> Why set a safety height rather than 0? It is easy for me to see, even at 80 or more miles out that when the computer says zero I can likely make it home.

But it never (practically speaking) *says* zero, does it? Are you going to abandon your final glide because the number goes to some small negative number (like hell...)?

The only good reason for a margin height so far articulated is to make the "green dots" meaningful on some moving map displays.

Evan Ludeman / T8

[Pat Russell[_2_]]

> The only good reason for a margin height so far articulated is to make the "green dots" meaningful on some moving map displays.

Good point. Perhaps some enlightened manufacturer could make a glide computer which features independent programming of the arrival height and the height at which the dots change color.

[[email protected]]

On Thursday, October 6, 2016 at 8:36:57 AM UTC-4, Pat Russell wrote:
> > The only good reason for a margin height so far articulated is to make the "green dots" meaningful on some moving map displays.
>
> Good point. Perhaps some enlightened manufacturer could make a glide computer which features independent programming of the arrival height and the height at which the dots change color.

My manufacturer gives me the tools to display the series of points where I will hit the ground and the series of points where I will reach the decision height I select.
FWIW
UH

[Martin Gregorie[_5_]]

On Thu, 06 Oct 2016 16:26:09 -0700, unclhank wrote:

>
> My manufacturer gives me the tools to display the series of points where
> I will hit the ground and the series of points where I will reach the
> decision height I select.
> FWIW UH
>
This is the strange ink-splot shape, also known as a 'glide amoeba',
that LK8000 can display. Whether it shows at all depends on how you
configure the program. If you have 'terrain' on, then the 'Glide Terrain
line' can be either 'Line' or 'Shade'. 'Line' shows an outline round the
area you can reach. 'Shade' colours the whole reachable area. If Terrain
is 'off' then you can only use 'Line' because 'Shade' changes the colours
used for terrain and terrain isn't being shown.

I fly in a flat area, so turn Terrain off to get a higher contrast
display and set the amoeba to 'Line'.

Similar glide-amoeba interactions may also happen in other programs, so
if you can't see it, try changing its representation and/or terrain
display.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Papa3[_2_]]

On Tuesday, October 4, 2016 at 7:32:34 AM UTC-4, RR wrote:
> I used to fly with a built in margin, I have now set it to 0. I think it was set to 800ft, so a normal pattern could be flown when I got home. Then on day my final glide got washed out (literally in the rain) and I really needed to know the correct arrival height. Was it 700ft reserve, 1000ft, what did 300 under glide realy mean? Now when I most needed the correct info, I was doing mental math. I switched that day.
>
> RR

A good friend and tremendous wit, Tony Benson used to have a great saying. "When the magic reads zero, I open the canopy and step out." His point was exactly yours Rick - trying to remember what margins/cushions etc he had set up sometimes required math at the least convenient times.

FWIW, I have the two glide amoebae (amoebas?) on the Clearnav set at 0 feet (Kellerman calls it the Crash Line) and 1,000 feet. So, I can instantly get a feel for where every known landable field sits relative to those two values.

P3

[JS]

On Thursday, October 6, 2016 at 7:56:45 PM UTC-7, Papa3 wrote:
> On Tuesday, October 4, 2016 at 7:32:34 AM UTC-4, RR wrote:
> > I used to fly with a built in margin, I have now set it to 0. I think it was set to 800ft, so a normal pattern could be flown when I got home. Then on day my final glide got washed out (literally in the rain) and I really needed to know the correct arrival height. Was it 700ft reserve, 1000ft, what did 300 under glide realy mean? Now when I most needed the correct info, I was doing mental math. I switched that day.
> >
> > RR
>
> A good friend and tremendous wit, Tony Benson used to have a great saying.. "When the magic reads zero, I open the canopy and step out." His point was exactly yours Rick - trying to remember what margins/cushions etc he had set up sometimes required math at the least convenient times.
>
> FWIW, I have the two glide amoebae (amoebas?) on the Clearnav set at 0 feet (Kellerman calls it the Crash Line) and 1,000 feet. So, I can instantly get a feel for where every known landable field sits relative to those two values.
>
> P3

Ah...
"The Basic Benson", as Captain Staubach would call him.
Too bad neither are around to defend themselves.
When in doubt, resort to Look Out The Window Dot Com.
Jim