Safety Altitude

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

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

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...ring/index.htm

Click on the "Safety Glides" button.

Cheers,
7Q

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.

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.

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.

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

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.

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

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 |