Safety Altitude

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.

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 |

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

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