Safety Altitude

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

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

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 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.

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

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, 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.

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, 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