Safety Altitude

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.

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"

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

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.

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

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.

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

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).

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

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.