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 5:24:22 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

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"

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).

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 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