Safety McCready Final Glide

[Philip Lee]

I don't understand the final glide symbol and behavior with respect to safety mccready.

The symbol shows the STF McCready value, but the arrival altitude seems to be calculated with the safety McCready? I can change the STF McCready to 10 kt, and the arrival altitude doesn't change.

This is so strange trying to optimize the final glide and surely not what anybody expects. Who do I email to fix this?

[Paul Remde]

Hi Philip,

I don't use the Safety Mc or the similar Safety Mc Offset.  I think the idea is that it can be used like a conservative safety buffer - similar to the "Safety Altitude" that is also found on the QNH and RES screen.  I keep the Safety Altitude set to 1000 ft - so I know exactly how much extra altitude I have for final glides.  Some pilots instead want to use the Safety Mc to give some safety margin.  For example, they may be flying on a MacCready 3 day with MacCready set to 3 for cruising speed indication, but they may set the Safety Mc to 5 knots.  The Safety Mc will be used to calculate final glide altitude required.  

Alternatively, on the QNH and RES screen, you can change from "Safety Mc" to "Safety Mc Offset" and enter something like +2 knots.  If you had the MacCready set to 3 and the Safety Mc Offset set to +2, the system would use 3 + 2 = 5 knots for the MacCready used to calculate final glides.  The goal being to have very conservative final glide calculations.

I think what you were seeing in your note above is correct.  The system is working as it is designed to work - no bug.  

You can see more information in the manual on page 34.

https://gliding.lxnav.com/wp-content/uploads/manuals/lx90xx-80xxUserManualEnglishVer900rev53.pdf

Best Regards,

Paul Remde

Cumulus Soaring, Inc.

[Steve Koerner]

I'm a believer in the idea of safety MC and always use it in addition to an arrival altitude setting.  Normally I keep the safety MC set higher than the operating MC because of the possibly very high consequence of not making a glide due to extraordinary or unexpectedly high sink in route.  

The OPs question raises an interesting point, however. When it occasionally happens that the operating MC is set to a larger value than the safety MC, wouldn't it be better that the arrival altitude calculations be based on the larger (more conservative) of the two settings? I haven't put a lot of thought to the matter, but off-hand, it seems like that would indeed be more useful.

Steve Koerner - GW

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[Morgan Hall]

I use the Safety MC Offset for this reason.  If I’m flying MC 3.0 and my offset is 2.4, my final glide calcs are at 5.4 so there is that increased buffer.  Since I rarely find myself on an MC0 final glide, I’m usually at an MC4.4 or so safety factor plus a 1000ft arrival.  

The problem with the Safety MC Offset is that you do need to understand very clearly what it is doing so that you keep your margins where you are happy.  For people new to the LX equipment and MC theory I usually recommend a straight safety MC so that they don’t fly around at MC0 all day with only a MC2 buffer or something insignificant like that.  Once you are flying faster and using higher MC values effectively I prefer the offset approach. 

Morgan

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[Dave Springford]

Take a look at my video about Safety MacCready and Offset here:

https://www.youtube.com/watch?v=hPDj59BgIk0&list=PLeAW0PUi2pqKAMJP7ETBplGNNlUYh0aKl&index=16&pp=iAQB

It does not answer your specific question, but should help understand what the computer is doing when using these functions.

One other "gotcha" to note  is  - if on the QNH & RES Page in the Setup menu - you have the Safety MacCready title showing in the selection field on the left side of the screen and any value including 0 is in the box on the right side - this will over ride any other MC setting you have for final glide and result in the behaviour you are describing where despite changes to your in flight MC the arrival altitude does not change because it is now associated with the Safety MacCready setting.  The in flight MC setting now only affects your speed to fly.

To fix this, edit the left side field to show the Offset option.    Now the offset is  being used instead of Safety MacCready.  If you want no offset set it to zero, if you want an offset set it as desired, but the Safety MacCready box can cause problems if it is set below your current in flight MC!

[Steve Koerner]

From Springford's comment, it seems like he agrees that it would be better if the glide were calculated based on whichever is the greater of the two MC settings. 

I'm worn out on making suggestions to LX, maybe somebody else can suggest that little change to them.

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[Philip Lee]

Thanks for the input all. The basic thing that is wrong is the UI. If it's designed to calculate the glide only on the safety mc, then the symbol should display the safety mccready value (or no value at all, just like the reach calcs on the map). It doesn't make any sense that the arrival altitude and displayed mccready have no connection to each other on the same UI element. XCSoar got this right. Anyway, somebody me an email off-list who I can bother about this.

[Ben Hirashima]

Philip, if safety MC is in use, the arrival altitude should always be calculated using the safety MC value, regardless of what your regular MC is set to. That's why the arrival altitude doesn't change when you change your regular MC value. This is intended, and XCSoar works in the same way. When using safety MC, your regular MC only affects speed to fly. The final glide symbol shows your regular MC, because regular MC can change often, and it's useful to have a reminder of what you set it to. Safety MC is not meant to change, and therefore is not displayed on the final glide symbol. You set it and forget it, just like the safety altitude. That being said, a side effect of using safety MC is that in order to calculate a racing-style final glide to an airport,  you have to set your safety MC to your climb rate in the final thermal. This is why safety MC is adjustable in the menu that appears when you press the center button on the stick; the one where you can adjust regular MC, ballast, bugs, etc. Personally, I never adjust safety MC in flight. On a recreational flight, I'm more concerned with having a generous margin than arriving as soon as possible. 

If you're racing, you should use your task arrival height to calculate your final glide, and that brings me to the one big problem with LXNav's implementation of safety MC.

They are using safety MC AND safety altitude to calculate the task arrival height. This is totally wrong. When you're racing, you want to ignore safety margins on final glide. With the improvements to safety practices at modern contests, you are racing to a finish that is typically not at ground level, or even at an airport. You're on final glide to the bottom of a finish cylinder with a floor well above the ground, and you don't care about having a safety margin above it. The safety margin is built in to the finish cylinder by the contest organizers. If the bottom of the finish cylinder is 1000ft AGL, you don't need a safety margin. For tasks, and ONLY for tasks, the arrival height should be calculated using your regular MC, regardless of what the safety MC is set to, and the safety altitude should not be included. I wish they would change this, but does take a lot of energy to convince LXNav to change things.

[Morgan Hall]

I think some of the confusion around Safety MC in the LX environment is that I believe it was an add-on based on input from Ramy.  So initially, the LX equipment did not have a concept of Safety MC.  Your glide was simply calculated based on your current MC settings and Bugs.  I think Ramy raised the Safety MC concept as an XCSoar user and eventually LX Implemented it as well as the Safety MC Offset option which is sort of a compromise between no padding or margin and the rigid Safety MC. So the fact that Safety MC and Safety MC Offset feel a bit wonky and their behavior catches people by surprise is not shocking to me.  

With more and more time in the LX ecosystem I think more along the lines of the three options represent different stages of experience with XC soaring and with LX equipment.

Option A) Minimal XC experience: I push people towards using a fixed Safety MC value of 4 or 5+.  This is for the person that maybe just barely understands MC theory and is spending a lot of mental energy on just exploring XC.  They aren't really thinking about speed and may not even vary their MC setting much.  So having a large static margin along with 1000ft arrival height will help them make it to landable options with minimal mental energy about the system.

Option B) Intermediate to Advanced XC but wanting built-in margins:  Use Safety MC Offset and an arrival height commensurate to comfort.  Maybe an offset of 2-3 depending on the terrain being flown over. But this type of pilot understands MC theory and changes their STF accordingly. Still wants a margin built in via an arrival height and some polar degradation.  

Option C) Advanced XC - No safety MC, possibly no arrival height margin.  The reality for this pilot type is that they are already doing all of the margin assessment continually.  They probably have some L/D Required value that they are comfortable with and are monitoring that in addition to the arrival height.  Adding in safety margins and arrival heights just complicates the mental load by performing math to achieve the "real" arrival height. +1000 means +1000 above the airfield not +1600 or some such padded amount.  

Personally, I'm still flying Option B. But I've been trending towards Option C and ditching Safety MC Offset and Arrival heights because I'm already evaluating lots of other data and the MC Offset and Arrival height just end up costing more mental math than anything.  If I'm trying to get final glide across generally unlandable terrain, I'm not really using the final glide advice from the glide computer.  I'm looking at the margins and the L/D required and deciding if they are fat enough.  The glide computer might think 32:1 is acceptable, but given my options I might be in a 20:1 mindset given terrain or anticipated winds.  

A large reason I have stuck with arrival heights and offsets is that I still fly a lot of mentoring flights in a Duo and having the safety margins built in is something that sets a good example and probably requires less explanation.  It prevents me from being in a "do as I say, not as I do" state.  I've always felt and stated that whatever method you choose doesn't matter so long as you understand what your margins are and how they are configured.  You don't want to think you have 1000ft arrival height built in and then find out it was zero. 

No wrong answers I suppose. But I think generally whatever option people use, they still hopefully "do the math" and validate the assumptions.  The flight computer never knows what air you are headed into or what terrain you are heading over, so we've all got to plan accordingly with that info.

Morgan

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[Ramy Yanetz]

Just catching up on this thread. All good answers. In my opinion everyone should use either safety MC or offset, and understand how it is calculated. I prefer safety MC4 and no arrival height margin which requires no additional calculation. Add offset or margin and you will need to pay more attention and do more math. So if it shows me arrival altitude of 100 feet it basically means I don’t have enough for even abbreviated  pattern just straight in. Also if you use offset as you increase your MC to fly faster, suddenly some airport labels will get out of glide, which personally I find confusing and misleading.  

I don’t think we need to request LXNav to change those things as it may cause further confusion. 

You can set Nav boxes  for arrival with safety MC, MC0 and I believe there is also one for  current MC. 

But reading Philip’s original question, I believe he refers only to the final glide chevrons, not to map labels or Nov boxes. I barely pay attention to it, but I believe he is right, the final glide chevron symbols which shows current MC should show arrival using this particular MC, or at least have an option to configure it to behave this way. 

Ramy

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