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How to (EN): understand the 'arrival height' indication on the SDI C4


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As of the update to 1.6 of the Aerosoft Discus, the SDI C4 flight computer now computes and displays a real-time calculation of your predicted arrival height all the way around the current flightplan.

As a reminder, you need a flightplan loaded into FSX for the SDI C4 to provide the full 'flight computer' functionality, i.e. it has to know where you are trying to fly to. This is exactly the same as the real SDI C4 - it relies on 'goto' waypoints being defined in the attached GPS, and the GPS providing 'distance to go' information.

The GOTO cursor position on the SDI C4 has 3 options that can be chosen via the right-knob:

(1) FG:OFF

In this setting the SDI C4 will ignore the distance-to-go reading from the GPS and the 'mid' numerical display on the C4 LCD panel will display current altitude. Unless you have altered the pressure setting, this will be MSL (i.e. above mean sea level), and will be in feet or meters according to your FSX preferences.

(2) FG: WP

Here's where the flight computer begins to get fancy. The SDI C4 will attempt to calculate your likely arrival height at the next waypoint, as illustrated in the picture below:


The C4 has to take into account a variety of factors to come up with a reasonable prediction of your arrival height, assuming a straight glide to the waypoint. These include:

  1. Current altitude - obviously how high you are now will directly affect how high you will be at the waypoint.
  2. Glider performance (also called the glider polar) - this gives the glide ratio at various speeds, i.e. the slope you glide down in still air. You can tell the flight computer you have altered the glider performance by changing the settings for Ballast and L/D (max glideslope) on other cursor positions.
  3. Distance to go - the further away the waypoint is, the lower you are going to be when you get there.
  4. Speed you are going to be flying - the faster you fly, the steeper you come down. You tell the flight computer how fast you will be flying via the Mccready Setting on another cursor setting.
  5. Wind - strength and bearing computed automatically by the C4, a headwind will make you come down more steeply (reduce your arrival height), a tailwind will extend your glide (increase your arrival height).
Actually the C4 uses the parameters in 2-5 to calculate height needed to go the distance required, and then subtracts that from your current altitude to get the arrival height.

You can view the same 'arrival height' reading either of two ways, which are actually synonymous (a good word for our German readers to look up...):

  1. it is the predicted altitude MSL you will arrive at the waypoint
  2. it is your current height above glideslope to arrive at the destination at zero feet/meters.
You can use either of these definitions, whichever you're comfortable with.

(3) FG:RTE

Here is the new feature in the updated firmware (update 1.6) in the Aerosoft Discus. FYI in the real C4, this capability was provided via a firmware update also...


This setting doesn't take a huge amount of explanation if you have fully grasped what the flight computer is doing in the GOTO/FG:WP setting. In the FG:RTE case, the flight computer is calculating the 'height needed' for each of the remaining legs of the task, and subtracting those from your current altitude to estimate the arrival height at your ultimate destination.

Note that each leg of the task has a different distance and bearing, so the effect of the wind will vary greatly around the task. In the example illustrated the pilot has a net headwind on the current leg, but will enjoy a tailwind on the final leg. The C4 can only use the current wind to compute the vectors necessary for the effect on your arrival heights, and a change in the wind around the task (perhaps as you get lower on the final glide) can have a significant impact on the computed altitude. E.g. on the task illustrated the wind may reduce as you descend on the final leg, and you find you have less of a tailwind than originally assumed using the wind at a higher altitude - in this situation the C4 will get more pessimistic about your arrival height as you progress.

This calculation is now truly for a final glide. Note that in the A.H (arrival height) cursor setting, you can specify an offset to be added to the displayed arrival height, to either add a safety margin or (more usually) to input the ground elevation of the destination airport. If you have a value in the 'A.H.' cursor setting other than zero, this will always be added to the true calculated MSL arrival height and displayed accordingly. This makes absolute sense for your arrival at the destination, but note the 'A.H. offset' figure will be added to the displayed arrival altitude in both FG:WP and FG:RTE settings (as the real C4) and makes a bit less sense for the waypoints.

Many FSX soaring tasks (e.g. 'Task 2' by Peter Luerkens) do expect you to have enough height to 'final glide' around the last turnpoint all the way home, and the FG:RTE setting will start giving you clues on the penultimate leg that you've almost made it home...

If you suspect a 'final glide around the last turnpoint' is likely to be possible, the normal technique would be to fly on 'FG:WP' for most of the task, but switch to 'FG:RTE' when you think that would suggest you've almost made it...

Note that elite pilots may well set off on final glide with the C4 indicating a negative arrival height. They will then attempt to improve their glide performance by pulling up in thermals (but not turning) along the final glide, so they gain pieces of altitude (relative to the still-air glideslope) as they progress and cross the finish line in the shortest possible time. This tends to lead to a higher risk of land-out though.

Good luck.


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Thanks a lot B21 for providing this manual :rolleyes:.

There is quite some information about the C4 but for me as beginner/n00b it is still hard to understand this masterpiece.

Isn't it a good idea to make a complete step by step manual with some examples as a pdf? (I'm even willing to pay for such a manual)

anyway, thanks for sharing your knowledge.



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  • 2 weeks later...

Yikes I should have kept an eye on this forum - apologies for the slow reply, I didn't expect replies to the 'tutorial' post.

Re the manual - I hope you've found the actual original SDI C4 user guide that is included with the Aerosoft package - I looked around on the web and found an online copy: C4 Competition Handbook, thanks to some Kiwi gliding crew...

Apart from that, and my tutorials that I've drafted here, really you need a book on cross-country soaring. The best one IMHO is by Helmut Reichman, called "Cross-Country Soaring". It's kind of in two halves - the first half is about the techniques of cross-country soaring (e.g. thermalling, dolphin flying, speed to fly) and the second is a technical explanation of the pressure sources and instruments, and all the different calculations and methods appropriate to soaring, e.g. how total-energy is calculated, or final glides. If you're more of an artist than a techie you can still appreciate the first part, and more browse the latter technical explanations, but it is a landmark book all the same.


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Thanks a lot for the link and the book title.

I will find out if I can found that book overhere in the Netherlands. I found 1 store but it is rather expensive... about 60 Euro....



By the way, isn´t it a good idea to put a copy of that book as a supplement in this forum? (that pdf file from your link)



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