OLC and Skylines Differences

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For 12 years now the soaring flight evaluation program Onlinecontest or OLC has become the worldwide standard for evaluation of flight performance providing almost instant insight into what is happening in the world of XC soaring. It went thru a number of changes and updates but is now officially endorsed by the International Gliding Committee (IGC) and has been a tremendous instigator of getting pilots to push their XC soaring skills. One of the more appreciated features of the OLC is that it fit in nicely with the advent of the GPS based digital loggers and that it uses the digital flight data to optimize the distances covered by the soaring flight to derive the greatest point count (now based on 6 legs for plain distance and an FAI triangle for an additional bonus). There is actually some mathematical wizardry involved, supplied by some soaring addicted mathematicians, which we take now for granted.

But there is now another XC evaluation program available, that some pilots prefer to the OLC – the SkyLines .5program. This program offers a few advantages but is supposed to use the same OLC rules and optimization for flight analysis. In most cases the results from these two programs are the same (or at least within a fraction of a point of each other). However lately there have been a few cases where the results for a given flight evaluated by these two programs were significantly different. Take the flight by Chris Young on May 18, 2015, his first flight in the Duckhawk from Arlington:

OLC: 214.5 points/204.6 km distance/ 126.0 km triangle

SkyLines: 176.2 points/ 196.2 km distance/ 64.5 km triangle (i. e. 38.3 points less).

Or take this flight by myself (and Kathleen) on the same day

OLC: 113.5 points/ 94.4 km distance/ 56.1 km triangle

SkyLines: 126.7 points/ 108.1 km distance/ 68.1 km triangle (i. e. 13.2 points more).

Examination of these flights shows that the differences are due to a misidentification of the release point by the SkyLines software. Identifying the release from tow (or engine shutdown) has been a weak point for both programs but much less so for the OLC. The usual indicator is a change in the energy rate, i. e. a sudden change in speed or rate of climb. However, if the tow pilot goes into “stretch mode” (i. e. power reduction) the reduction in climb rate may be interpreted as release (apparently the cause for the differences in Chris Young’s case). Likewise, releasing in a strong thermal the speed and the rate of climb may not change significantly enough to indicate release (as in my case). An additional indicator would be a drastic direction change into circular flight (i. e. thermalling) but all these indicators have limitations and in its current form SkyLines seems to be the weaker program in identifying the beginning of the soaring flight. This may cause more than a little difference in distance; in the case of Chris Young’s flight it meant the difference between closing a larger triangle or not (thus the large difference in triangle distances).

The gist of it all? The flight evaluation results from both programs have to be scrutinized for proper identification of the release point and edited if necessary. And maybe the programs will be made more foolproof for this purpose.