Altitude is only half of your energy.
A glider carries its energy in two accounts. Potential energy is height and the altimeter reads it directly. Kinetic energy is speed, and the exchange rate between them is one line of physics: h = v²/2g. At 200 km/h that is 157 metres of height hiding in the airspeed indicator. Slow to 100 km/h and 118 of those metres reappear on the altimeter, minus what drag takes on the way.
This is why a final glide flown fast and one flown slowly can arrive at the same place with wildly different altimeter readings and still be, in the only sense that matters, equally successful. The fast one banked its margin in speed. The total energy height — height plus the speed converted into height — is what the glide is really about, and it is the number your computer is quietly managing when it tells you that you are 200 metres above the glide.
Ninety-seven kilometres an hour, by assumption.
A final-glide computation has to assume something about how the glide ends. Assume you arrive at zero speed and the arithmetic is nonsense. Assume you arrive at whatever you happen to be doing and the number jumps every time you move the stick. LX computers take the sensible third option: they assume you cross the finish at a fixed reference speed of 27 metres per second — 97 km/h — and that everything you are carrying above that is available to you as a pull-up.
So when the display says you will arrive with your reserve intact at 180 km/h, it has already counted the ninety-odd metres of excess speed as height. Cross the line at that speed and hold it, and you are physically lower than the reserve suggests. Pull up as the model assumes, and the height appears — about 85% of it, because the zoom is not free. If you never pull up, you finish below the computed glide, and the computer was not wrong. It simply expected a manoeuvre you declined to fly.
What the recorder writes down.
The default is the trap. Two hundred metres of reserve sounds generous, but at 180 km/h ninety of those metres are speed, so the recorder sees you cross at 110 — and from a 1200 metre release that is more than a kilometre of height lost. Slow the crossing to 110 km/h and watch the marker climb without changing the reserve at all — it stays red until the reserve itself clears the floor. Then raise the reserve until the verdict turns: that reserve is the one to set on a badge day.
What the file will say.
The IGC file is pressure altitude and position at a fixed interval — every second, or every few. Whatever else the recorder appends, the evaluation reads no airspeed, no energy and no opinion about what you were about to do. So when a claim is evaluated, the finish is a barometric altitude at a moment in time — and the Sporting Code limits how far below your start or release point that altitude may be. For badge and record flights the limit is 1000 metres of height loss, and the wording differs between badge tasks and records, so the current edition and your national association have the final word.
The practical consequence is uncomfortable and worth internalising: a finish that is legal by the display can be illegal by the file. The display is counting your speed. The file is not. On a day when the claim matters, the reserve altitude in the computer stops being a safety number and becomes a paperwork number as well, and it needs to cover both jobs at once.
MacCready sells height for speed at the line, too.
Raising the MacCready setting on a final glide does exactly what it does anywhere else: it buys speed with altitude. The glide gets steeper, you arrive faster, and the display barely flinches, because the extra speed is being counted as height. What falls first and furthest is the barometric crossing altitude. Fly the last thirty kilometres at MC 4 instead of MC 2 and you will finish measurably lower on the altimeter while the computer insists nothing has changed.
Wind works on the geometry rather than the energy. A headwind on the final glide steepens the ground track and demands more height at the start, but it does not change what your speed is worth once you are there — the conversion between speed and height is the same in any airmass. That is a useful separation to hold on to when the last thermal is weak and the decision is whether to take it.
In your cockpit.
Three habits follow from all of this. Know what your crossing speed is worth in metres, because it is the difference between the display and the altimeter. Set the reserve for the job the day actually has — a safety margin on a normal day, and a margin plus the badge floor on a claim day. And if you do pull up at the line, pull up gently: the gust-loads article explains what an abrupt one costs the wing, and most competitions now put a minimum height on the finish ring precisely so that nobody needs one — for reasons that have nothing to do with structure and everything to do with the gliders behind you. For the arithmetic of the glide itself, start at the final-glide article; for what the file contains, see the flight-recorder article.
Final glide, energy and reserve on one page — big enough to read honestly at 200 km/h.
Arrival altitude with the finish convention built in, and a reserve field worth setting deliberately.
Runs the finish arithmetic once, on the Sense's air data, and every NAVIA screen shows the same arrival.