Last week I noted an incredible video of an Italian pilot who had kept a glider aloft for more than 11 hours, over the sharpest peaks of the Alps, relying strictly on wind current rather that with any kind of motorized support.

The letters I've received have been in two categories.They ask either, "how could he stay up this long? Did he start somewhere near the moon?" or else "Why on Earth does the video showing flying right along the faces and steep escarpments of the mountains, rather than keeping a prudent distance?" Eg:


Others have wondered why some of the flight was on the shaded side of the mountain ridge, rather than in the sun. Michele Travierso, an Italian writer and glider pilot based for many years in China (where, he points out, he can't fly very much), sent me the original video and now sends in this explanation for the seemingly dare-devil path right next to the cliffs:

To answer to your reader on why in many scenes of the video the glider is flying on the dark side of the mountain, while usually one would expect to find rising air (lift) on the sunny side of it: it has to do with the steepness of the mountain face, the wind strength and orientation and what glider pilots call the optimal "working layer."

This flight was mostly conducted using ''orographic lift', or lift generated by wind hitting a mountain or hill face, ideally at a 90 degree angle. That day, the wind was prevalently from the north, which generated lift on north-facing side of the Alps, hence parts of the footage are in the shade. Not all the time however. The route was not a perfect east-west line and the ridges follow a sinuous path that needs to be mimicked accurately, as it's shown in the video. [And the map below.] Also, the flight lasted 11 and a half hours and the sun position changed.


The pilot, Alberto Sironi, stayed close to the face of the mountain because that's where the strongest lift is. Steeper faces require the pilot to fly closer to them, in terms of horizontal position relative to the mountain, while shallower ones produce lift a bit further away from the face of the peaks. In regards to the altitude at which he flies, the pilot made "an energetic bargain." More accurately: it's certainly possible to gain more altitude than that in those conditions, but he must have learned that flying at that particular altitude and position relative to the range gave him the best ratio of speed and lift. The name of the game for breaking a record is speed. In other words, he could fly faster, without either stopping or losing altitude, if he stayed close to and at pretty much the same height of the crests.

Ridge soaring is the way of gliding that allows higher speeds since a pilot rarely has to stop and climb. It's also the most challenging and, many say, the most rewarding. The first way one learns involves flying in thermals, which are bubbles/columns of rising air, warmer than their surrounding air mass, that usually culminate in those white, fluffy 'good weather cumulus' clouds - to gain altitude in a thermal, a pilot has to fly in a circle inside them, thus losing time. Most good days involve a mix of ridge and thermal flying. See this source on ridge soaring.

I share the tremendous respect that your readers have towards those that can accomplish such great distances at these speeds. Hats off to Mr Sironi then!