3/5 – Gliding and volcanic ash

Patagonia 2011

To all things misfortune is good, it is said. If ash is actually a poison for the engines and prevents all take-offs, the situation changes when the motor is put back.

Once in flight, we discovered two types of volcanic ash:

Primary ash, which is the fresh ash leaving the crater and materialized in the plume, isolated and easily visible in a clear sky (January 2012) but hard to identify when absorbed into a cloud layer.

Primary ashes embedded in the cloud layer, November 2011

In flight at 14.500ft above the erupting crater

Its extent perpendicular to the axis of the plume is quite limited, about 10 km at a distance of 100 km away from the volcano, but its density and toxicity are very high. Better to stay home, wearing the mask if required to go out.

The photo shows the position of the plume of primary ash on January 29, 2012. When the wind turns quickly, the ash front penetrates into the air mass like a cold front.

Front of ashes arriving from NW

Luckily, we have never been forced to fly under these conditions where the visibility is only a few tens of meters. The secondary ash is the consequence of wind erosion and lifting of the primary ash deposited onto the ground by the plume, carried away toward the East, day after day. The photo shows the extent of the damaged zone, a triangle whose leg is more than 600 km long.

Plume and affected area, 29.1.2012

The photo shows, on the one hand, the total desertification of the territory (we are flying abeam Caleufu, the wing pointing east, the territory visible on the photo is about 80 x 80 km), and, on the other hand, the intensification of the density of ash in suspension while going eastward.

Rio Caleufu covered with ashes

The photo, taken 50 km north of Bariloche at 6700 m, looking south towards the lake and the city, shows the position of the wind erosion ash front, the mountains located to the west appearing clean whereas, everything that is downwind of the front is totally unflyable, at least with an engine.

Wind ashes in movement

We had twice been forced to fly down home while crossing the secondary ash cloud. The top of this cloud doesn’t exceed 3000m and the visibility is around a few hundred meters, in all directions. Considering the absence of icing risk, the presence of three redundant GPS navigation systems and of two gyroscopes supplied by three independent electric circuits, the IMC descent didn’t pose any problem except that the time it took to come down from 3000m in this mash seemed to me an eternity that will not quickly fade from my memory.

But not everything has to be thrown away: the ashes lifted by the wind are so light, that they gorgeously materialized the hydraulic jumps and the classic rebounds. Very useful at the start, allowing us to easily identify the heart of the lift! The photo, taken at 3000m and 10km north of the airfield looking north, shows the perfect materialization of a small hydraulic jump, with the presence of Kelvin Helmholtz rolls while the particles of air and ash were falling at a super critical speed, which means a speed higher than the average speed of the surrounding air mass, consisting in a practically vertical rise of the particles from the ground until the altitude at which the speed becomes, once again, equal to that of the surrounding air mass, in the present case, around 3500m. One can observe that, about 50 km north, this system gives way to a classic, sinusoidal, rebound wave system without Kelvin Helmholtz rolls.

Hydraulic jump materialized by the ashes

The photo below, looking eastward, shows the materialization of the lifted ash trapped in a magnificent hydraulic jump located downwind, right in the middle of the Pampa about 50 km away from the mountains. This photo shows three important features of the hydraulic jump: 1) the presence of rolls aligned with the axis of the wind ending precisely at the front, 2) a unique, practically vertical front, in which the particles of air and ash rise to the altitude of the laminar layer and 3) a high altitude cloud having a leading edge located headwind (into wind??) with respect to the ground line of the front (the shadow can be seen). In this case also, it can be observed that the ash does not pass into the laminar layer and remains caught between the ground and about 3000m. A disaster for power planes, light or commercial aviation.

Hydraulic jump materialized on the pampa

The photo below, taken at 5000m and 50km North of the airfield, looking Northeast, shows, on one hand, the total absence of exchange between the two air masses and, on the other hand, the vertical amplitude of the sinusoidal displacement of an elementary particle, which is in the order of only 200 to 300 m. This characteristic represents the fundamental difference between thermal and wave lift. In the first case, each elementary particle travels the whole distance from the ground to the cloud, while, in the second case, it travels vertically only a few hundred meters and the movement is transmitted from one particle to another, similarly to the game of colliding pendula. It is therefore necessary to forget the classic diagram that can be found in the best books, in which one sees a sinusoidal movement of the fluid lines having an amplitude of the same order of magnitude as that of the mountain, whereas, the actual slant of the upward and downward fluid lines is only a few degrees, hardly visible on a drawing at scale 1/1.

Materialization of the rebounds by the ashes

In conclusion, whatever the wave mode, no air particle is transferred from the turbulent layer into the laminar layer, which was always completely devoid of the least dust.