On Oct. 7, an A330-330 “VH-QPA”, flying from Singapore to Perth as Qantas 72 with 303 passengers and 10 crew members on board, made an emergency landing in Learmonth Western Australia after it suffered a sudden change in altitude that caused 33 (still unconfirmed figure) injuries. It is not clear what actually happened to the QF72 flight, even if a mid-air collision or a tech-issue could not be excluded, the most probable cause of the altitude change was some sort of severe turbulence.
According to my personal experience, turbulence is one of the aviation-related things that causes more worries to the passengers. Most of my friends are scared of flying just because of turbulence. Various kinds of turbulence exist, in accordance with the meteorological phenomenon that provokes it. However, in macrocospic terms the turbulence can be defined as the vibrations suffered by an airframe because of the movement of air masses. Based on this first definition, all those things that cause vibrations, prejudicing the quality of the flight (if not quite putting it in danger), can be considered turbulence. The vertical wind shear, a vertical stream caused by a quick variation of the direction and intensity of the wind (that can be particularly dangerous at low altitude when the aircraft is in short final for landing) can be considered a sort of turbulence. The turbulence is measured with a scale that takes into consideration the effects on the aircraft: Light Turbulence, if the aircraft is subject to light impulses to change altitude and attitude; Moderate; if the intensity is higher but still under control e completely manageable by the crew; Severe, if the variations are ample and violent and lead the aircraft momentarily out of control; Extreme, if the aircraft suffers violent airframe shock, is damaged and goes out control. Obviously, such a scale, that measures the effect of the vibrations on the aircraft, enables to quantify correctly the turbulence experienced by aircraft of the different type. A Moderate Turbolence is quite different between a B.777 and a Tampico. The kind of turbulence the majority of you will encounter travelling around the world, should not exceed the Moderate level: should a flight cross an area of turbulence it would request the Air Traffic Control a course deviation or an altitude change that should bring the aircraft in more tranquil air. However there are a few cases of aircraft encountering Severe Turbolence (like, maybe QF72) and in those cases the result is always the same: there are a few injured, most probably those who hadn’t their seat belt fastened. All the aircraft transiting through the same area have to inform the ATC unit that they are experiencing turbulence at that particular Flight Level, as to permit the radar controller to inform all the other aircraft approaching the area of the intensity of the turbulence at a particular altitude. All the aircraft, from the C-150 to the Airbus 380 can be interested by turbulence, even if the type and intensity of the phenomenon that can interest a civilian wide-body flying at FL330 above the Ocean, is not even comparable to the one affecting a small private plane. I perfectly remember the turbulence that invested my little C-152 when I flew abeam Monte Soratte (to the North of Rome), in bound Capranica. The aircraft began to vibrate because of the intense (for that kind of aircraft) winds that were caused by the mountains and I had to reduce the speed in order to decrease the intensity of the forces acting on the surfaces of the aircraft. Other thing are the vibration that I experienced in a B747-400 (VB-OJJ) flying at FL330 between BNE and LAX over the Pacific. The origins of the two kinds of turbulence are quite different. In the first case, the turbulence was caused by an obstacle (a mountain) that broke the airflow causing the so-called Mountain Waves that in particular weather conditions can generate the characteristic lenticular clouds. In the other case, the turbulence was caused by the bad weather that the QF175 encountered enroute. This kind of turbulence is often linked to the jetstreams, that are generated by the difference of pressure between troposphere and tropopause. At the border between these two atmospheric areas, these stremas move large mass of air that cause turbulence. Jetstream can be particularly strong and travel parallel to the ground at heights around 11 chilometers in a West-East direction (in both emispheres because of the Coriolis acceleration). There are two major jetstream flows in both emispheres at polar latitudes and two minor subtropical flows next to the Equator. Since in the Northern Emisphere the jetstream appear at high latitudes (up to 60° – 70° N), they interest many transatlantic routes: even for this reason flights from the US to Europe last 2 hours less than flights on the opposite direction. It is connected to the jetstreams the Clear Air Turbulence, a kind of turbulence that is not associated to any cloud. Anyway, no worry, the flights are planned taking into consideration the possibility to encounter a jetstream and aircraft are projected to resist to any CAT. Just keep your seat belt fastened; many injuries could have been avoided and could be avoided in the future.
