Clear Air Turbulence
The clear air turbulence or clear air turbulence (abbreviated CAT ) to German turbulence in cloud-free air , is a strong movement of air in areas with no visible clouds phenomena. With strong accelerations it leads i. d. Usually to an unintentional change in height of an aircraft , which is sometimes perceived as an " air hole " by aircraft occupants . It occurs during flight without the pilot being able to foresee it by intensively observing the airspace. In clear air, the pilot has no visible indication of the movement of the air masses .
The Clear Air Turbulence is caused by the meeting of larger air masses that move at very different speeds at heights of 7,000 to 12,000 meters. The phenomenon often occurs in the area of the jet stream , sometimes also near mountain ranges. This type of turbulence can pose a significant threat to aviation , since, in contrast to other weather phenomena such as thunderstorms , it cannot be located with the naked eye or with radar and therefore cannot be avoided by flying around it in time. Clear air turbulence can be measured with a scintillometer (measurement of the refractive properties of the air) or with a Doppler lidar .
The very rapid change in direction and speed of the air leads to rapid and unpredictable fluctuations in the aircraft's lift .
Larger aircraft are not shaken as strongly by these turbulences due to their large wingspan and their high mass and the associated inertia. On the other hand, the airframe of these machines is particularly stressed by turbulence.
Serious incident
On December 28, 1997 fell Boeing 747 of United Airlines (flight 826) on a flight from Tokyo to Honolulu after a two hour flight over the West Pacific in 33,000 feet altitude 1,000 Miles east of Tokyo in heavy turbulence. Shortly beforehand, the pilots had received a warning about strong clear air turbulence and switched on the seat belt sign. The aircraft was shaken very badly, the inside cabin was devastated, passengers and drinks carts hit the ceiling. One passenger died in the incident and 110 passengers were injured. The plane immediately returned to Tokyo. Twelve passengers were admitted to the hospital. The flight recorder showed that the aircraft had been lifted from the turbulence and sagged 100 feet six seconds later.
This incident underscores the recommendation made by many airlines and the FAA in 1995 to stay buckled up during the entire flight, even if the seatbelt sign has gone out. In any case, the pilots have to stay buckled up with their lap belt during the entire flight . The shoulder straps, which are only required for take-off and landing, are also put on again for safety in the event of expected turbulence.
On December 5, 1996, 16 passengers on an American Airlines flight were injured when the plane struck Clear Air Turbulence over Colorado .
On January 10, 2008, 14 passengers on Air Canada Flight AC190 were injured when the Airbus 319 unexpectedly encountered severe turbulence on its way from Toronto to Victoria at an altitude of 35,000 feet. Six passengers were seriously injured and had to be hospitalized after the emergency landing in Calgary .
On April 12, 2016, a Boeing 777-200 with the flight number Thai Airways International TG434 on the way from Jakarta to Bangkok got into violent turbulence which injured several people.
On May 1, 2017, 27 passengers in a Russian passenger plane were injured in severe turbulence on a flight from Moscow to Thailand. According to the authorities, the aircraft of the airline Aeroflot was already approaching Bangkok when it suddenly got into turbulence on Monday night. The aircraft was shaken so violently that passengers who were not wearing seat belts were thrown against the ceiling. According to the Russian Embassy in Bangkok, 24 Russians and three Thais were injured. Most of the victims suffered bruises or broken bones, so several passengers had to be operated on.
According to the FAA, from 1981 to 1996 there were exactly 252 reports of turbulence affecting large aircraft. Two passengers died (both not wearing seat belts), 63 were seriously injured, 863 were slightly injured. Two thirds of the accidents occurred at altitudes over 30,000 feet. Every year around 60 passengers in the USA are injured by turbulence because they are not wearing seat belts.
Even at the slightest sign of Clear Air Turbulence ( e.g. pilot report , PIREP) from other aircraft, the pilots switch on the seat belt sign for the passengers and may change their altitude or flight route slightly and reduce their airspeed.
For smaller aircraft, the maximum speed for flights in turbulent air is known as V NO (Velocity Normal Operations, Normal Operating Speed, or Maximum Structural Cruising Speed) and is the end of the green arc on the airspeed indicator . Then the yellow arc begins. This speed range may only be flown in calm air. Furthermore, the pilot will limit his rudder deflections (steering deflections ) and no longer make full rudder deflections. Otherwise, the loads on the aircraft structure from the rudder deflections and the turbulence could add up to a critical force.
hazards
It is assumed that clear air turbulences alone are not able to damage or even destroy aircraft.
On the other hand, in connection with other circumstances, weather phenomena or static material weakness (see hairline cracks ), it is considered certain that clear air turbulences can contribute to damage or even to the complete destruction of an aircraft, see BOAC flight 911 .
However, it must also be taken into account that the BOAC Boeing 707 accident occurred at the early start of jet aviation, 1966, and since then extensive material changes have fundamentally improved the stability of new aircraft.
literature
- Richter, Jan-Arwed; Wolf, Christian - Mayday! Flug ins Unglück , GeraMond Verlag, 2006, ISBN 3-7654-7044-9
Individual evidence
- ↑ Aircraft accident data and report in the Aviation Safety Network (English)
- ↑ Thai B772 near Singapore on Apr 12th 2016, turbulence injures 6. In: The Aviation Herald. Retrieved April 16, 2016 .