Clear Air Turbulence

The Clear Air Turbulence ( CAT for short ) to German turbulence in cloud -free air referred to air turbulence in areas without visible clouds phenomena. They lead to an unwanted change in height of an aircraft and are perceived by occupants of the aircraft as an air hole. They occur during the flight without the pilot can anticipate this through close monitoring of the airspace. In clear air the pilot has no visual indication of the movement of air masses.

The Clear Air Turbulence is caused by the clash of major air masses that move at very different speeds at altitudes 7000-12000 meters. Sometimes also in the vicinity of mountain ranges, the phenomenon often occurs in the region of the jet stream. This type of turbulence can be a significant threat to aviation, as it can be in contrast to other weather phenomena such as thunderstorms, neither tracked with the naked eye or with radar and therefore can not be dodged in time by flying around. To some extent clear air turbulence can be measured with a scintillometer (measurement of Refraktionseigenschaft the air) or with a Doppler lidar.

The very rapid change of direction and speed of air leads to rapid and unpredictable fluctuations in the lift of the aircraft.

Larger aircraft are shaken not as strong this turbulence due to their large wingspan and their high mass and associated inertia. On the other hand, the airframe of these machines of turbulence is very heavily loaded.

Severity of incidents

On 28 December 1997, a Boeing 747 became the United Airlines ( flight 826 ) on the flight from Tokyo to Honolulu to two-hour flight over the western Pacific in 33,000 feet altitude, 1000 miles east of Tokyo in heavy turbulence. The pilots had just previously received a warning of strong turbulence Clear Air and turned on the seatbelt sign. The aircraft was very difficult to shake the inside cabin was devastated, passengers and beverage cart flew to the ceiling, a passenger died while 110 passengers were injured. The aircraft immediately returned to Tokyo. Twelve passengers were taken to the hospital. The flight data recorder showed that the plane was lifted from the turbulence and six seconds later by 100 feet ( about 30 meters) by sagged.

This incident once again underlines the recommendation of many airlines and the FAA of 1995, to remain as fastened during the entire flight, even if the seatbelt sign is off. Pilots must remain buckled anyway during the flight with her lap. The shoulder straps that are prescribed only for starting and landing shall be reinvested also to the safety of the expected turbulence.

In the U.S. about 60 passengers injured by turbulence annually because they are not strapped.

On December 5, 1996 16 passengers were injured on a flight operated by American Airlines as the plane came over Colorado in Clear Air Turbulence.

On January 10, 2008 14 passengers were injured on the Air Canada flight AC190, as the Airbus 319 suddenly fell on his way from Toronto to Victoria at an altitude of 35,000 feet in heavy turbulence. Six passengers were seriously injured and had to be stationary supplies after an emergency landing in Calgary.

According to the FAA, there were 1981 to 1996 exactly 252 reports of turbulence, the large aircraft concerned. Two passengers (both not wearing ) died, 63 were seriously injured, 863 were slightly injured. Two thirds of the accidents occurred at flight altitudes above 30,000 feet.

Even the slightest signs of clear air turbulence (eg pilots report, PIREP ) from other aircraft, the pilots will turn on the seatbelt sign for the passengers may change their altitude or flight route easily and reduce their flight speed.

For smaller aircraft, the maximum speed for flights in turbulent air than VNO (Velocity Normal Operations, Normal Operating Speed ​​or Maximum Structural Cruising Speed) is called and is on the airspeed indicator, the end of the green arc. Then the yellow arc starts. This speed range only allowed to fly in calm air. Furthermore, the pilot will limit its surface deflections ( control deflections ) and make any full rudder deflections more. Otherwise, the loads on the aircraft structure through the control surface and the turbulence could add up to a critical force.

Conclusion

It is believed that Clear Air Turbulences alone are not capable of damaging or even destroying aircraft.

However, in conjunction with other factors, such as Weather phenomena or static material weakness (see hairline cracks ), it is well established that can (see BOAC Flight 911) contribute Clear Air Turbulences to damage or even complete destruction of an aircraft.

However, this is also taken into account that the misfortune of the Boeing 707 BOAC in the early start of the Jet Aviation, 1966, occurred, and since then extensive material changes the stability of new aircraft have generally improved.