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AirAsia Flight QZ8501: How passenger jets avoid bad weather

12/31/2014 05:00 EST | Updated 03/01/2015 05:59 EST
Aviation experts say that poor weather alone is almost never the sole cause of a commercial airliner crash, but pilots will try to avoid nasty systems just to be safe. 

Indonesia AirAsia's Flight QZ8501, an Airbus A320-200 carrying 162, lost contact with air traffic control early on Sunday during bad weather on a flight from the Indonesian city of Surabaya to Singapore. 

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Wreckage and bodies were pulled from the sea off the coast of Borneo on Tuesday and the investigation into what caused the crash has only just begun. 

What types of weather pose a risk to planes?

Bruce Carmichael, director of the aviation applications program at the National Center for Atmospheric Research in Boulder, Co., said thunderstorms combine a variety of weather conditions that can affect the performance of a plane. 

These include severe turbulence, structural icing that can increase drag, wind sheer, poor visibility and lightning strikes. 

"For commercial airlines, the thunderstorm embodies almost every hazard known to aviation in one package," Carmichael said. 

There can also be a large amount of ice crystals that can build up inside engines and cause a flameout. Carmichael said pilots are almost always able to restart engines before reaching the ground. 

Edward McKeogh, president of Canadian Aviation Safety Consultants, said avoiding poor weather is often a question of ensuring a smooth flight. 

Perhaps one of the most nerve-wracking weather phenomena for worried flyers poses very little risk to the actual aircraft.

"For commercial airlines, the truth is that turbulence is not much of a hazard if people are belted in and not up and about," Carmichael said, adding that airframes on commercial airliners are designed to handle the stress. 

How do pilots detect bad weather?

Pilots rely on a number of different sources for detecting bad weather, including onboard radar systems that give an indication of conditions about 160 kilometres directly in front of the aircraft, says Edward McKeogh, president of Canadian Aviation Safety Consultants. 

They also depend on information provided by dispatchers from their individual carrier to find a safe path around or above the bad weather. Air traffic controllers can also provide additional weather conditions.

Some of this info can come from pilots who have already passed through a weather system who are themselves communicating with controllers and company weather dispatchers.

How do pilots avoid bad weather?

Pilots have a few options for steering clear of bad weather. 

One option is to fly above the system, but this is not always possible, McKeogh said, because some storms can reach higher than 45,000 feet (13,700 metres) — above the maximum height of commercial aircraft. 

Pilots will also try to plot a course through the bad weather by looking for areas where the storm is less intense or gaps between adjacent systems. 

"These are often like picket fences, you got a little right and a little left and between the cells," McKeogh said.   

McKeogh said sometimes pilots will try to fly around bad weather systems if there is no clear way through, usually staying around eight kilometres away. 

A U.S. Federal Aviation Administration document entitled "Thunderstorms — Don't flirt...skirt 'em" also suggested that planes fly no closer than eight kilometres from thunderstorms, and to generally stay 30 kilometres away if possible. 

How do they plot a course?

Carmichael said the decision on how to avoid a thunderstorm is based on a number of factors, including the amount of fuel on board and the number of planes in the area. 

Flight QZ8501 was flying at 32,000 feet (9,753 metres) and had asked to fly at 38,000 feet (11,500 metres) but failed to get permission due to heavy air traffic before controllers lost contact. 

"It`s a very collaborative process that takes place between the pilot flying the aircraft, the dispatcher sitting in their airline office area and the controller and traffic manager on the side of the air navigation service provider," he said.

The latter is trying to find a safe path for multiple aircraft that are all trying to traverse the same weather system. 

However, pilots do have the ultimate authority to change course in the event of an emergency, Carmichael said.

Do planes turn back because of weather?

Experts say it is extremely rare for commercial airliners to turn back because of weather although it can happen with smaller craft. 

"It typically happens for some mechanical reason or some other reason," Carmichael said. "Usually a commercial aircraft when they take off, they know what they`re going to be doing."

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