Just one week ago, on Wednesday, 40 people were killed after a TransAsia Airways flight crashed into a river in Taipei, Taiwan's capital, after hitting an overpass.Footage of the doomed plane clipping an elevated section of the road before plunging into the Keelung River has been imprinted on people's minds.In apparent defiance of statistical probability, this was the second fatal air accident for the same airline within the last seven months. On July 23, another TransAsia flight crashed in Taiwan's western Penghu Islands during an aborted landing, claiming more than 40 lives.Last Wednesday's crash also follows three Malaysia-related aviation disasters in the last 11 months – including the unknown fate of Malaysia Airlines Flight MH370, which went missing last March en route to Beijing with 239 passengers and crew aboard not long after leaving Kuala Lumpur.Despite the most intensive search in commercial aviation history, no trace of the plane has been found so far. In total, there were 990 fatalities in 21 fatal airliner accidents in 2014, according to an accident report of the Netherlands-based Aviation Safety Network.Reasons behind aviation accidents vary, but it is often a series of risk factors coming together simultaneously that can spell disaster, say aviation experts speaking to Anadolu Agency."These could be everything – technical failures, pilot errors or unexpected weather conditions, but air accidents generally occur when many failures come together," said Behçet Kömürcü, general coordinator of the Association of Turkish Airline Pilots.According to the Transport Ministry, pilot error is the primary cause in 55 percent of all fatal air accidents. Mechanical failures follow with 20 percent rate and weather conditions with 13 percent. The U.S.-based website PlaneCrashInfo.com claims the pilot's role is the primary cause in 57 percent of plane crashes.Kömürcü, a retired pilot who flew around the world for more than 40 years, said those flying aircraft have to exercise split-second decision-making. "For example, in the Indian Ocean, weather conditions change often. In this situation, as a pilot, you should make quick decisions in order to prevent any unexpected situations."However, there are some unpredictable weather incidents like lightning, fog and wind shear that make things difficult for pilots."Lightning can cause a plane's engine to breakdown or fog can affect the visibility range negatively," the retired pilot says.Turkish meteorologist Mikdat Kadıoğlu describes the importance of weather estimates for a safe flight."What will be the direction of the wind during the flight? At what altitude will a plane need to fly in order not to hit turbulence? How much fuel will a plane need? All of these are about weather estimates and should be calculated properly."Birds – even if they are small – are also another threat."You cannot predict a bird strike. These kinds of incidents happen while taking off and landing. Airport officials take measures against them, but you may encounter a bird regardless and it can cause damage to the plane," Kömürcü added.In this situation, the skills and experiences of pilots are crucial, say experts."Pilots should be ready for every kind of failure and should evaluate every situation in the right way in order to avert any dangers," said Bekir Er, a flying instructor at the Ankara-based Turkish Aeronautical Association."Experienced pilots can decrease the risks for accidents if they have a lot of flight experience," Er said; however, he is also cautious about this. "But some accidents still occur with these pilots as some of them are so self-confident and do not care about certain warnings."Aircraft maintenance checks are another risk factor."Some technical checks can be lacking at times or conducted in a wrong or poor way, which can cause an accident," Er said. "Again, planes are mechanical vehicles – you cannot guarantee that they will not break down even if checks are completed properly."On Friday, Taiwan's Aviation Safety Council revealed that data from the two black boxes retrieved from the TransAsia Airways ATR72-600 aircraft indicated that both of its engines had lost power before it crashed into the Keelung River.According to Er, "More engines mean more safety … If I could choose, I would always select four-engine planes to travel with. But again, this does not mean two-engine planes are not secure. You can travel comfortably with them as well."Although all of these risk factors exist, air travel is still accepted as the safest means of transportation, compared to driving, and its safety is getting better by the day with advances in technology.The reality is that despite the recent high-profile crashes, there has been a reduction in the overall numbers of aviation accidents over the past years, according to a report of the United Nations' International Civil Aviation Organization (ICAO).The global accident rate decreased by around 40 percent, from 4.1 accidents per million departures in 2009 to 2.8 accidents in 2013, the report said.