A Kam Air passenger jet carrying 104 people crashed into a mountainside in Afghanistan this week, apparently killing everyone on board, in the countrys worst aviation accident. Six American civilians were among the dead. A heavy snowstorm likely contributed to the crash, although landing at the Kabul airport is a challenge even during good weather, because the city is on a high plateau surrounded by mountains. Aging air fleets and poor air-traffic control are also concerns. The U.S. government and many international aid agencies do not allow their employees to fly Afghan airlines.