Why Can’t Some Planes Fly in Hot Weather?
It is sometimes more difficult to get airplanes off the ground in extremely hot weather. But the problem is not the temperature itself or the humidity.
It is the air density that can get in the way.
Hotter air gets thinner. Thin air makes it more difficult for planes to lift and land safely. This is especially the case with smaller planes.
That is why more than 50 flights were canceled in the southwestern United States earlier this week. Temperatures there have climbed to 49 degrees Celsius.
Airplanes take off and stay in the air because of lift -- the force from the movement of air under the plane's wings that push it upward.
Lou McNally is a professor of meteorology at Embry-Riddle Aeronautical University.
When pilots try to take off in extreme heat the planes sometimes cannot lift before the runway ends. And, when pilots try to land when the air is too thin and hot, they can also run out of runway, McNally said.
"As air warms up, it expands and there's fewer molecules to be under your wing," he said. With less lift, "you need more of everything. You need more thrust to take off. You need more distance (on the runway) to take off. You need more distance to land. You need more speed to land."
To fly in high heat, planes must create more power and have larger wings.
At Dubai International Airport and other Gulf airports many flights arrive late at night and early in the morning when it is cooler. Gulf airlines also often use larger planes that are not as affected by high heat.
In Phoenix, Arizona, though, airlines still use small jets for shorter trips. This week's extreme heat is rarer there than in Persian Gulf cities.
Airlines can take steps to be able to safely fly in extreme heat. One step is to reduce the plane's weight by selling fewer seats. That is what American Airlines did during the recent Phoenix heat wave. Airlines can also reduce weight by lowering a plane's fuel level. Then the plane can stop somewhere cooler to refuel and take off again.
Before they fly, pilots examine measures of what is called density altitude. These include temperature and humidity.
The density altitude is the main issue, but there are other risks that come with flying in hot temperatures. R. J. Hansman, an aeronautics professor at MIT, says the plane's parts may become too soft or even melt.
And pilot Patrick Smith says the plane's brakes and machinery can overheat. At that point, "things begin to break down," he said.
I'm Ashley Thompson.
1.take off 起飞
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2.break down 分解；发生故障
Talks with business leaders broke down last night.
3.push upward 往上推动
Water exerts an upward push on all objects in the water.
4.run out of 用完
They have run out of ideas.
1.It is the air density that can get in the way.
get in the way 妨碍；阻碍
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Cockburn engaged in a flight of fancy, never once allowing facts to get in the way.
2.At that point, "things begin to break down," he said.
At that point 在那时
At that point, anything could happen
At that point the radio handset fell to pieces.
有时，飞机在极度高温的天气中是很难起飞的 。不过，原因并不是温度或空气湿度 。
天气炎热空气较稀薄 。空气稀薄会影响飞机正常起飞和降落 。这种情况对于小型飞机更为常见 。
本周早些时候，美国西南部超50多班飞机因为高温被迫停飞 。那里的气温已飙升至49摄氏度 。
卢·麦克纳利（Lou McNally）是安伯瑞德航空大学的气象学教授 。
他表示：在高温天气下，有时飞机滑到跑道尽头仍然无法起飞 。在高温且空气稀薄的状态下降落时，飞机也容易冲出跑道 。
他指出，“随着空气温度上升，气体膨胀，机翼下的空气分子量会减少” 。一旦飞机的升力减弱，那么就需要“万事俱备” 。起飞时，飞机需要的动力更足，起飞距离更长 。而降落时，飞机需要的跑道更长，降落速度更快 。
迪拜国际机场和其他海湾地区机场的飞机通常会在温度较低的夜间或凌晨时降落 。海湾地区的航空公司也常使用大型飞机，避免受高温天气的影响 。
在美国亚利桑那州的凤凰城，短途航线仍然由小型客机执飞 。该市本周的极端高温天气比波斯湾地区更为罕见 。
航空公司可采取措施保证高温天气下飞机的安全飞行 。例如，对航班进行限售来减轻飞机载重量 。近日，在凤凰城的高温天气下，美国航空公司纷纷采取这一措施 。航空公司亦可通过降低飞机的燃油水平来减轻载重 。之后飞机可降落在气温低的地方加油，然后再起飞 。
起飞前，飞行员会检测所谓的“密度高度” 。检测的内容包括温度与湿度 。
虽然密度高度是主要问题，不过高温下飞行还存在其他风险 。麻省理工学院航空学教授R. J. Hansman指出，飞机的部分零件可能会变软甚至融化 。
飞行员帕特里克·史密斯（Patrick Smith）表示，飞机制动与机械装置可能会过热 。到那时，“形势就不容乐观了” 。