Airplanes retract their landing gear while in flight for the same reason.

出于同样的原因，飞机在飞行时收回起落架。

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The more difficult second level requires a flight of 180 seconds followed by a landing on simulated lunar terrain, complete with rocks and craters.

更困难的第二个级别，要求飞行180秒，接着在遍布岩石和陨石坑的模拟月球地形上降落。

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Although this flight ended in a successful landing, the graph in Figure 2 shows several large spikes (the blue line) in height measured by radar.

尽管这次飞行以成功着陆结束，图 2 中的图表还是显示出一些陡峭的峰值（蓝线）。

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Everything worked as planned. Everyone expected the third moon-landing flight, Apollo Thirteen, would go as well as the first two.

VOA: special.2009.07.22

It was the countdown for Apollo Eleven the space flight that would carry men to the first landing on the moon.

VOA: special.2009.07.15

They were more than two-thirds of the way to the moon on a flight path that would take them to a moon landing.

VOA: special.2009.07.22

Sullenberger, who was hailed as a hero for successfully landing the flight, also testified Tuesday before a House subcommittee, telling lawmakers the state of the airline industry is in disarray.

CNN: Controller thought Hudson landing would be 'death sentence'

Before landing, the flight crew were not aware the nose wheel steering system was not working, as the hydraulic fluid level was low.

BBC: NEWS | Europe | Isle of Man | Air crew criticised for fire risk

Last month, safety regulators grounded the entire fleet of Boeing 787 Dreamliners after a battery on a plane operated by Japan Airlines caught fire, while a battery malfunction forced an emergency landing of a flight operated by All Nippon Airways.

BBC: Japan Airlines raises full-year profit forecast