同学你好，该知识点来自沪江网校《新概念英语2、3、4册连读【随到随学班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
这里用had touched down，是表示这件事一发生，就要发生keep calm and to get off the plane这两个动作了，所以touch down相当于一个过去的过去。但是这里用一般过去时也是可以的。

as soon as 指紧接着发生的两个短动作，主从句都用一般过去时
如 He took out his English books as soon as he sat down.

有时，为了特意表达刚刚完成某事就如何如何，需要用完成时。
如：I will go with you as soon as I have washed my face.


祝同学学习进步~