Ali the planes had retractable undercarriages .

所有飞机都有收放式起落架。

A machine of this sort often contains up to six cutting heads fashioning simultaneously , under computer control and at breakneck speed , small pieces of metal that are later assembled into larger items such as undercarriages for aircraft or vehicle engines .

一台机器往往有6个切割头，在电脑控制下，它们以飞快的速度同时对小块金属进行加工，随后，这些小块金属被组装成较大的产品，比如飞机起落架或汽车引擎。