when 的用法，下面这句里的when用法该怎么解释啊，要是我光看翻译，不会想到会用when。谢谢！！
After taking off, we were flying low over the city and slowly gaining height, when the plane suddenly turned round and flew back to the airport. 起飞之后，我们在城市上空低低地飞行，然后慢慢爬高。这时飞机突然调转头来，飞回了机场。
英语中，连词when表示"当……时候"，引导的时间状语从句表示与主句动作同时或不同时，从句动词用延续或非延续性都可以。如： I'll telephone you when I get there tomorrow.明天我到那里的时候就给你打电话。 但是在高一新教材Unit 15 "The necklace"中，就出现了when用作并列连词，表示"就在这时"(=and this time/just at that time)的用法。如： Tom was sitting near the fire when he heard a knock at the door.汤姆正坐在火炉边，这时他听到一阵敲门声。 I was just going to speak when the doorbell rang.我正准备讲话，这时门铃响了。 当并列连词when用于"就在这时，正在这时"的用法时，它所引导的并列句表示动作发生后，另一个并列句所表示的动作之后，一般不可以用其他词代替，常用于以下结构： 1.was/were doing +when意思为"就在这时，正在这时"。如： I was walking in the street when someone called me from behind.我正在街上走，就在这时有人从后面叫我。 They were discussing the problem when the teacher came in.他们正在讨论问题，就在这时老师进来了。 2.was/were about to do...when意思为"正要/正准备做某事，就在这时……"。如： She was about to go out when an unexpected visitor came.她正准备出去，这时来了一位不速之客。 We were about to set out when it began to rain.我们正要动身，就在这时开始下雨了 3.be+表示状态的介词短语+when意思为"正在做某事，就在这时……"。如： They were at work in the office that afternoon when a big fire broke out.那天下午他们正在办公室工作，就在这时一场大火发生了。 We were on duty in the classroom when a boy rushed in.我们正在教室里值日，就在这时一名男孩冲了进来。 Li Ping was on his way home when two boys stopped him.李平正往家走，突然两个男孩拦住了他。
首先，上面这句英语有问题，句子结构不对，表达的意思乱.when通常作状语从句引导词，表示当...时候，一般放在句中，也可以放在句首。上面的句子应该这样改：After taking off, we were flying lowly over the city. When the plane suddenly turned round,we found it flewing back to the airport.每个句子都是一个意思，不同意思的句子通常放在两个独立句子里，这样是否更清楚呢？