One of these can be a spare  engine.

其中之一可以是备用引擎。

It would simply attach the  spare engine to the wing of an existing 747.

它只需将备用发动机连接到现有 747 的机翼上。

So why doesn't the practice of ferrying a  spare engine under the wing happen anymore?

那么，为什么在机翼下运送备用发动机的做法不再发生了呢？

However, to have shipped a spare engine over to South Africa would  have cost time and, subsequently, more money.

然而，将备用发动机运到南非会花费时间，随后会花费更多的钱。

We've discussed before how Qantas did this in 2016 as a way of ferrying a spare  engine to one of its stranded jumbo jets.

我们之前已经讨论过澳航如何在 2016 年将备用发动机运送到其中一架搁浅的大型喷气式飞机上。