这句话是由when引导的定语从句吗？

Question

这句话是由when引导的定语从句吗？The sound of planes could be heard outside the city of Tangshan even when on planes were in the sky.

Answer 1

不是定语从句,是由when引导的时间状语从句.
The sound of planes could be heard outside the city of Tangshan even when no planes were in the sky.
即使天空没有飞机，在唐山城外也可以听到飞机声。

Answer 2

　　不是定语从句，是时间状语从句。因为定语从句需要有一个先行词，在主句The sound of planes could be heard outside the city of Tangshan 中没有可以做先行词的词。本回答被提问者和网友采纳

Answer 3

when这里引导时间状语从句, 甚至当没飞机在天空时