" Does a Plane have a Horn?" (yes & no) - KLM Blog

not the kind that can warn public people of danger: 

"So, the signalling system aboard an aircraft isn't really used in the same way as the hooters and horns aboard other vehicles and vessels. A pilot can't hoot at other aircraft. In fact, the horn doesn't work when the aircraft is in flight, because the signalling system is turned off.
Whereas horns in cars are mainly used as a warning system, the horn ..."

https://blog.klm.com/does-a-plane-have-a-horn/
Vk