Today’s invention is a way for aircraft to communicate with the ground if their radios are jammed or otherwise interfered with.
Pipework would lead from the aircraft’s waste water system to each wingtip.
In a radio breakdown situation, an Air Marshall, for example could send a text to a receiver on the plane which would translate this into morse code.
The code would be interpreted by the waste water pump and generate bursts of fluid from each wingtip.
These might alternate, thus forming a pattern of water vapour tracks in the sky which could be read from the ground without eg a hijacker becoming aware of the communication (…LLRRRLLLLRLRLL…)