Quote:
Originally Posted by Ouisch Your question was phrased just fine.
The sign doesn't use an article mainly because on road signs they want to keep the text as concise and brief as possible. People are driving by and have to be able to read and understand the text on a sign quickly.
Also, because there might be more than one low flying aircraft, no article is necessary. "Aircraft" is plural in this instance. |
It wan't written on a road sign, in fact. A traffic sign with a picture of an airplane is explained by the sentence (at least in the book).
Then, the reason for not using the article is probably the second fact you mentioned. However, I don't understand how can (one) aircraft be a plural... But you are a native speaker - not me

... you must understand it better.