Sentence:  A Huskie helicopter was whirring over the Gulf of Tonkin in search of Cullen when Vietnamese 1st Lieut. Nguyen Van Phu, who ditched his flaming Skyraider near the spot where the U.S. pilot went down, fired a <WSD>smoke signal</WSD> to attract its attention. 
Meaning of the word marked with <WSD> tags:  A type of flare or combustion device sometimes used as a distress signal.