Green’s Dictionary of Slang

(US) a helicopter; also attrib.

1965		L.A. Times 30 Jul. I 22: Then they told me on the radio that the helo (helicopter) was on the way [HDAS].
1971		B. Adler Vietnam Letters (2003) 17 Feb. 155: They hauled them up to the dust-off helo like a sack of potatoes.
1977		J. Langone Life at the Bottom 53: That helo pilot who parked his bird on the top of Erebus and took a piss into the crater.
1989		J. Joss Strike 45: Lifesaving SH-3 helo alights for mere seconds.
2018		‘Aidan Truhen’ Price You Pay 232: Even the helo’s gone. Got a system failure, engine trouble, the whole damn nine.