Skyraiders were often time used for RESCAP in SAR operations and where known as Sandy, thus the call sign. It may have had something to do with this definition of the word 'Sandy':
SANDY: the navigational name of the northeastern-most corner of the Saigon Flight Information Region (FIR), of which flight past assured the crew of combat pay and combat income tax exemption.
I dunno if this is all really connected, I wasn't there! But in makes sense...
Some other info on the names given to the Skyraider:
'Twas originally designated and named the BT2D Dauntless II
When the USN designation system changed in 1946, it was then known as the AD (Attack, Douglas) (hence the nickname Able Dog)
The unofficial name 'Spad' was also connected with the AD designation; someone along the line coming up with the acronym SPAD (Single Place Attack Douglas)
The Skyraider was also known as 'Hobo' (callsign?), 'The Flying Dumptruck' and 'Trau Dien', which is Vietnamese for 'Crazy Water Buffalo'.
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