The salutation is not encrypted because it's designed to attract the attention of the radio operators on the plane. They need a few moments to grab their pens t

The salutation is not encrypted because it's designed to attract the attention of the radio operators on the plane. They need a few moments to grab their pens to write down the message that follows so they aren't going to miss any characters. So far as leaking information - by the time these are sent as real-world communications (i.e. not as a test or during training), the planes are nearing their target. So it won't matter for much longer if the enemy knows, since the positions of the planes are (hopefully) unknown.