Ralph has wished for rescue. He assumed this rescue would come in the form of aircraft overhead or a ship out in the distance who would see their fire.
IRONICALLY, when the sound of some other lifeform is near, their fire is completely out meaning chance for rescue at that point was impossible. This is truly depressing.
Furthermore, in chapter 6, the airflight above meant a pilot ejected himself and his dead body was stuck in the trees. This pilot could have been rescue, but ironically, this pilot serves to feel like the beast. Samaneric find him and believe him to be the beast bringing the report to everyone else. (Later in chapter 7, this is confirmed by bigger boys as "the Beast" although it actually isn't)
In addition to the irony that the fire on the top of the mountain, meant for rescue by a plane is out and the mountain is the receptor of a dead parachutist, there is the sad irony of the parchutist's body being
a sign [that]came down from the world of grown-ups, though at the time there was no child awake to read it.
Rather than interpreting the parachutist as representative of the evil and savagery in man that wages war, Samneric interpret him as the beast when they climb the mountain to stoke the signal fire that has burned out. Only Simon, who when he thinks of the beast, has rise
before his inward sight the picture of a human at once heroic and sick.
An adult has come from the civilized world, but he is a sign Ralph has not sought. It represents man's intrinsic illness as he is a man from battle, and, ironically, Simon is the only one who can recognize it.
At the end of Chapter 5, Ralph and Piggy are discussing the meeting that had ended in chaos. Piggy was encouraging Ralph to call the kids who had run off back, but Ralph refuses saying that they would not obey the conch. Ralph wants to give up being chief. Desperately, he cries,
"If only they could get a message to us."
This desire is fufilled in Chapter 6, when the "beast from air" lands on the island--the dead man with the parachute. Irony is created in two ways. Sam and Eric spot the parachute, and without investigating assume it is the beastie that the littluns have been discussing. As fear grows on the island, the boys become more savage. The long-for sign from the adult world, does not bring the peace and stability that Ralph longed for. Instead it produced more chaos and the eventual splintering of the group with Jack taking control of the vast majority of the boys.
But irony is also created by the fact that the sign from the adult world is a violent sign. The dead man was killed by the war that the adults are engaging in. Golding reminds his readers that the adults are faring no better in their world than the children are in theirs. The adults cannot bring peace to the island; they are engaged in a world war themselves.