Five Go to Billycock Hill (The Famous Five #16) by Enid Blyton

Hurrah! It’s holiday time, and the Famous Five are spending it at Billycock Hill. Most exciting of all, they’ve made a new friend – a real pilot! But now he has disappeared, with top secret equipment …The Five are puzzled, and concerned, too. Could their new friend by a spy – a traitor!. Whatever his motives are, the gang feel duty-bound to investigate