A nice, quick little adventure story. Half the chapters are from the perspective of the girl's grandfather, a colonel and pilot who flew in WWII and Korea -- that's the kind of fun detail you can have in books of this era -- raising the initial alarm on the missing plane and volunteering to fly in the searches himself. While unique, that part isn't as interesting as the kids scrambling their way down unfamiliar mountainside (in the dark! they have a flashlight, but even so, everyone here should have died at least twice), tumbling down ravines and getting lost in abandoned mines and such.
Speaking of random dangers: this is the second '70s teen book I've seen that includes a pack of aggressive wild (stray) dogs. Was this really such a concern in wilderness areas back in the day??