A missing plane turns out to be the tip of a smuggling mystery. The team fears that Bertie has crashed, and end up finding the mystery revolves around diamonds.

goodreads summary

Biggles is called in to investigate the disappearance of a plane from Kingsmead Flying Club. Taffy Welsh, the club's chief pilot was giving a half hour 'joy ride' to a paying member of the public in a Piper Cub aircraft and has not been seen since. Biggles, Ginger and Bertie set out on the trail (Algy being in India) and after a week or so, their diligent enquiries give them a lead.