In The Case of the Poisoned Crops, Felix Harley was a South African pilot who flew for the research station at Klookerstein, experimenting with methods for destroying locust plagues. Biggles knew Harley from the time he served with the R.A.F. during the war and declared him to be a "good chap" and that there was "nothing phoney" about him.
At the time of the story, someone had been destroying crops in Africa using a spray made from a stolen insecticide designed to destroy locust plagues. Coincidentally, a weak radio signal had been sent from an aircraft asking for someone to go to the base at Kllokerstein. When the base was visited by the British authorities, Harley was found to be missing. General de Groot, who was in charge of the base, told the visitors that Harley had absconded with the staff payroll, taking a De Havilland Gipsy Moth with him.
Biggles set out to investigate the crop poisoning and also the disappearance of Harley. He sent Algy and Bertie in the Auster to look for Harley's aircraft. Biggles reasoned that Harley would have flown east from Klookerstein towards the nearest settlements, and that if he had been wounded, he couldn't have flown very far.
Biggles was correct in his assumptions and Algy and Bertie soon found the Gipsy Moth with Harley's body in it. He had been murdered with at least two bullets. Harley had obviously discovered that de Groot was up to no good with the stolen insecticide and had tried to escape to report but had been shot in the process.