The challenging production schedule meant that flight tests of the DH.88 began just six weeks prior to the start of the race. Hamilton-Standard hydromatic variable-pitch propellers were initially fitted. During testing, the propeller blade roots were found to interfere unacceptably with the airflow into the engine. Instead, a French two-position pneumatically actuated Ratier type was substituted. Its blades were manually set to fine pitch before takeoff using a bicycle pump, and in flight they were repositioned automatically to coarse (high-speed) pitch via a pressure sensor. A drawback was that the propellers could not be reset to fine pitch except on the ground. Other changes included the installation of a large landing light fitted in the nose and a revised, higher profile to the cockpit to give the pilot marginally improved visibility.