It was also discovered that when the landing gear was deployed, the large door in front of each main wheel caused turbulence that resulted in severe vibrations of the tail section. The doors were reduced in size, but the problem persisted. Eventually, the doors were removed altogether. During the flight test program, the engine nacelles were lengthened to reduce drag. The flight test program, the airworthiness trials, and the acceptance flights were completed over the course of four months, encompassing 62 flights that totaled 37 hours in the air. On December 18, 1933, the Fokker F.XX made its KLM debut on a special Christmas mail flight to the East Indies. The objective was to fly as fast as possible to the Dutch colonies in competition with another aircraft, the Pander S-4 Postjager, to inaugurate a special mail service.