 In the rear of the aircraft were three RAF crewmen and a civilian technical advisor from the Avro company.Due to the heavy rain and visibility reduced to , "XA897" was on a ground controlled approach (GCA) and was informed by Heathrow's air traffic controller that they were above the glide slope (GS) and needed to lose altitude.However, the crew reduced their height too much, with their air speed close to the minimum drag point for a gear down configuration.