01/31/2010. Remarks by Johan Visschedijk: "Designed by Roy Chadwick the 621 was of all-metal structure covered with fabric. It was chosen to succeed the Avro 504N in the RAF's Flying Training Schools in 1932, after comparative trials with various elementary trainers. It thus carried on the tradition of Avro trainers in the RAF, which started with the classic Avro 504K and 504J biplanes of the First World War. A trial batch of 26 aircraft was produced, designated 621 Trainer, which were to Specification 3/30, fitted with five-cylinder Siddeley Mongoose engines, these Trainers entered service in 1930.
The large-scale production version was designated 621 Tutor, these were to Specification 18/31, and fitted with Siddeley Lynx seven-cylinder engines. The Tutor also differed from the earlier Trainer in having a Townend ring on the engine (though some early production Tutors were uncowled), low-pressure tires and a redesigned fin and rudder. About 400 Tutors were built for the RAF, this included fourteen Avro 646 Sea Tutors, which were twin-float seaplane versions to Specification 26/32."