It began as a training flight and nearly became a very strange accident report.
On 29 September 1940, two Avro Anson trainers of No. 2 Service Flying Training School, Royal Australian Air Force, took off from Forest Hill near Wagga Wagga, New South Wales. Over the town of Brocklesby, at about 1,000 feet, they collided. The lower aircraft's turret wedged into the port wing root of the upper one, and its fin and rudder jammed against the upper plane's tailplane. They did not come apart. They stayed locked, one riding piggyback on the other.
The collision knocked out both engines of the upper Anson. The engines of the plane underneath kept turning at full power. So the two aircraft, jammed together, began to circle slowly. The pilot of the upper plane, 22-year-old Leading Aircraftman Leonard Graham Fuller, described the "freak combination" as "lumping along like a brick." He found he could still steer it with his ailerons and flaps.
The two navigators bailed out, and then the lower plane's pilot, whose back had been injured when the other aircraft's blades cut through his fuselage. Fuller stayed with the tangled pair. He flew on for about 8 kilometres, then made a belly landing in a paddock.
All four crewmen survived. The upper Anson was repaired and returned to flight service. The lower one was used as an instructional airframe.
Two aircraft became one, briefly. The one on top did the flying.
Sources: - Wikipedia, "1940 Brocklesby mid-air collision": https://en.wikipedia.org/wiki/1940_Brocklesby_mid-air_collision