Exact date is conjecture. Adding 20 years for a generation to pass after Yetaxa's death in 1430 arrives at this date. Leonardo Da Vinci is born in the small Tuscan town of Vinci, near Florence, Italy. The Meddling Monk discusses the possibility of powered flight with Leonardo Da Vinci. The Time Meddler.
Exact date is conjecture.
This is the year that Leonardo is first believed to have drawn plans for a flying machine. In the Dukedom of San Martino, Italy, court astrologer Hieronymous predicts the Duke's imminent death. The 4th Doctor and Sarah are brought to San Martino, Italy by energy from the Mandragora Helix. Acting through the Cult of Demnos, the Helix attempts to gain control of the Earth by destroying several prominent thinkers of this time, thus plunging the world into a new Dark Age. This date is given only in the novelization, and is used by most other chronologies.