Final answer:
The document approving a design change in the type design of an aircraft by Transport Canada is the Supplemental Type Certificate (STC).
Step-by-step explanation:
The document issued by Transport Canada that approves a change in the type design of an aircraft is known as a Supplemental Type Certificate (STC). The STC is significant because it certifies that the modification proposed (which could involve structural, powerplant, or other changes) complies with the safety and regulatory standards set by Transport Canada. It is a required document for any modification that alters the aircraft's original type certificate.