Plain English Meaning
An FAA safety notice — not a rule, but a strong recommendation — that explains why propellers can overspeed in piston-engine aircraft and what pilots and mechanics can do about it.
Full Definition
A Special Airworthiness Information Bulletin issued by the FAA's Small Airplane Directorate (the 'CE' prefix), numbered 10-21, addressing propeller overspeed events in piston-engine aircraft. The bulletin is non-regulatory and provides safety information, recommendations, and awareness to owners, operators, and mechanics regarding causes of propeller overspeed and steps to prevent or respond to it.
Context Anchor
Seen in FAA training material and safety references when discussing propeller overspeed in piston-engine aircraft.
Derivation
The 'CE' identifies the FAA's Small Airplane Directorate (formerly headquartered in Kansas City, the 'Central' region), '10' is the year of issue (2010), and '21' is the sequence number for that year. So 'CE-10-21' means: Small Airplane Directorate, 21st bulletin of 2010.