Definition
A preflight or operational checklist produced by someone other than the aircraft manufacturer — for example, a flight school, training organization, instructor, or commercial publisher — used in place of or alongside the manufacturer's checklist. Because it is not produced by the manufacturer, it is not automatically authoritative and must be cross-checked against the Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM) to confirm it is accurate, current, and appropriate for the specific aircraft.
Plain English
A checklist for the airplane that was made by someone other than the company that built the airplane. It can be useful, but the pilot still has to make sure it matches what the airplane's official handbook says.
Context Anchor
Seen during preflight planning and visual preflight inspection, when a pilot chooses which checklist to use before starting or flying the airplane.
Derivation
Third party' means someone other than the two main parties involved — here, the aircraft manufacturer and the pilot/operator. So a third party checklist is one written by an outside source rather than by the company that built the aircraft.
Why Pilots Care
Reliance on an incomplete third-party checklist can result in overlooked preflight items, increasing the risk of mechanical failure or loss of control after takeoff.
Intuition Check
Third party does not mean automatically approved or automatically wrong. It means the checklist came from an outside source, so the pilot must verify it before trusting it.
Example Sentence 1
Before using the laminated third party checklist his school provided, the student compared it to the POH and noted two items that had been left out.
Example Sentence 2
The student pilot learned to avoid third-party checklists after discovering one lacked the fuel strainer drain procedure.