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The Lockheed McDonnell 3-12 is a fictional aircraft model of which G-ERTI is the only known example.
The fictional 'Lockheed McDonnell' make is a combination of real aircraft manufactureres, 'Lockheed Martin' and 'McDonnell Douglas' (with the unused names from these makes being 'Martin' and 'Douglas').
The plane used most often to represent G-ERTI is a McDonnell Douglas MD-80, despite this plane being much bigger than G-ERTI's 16-seat capacity. She is more likely to resemble a McDonnell 220 in size, though that aircraft has four engines to the 3-12's two.