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Substantial damage to an aircraft is defined as damage that adversely affects the structural strength, performance, or flight characteristics of the airplane. This includes any form of structural damage, such as significant deformation of the airframe, fuselage, or critical components that could compromise the safety and airworthiness of the aircraft.

Structural damage that affects flight characteristics directly impacts how the aircraft behaves in the air, which can lead to a loss of control or an inability to safely maneuver. When the integrity of key structural components is compromised, it raises serious concerns about the overall safety of the flight.

While a gear up landing can lead to damage, it might not always be classified as substantial if the structural integrity remains intact. Similarly, a single engine failure, although serious, may not necessarily result in substantial damage, especially if the aircraft is designed to operate safely on one engine. Counting fuel levels during flight is a routine operational task that does not relate to physical damage to the aircraft at all. Thus, when considering which situation would constitute substantial damage, it is clear that structural damage affecting flight characteristics is the most critical to the overall safety and functionality of the aircraft.