What qualifies as substantial damage according to NTSB regulations?

Substantial damage, as defined by NTSB regulations, specifically involves damage that affects the structural integrity or operational characteristics of an aircraft. This includes any alterations that could compromise the safety, performance, or functionality of the aircraft in flight. Damage that affects structural strength refers to any changes that could weaken the aircraft frame or components, while impacts on flight characteristics could involve issues with control surfaces or aerodynamics that could make the aircraft difficult or unsafe to operate.

The other options do not align with the NTSB definition of substantial damage. For instance, damage that merely requires repairs does not necessarily imply that the structural integrity is compromised. Minor cosmetic damage lacks significance in terms of operational safety and structural concerns, and while engine malfunctions might be critical, they don't inherently indicate substantial damage either, as they may not affect the overall structure or flight characteristics directly. Thus, the definition in this context focuses on more serious impacts that compromise safety during operation.