Within what time period must the nearest NTSB field office be notified of an aircraft accident resulting in substantial damage?

The nearest National Transportation Safety Board (NTSB) field office must be notified immediately after an aircraft accident resulting in substantial damage. This requirement emphasizes the importance of prompt reporting, as it allows for quick investigation and assessment of the accident scene. Timely notification ensures that necessary resources can be deployed effectively, evidence can be preserved, and critical information can be gathered to enhance aviation safety and prevent future accidents.

While other options suggest longer timeframes for notification, they do not align with the NTSB's emphasis on immediacy in incidents categorized as accidents that involve significant damage. The NTSB prioritizes the swift collection of facts and data, which can be instrumental in understanding the causes of accidents and shaping regulatory responses or improvements in safety measures.