When is a second alternate required for flight dispatch under Part 121?

A second alternate is required for flight dispatch under Part 121 when the weather conditions for both the destination and the first alternate are marginal. Marginal weather conditions imply that the visibility or cloud cover at these locations may not meet the minimum standards for a safe approach and landing, increasing the necessity for an additional backup plan. By having a second alternate, flight dispatch ensures that there is an additional option available if the first alternate also has unsuitable conditions, thus providing enhanced safety and operational reliability.

In contrast, favorable destination conditions do not necessitate a second alternate, as the primary destination is expected to be reachable and safe. The requirement for a second alternate is not automatically related to flight distance, so the length of the flight being over 1000 nautical miles is not a grounds for requiring an additional alternate. An engine failure during takeoff, while serious, impacts in-flight safety and operational procedures but does not set criteria for dispatch planning concerning alternates prior to departure.