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 the destination and the first alternate are marginal. This requirement is in place to ensure that there is an additional option for safe landing in case the first alternate becomes unsuitable due to unexpected weather changes. If the destination or the first alternate has weather conditions that do not meet certain minimums, having a second alternate provides added safety and reliability for the flight's operational plan.

In this context, marginal weather conditions could refer to scenarios where the visibility or cloud cover is close to meeting the regulatory minimums, leaving little room for error. A second alternate allows for continued adherence to safety standards and operational integrity. When destination and first alternate conditions are uncertain or not optimally safe, implementing a second alternate becomes critical to avoid potential issues upon arrival.

The other options such as favorable destination conditions, flight distance exceeding 1000 nautical miles, or engine failure during takeoff do not inherently necessitate a second alternate under Part 121 regulations. Those situations do not reflect the need for additional contingency planning centered around weather-related landing capabilities.