What scenario triggers the need for an Application and Release Air Brake Test (Class 3)?

The correct answer is the scenario involving changing the lead engine, which triggers the need for an Application and Release Air Brake Test (Class 3). This requirement is in place because when the lead engine is changed, it can impact the air brake systems, necessitating a test to ensure that they function correctly both in terms of application and release of the brakes. This test is crucial for safety, as it verifies the operational integrity of the air brake system after any modifications that could affect performance.

In contrast, other scenarios such as those involving initial terminal departures or sequence numbers do not inherently affect the air brake systems to the same extent. Conducting a safety inspection is critical but does not specifically dictate the need for a Class 3 test unless there are changes that impact the brake functionality. Therefore, changing the lead engine stands out as a direct trigger for this specific brake testing requirement.