How long must the engine be run at idle during warm-up?

The correct approach to engine warm-up emphasizes the importance of allowing the engine oil to circulate and reach normal operating pressure before advancing the thrust lever. By running the engine at idle until the oil pressure reaches normal, you ensure that the engine components are adequately lubricated and that the system is functioning correctly. This is crucial for preventing engine wear and potential damage that could arise from operating with cold oil, which may not provide sufficient protection.

In this context, the other options specify arbitrary time frames that do not take into account the actual condition of the oil pressure. Relying solely on a set number of seconds or minutes may not guarantee that the engine is adequately warmed up, which is why waiting for normal oil pressure is the most appropriate and recommended practice. This method aligns with the operational guidelines and maintenance best practices for ensuring optimal engine performance and safety.