When should the APU be restarted if the RPM is under 30%?

The APU (Auxiliary Power Unit) should be restarted after 30 seconds if the RPM is under 30%. This timeframe allows for an adequate cooling and stabilization period, ensuring that the APU has the necessary conditions for a successful restart. Restarting too soon may lead to increased wear on the engine components due to inadequate cooling or other issues that could impact the APU's performance and reliability. By waiting for 30 seconds, operators can help minimize potential damage and ensure that the APU has an increased chance of restarting successfully.