Products such as ASTO 390 are commonly used in the APUs of four-engine intercontinental aircraft, but are now being considered for use in twin-engine aircraft.

If a main engine fails, you need to have confidence that your APU will start reliably. An aircraft’s extended range twin operations (ETOPS) certification^ is partly based on the reliability of back-up systems such as the APU. In the event of engine failure, the APU must make up for any reduction in electrical power.

APUs are typically shut down when cruising. During this time, the oil can be cooled to less than –40°C. At this temperature, the viscosity of standard 5-cSt oil increases to about 10,000 cSt. This causes a large viscous drag that may result in a hung start.

ASTO 390, with its much smaller viscosity increase of typically 2,000 cSt at –40°C, can help to improve cold-soak start-up reliability and thus maximise ETOPS times, as it has significantly lower viscosity at cold-soak start-up temperatures (–40°C) than a standard 5-cSt oil.

^ The time a fully loaded twin-engined aircraft requires for single-engine flight to the nearest suitable airport.