The ISFD provides third attitude data to the two autopilots so that in case one attitude data is supplying corrupted or no data the system can still rely on two attitude sources. Also, during normal autopilot operation with the failure of one ADIRU and the old school analog sby att. ind. it is not possible to reangage the autopilot, which is possible having the ISFD, since it is providing digital att. data to the autopilot in use.