What data does an INS provide when its navigation function is inoperative and the control switch is set to ATT?

When an Inertial Navigation System (INS) operates in ATT (attitude) mode, it primarily provides information related to the aircraft's orientation and attitude in space. This mode is typically used when the navigation functions of the INS are not available or inoperative.

In ATT mode, the INS can track the aircraft's position in terms of its pitch, roll, and yaw, which collectively represent the aircraft's attitude. This means that the system can tell the pilot how the aircraft is oriented relative to the horizon and its flight path.

While heading information is crucial for navigation, the system's role when in the ATT mode does not include updating heading data, as this requires a functioning navigation source. Instead, the focus is on the relationship of the aircraft to the earth’s surface through its attitude, without real-time positional updates. Thus, the correct interpretation is that the INS provides attitude and heading data, making options that include ground speed irrelevant when the navigation function is inoperative.

In summary, the correct answer highlights the operational capacity of the INS during non-navigational states, focusing on critical data like attitude and, importantly, the associated heading, which is derived directly from the attitude information.