What is the primary function of the inner marker (IM) in ILS?

The primary function of the inner marker (IM) in the Instrument Landing System (ILS) is to signal the decision height point. This point is critical for pilots as it indicates the altitude at which they must make a decision to either continue the approach and land or execute a go-around if visual contact with the runway has not been established.

When a pilot crosses the inner marker, it indicates that the aircraft is at a specific point in the approach sequence, usually just before reaching the runway threshold. The decision height serves as a safety measure, allowing pilots to assess whether they can see the runway and safely land. If visibility is insufficient at that moment, the pilot is trained to initiate a go-around, ensuring safety is prioritized.

Considering the roles of the other choices, while the final approach fix is important in the approach sequence, it is actually indicated by other markers in the ILS system, namely the outer and middle markers. The inner marker does not help in determining altitude to descend in the same manner, as it serves primarily to indicate the decision height. Finally, visual approach slope indication is not a function of the inner marker but is typically managed by visual aids such as the visual approach slope indicator (VASI) lights. Therefore, the inner marker’s