Final answer:
On an ILS approach, full scale deflection on the CDI indicates that the aircraft is 2.5 nautical miles off course laterally.
Step-by-step explanation:
In an ILS approach, when you are 10 miles from the end of the runway, full scale deflection on the course deviation indicator (CDI), also known as the localizer, indicates that the aircraft is 2.5 nautical miles off course laterally. The CDI is the instrument that provides information about the lateral position of the aircraft in relation to the ILS localizer course. The full scale deflection represents the maximum deviation from the desired course. For example, if the aircraft is fully deflected to the left on the CDI, it means that it is 2.5 nautical miles to the left of the desired course. If it is fully deflected to the right, it means that it is 2.5 nautical miles to the right of the desired course.