generator checks the First Officer's, the First Officer's checks the Captain's. Whichever symbol generator detects a fault, puts up a warning on its own display. The external monitoring channel also checks sensor inputs (to the symbol generator) for reasonableness. A spurious input, such as a radio height greater than the radio altimeter's maximum, results in a warning. Human factors Clutter At various stages of a flight, a pilot needs different combinations of data. Ideally, the avionics only show the data in use—but an electromechanical instrument must be in view all the time. To improve display clarity, ADIs and HSIs use intricate mechanisms to remove superfluous indications temporarily—e.g., removing the glide slope scale when the pilot doesn't need it. Under normal conditions, an EFIS might not display some indications, e.g., engine vibration. Only when some parameter exceeds its limits does the system display the reading. In similar fashion, EFIS is programmed to show the glideslope scale and pointer only during an ILS approach. In the case of an input failure, an electromechanical instrument adds yet another indicator—typically, a bar drops across the erroneous data. EFIS, on the other hand, removes invalid data from the display and substitutes an appropriate warning. A de-clutter mode activates automatically when circumstances require the pilot's attention for a specific item. For example, if the aircraft pitches up or down beyond a specified limit—usually 30 to 60 degrees—the attitude indicator de-clutters other items from sight until the pilot brings the pitch to an acceptable level. This helps the pilot focus on the most important tasks. Color Traditional instruments have long used color, but lack the ability to change a color to indicate some change in condition. The electronic display technology of EFIS has no such restriction and uses color widely. For example, as an aircraft approaches the glide slope, a blue caption can indicate glide slope is armed, and capture might change the color to green. Typical EFIS systems color code the navigation needles to reflect the type of navigation. Green needles indicate ground-based navigation, such as VORs, Localizers and ILS systems. Magenta needles indicate GPS navigation. Advantages EFIS provides versatility by avoiding some physical limitations of traditional instruments. A pilot can switch the same display that shows a course deviation indicator to show the planned track provided by an area navigation or flight management system. Pilots can choose to superimpose the weather radar picture on the