How many sources of altimeter setting information are required in most terminal facilities?

• A. 3
• B. 4
• C. 5
• D. 2

Answer: (D)

Two sources of altimeter setting information are provided in most terminal facilities to ensure reliable, up-to-date data for safe altitude control. The primary source comes from the weather observation network (such as METAR/ASOS/ATIS), which gives the current altimeter setting for the area. The secondary source is the field’s own altimeter setting instrument in the control tower (or another independent local source at the field). Having these two sources lets controllers cross-check and use a backup if one data stream is unavailable or questionable, reducing the risk of altitude errors. In practice, both values are available for reference, and the more current or conservative value is used as needed.