When are two periods used in flight plan routes in FDIO messages?

• A. Between like elements
• B. Between unlike elements
• C. At the start
• D. Within every segment

Answer: (A)

Two periods act as a delimiter that clearly separates successive items of the same type in a flight plan route. When you list multiple items in a row that are all the same kind—such as consecutive fixes or consecutive airways—the route uses two periods between them to show that each item is a new element of the same category. If the route moves from one type of element to another (for example, a fix to an airway), a single period marks that boundary. This prevents ambiguity in how the route is parsed and interpreted. So the double-period rule is specifically about boundaries between like-type elements, which is why that option best fits the behavior described.