It doesn't. It prevents the last digit from being cut off =unawares=. It
acts as a checksum of sorts. If all runways are two digits, then any
transmission missing a digit is suspect. If some are one and some are
two, then there are some cases where an error would not be noted; this is
especially the case with runway 2-20, both of which exist on most airports
for which one exists.


Wouldn't the gap in the transmission do that?

Maybe. But not certainly. People don't always speak with the same
cadence, so time is a poor checksum.

Jose
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