Aircraft Call Signs. Improper or abbreviated call signs can result in a pilot's executing a
clearance intended for another aircraft. As an example, assume that a controller issues an
approach clearance to an aircraft at the bottom of a holding stack and an aircraft with a similar
call sign (at the top of the stack) acknowledges the clearance with the last two or three numbers
of his call sign. If the aircraft at the bottom of the stack did not hear the clearance and failed to
intervene, flight safety would be affected. This kind of "human" error can strike swiftly and is
extremely difficult to rectify. You must be certain that aircraft identification is complete and
clear before taking action on an ATC clearance. FAA personnel will not abbreviate call signs of
aircraft having authorized call signs. FAA may initiate abbreviated call signs of other aircraft by
using a prefix and the last three digits or letters of the aircraft identification after
communications are established. Controllers, when aware of similar or identical call signs, will
take action to minimize errors by emphasizing certain numbers or letters, by repeating the entire
call sign, or by repeating the prefix.
The 24-hour clock system is used in radio-telephone transmissions. The hour is indicated by the
first two figures and the minutes by the last two figures. FAA uses Greenwich Mean Time
(GMT), Coordinated Universal Time (UTC) or Zulu ("Z") for all operations.
Example: 0000 ZERO ZERO ZERO ZERO
Example: 0920 ZERO NINER TWO ZERO
Time may be stated in minutes only (two figures) in radio-telephone communications when no
misunderstanding is likely to occur. When two figures are used, the current hour (within 60
minutes) is understood to be the time being referenced.
Example: "Mobile Radio, [Call sign], low-level, entering Victor Romeo One Zero
Two Zero at Point Alpha at Four Three, exit Point Foxtrot."
"Four Three" is understood to mean 43 minutes past the current hour. If the call was made at
0935 local time, the aircraft will enter the low-level route at 0943 local.
Current time in use at a station is stated in the nearest quarter minute in order that pilots may use
this information for time checks or to set their clocks. Fractions of quarter minutes less than
eight seconds are stated as the preceding quarter minute; fractions of quarter minutes of eight
seconds or more are stated as the succeeding quarter minute.
Example: 0929:05 TIME: ZERO NINER TWO NINER STRAIGHT UP 0929:10
TIME: ZERO NINER TWO NINER AND ONE QUARTER
Digits indicating hundreds and thousands in round numbers, such as ceiling heights and upper
wind levels up to 9999, shall be spoken as follows: