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4. Numeric v. alphanumeric call signs4.1. Many airlines continue to use their IATA commercial flight numbers as call sign suffixes. However, becausethey tend to be allocated in batches of sequential and very similar numbers, call sign confusion occurs.4.2. Several airlines have switched to alphanumeric call signs reasonably successfully in recent years. However,if every operator adopts alphanumeric call signs, the limited choices available within the maximum of 4 elementsallowed within a call sign suffix means that call sign confusion, similar to the existing numeric system,is likely to result.4.3. Before changing to an effective all-alphanumeric call sign system, which involves a significant amount ofwork, it is recommended that operators review their existing numeric call sign system to deconflict any similarnumeric call signs. Where there is no solution to those call signs that have a potential for numeric confusion,alphanumeric call signs can be adopted.
So far the only North American airline I've noticed that uses alphanumeric callsigns on a regular basis is SkyWest.http://flightaware.com/live/flight/SKW82C/history/20091208/1212Z/KPHL/KMKE